PVWMA Salt and Nutrient Management Plan (SNMP) Development
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TABLE OF CONTENTS
1.0
Introduction ..................................................................................................................................................... 3
1.1
SNMP development Objectives ...................................................................................................................... 3
2.0
Hydrogeologic Setting ....................................................................................................................................4
3.0
Groundwater Budget ...................................................................................................................................... 7
4.0
Salt and Nutrient Groundwater Conditions ................................................................................................. 10
4.1
Constituents of Concern and Relevant Sources .......................................................................................... 10
3.1.1
Total Dissolved Solids (TDS) ................................................................................................................. 10
3.1.2
Chloride ................................................................................................................................................. 12
3.1.3
Nitrate – NO3 ........................................................................................................................................ 12
4.2
Existing Conditions.................................................................................................................................... 13
3.1.4
Groundwater ......................................................................................................................................... 13
3.1.5
Surface Water ....................................................................................................................................... 24
3.1.6
Delivered Water Quality .......................................................................................................................26
5.0
Loading Risk Analysis .................................................................................................................................... 31
5.1
Data and Methods ......................................................................................................................................... 31
4.1.1
Soil Water Holding Capacity ................................................................................................................. 32
4.1.2
Agricultural Land Use ...........................................................................................................................34
4.1.3
Irrigation Intensity ................................................................................................................................34
4.1.4
Crop Fertilizer Use ................................................................................................................................ 37
4.1.5
Sewer Lines and Septic Systems ......................................................................................................... 40
4.1.6
Groundwater Recharge Locations .......................................................................................................43
5.2
Salt Loading Risk Analysis .........................................................................................................................43
4.1.7
Seawater Intrusion ...............................................................................................................................43
4.1.8
Irrigation Salt Loading ......................................................................................................................... 46
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4.1.9
5.3
Surface Water Infiltration Salt Loading .............................................................................................. 49
Nutrient Loading Risk ............................................................................................................................... 51
4.1.10
Agricultural Fertilizer ............................................................................................................................ 51
4.1.11
Sewer Exfiltration ................................................................................................................................. 53
4.1.12
Septic Exfiltration or Failures ............................................................................................................... 53
4.1.13
Surface Water Recharge Nitrate Loading............................................................................................ 57
4.1.14
Subordinate Nitrate Sources ............................................................................................................... 60
4.1.15
Nitrate Loading Comparisons .............................................................................................................. 61
5.4
6.0
Key Loading Analysis Findings ................................................................................................................. 64
References .................................................................................................................................................... 66
6.1
Literature Cited ............................................................................................................................................. 66
6.2
Datasets Used ...........................................................................................................................................67
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1.0 INTRODUCTION
The California State Water Resources Control Board (SWRCB) adopted a Recycled Water Policy in 2009 that
requires every groundwater basin or sub-basin in California to develop Salt and Nutrient Management Plans
(SNMPs) to manage salts, nutrients, and other significant chemical compounds. SNMPs are intended to help
streamline permitting of new recycled water projects while ensuring attainment of water quality objectives
and protection of beneficial uses. As part of the Integrated Regional Water Management (IRWM) Plan, SNMPs
will be completed for the entire Pajaro River Watershed, which encompasses three counties and the
jurisdictions of three water districts: Santa Clara Valley Water District (SCVWD), San Benito County Water
District (SBCWD), and Pajaro Valley Water Management Agency (PVWMA). SCVWD is developing the SNMP for
the Llagas sub-basin, SBCWD is developing the SNMP for the Bolsa, Hollister, San Juan Bautista Area, and Tres
Pinos sub-basins, and PVWMA is developing the SNMP for the Pajaro Valley Groundwater Basin. Stakeholder
involvement and input is a critical component of each SNMP.
In order to facilitate stakeholder input, a series of draft documents will be provided for review and feedback at
critical milestones, allowing for an efficient and incremental development approach to the final PVWMA SNMP.
This initial draft contains 5 preliminary sections of the SNMP; including the hydrogeologic setting, an overview
of current groundwater and surface water quality with respect to salts and nutrients, recommended approach
and draft products supporting groundwater loading analysis. These sections will be revised based on
stakeholder comments and will be resubmitted with the next series of draft SNMP sections and supporting
deliverables. Key tasks and current schedule for completion of PVWMA SNMP are provided in Table 1.1.
Table 1.1: PVWMA SNMP schedule for development. Additional stakeholder meetings may be
held as necessary.
Primary Tasks
Task 1. Stakeholder Outreach
Schedule
Stakeholder Meetings at critical milestones
Task 2. Conceptual Model
Draft included
Task 3. Salt and Nutrient Loading Analysis
Draft included
Task 4. Assimilative Capacity Estimate
Draft Fall 2013
Task 5. Develop or update objectives
Task 6. Develop or update priority
program/projects
Task 7. SNMP Monitoring Plan
Draft Fall 2013
Draft Spring 2014
Task 8. Conduct anti degradation analysis
Draft Spring 2014
Task 9. Complete SNMP
1.1
Draft Fall 2013
Summer 2014
SNMP DEVELOPMENT OBJECTIVES
The development of the PVWMA SNMP has a number of objectives:
1.
Maintain consistency with, and leverage complementary water management plans completed either
by PVWMA or within the Pajaro River Watershed to the extent practical, including, but not limited to,
the 2012 PVWMA Basin Management Plan Update and SNMPs under development by SBCWD and
SCVWD on the upper portions of the Pajaro River Watershed.
2.
Consider and incorporate stakeholder input into the SNMP.
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3.
Contents and recommendations that is consistent with regulatory requirements to protect
groundwater beneficial uses: Agricultural Water Supply (AGR), Municipal and Domestic Supply (MUN)
and Industrial Service Supply (IND).
4.
Acceptance by the Central Coast Regional Water Quality Control Board (CCRWQB).
2.0 HYDROGEOLOGIC SETTING
The PVWMA SNMP focuses on the approximately 120 square miles within PVWMA’s service area (Figure 2.1),
located at the coastal boundary of Pajaro River Watershed. The San Andreas Fault runs through the eastern
portion of the PVWMA service area and is a hydrogeologic boundary within the aquifer system. As a result,
groundwater conditions in the relatively small area east of the San Andreas Fault are not included in the SNMP
analysis of the Pajaro Valley Groundwater Basin (PVGB). Flows of the Pajaro River are gauged by the United
States Geological Survey (USGS) at Chittenden Gap, the eastern boundary of the Lower Pajaro River
Watershed. PVWMA collects water quality samples at three sites along the Pajaro River and at over 25 sites
along its tributaries.
PVWMA recently completed an update to their Basin Management Plan in late 2012 that included a detailed
summary of the State of the PVGB (Chapter 2, PVWMA 2012). Rather than drafting similar content, relevant
sections of Chapter 2 (PVWMA 2012) are inserted below to provide an overview of the current hydrogeologic
setting, precipitation patterns, current and historic land use, local water supplies, and water usage. PVWMA
continues to compile and manage a breadth of diverse physical and chemical datasets that will be utilized
extensively to complete this SNMP.
Supplemental to the information extracted from the Basin Management Plan (PVWMA 2012), a historical
precipitation frequency analysis was conducted to define water year types based on water year precipitation
totals within the PVWMA boundary (Figure 2.2). This simple and defensible classification of water year type
provides climatic context to expected year to year variability of many key processes potentially influencing
annual constituent loading to the groundwater basin such as groundwater recharge, groundwater extractions,
irrigation volumes, groundwater quality, seawater intrusion migration, etc.
The following 17 pages are a direct insert of the existing draft section of the PVWMA Basin Management Plan
Update (PVWMA 2012). Once the PVWMA Basin Management Plan is finalized in 2013, the content of the relevant
sections will be integrated directly into the SNMP.
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3.0 GROUNDWATER BUDGET
PVWMA contracted with the USGS in 2005 to develop the Pajaro Valley Hydrologic Model (PVHM), a robust
hydrologic model of the Pajaro Valley Groundwater Basin (PVGB). The PVHM incorporates past and current
land use and groundwater pumping data (available as a result of PVWMA’s programs established in the mid1990s) to use as a tool to estimate the water budget of the PVGB and to compare the impacts of various
proposed water management scenarios during the preparation of the 2012 BMP Update.
The PVHM was designed to reproduce natural and human components of the hydrologic system and related
climatic factors, so that the components of the Basin Management Plan are adequately represented with
relatively reasonable accuracy. In developing the PVHM, the conceptual model and the hydrogeologic
framework of the Pajaro Valley were both refined from what had been used in previous modeling efforts. The
conceptual model identified inflows and outflows that include the movement and use of water from natural
and human components. The groundwater flow system is characterized by a layered geologic sedimentary
system that results in vertical hydraulic gradients resulting from the combined effects of the application of
irrigation water at the land surface plus groundwater pumpage. Agricultural pumpage is a major component
to simulated outflow that is often poorly recorded; therefore, a coupled farm-process model is used to
estimate historical pumpage for water-balance subregions as well as the delivery of surface water to and from
the Harkins Slough Aquifer-Storage-and-Recovery System (HS-ASR) and related Coastal Distribution System
(CDS) since 2002. The new integrated hydrologic model includes new water-balance subregions, delineation of
natural, municipal, and agricultural land use, streamflow networks, and groundwater flow systems. Redefining
the hydrogeologic framework (including the internal architecture of the deposits) and incorporation of these
units into the simulation of the regional groundwater flow system indicate the importance of the basal
confining units in the alluvial deposits and between the upper and lower Aromas sands with respect to regional
groundwater flow, locations of recharge, and the effects of development.
Table 3.1 shows a preliminary water budget (inflows and outflows) within the active model grid (Figure 3.1)
comparing average annual groundwater flux estimates for a recent five year (2005-2009) period, and
representative recent wet (2006) and dry (2008) water years. The PVHM output suggests the primary inputs to
the PVGB are direct infiltration and recharge occurring in surface water streams, with a relatively lower
contribution via stream recharge during dry years. Groundwater extraction is the greatest outflow from the
PVGB, with minimal volume differences between wet and dry years. The groundwater budget suggests the
difference between inflows and outflows during a simulated dry year may exceed 22,000 AFY. The preliminary
water budget simulated over the five year period 2005 – 2009 resulted in an average deficit of 15,000 AFY.
PVHM results suggest net recharge is possible during wet years.
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Based on the PVHM hydrologic modeling results, PVWMA established a target of reducing groundwater
production in the Pajaro Valley groundwater basin by approximately 12,000 acre-feet per year (AFY). An update
to the PVWMA Basin Management Plan (2012 BMP Update) was developed by an Ad Hoc Basin Management
Plan committee, established by the Board of Directors in 2010. The BMP Update identifies three main projects
and programs that meet the goal of 12,000 AFY:
1.
Establish conservation programs in an effort to use existing supplies more efficiently, with a target of
saving 5,000 AFY.
2.
Optimize the use of existing water supply facilities, such as the water recycling facility and the Harkins
Slough Managed Aquifer Recharge and Recovery Facility. The target is to gain an additional 3,000 AFY
from these two facilities.
3.
Construct new water supply facilities capable of producing 4,100 AFY.
Table 3.1: PVHM preliminary groundwater budget summary. Average flows reported in in acrefeet per year and rounded to the nearest hundred. A water year is defined as October 1st
through September 30th.
Source
PVHM
PVHM
PVHM
Time Period (Water Years)
2005-2009
2006 (Wet)
2008 (Dry)
4,000
3,600
4,100
Inflows:
Landward Underflow3 (LU)
Net Direct
Infiltration1
(DI)
24,100
26,400
22,400
Streamflow Infiltration1(SI)
18,300
25,700
16,400
Total Recharge (DI+SI):
42,400
52,100
38,800
Total Onshore Inflows:
46,300
55,600
42,900
1,900
-7,200
3,500
2,800
4,100
4,900
300
200
0
1,500
1,400
1,500
9,600
9,500
9,600
Outflows:
Storage Depletion1,2
Storage
Depletion2,3 (SWI)
Outflow to Bay1,3 (OB)
Rural Residential
Water Supply
Pumpage1
Pumpage5
Agricultural Pumpage
45,800
45,100
45,800
Total Pumpage4
56,900
56,000
57,000
Total Discharge:
61,800
53,100
65,400
Inflows – Outflows7 =
-15,500
2,500
-22,500
1
Estimated from (farm Net Zone & WtrYr) from spreadsheet WBS_PVHM_03102011.xlsx
Estimated for Onshore Portion of the Active Model Grid
3
Estimated from (gwswi Net Zone & WtrYr) from spreadsheet WBS_PVHM_03102011.xlsx
4
Estimated from (FDS AFY by Farm and Water Year) from spreadsheet WBS_PVHM_03102011.xlsx
5
Estimated from (FDS AFY by Farm and Water Year) from spreadsheet WBS_PVHM_03102011.xlsx
6
Excludes water-balance subregions for Soquel Creek (21) and Central (23) Water Districts
7
Negative numbers indicate storage depletion and positive number indicates storage accretion
2
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4.0 SALT AND NUTRIENT GROUNDWATER CONDITIONS
An evaluation of current groundwater salt and nutrient conditions within the PVWMA service area was
completed using the best available data. The existing conditions of the PVGB were compared to basin water
quality standards established by the CCRWQCB in order to assess and document current conditions.
Groundwater quality varies significantly both spatially and vertically throughout the PVGB, requiring
reasonable data integration techniques to document a practical representation of existing conditions. Prior to
presenting these data, salt and nutrient constituents focused upon by this SNMP are identified and the
rationale for their selection as representative constituents is provided.
4.1
CONSTITUENTS OF CONCERN AND RELEVANT SOURCES
For the purpose of establishing a baseline and tracking progress in this SNMP, the use of a finite number of key
indicator constituents was chosen and evaluated through the stakeholders. A broader constituent evaluation
was conducted initially in order to identify which salt and nutrient compounds would be most effective in
meeting the SNMP development objectives. A range of salt and nutrient compounds were considered for
inclusion based on their respective fate and transport in the environment, availability of local groundwater
datasets, and biogeochemical behavior relative to other salt or nutrient compounds. After consideration and a
discussion with the stakeholders in attendance at SNMP Stakeholder Meeting #1 the final priority constituents
selected for the PVWMA SNMP are TDS, chloride and nitrate-NO3.
3.1.1
TOTAL DISSOLVED SOLIDS (TDS)
TDS is the combined content of organic and inorganic substances that remain in the water column following
filtration through a 2 µm sieve. The use of TDS as a water quality parameter representing the relative
concentration of salts in a water sample is a very common, cost effective and acceptable way to classify
salinity. TDS is composed of the following primary constituents, making it a simple and universal proxy for the
bulk salt content of water: calcium, phosphates, nitrates, chloride, sodium, and potassium. PVMWA staff
conducts extensive groundwater and surface water monitoring efforts on various time scales that range from
monthly to semi-annually. TDS sampling of surface and groundwater within the Pajaro Valley is extensive,
providing a breadth of both spatial and temporal TDS concentration data.
Within PVGB the primary source of salts in groundwater is from seawater intrusion resulting from long-term
groundwater overdraft. Additional sources of salts include direct infiltration from the land surface to
groundwater and stream flow infiltration. Irrigation practices may result in soil salt accumulation and potential
vertical infiltration to groundwater. Stream flow infiltration of poor quality is of particular concern in upstream
reaches of the Pajaro River near Murphy’s Crossing. Chronic groundwater overdraft of the PVGB continues as
more water is extracted for agricultural irrigation and household water supply than is recharged in most water
years (Table 3.1). In order to better separate the spatial distribution, impacts and strategies relevant to these
two sources of TDS (irrigation accumulation and seawater intrusion), chloride has been included as a
constituent of concern (see Section 4.1.2). Figure 4.1 presents the salt cycling and associated effect on aquifer
salt impacts from crop irrigation practices. The salt content (measured as TDS concentration) of irrigation
waters has a significant impact on the relative salt leaching to the adjacent aquifer due to plant sensitivity to
elevated TDS content, requiring greater water volume applications to achieve the desired plant growth.
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For sensitive crops, such as strawberries, ideal TDS concentrations in irrigation water are < 450 mg/L. A
PVWMA objective for delivered irrigation water is below 500 mg/L TDS (PVWMA 2012). The Central Coast Water
Quality Basin Plan (RWQCB 2011) has a median groundwater quality objective for PVGB TDS concentrations set
at 1200 mg/L.
3.1.2
CHLORIDE
Chloride is a negatively charged anion and as sodium chloride (NaCl), it is the most common salt contained in
seawater. A significant source of salts in the basin groundwater is due to seawater intrusion as a result of longterm groundwater overdraft. Given that TDS is a bulk measure of all salts and a significant amount of existing
local surface water and groundwater data, chloride has been included as a priority constituent of concern to
provide additional data to inform comparisons of potential land use sources verses seawater intrusion sources
of salts. Seawater has a total dissolved solids (TDS) concentration of about 35,000 mg/L, of which dissolved
chloride comprises approximately 19,000 mg/L. Ambient concentrations of chloride in groundwater are
typically < 20 mg/L, so there is a large contrast in chloride concentrations between freshwater and seawater.
PVWMA uses a chloride concentration threshold of 100 mg/L in groundwater to define the inland extent of
seawater intrusion (PVWMA 2012). The Central Coast Water Quality Basin Plan (RWQCB 2011) has a median
groundwater quality objective for chloride of 150 mg/L.
3.1.3
NITRATE – NO3
Nitrogen and phosphorous are the two most universal nutrients in the environment and are both essential for
primary production via photosynthesis. Environmental fate and transport of these two nutrients are very
different. Phosphate in water can exist in four different forms that are controlled by the pH of water. Three of
these phosphate salts that are common in natural waters adhere to clay particles, making phosphate ions
relatively less mobile in subsurface environments than key forms of dissolved nitrogen.
In contrast, nitrogen can exist in five different oxidation states and nitrogen cycling is very complex. Nitrate is
the most common dissolved form in oxygenated environments and typically is the primary nitrogen (N)
constituent used to identify surface or groundwater nitrogen concentrations. Dissolved inorganic nitrate is a
conservative constituent in the environment, meaning it readily moves through the subsurface and its
migration is not limited by interactions with soils to the same extent as phosphate. Selecting nitrate as the
nutrient constituent proxy for the PVWMA SNMP was based on the following key considerations:
1.
The conservative nitrate compound is a potential worst case scenario of the nutrient conditions in
local groundwater;
2.
Local surface and groundwater datasets for nitrate–NO3 are extensive;
3.
Excessive nitrate in water can result in human health concerns making nitrate concentration standards
in local waters more stringent than phosphate;
4.
There are not assumed to be any additional significant and controllable sources of phosphate in the
Pajaro Valley that are not also nitrate sources;
5.
The choice was vetted and accepted by the stakeholder committee.
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A number of analytical methods are available to quantify the amount of nitrate in water and, in order to ensure
consistency, dissolved nitrate measured as NO3 is the representative nutrient compound for the PVWMA
SNMP.
Figure 4.2 is a simplified schematic of the nitrogen cycle relevant to the PVWMA SNMP and groundwater
nitrate-NO3 concentrations, with controllable anthropogenic sources noted in red boxes and the critical
nitrogen biogeochemical processes provided between each reservoir. The form, levels, and distribution of
nitrogen within the PVGB varies significantly both spatially and temporally and representation of this
complexity is not intended with Figure 4.2, nor is it necessary to achieve the primary objectives of the SNMP.
There are a number of state and federal water quality standards for nitrogen species potentially relevant to
this SNMP. Table 4.1 presents the variety of standards and sources.
Table 4.1: Summary of relevant federal and state N standards.
Regulation / document
EPA National Primary Drinking
Water Regulations (2009)
Nitrogen
Standard
constituent
Nitrate – N
Drinking water
Nitrate –
Groundwater median
water quality objective
NO3a
Total nitrogen as N
Value
< 10 mg/L
< 45 mg/L
< 5 mg/L
Beneficial Use Standards
Water Quality Control Plan for the
Nitrate-N
< 30 mg/L
Nitrate-NO3a
< 132 mg/L
Nitrate – NO3
< 90 mg/L
MUN
Nitrate – NO3
a Converted documented standard to Nitrate-NO3 using molar mass.
< 45 mg/L
Central Coast Basin (June 2011)
AGR (irrigation)
AGR (livestock)
4.2
EXISTING CONDITIONS
3.1.4
GROUNDWATER
Groundwater quality datasets from PVWMA and the City of Watsonville were compiled to generate spatial
representations of existing groundwater conditions for the three constituents of concern (TDS, chloride and
nitrate-NO3). PVWMA possesses an extensive water quality dataset that is the culmination of decades of
sampling monitoring and production wells within the area. The temporal sampling of specific wells ranges from
a single sampling event to long-term sampling programs. The City of Watsonville maintains and annually
samples 14 groundwater production wells within the City limits and surrounding area. The opportunities and
constraints of different data integration options were extensively evaluated prior to the selection of the
recommended approach. The intent was to create representative maps that reasonably reflect existing
groundwater conditions for the constituents of concern given the inherent seasonal and inter-annual variability
of groundwater quality while maximizing the breadth of the available data. The respective data from over 300
sites sampled over the last 10 years were integrated (2002-2011) to ensure a range of water year types were
included. The period 2002 - 2011 consisted of four dry, two average and four wet water years (Figure 2.2).
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Many of the wells sampled are privately owned and require methods of display that protect the anonymity of
these locations and the confidentiality of the data. This led to the use of the inverse distance weighted (IDW)
toolset in geographic information systems (GIS) software to generate an interpolated surface based on the
nearest (in proximity) five well locations. For each constituent, the decadal average and maximum
concentration from each sampling location were integrated to summarize the existing groundwater
conditions. It is assumed the average and maximum provide a representative range of existing conditions over
the last decade. While the maximum constituent concentration maps do represent the “worst case” scenario,
they provide an excellent opportunity for the SNMP objective development for the PVGB due to the expected
sensitivity of maximum, worst case groundwater concentrations to the implementation of effective strategies
to reduce sources of salt and nutrient sources to PVGB groundwater. Documentation of the average decadal
conditions is consistent with water quality standards as set by the RWQCB, which are typically based on
average or median values from large datasets.
A similar analysis on historical datasets prior to 2002 was not conducted for two primary reasons: 1. the focus
of the SNMP is to identify current conditions, existing sources and develop strategies to protect and
potentially improve future groundwater quality, reducing the value of evaluating past conditions and 2. the
recently completed PVWMA Basin Management Plan (2012) provides an adequate summary of historic
groundwater quality in Chapter 2 (State of the Basin) and can be referenced as necessary.
The existing groundwater quality conditions are presented in Figures 4.3-4.8 and discussed below. There are a
number of known limitations associated with the selected approach to integrate available data to summarize
the existing PVGB conditions.

Some wells were only sampled once, but these single concentrations were included with the
assumption that this temporal limitation was worth the increased spatial resolution. The values
assigned to these sites may be influenced by the conditions at the time of sampling. However, the IDW
interpolation creates concentration contours using the five nearest monitoring wells, thus limiting the
influence of these infrequently sampled locations on the final results.

The hydrogeology of PVGB is complex and the population of wells sampled is screened across a range
of depths representing different areas within the aquifer system. The existing condition maps
produced for this SNMP have integrated and smoothed these vertical and spatial differences on to a
single map that represents average groundwater quality conditions.

The temporal variability of groundwater quality is significant and can be influenced by climate,
variations in groundwater inputs or outputs, and/or changes in the source loading. The summary of
PVGB existing conditions is intended to represent the reasonable range of groundwater conditions
that will adequately inform the development of focused and relevant SNMP strategy implementation
objectives and identification of effective management strategies and priority locations to achieve the
stated objectives.
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3.1.4.1
TDS
The TDS groundwater concentration distribution maps were created using 310 monitoring and production well
locations and includes 1697 discrete water quality samples collected by PVWMA and City of Watsonville from
2002 – 2011. The individual sample TDS concentrations ranged from a minimum of 45 mg/L to a maximum value
over 27,000 mg/L. Using the IDW method as described above, the average value from each monitoring site was
used to create Figure 4.3 and the maximum value was used to create Figure 4.4. Results were grouped into
four discrete concentration categories and acreage summaries provided in Table 4.2.

TDS concentrations < 450 mg/L are considered ideal for irrigation waters to minimize potential toxicity
issues with crops as well as reduce the rate of soil salt accumulation and potential vertical leaching to
the groundwater.

TDS concentrations > 450 and < 1000 mg/L are considered not optimal for long term irrigation use
without mitigation, but still adequate for crop generation.

TDS concentrations > 1000 and < 1,800 mg/L are considered excessive for irrigation use and
inadequate for crop generation.

TDS concentrations > 1,800 mg/L are considered excessive in groundwater.
Table 4.2: Acreage and percent of total summaries for data displayed in Figures 4.3 and 4.4.
Decadal MEAN TDS
Decadal MAX TDS
Figure 4.3
Figure 4.4
(acres / % of total)
(acres / % of total)
0-450 mg/L
35,300 / 52%
29,300 / 43%
>450 mg/L – 1000 mg/L
27,900 / 41%
30,500 / 45%
>1000 mg/L – 1800 mg/L
3,550 / 5%
6,950 / 10%
>1800 mg/L
637 / 1%
900 / 1%
TDS
Concentration Range
Locations in the PVGB where TDS concentrations are highest include the western boundary, consistent with
the mapped seawater intrusion front. Eastern areas are elevated including Murphy Crossing and the East Area
where stream flow infiltration of high TDS water originating in the upper reaches of the Pajaro River
Watershed occurs.
3.1.4.2
Chloride
The maps representing the groundwater chloride concentration distribution were created using 309
monitoring and production well locations and includes 1717 discrete water quality samples collected by PVWMA
and City of Watsonville from 2002 – 2011. The individual sample chloride concentrations ranged from a
minimum of 3 mg/L to a maximum of 13,705 mg/L. Using the IDW method as described above, the average
value from each monitoring site was used to create Figure 4.5 and the maximum decadal values were used to
create Figure 4.6, summarized in Table 4.6. Results were grouped into four discrete concentration categories:

Chloride concentrations > 100 mg/L are used by PVWMA to identify the inland extent of seawater
intrusion within the groundwater aquifer.

Chloride concentrations < 250 mg/L are used as a common surface water standard.

Chloride concentrations < 500 mg/L used as a threshold between high and very high chloride.
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Chloride concentrations > 500 mg/L used as indicator of locations with persistent seawater intrusion
Table 4.3: Acreage and percent of total summaries for data displayed in Figures 4.5 and 4.6.
Decadal MEAN Cl
Decadal MAX Cl
Figure 4.5
Figure 4.6
(acres / % of total)
(acres/% of total)
0-100 mg/L
55,500 / 82%
52,900 / 78%
>100 mg/L – 250 mg/L
8,870 / 13%
10,400 / 15%
>250 mg/L – 500 mg/L
2,600 / 4%
3,170 / 5%
>500 mg/L
644 / 1%
1,200 / 2%
Chloride
Concentration Range
The seawater intrusion interface is a complex three-dimensional surface and the actual location of this dynamic
boundary varies with elevation, seasonally and annually. PVWMA has generated a best approximation of the
current (2011) inland extent of seawater intrusion as displayed on Figures 4.5 and 4.6. Inland of the seawater
intrusion boundary groundwater possesses an average chloride concentration below 100 mg/L, with the
exception of the decreased groundwater quality west of Chittenden Gap. According to the USGS
hydrodynamic model, this may be in part attributable to the poor surface and groundwater quality
contribution from upstream locations.
3.1.4.3
Nitrate-NO3
The nitrate-NO3 groundwater concentration distribution maps were created using 310 monitoring and
production well locations and includes 1718 discrete water quality samples collected by PVWMA and City of
Watsonville from 2002 – 2011. Individual sample nitrate-NO3 concentrations ranged from a minimum of 0.5
mg/L to a maximum of 1830 mg/L. The average value obtained from each sampling location was spatially
integrated using IDW method to create Figure 4.7 and the decadal maximums were used to create Figure 4.8.
Acreage summaries from both figures are summarized in Table 4.4 below. Results were grouped into four
discrete concentration categories:

Nitrate-NO3 concentration <10 mg/L was chosen as a minimum threshold to identify locations of
relatively minimum nitrate concentrations.

Nitrate-NO3 concentration <45 mg/L are used by Environmental Protection Agency (EPA) as a drinking
water standard.

Nitrate-NO3 concentration <100 mg/L used as a threshold between high and very high nitrate.

Nitrate-NO3 concentration >100 mg/L.
Table 4.4: Acreage and percent of total summaries for data displayed in Figures 4.7 and 4.8.
Decadal MEAN NO3
Decadal MAX NO3
Figure 4.7
Figure 4.8
(acres / % of total)
(acres/% of total)
0-10 mg/L
25,500 / 38%
24,600 / 36%
Nitrate-NO3
Concentration Range
>10 mg/L – 45 mg/L
28,400 / 42%
23,100 / 34%
>45 mg/L – 100 mg/L
10,500 / 16%
12,600 / 19%
>100 mg/L
3,260 / 5%
7,410 / 11%
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Elevated groundwater concentrations of nitrate-NO3 are found in the sand dunes of the San Andreas Terrace
as well as in the eastern area between highways 129 and 152 (Figure 4.7). Locations of elevated nitrate-NO3
concentrations expand when maximum concentration values are displayed (Figure 4.8). Another area with
elevated concentrations nitrate-NO3 is just south of Corralitos, as seen on Figure 4.8 displaying the maximum
nitrate-NO3 concentrations of the PVGB.
3.1.4.4
Groundwater Existing Conditions Summary
A summary of groundwater existing conditions is provided in Table 4.5 below for each constituent of concern
by documenting the area of the PVGB exceeding selected thresholds representing the transition to degraded
water quality.
Table 4.5 PVGB groundwater existing conditions summary.
Constituent
3.1.5
Fraction of area
Fraction of area
Selected Threshold
exceeding threshold of
exceeding threshold of
(mg/L)
MEAN values
MAX values
(acres / % of total)
(acres / % of total)
TDS
1,000 mg/L
4,187 / 6%
7,850 / 11%
chloride
100 mg/L
12,114 / 18%
14,770 / 22%
nitrate-NO3
45 mg/L
13,760 / 21%
20,010 / 30%
SURFACE WATER
Surface water plays a large role in recharging the aquifers in PVGB with the PVHM results suggesting that
approximately 40% of the annual recharge occurs via steam infiltration (Table 3.1). For this reason, a surface
water quality existing conditions analysis was conducted. The concentrations of salts and nutrients in surface
water systems are highly variable, depending on the time of year and the climate and recharge from surface
water systems to groundwater aquifers may improve or degrade water quality in the aquifer depending on the
concentration of salts and nutrients in both systems.
PVWMA monitors surface water quality at 30 sites throughout the basin on a monthly to quarterly frequency
since 1994 (Figure 4.9). As a result, the PVWMA possesses a diverse surface water monitoring dataset that
includes sites along the Pajaro River, Corralitos Creek, Carneros Creek, College Lake, Green Valley Creek, Pinto
Lake Outflow, Corncob Canyon, as well as the Harkins and Watsonville Slough systems. In addition, various
other surface water sampling efforts have been conducted over the years to address a variety of questions
including water quality concerns or to inform model development, and/or management decisions.
The PVWMA water quality dataset for the period 2002-2011 was queried to provide constituent concentration
statistics on a quarterly basis. Surface water concentrations vary significantly by season with high winter flows
typically possessing much lower concentrations than dry season base flows. In an effort to document this
seasonal variation by constituent, the average decadal concentrations by quarter were evaluated for each
constituent to identify the values with the greatest seasonal range.
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Sample results below the analytical detection limit for nitrate-NO3 of 1 mg/L was common and therefore these
samples were assigned a value of 0.5 mg/L as to not exclude valuable data (Helsel 2005). Samples below TDS
and chloride analytical detection limits were infrequent and therefore no data adjustments were made. In all
instances, an average of 15 data points were used to generate both the Q1 (January 1 – March 31) and Q3 (July 1
– September 30) averages by site for each of the three constituents of concern.
Figures 4.10 and 4.11 summarize the existing TDS and chloride surface water conditions given the spatial
distribution of the PWMA surface water sampling program. Sites in the coastal proximity and downstream of
Chittenden Gap on the Pajaro River (PR1, PR2 and PR3) possess relatively higher salt content, particularly
during base flow conditions. Approximately 80% of the sites have average surface water TDS concentrations
over 450 mg/L in Q3. Surface water systems devoid of significant agricultural influence are expected to have
TDS concentrations below 200 mg/L, though natural TDS levels will vary depending upon local geology
(RWQCB 2010).
Figure 4.12 presents the first and third quarterly average nitrate-NO3 concentrations, with 17% of sites
exceeding 45 mg/L in the winter and 23% exceeding 45 mg/L in the summer. The best water quality is found at
sites located in the upper watershed, including Upper Corralitos Creek where the density of potential nutrient
sources is relatively low.
3.1.6
DELIVERED WATER QUALITY
The PVWMA has constructed several water supply facilities and over 20 miles of underground distribution
pipeline in an effort to reduce coastal groundwater pumping to stop seawater intrusion and eliminate
groundwater overdraft. Water delivered via the Coastal Distribution System (CDS) is used for agricultural
irrigation supply in lieu of pumped groundwater and therefore serves as “in-lieu recharge.” The area served by
the CDS is known as the Delivered Water Zone (DWZ) – Figure 4.13. PVWMA staff closely monitors the quality
of delivered water, which consists of disinfected, tertiary treated, recycled water, City of Watsonville potable
water, water recovered from the Harkins Slough Managed Aquifer Recharge and Recover Facility, and
additional supply from PVWMA blend wells. Since recycled water deliveries commenced in 2009, the volume of
delivered water has increased from 2,400 AFY to over 3,500 AFY. Over 360 water quality samples have been
collected from active turnouts on the distribution system since April of 2009. Though water quality varies, the
average concentration of the constituents of concern of the DWZ water is TDS 605 mg/L, chloride 102 mg/L,
and nitrogen-NO3 23 mg/L. Total water use in the DWZ can exceed 10,000 AFY. In addition to delivered water,
pumped groundwater is a primary source of irrigation supply to growers in the PVWMA.
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5.0 LOADING RISK ANALYSIS
A key element of the SNMP, as defined by the State Water Resources Control Board, is to conduct a salt and
nutrient loading analysis that identifies primary sources of salts and nutrients and estimate the loading to the
local groundwater basin. Little guidance is provided on the recommended methods, data needs, analysis
approach, temporal or spatial resolution, and other key elements of the loading analysis. A SNMP loading
analysis could include complex salt and nutrient fate and transport models to generate quantitative
constituent mass loading estimates for some specified time interval. However, this level of increased in
complexity and associated increased costs is not necessary to obtain an adequate understanding of the salt
and nutrient loading and identify priority sources and locations where pollutant source control actions are
needed.
It is assumed that the desired outcome of the loading analysis is to provide guidance to the local stakeholders
to identify priority locations and land use practices that can be modified to most effectively reduce existing and
future annual loads of salts and nutrients to groundwater. As a result, a loading risk analysis approach was
undertaken for this SNMP. The risk analysis approach incorporates the critical factors expected to be most
influential in relative constituent loading (which are typically the parameters that complex model outputs are
most sensitive to) and integrate these factors using categorical scales of relative impact that are supported by
literature and basic scientific principles. The risk analysis relies heavily on existing water quality, land use
mapping, and other readily available local datasets. The risk analysis generates relative priority sources and
locations of salt and nutrient loading, which is ultimately how the absolute quantitative outputs from more
complex approaches would be used. Sources with the highest mass per unit time loading rates and the
greatest potential to impact groundwater would be priority locations where management strategies would be
recommended.
The approach and products from a risk analysis is more transparent and easier for a broad range of
stakeholders to understand. It utilizes available datasets with clearly stated assumptions, preserves our
technical understanding of system dynamics, and achieves the desired outcomes of the loading analysis task as
required by the CCRWQCB. This approach and resulting recommendations can also be updated as changes to
land use, crop rotations, and agricultural practices occur in the future. In addition, these products will directly
inform SNMP objective development and guide identification of management strategies that will address
relative priority locations and sources of the constituents of concern.
5.1
DATA AND METHODS
For each of the key potential sources of salts and nutrients to the local groundwater as identified in Section
4.2, a spatial risk analysis was conducted using available local datasets and well documented assumptions,
supported by references where applicable. The risk analysis is based on a categorical designation of critical
spatial factors or conditions (i.e. soil type, land use management, measured water quality, etc.) on a relative
scale using indicative thresholds that are supported by existing standards, practices or best professional
judgment. Below we detail data sources, methods, and rationale for the categorization and display of the
spatial datasets used.
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4.1.1
SOIL WATER HOLDING CAPACITY
The relative permeability, or water holding capacity of soil in the unsaturated zone, controls the rate at which
water can infiltrate a soil horizon and is a critical factor in determining the likelihood of dissolved constituents
derived from land use practices reaching shallow groundwater. A linear regression model was created by Nolan
et al. (2002) to predict (with 75% confidence) the likelihood of recently recharged shallow groundwater
aquifers within the U.S. that possess elevated nitrate levels, which was found to be highly sensitive to land use
(both urban and agriculture) and distribution of well-drained soil types. The soil water holding capacity of soils
within the Pajaro Valley is incorporated as a critical component of the risk analysis for controllable sources that
are introduced to the environment in a location that requires vertical infiltration and transport to reach
groundwater. The likelihood of nutrients leaching vertically from the unsaturated zone into shallow
groundwater is greater in locations of low soil water holding capacities, with the known limitation that depth
to shallow groundwater, subsurface hydrogeology and stratigraphy and localized heterogeneities will greatly
influence the actual hydrologic connection between dissolved constituents introduced to surface soils and the
subsurface aquifer.
The water holding capacity of the soil is dependent on a number of factors including clay content, compaction,
drainage, and slope. The NRCS soil maps (SSURGO 2009) classify agricultural soils and provide data on the
water holding capacity, drainage class, and slope. The soil classification data were reviewed and categorized
into 4 categories. The NRCS database provides 3 sets of water holding capacities: 0-25 cm, 0-50 cm and 0100 cm. Higher variability exists within the shallower depth capacities. In addition, the depth of deep tillage
and rooting depth of certain crops grown in the Pajaro Valley is approximately 90 cm. These factors mean that
the water in 0-100 cm can be accessed for crop uptake and that tillage to overcome compaction is affecting
this depth. Water holding capacity categories were based on discrete soil types and on the soil’s ability to hold
the potential irrigation volumes applied to crops in the Pajaro Valley. The categories are:
Low: Water holding capacity defined as 0 – 8.9 inches of water per meter of soil profile, equating to a
retention range of 0 to 0.75 acre feet (AF) of water applied per acre per year. These soils are
characterized as relatively unconsolidated with high sand and low organic content, located near the
coast and the steepest slopes above the river bottom. These soil types are expected to have the
highest risk of vertical leaching within the basin.
Moderate: Water holding capacity defined as 9 – 14.9 inches of water per meter of soil profile,
equating to a retention range of 0.75 – 1.25 AF /acre/yr. These soils include the silty/sand loams on
various slopes.
High: Water holding capacity of 15 – 17.9 inches of water per meter of soil profile, or 1.25 –
1.5 AF/acre/yr. These soils include finer silty/sand loams.
Very high: Water holding capacity of 18 – 28 inches of water per meter of vertical soil profile per year;
or 1.5 – 2.3 AF/acre/yr. These soils are predominantly clays as indicated by the extensive deposit along
the historic Pajaro River floodplain. These soil types are expected to have the lowest risk of vertical
salt and nutrient leaching.
The relative water soil holding capacity distribution within the PVWMA area is presented in Figure 5.1.
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4.1.2
AGRICULTURAL LAND USE
Agricultural land use is known to be a significant source of salts and nutrients to local groundwater basins from
irrigation and fertilizer application practices, respectively. The Pajaro Valley is one of the most productive
agricultural regions in the country and future sustainability of these lands will likely require modification of
irrigation and fertilizer practices. Existing agricultural land use distribution within the basin is presented based
on annual crop mapping conducted by PVWMA in 2012. Given the regular rotation of crops, the 2012 PVWMA
crop mapping could potentially misrepresent the land use designation as fallow for some parcels. Therefore,
2011 was used as supplemental layer to verify if a parcel was fallow two sequential years prior to fallow
designation. If a parcel was fallow in 2011 but mapped by the PVWMA as an active crop in 2011, the parcel was
assigned the crop type present in 2011. Figure 5.2 and Table 5.1 present the crop type category designations and
resulting total acreage under each land use that collectively comprise the nearly 26,800 acres of cultivated land
within the basin or 38% of the land west of the San Andreas Fault (also referred to herein as the Pajaro Valley
Groundwater Basin; PVGB). Note that the total crop type mapping area conducted by the PVWMA within the
PVGB is 69,957 acres, which is less than the total area of land within the PVGB boundary (72,127 acres).
Table 5.1: Acreage summary of Figure 5.2.
Land Use Category
4.1.3
Acres
% of total PVGB
Vegetable Row Crops, Artichokes
9,138
13%
Strawberries
7,994
11%
Horticultural Nurseries
1,343
2%
Caneberries
5,003
7%
Deciduous (Apple Orchards)
2,179
3%
Other, Unknown Agriculture, Vine/Grapes
1,142
2%
Total Agricultural Land Use (2012/2011)
26,799
38%
Fallow or other non-agricultural land uses
43,158
62%
Total PVGB area
69,957
100%
IRRIGATION INTENSITY
Literature values and local irrigation water use datasets were integrated with agricultural land use described
above to create a map of the relative distribution of irrigation intensity within the Pajaro Valley (Figure 5.3).
Relative annual irrigation volumes are assumed to influence the risk of salt and nutrient vertical migration from
agricultural lands to the local groundwater table. Agricultural land comprises approximately 38% of the land
area within the PVWMA area, with 43% Native Vegetation/Riparian and 17% Urban/Turf, and associated practices
are identified as potential critical sources of salts and nutrients (Section 1.2 of PVWMA BMP 2012). Annual
water use for a particular crop type is dependent on a number of factors including winter rainfall, climate, and
soil type with the assumption that general crop types and/or irrigation practices can be categorized into
relative categories of anticipated average use rates. A number of data sources were considered to obtain a
range of rates which should encompass the variability caused by the different factors listed above. These
sources were:
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5.2
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1.
PVWMA water usage data collected in 2011,
2.
MCWRA water usage data collected in 2010 (MCWRA 2010),
3.
Data collected by Central Coast U.C. Irrigation Farm Advisor (Cahn et al. 2011 and 2012) and
4.
Published documents (Prichard et al. 1989, Righetti et al. 1998, Mahler and Barney 2000, Pettygrove
2003, Hart et al. 2006a and 2006b, Smith et al. 2008, LeStrange et al. 2010, Turini et al. 2010).
The categorical thresholds were created to encompass the range of potential irrigation volumes applied to
typical crops cultivated in the Pajaro Valley under the cool climatic conditions of the Central Coast. The same
crops grown in other parts of California will have different potential irrigation volumes due to climate and soil
types. The irrigation intensity categories are:
High: Crop type and irrigation practice that uses an estimated 2.4 – 3.0 AF of water per land acre per
year. High usage crops in the Pajaro Valley are multiple cropped annual acres of cool season vegetable
row crops, artichokes, and certain types of nurseries. The majority of these crops are grown with
sprinkler or some type of overhead irrigation.
Moderate: Crop type and irrigation practice that uses an estimated 1.8 – 2.3 AF of water per land acre
per year. Moderate usage crops are strawberries and cane berries irrigated using drip irrigation.
Low: Crop type and irrigation practice that uses an estimated 0.5 – 1.7 AF of water per land acre per
year. Low usage crops include grapes and tree fruit orchards.
None: Fallow and non-agricultural land that receive rainfall only, but contribute water via percolation
to groundwater and runoff to surface waters.
PVWMA agricultural land use surveys from 2012 (and supplemented with 2011 surveys) were used to identify
the spatial distribution of crop types. These crop type designations were assigned irrigation intensity per the
categories above resulting in a mapped distribution of agricultural irrigation intensity within the basin (Figure
5.3) and acreage summarized in Table 5.2 below.
Table 5.2: Acreage summary of agricultural land use distribution by annual irrigation intensity
rate per acre of land as presented in Figure 5.3.
Annual irrigation intensity
Figure 5.3
% of agricultural
category
(acres)
land
High
10,481
39%
Moderate
12,997
48%
Low
3,321
12%
26,799
100%
Total agricultural land use
Total non-agricultural land use
4.1.4
43,158
CROP FERTILIZER USE
Similar to irrigation intensity, literature values and local annual fertilizer application data were integrated with
agricultural land use distribution described in Section 5.1.2 to create a map of the relative distribution of the
pounds of nitrogen applied per acre within the PVWMA area (Figure 5.4). The annual mass of nitrogen applied
to a crop is dependent on a number of factors including cultivar, residual soil N, winter rainfall, climate, and soil
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type. Multiple sources of data were used to develop reasonable range of annual N loading rates which capture
the expected seasonal and site variability expected in the Pajaro Valley. Primary sources used to identify the Nloading categories were:
1.
PVWMA nitrate survey data from growers collected 2009 – 2011 (Platts 2011),
2.
A compilation of references detailing N use for different crops in California, Oregon and Idaho
(Righetti et al. 1998, Mahler and Barney 2000, Pettygrove 2003, Hart et al. 2006a and 2006b, Smith et
al. 2008, LeStrange et al. 2010, Turini et al. 2010), and
3.
USDA NASS (2011a and 2011b) data for crops grown in California, Washington and Oregon. Including
Washington and Oregon was necessary in order to get an adequate amount of data relevant to the
diversity of cane berry crops grown in Pajaro valley.
The rate of N application includes all types, nitrate N, ammoniac N, and organic N. The categories are based on
the actual annual rates of N applied to high production cost crops grown in the Pajaro Valley.
High: Crop type that typically is managed by applying 150 – 250+ lbs. of N per land acre per year. High
usage crops are annual vegetable crops, certain types of nurseries, and strawberries.
Moderate: Crop type that typically is managed by applying 76 – 149 lbs. of N per land acre per year.
Moderate usage crops are cane berry crops (raspberry, ollalieberry, and blackberry) and tree fruit
crops (orchards).
Low: Crop type that typically is managed by applying 35 – 75 lbs. of N per land acre per year. Low
usage crops are grapes, various horticultural plant production facilities, and certain types of
nurseries.
None: Fallow and non-agricultural land use that do not receive fertilizer applications and make
minimal contributions of nutrients via percolation to groundwater and runoff to surface waters.
The above classification of crops is based on average reported amounts for the various crop types. However,
depending on the business strategy of the individual agricultural producer and certain factors of crop
production, there may individual operations that have significantly different practices. As the SNMP develops,
more data will be necessary to identify acreage and operations that may be categorized differently than the
crop type designations above.
Crop type designations presented in Figure 5.2 were categorized based on the definitions above resulting in
agricultural fertilizer use distribution within the PVWMA area (Figure 5.4).
It must be noted that it was desired to include an additional Very High category to the fertilizer use analysis,
defined as users that apply > 250 lbs. of N per land acre per year. Applying the results of annual grower surveys
conducted by Belinda Platts on behalf of PVWMA from 2009 to 2012 indicated that an estimated 29% of
agricultural land in the PVWMA area is subject to annual fertilizer applications of >250 lbs N/acre/year. Given
that there is approximately 26,800 acres currently under agricultural land use within the basin, approximately
7,772 acres should be categorized as Very High (Table 5.3). A limitation for including the very high category in
Figure 5.4 is due to the lack of spatially explicit knowledge of which strawberry and vegetable crops within the
Pajaro Valley are currently implementing these very high fertilizer application practices.
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Table 5.3: Comparison of the agricultural land use distribution by annual fertilizer application
rate per acre of land as presented in Figure 5.4 and expected distribution with the inclusion of
very high usage category (>250 lbs N/acre/year) given results of PVWMA 2009-2012 grower
surveys by Belinda Platts.
Annual fertilizer application
Figure 5.4
% of ag
category
(acres)
land
Very High (> 250 lbs N/acre/yr)
High
4.1.5
Expected
distribution
(acres)
n/a
% of ag
land
7,772
29%
(150-250+ lbs N/acre/yr)
18,475
69%
10,703
40%
Moderate (76-149 lbs N/acre/yr)
7,182
27%
7,182
27%
Low
1,142
4%
1,142
4%
(35-75 lbs N/acre/yr)
Total agricultural land use
26,799
Total non-agricultural land use
43,158
SEWER LINES AND SEPTIC SYSTEMS
Human waste can be a significant source of nitrate to the subsurface. Aging infrastructure and insufficient
maintenance of sewer systems can result in leakage from sewer pipes, leading to infiltration of raw sewage
into the surrounding soil and ultimately into underlying groundwater. Septic systems are designed to treat
domestic wastewater and for the prevention of human exposure to pathogens, but even functioning septic
units will discharge nitrogen to the subsurface.
Spatial data documenting existing locations of sewer lateral and transfer lines within the PVWMA area were
obtained from the City of Watsonville GIS department and Pajaro County Sanitation District (PCSD). There were
no GIS data associated with PCSD sewer lines, so existing AutoCAD layout sheets were interpreted in order to
digitize sewer line locations in GIS by 2NDNATURE. Figure 5.5 presents the location of existing sewer lines
within the basin overlain with the water holding capacity of the local soils. All sewerage in the basin is routed
to the Watsonville Wastewater Treatment Plant located on West Beach Street servicing 21 square miles,
approximately 55,000 people, treating an average of 6.6 million gallons per day.
Locations of septic systems were obtained from the County of Santa Cruz Environmental Health Services
Department (EHS) and Monterey County EHS Department. The level of ongoing data management and quality
differs between the two counties, with Santa Cruz having an active data management program including
locations, failures, age, and pump dates. In contrast, Monterey County retains spatial information using a
private consulting company and did not have access to the data at time of request. Maps provided by
Monterey County EHS using database outputs illustrated which parcels in the county contained septic system
permits. These maps were used by 2NDNATURE to digitize these locations in GIS to represent septic locations.
The Monterey County EHS database may only have records for 50-60% of the systems that are actually installed,
so the resulting septic maps would show trends more so than an actual complete inventory. Figure 5.6
presents the distribution of the 4,472 existing septic systems within the Pajaro Valley (3,289 in Santa Cruz Co
and 1,183 in Monterey County), overlain with the soils water holding capacity. Septic datasets provided by the
two counties were adjusted to remove any septic systems within the City of Watsonville sewer boundary as
well as points located within open water bodies. Additional QA/QC of these data should be performed by the
respective counties.
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5.5
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GROUNDWATER RECHARGE LOCATIONS
The USGS Pajaro Valley Hydrologic Model (PVHM) identifies two primary inputs to the PVGB as direct recharge
(or distributed recharge direct from the land surface) and stream recharge via the stream bed of the local
surface water stream network (see Table 3.1). The spatial distribution of these recharge zones varies
throughout the PVWMA area and a large amount of past research has been conducted to identify locations
where groundwater recharge to the local aquifers occur. In an effort to compile existing documentation on
recharge zones in the PVGB area, Figure 5.7 summarizes recharge zone mapping by Hecht et al. (2007) and the
County of Santa Cruz. Also included in Figure 5.7 is the low soil water holding capacity soil layer mapped and
presented in Figure 5.1. All three of these spatial data sources indicate the high relative recharge occurring the
area along San Andreas Road in the northwestern portion of the PVGB, land surrounding the town of Corralitos
and the Las Lomas area to the south. The spatial distribution of primary recharge zones generally align with the
location of the low water holding capacity soils, providing additional evidence in support of critical role soil
properties play in constituent leaching to groundwater.
Figure 5.8 is a stream-bed vertical conductivity classification based on PVHM parameters and field
measurements conducted by the USGS. The darker blue reaches are where groundwater recharge occurs at a
greater frequency and rate, notably the upper reaches of the Pajaro River and reaches located on Corralitos
Creek, that are assumed to be frequently net losing streams.
5.2
SALT LOADING RISK ANALYSIS
The priority potential sources of salt to the local groundwater basin are:

Seawater intrusion

Irrigation practices, particularly with waters containing elevated salts (>1000 mg/L TDS)

Stream recharge of water with elevated salts (>1000 mg/L TDS)
4.1.7
SEAWATER INTRUSION
The existing condition maps for TDS and chloride (Section 4.2.1.1 and 4.2.1.2) indicate that seawater intrusion is
the greatest source of salts to the aquifer system, with approximately 20% of the groundwater within the basin
observed to have chloride concentrations in excess of 100 mg/L. While groundwater extractions for irrigation
and municipal water supply purposes continue in areas inland of the current seawater intrusion boundary
(Figure 4.5), PVWMA began water deliveries in 2002 through the newly constructed Coastal Distribution
System (CDS). Water delivered to coastal ranches through the CDS replaces groundwater that would
otherwise be pumped and is expected to reduce the magnitude of annual groundwater overdraft as well as the
rate of seawater intrusion (PVWMA 2012).
While the hydrogeology of the groundwater basin is complex, seawater intrusion will generally continue to
migrate inland if groundwater levels remain at or below sea level. Seawater intrusion is sensitive to
groundwater extraction rates relative to recharge and rate of inland seawater migration has historically varied
between wet and dry periods. A USGS report indicated that sustained future drought occurrence and
projected climatic changes that may increase their frequency pose a high risk of continued seawater intrusion
into regional groundwater basins if appropriate management actions are not implemented (Flint et al. 2012).
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5.8
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In order to remain consistent with the spatial representation of loading risk by source and constituent
presented below, the spatial loading risk from continued seawater intrusion is estimated using existing
locations of elevated chloride levels as high risk, areas within 0.5 miles of the current estimated boundary of
seawater intrusion as moderate, and areas beyond 0.5 miles as relatively low risk (Figure 5.9).
4.1.8
IRRIGATION SALT LOADING
The likelihood of irrigation water leaching nutrients below the crop roots, and thus the relative risk of salt
loading to the groundwater basin from agricultural irrigation is dependent on a number of factors. These
factors include: crop plant uptake rates, irrigation volume applied, soil moisture status, and soil water holding
capacity.
The sources of agricultural irrigation supply within the PVWMA area is either groundwater produced from
privately owned production wells on site, or delivered water produced by PVWMA water supply facilities such
as the Recycled Water Treatment Facility and the Harkins Slough Managed Aquifer Recharge and Recovery
(HS-MAR) Facilities. From 2009-2011 the PVWMA delivered an average of 2600 AF/yr to coastal ranches. Figure
4.13 presents the shows the Delivered Water Zone, within which growers are able to irrigate with water
provided through the Coastal Distribution System.
Irrigation TDS content is either the existing average annual TDS concentration of the local groundwater as
applied by user on local well water (Figure 4.3) or assumed to be of moderate quality 1 (450 mg/L – 1,000 mg/L)
for all agricultural users located within the delivered water zone (Figure 4.13). Based on the interaction of
these factors an irrigation salt loading matrix was created (Table 5.4) and used to generate Figure 5.10. Table
5.5 summarizes the acreage of results displayed in Figure 5.10.
Table 5.4: Risk matrix to determine relative irrigation salt loading risk within PVGB.
Water Holding Capacity (AF)
Water USE
(AF/acre/year)
All categories
High
(2.4 - 3.0+)
Mod
(1.8 - 2.3)
Low
(0.5 - 1.7)
All categories
1
Irrigation
Water TDS
(mg/L)
High
(>1000)
Mod
(450-1000)
Mod
(450-1000)
Mod
(450-1000)
Low
(<450)
Very High
High
Mod
Low
(>1.5)
(1.25 - 1.5)
(0.75 - 1.25)
(< 0.75)
MOD RISK
HIGH RISK
HIGH RISK
HIGH RISK
MOD RISK
MOD RISK
HIGH RISK
HIGH RISK
LOW RISK
MOD RISK
MOD RISK
HIGH RISK
LOW RISK
LOW RISK
MOD RISK
MOD RISK
LOW RISK
LOW RISK
LOW RISK
LOW RISK
Based on the average TDS concentration of hundreds of delivered water samples collected by PVWMA staff.
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Table 5.5: Acreage summary of categorical salt loading risk of agricultural lands (Figure 5.10).
(Note that the total land use mapped in Figure 5.10 is limited by the extent of the available PVWMA
groundwater monitoring network.)
Irrigation salt loading risk
Acres
% of total Ag-Land
HIGH
6,647
25%
MODERATE
9,669
36%
LOW
10,285
39%
Total agricultural land use
26,601
100%
Non-agricultural land use
4.1.9
39,400
SURFACE WATER INFILTRATION SALT LOADING
The locations of greatest risk of surface water recharge as a source of salts to the PVGB are identified by
intersecting the PVWMA surface water salt concentrations for Q3 with the USGS stream permeability
distribution. The third quarter represents the dry season and typically poorest surface water quality conditions.
The summer months is also the time of year when shallow groundwater elevations are at seasonal lows and
recharge via stream channels is expected to be the greatest, though in some instances streams run dry during
this time. Table 5.6 is the risk matrix used to identify the relative expected impact to local groundwater salt
content for each of the PVWMA sampling locations. Low risk sites are either locations where stream recharge
is relatively low or water quality data indicate infiltration of waters with very low salt content, which is desired
for recharge. Figure 5.11 indicates that areas where high salt surface waters are introduced to losing stream
reaches are the Pajaro River between Chittenden Gap and Murphy Crossing. Intrinsic tracers such as boron and
chloride that have high groundwater concentrations in the area support this assumption (Ref). The persistent
poor Pajaro River water quality at the eastern PVWMA boundary through time and the high vertical
conductivity of this upper reach has been attributed as the cause of the elevated TDS in groundwater in the
East Area (Munster, Hecht, Lockwood – GRA 2010 and Figure 4.4). Reaches of moderate salt loading risk are
located in the north eastern areas on Corralitos Creek and other tributaries.
Table 5.6: Risk matrix to determine stream infiltration salt loading risk within PVGB to produce
Figure 5.11.
Q3 TDS MEAN concentration (Figure 4.10)
Stream bed vertical
Very Low
Low
Mod
High
conductivity
< 450 mg/L
1000 > 450 mg/L
1800 > 1000 mg/L
> 1800 mg/L
LOW RISK
MOD RISK
HIGH RISK
HIGH RISK
LOW RISK
MOD RISK
HIGH RISK
HIGH RISK
LOW RISK
MOD RISK
MOD RISK
MOD RISK
LOW RISK
LOW RISK
LOW RISK
LOW RISK
LOW RISK
LOW RISK
LOW RISK
LOW RISK
Very High
(>3.0 m/day)
High
(1.0-3.0 m/day)
Mod
(0.25-1.0 m/day)
Low
(0.01-0.25 m/day)
Very Low
(<0.01 m/day)
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NUTRIENT LOADING RISK
Figure 4.2 summarizes potential sources of nitrogen-NO3 to the PVGB. Additional evaluations of the potential
spatial distribution and potential magnitude resulted in the identification of priority potential sources of
nutrients to the PVGB listed below. The other sources noted in Figure 4.2 are expected to be relatively
insignificant annual contributors of nitrogen-NO3 to PVGB and the supporting rationale is provided at the end
of this section.

Fertilizer applications associated with agricultural practices

Sewer exfiltration

Septic exfiltration or failures

Localized recharge of surface waters
4.1.10 AGRICULTURAL FERTILIZER
Relative risk of nitrate loading to the groundwater basin from fertilizer applications on agricultural lands is
assumed to be dependent upon three critical factors:
1.
Soil water holding capacity (Figure 5.1),
2.
Annual amount of nitrogen applied as fertilizer (Section 5.1.4/Figure 5.4), and
3.
Annual irrigation volumes (Section 5.1.3/Figure 5.3) as a function of crop type.
N removal rates from agricultural lands by crop harvest are not considered in this analysis since it is only
determining relative risk of nitrate loading. Lands not agriculturally cultivated based on the available PVWMA
land use datasets (2011 and 2012) are assumed to be relatively insignificant agricultural fertilizer sources. These
critical factors are integrated based on Table 5.7 and presented in an agricultural fertilizer risk map (Figure
5.12). An acreage summary of the relative nitrate loading risk is presented in Table 5.8.
Table 5.7: Risk matrix to determine relative agricultural fertilizer nitrate loading risk within
PVWMA area.
Water
(AF/acre/year)
High
(2.4 - 3.0+)
Mod
(1.8 - 2.3)
Mod
(1.8 - 2.3)
Low
(0.5 - 1.7)
Low
(0.5 - 1.7)
Fertilizer
(lbs. N/acre/year)
High
(150 – 250+)
High
(150 – 250+)
Mod
(76 - 149)
Mod
(76 - 149)
Low
(35 - 75)
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Very High
(1.5 - 2.3)
Water Holding Capacity (AF)
High
Mod
(1.25 - 1.5)
(0.75 - 1.25)
Low
(< 0.75)
MOD RISK
HIGH RISK
HIGH RISK
HIGH RISK
MOD RISK
MOD RISK
HIGH RISK
HIGH RISK
LOW RISK
MOD RISK
MOD RISK
HIGH RISK
LOW RISK
LOW RISK
MOD RISK
MOD RISK
LOW RISK
LOW RISK
LOW RISK
LOW RISK
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Table 5.8: Acreage summary of Figure 5.12.
Agricultural nitrate loading risk
Acres
% of total Ag-Land
HIGH
14,312
53%
MODERATE
10,224
38%
LOW
2,263
8%
Total agricultural land use
26,799
Non-agricultural land use
100%
43,158
4.1.11 SEWER EXFILTRATION
The relative risk of nitrate loading to the local groundwater from sewage exfiltration is assumed to be
dependent on the probability and/or annual amount of leakage and the soil water holding capacity, with low
saturation soils having a high risk of vertical migration of nutrients released from sewage. The probability of
leakage or exfiltration could be estimated based on age of lines, time since last retrofits or repairs, recent
integrity test results, or other spatially explicit information on relative condition of sewer lines. Therefore,
sewage risk is determined using the matrix in Table 5.9 and presented in Figure 5.13. Of the total 145 miles of
sewer lines within the PVWMA area, 19 miles are located in highly permeable soils and expected to be relatively
higher priority locations to protect against leaks.
Table 5.9: Sewer nitrate loading risk matrix.
Water Holding Capacity (AF)
Sewer Line
Very High
(1.5 - 2.3)
High
(1.25 - 1.5)
Mod
(0.75 - 1.25)
Low
(< 0.75)
LOW RISK
LOW RISK
MOD RISK
HIGH RISK
Information regarding sewer system age, integrity or other potential information was not available. Figure 5.14
shows reported Sanitary Sewer Overflows (SSOs) in the urban area from 2007-2013, indicating a number of
recent sewage leakages have occurred. SSOs include any leakage, spill, overflow, or other discharge of sewage
from sanitary sewer systems. Category 1 SSOs are those that spill at least 1000 gallons, or result in a discharge
to surface water or a storm drain that does not return to the sanitary sewer system. Category 2 SSOs are all
other overflows (State Water Resources Control Board 2011).
4.1.12 SEPTIC EXFILTRATION OR FAILURES
The relative risk of nitrate loading to the local groundwater from septic exfiltration is assumed to be
dependent upon the soil water holding capacity, the density of septic systems within a given area and the
probability of leakage of existing septic systems. The U.S. EPA (USEPA 1977) reports a threshold of 40 septic
systems per square mile or more as “relatively high”. In comparison, the average septic density within the
PVWMA area is 30 septic units per square mile, with the densities as high as 425 septic units per square mile
within Freedom, Corralitos and Las Lomas.
Given the available information, the relative risk of septic systems to leach nitrate to local groundwater is
based on the soil water holding capacity (Table 5.10) and presented in Figure 5.15. High septic density, high risk
locations are identified in Corralitos, Freedom and Las Lomas areas and Table 5.11 summarizes the distribution
of septic units in each risk category.
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Table 5.10: Septic system nitrate loading risk matrix.
Water Holding Capacity (AF)
Septic System
Very High
(1.5 - 2.3)
High
(1.25 - 1.5)
Mod
(0.75 - 1.25)
Low
(< 0.75)
LOW RISK
LOW RISK
MOD RISK
HIGH RISK
Table 5.11: Summary of Figure 5.15.
Number of
SC / Mo
septic systems
County
HIGH
1889
1023 / 866
42%
MODERATE
2152
1955 / 197
48%
LOW
431
Septic nitrate loading risk
Total
311 / 120
4472
%
10%
100%
According to the septic failure records maintained by Santa Cruz County EHS, septic systems in the PVWMA
have a recent failure rate of 1% per decade, or less than 10 units per year. No comparable data was available for
Monterey County septic systems.
4.1.13 SURFACE WATER RECHARGE NITRATE LOADING
The locations of greatest risk of surface water recharge as a source of nitrogen-NO3 to the PVGB are identified
by intersecting the PVWMA surface water NO3 concentrations for Q3 with the USGS stream permeability
distribution. The third quarter represents the dry season and typically poorest surface water quality conditions.
The summer months is also the time of year when shallow groundwater elevations are at seasonal lows and
recharge via stream channels is expected to be the greatest, though in some instances streams run dry during
this time.
Table 5.12 is the risk matrix used to identify the relative expected impact to local groundwater nitrogen-NO3
content for each of the PVWMA sampling locations. Low risk sites are either locations where stream recharge
is relatively low or water quality data indicate infiltration of waters with low NO3 content, which is desired for
recharge. Figure 5.16 indicate the relative distribution of nitrate stream infiltration risk, noting the issue
associated with inherited poor water quality from the upper Pajaro River watershed that is transported to a
reach with very high stream bed vertical conductivity. Reaches of moderate concern are located on Corralitos
Creek.
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Table 5.12: Risk matrix to determine stream infiltration nitrate loading risk within PVGB.
Q3 nitrate-NO3 MEAN concentration (Figure 4.7)
Stream bed
vertical
conductivity
Very High
(>3.0 m/day)
High
(1.0-3.0 m/day)
Mod
(0.25-1.0 m/day)
Low
(0.01-0.25 m/day)
Very Low
(<0.01 m/day)
Very Low
Low
Mod
High
< 10 mg/L
10 > 45 mg/L
100> 45 mg/L
> 100 mg/L
LOW RISK
MOD RISK
HIGH RISK
HIGH RISK
LOW RISK
MOD RISK
HIGH RISK
HIGH RISK
LOW RISK
MOD RISK
MOD RISK
MOD RISK
LOW RISK
LOW RISK
LOW RISK
LOW RISK
LOW RISK
LOW RISK
LOW RISK
LOW RISK
4.1.14 SUBORDINATE NITRATE SOURCES
The conceptual model of simplified nitrogen cycle (Figure 4.2) contains a number of nitrogen sources for which
a risk analysis was not conducted. Available local information, other groundwater pollutant loading studies and
best professional judgment were used to determine that the sources below are expected to be insignificant
contributions (< 10%) to the annual PVGB nutrient budgets. Below provides the rationale to support these
determinations.
Irrigation water: It is expected that the nitrate content of irrigation water within the PVGB can range
significantly depending upon the source. The average nitrogen-NO3 concentrations of the Delivered Water
Zone is 23 mg/L (Section 4.2.3) and growers using local groundwater can irrigate with water ranging from 10 to
100+ mg/L depending upon the location and season (Figures 4.7 and 4.8). Given that optimal crop cultivation
can be achieved by managing a delicate balance of needed biologically available nitrogen by the plants, nitrate
enriched irrigation waters is considered an opportunity for growers to implement practices to manage fertilizer
additions rather than a risk to groundwater leaching.
Riparian zone land use risk. Lands in close proximity to riparian zones can be hydrologically connected to
surface water systems, posing a potential for greater risk of surface water constituent loading during storm
runoff events. A simple riparian zone risk layer was created that identifies high risk lands located within 150 ft
of the surface water stream and moderate risk within 500 ft (Figure 5.17). The riparian areas upstream of highly
permeable stream reaches may be further prioritized as locations to reduce potential surface water sources.
Urban stormwater runoff. Stormwater runoff can possess nitrate levels above natural concentrations as a
result of atmospheric deposition of nitrous oxide from automobile exhaust and industrial inputs. Accumulation
of these constituents on impervious surface can result in stormwater concentrations of nitrate, but the USEPA
reports average nitrate concentrations in highly urbanized stormwater to be < 1 mg/L nitrate as N. These
relatively low nitrate concentrations, combined with relatively low impervious area within the PVMWA
boundary (< 15%) and the rural character of this area, led to the conclusion that potential contribution of
stormwater runoff to local groundwater is likely insignificant compared to other sources and therefore was not
analyzed.
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Atmospheric Deposition: Nitrous oxide from automobile or industrial exhaust is the primary source of nitrogen
in atmospheric deposition. The relatively rural land use within the PVGB and the localized and small area of the
City of Watsonville suggest atmospheric deposition is not a significant source of nitrate to the local
groundwater. However, the implementation of local mitigation strategies to reduce greenhouse gas emissions
will have some complementary benefits of reducing local atmospheric deposition of nitrogen and could reduce
the nitrate concentrations in local stormwater runoff.
Animal Waste: A review of the existing land use within the PVGB indicates no significant presence of dense
animal feed lots or other types of dense livestock operations. A number of rural properties possess modest
numbers of horses, cattle or other livestock, but based on the sparse distribution and density of livestock in the
PVGB animal waste is not considered a significant or priority source of nitrate to groundwater. Livestock and
pet best management practices that include keeping livestock and waste out of riparian zones and
encouraging proper disposal of animal waste can reduce the localized nitrogen inputs of this source.
4.1.15 NITRATE LOADING COMPARISONS
A series of risk maps have been created based on best available local data, research and expected relative risk
of nitrate loading to groundwater from agricultural practices, sewer and septic systems and surface water
recharge independently. Findings from a recent and extensive nitrate groundwater loading analysis conducted
on Tulare Lake Basin and the Salinas Valley (Viers et al. 2012) are used to provide a simple estimate of average
annual nitrate loading to the PVGB from agriculture, sewer and septic system inputs, and wastewater
treatment plant (WWTP). Nitrate loading from stream recharge is estimated using relevant values from the
USGS groundwater budget (Section 3.0) and the PVWMA surface water quality dataset (4.2.2).

Agriculture (1,742 t N/yr): Using a similar approach to assign a typical N fertilizer application rate by
crop type, Viers et al. (2012) integrated agricultural land use maps to estimate the annual fertilizer
inputs within the area of interest. Using a mass balance approach of annual N losses from agricultural
lands including harvest and runoff, Viers et al. (2012) estimates a project area average of 137 lbs
N/acre/yr leaches from agricultural lands to groundwater. The Viers et al (2012) project area
encompasses over 3 million acres of agricultural land within the Tulare and Salinas Groundwater
Basins. A portion of the project area is located within Monterey County within the Salinas Valley, and
the Monterey County groundwater nitrate loading intensity is estimated to be 123 lbs N/acre/yr. The
midpoint between the Viers et al (2012) project average and Monterey County estimates is 130 lbs
N/acre/yr.
In order to verify an estimated 130 lbs N leaches to groundwater per acre of agricultural land per year
is applicable to the PVGB we compare of the crop type distribution and the annual fertilizer application
rates between the Tulare/Salinas Basin and the PVGB. Table 5.13 summarizes the crop type distribution
between the Viers et al (2012) and the PVGB and suggests that PVGB has a much higher relative
distribution of high fertilizer demand crop types than the Tulare/Salinas Basin, approximately 40% to
67% respectively. A comparison of the spatially weighted average fertilizer application rate per acre of
agricultural land per year is also made. In the Tulare and Salinas Basins 225,000 t N/yr is assumed to be
applied to 3.1 million acres or 145 lbs N/acre/yr. Using the midpoint application rate for each of the
fertilizer application categories (Table 5.3), an estimated 2,432 t N/yr is applied as fertilizer on over
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26,800 acres of agricultural land in the basin or an average fertilizer application rate per acre of
agricultural land of 180 lbs N/yr (Table 5.14). Given that the high fertilizer demand crop type
distribution and the annual average fertilizer application rate are both higher in PVGB than
Tulare/Salinas Basin, the estimate of 130 lbs N leaching to groundwater/acre/year appears
conservative.
Viers et al. (2012) assumes a number of other N inputs to agricultural lands including manure
applications, atmospheric deposition, and irrigation waters that are not addressed as potential N
sources to Pajaro Valley agricultural lands. However, synthetic fertilizer is determined to be greater
than 50% of these sources in the Tulare and Salinas Basins and some of these other source
contributions to Pajaro Basin lands may be relevant. Given the crop distribution, land management
and geographic proximity to the Pajaro Valley, 130 lbs N/acre/yr is used, yielding an estimated annual
nitrate load leaching to groundwater from agricultural lands of 1,742 t N/yr. Given the significantly
higher density of high fertilizer and water demand crop types in the Pajaro Valley it is likely that the N
groundwater leaching rate is higher than the assumed value of 130 lbs N/acre/yr.
Table 5.13: Comparison of crop group distribution typically receiving relatively high annual N
fertilizer applications for area analyzed by Viers et al. (2012) and the PVWMA area, indicating a
much higher proportion of high fertilizer demand crops within the PVWMA. High fertilizer
demand crop types by Viers et al. (2012) grouping include field crops, haylage and vegetables.
PVWMA high fertilizer crops include vegetables, strawberries and nurseries.
Crop type area in Tulare/Salinas Basins
Table 6 (b) p. 29 from Viers et al (2012)
Crop Group
ACR [acres]
CAML [acres]
Subtropical
219,173
251,299
Treefruit
175,393
217,705
Nuts
313,525
459,833
Cotton
640,705
604,479
Field Crops
356,024
Haylage
Crop type area in PVGB
Figure 5.2
PVWMA
Crop Group
land use
data [acres]
Vegetable row crops
9,138
Strawberries
7,994
496,467
Horticultural Nurseries
1,343
586,700
385,561
Caneberries
5,003
Alfalfa
360,450
368,375
Rice
5,184
12
Deciduous
(Apple Orchards)
2,179
Vegetables
648,889
441,288
Other, Unknown
Agriculture, Vine/Grapes
1,142
Grapes
399,572
518,548
Pastureland
(2006 land use survey)
761
3,705,617
3,743,567
1,591,613
1,323,316
43%
35%
TOTAL agricultural land
TOTAL high fertilizer
demand crops
% high fertilizer crops of
total
TOTAL agricultural land
TOTAL high fertilizer
usage crops
% high fertilizer crops of
total
27,560
18,475
67%
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Table 5.14: Calculation of spatially weighted average of N applied per year per acre of
agricultural land in PVWMA.
Fertilizer application rate
category
(Table 5.3)
Fertilizer application rate
used in estimate
(N lbs/acre/yr)
Acreage in
PVWMA
(acres)
t N /yr per
application rate
category
Very High
250
7,772
971
High
200
10,703
1070
Moderate
100
7,182
359
Low
55
1,142
31
26,799
2,432
Totals
Average fertilizer application rate in PVWMA (t N/acre/yr)
0.09
Average fertilizer application rate in PVWMA (lbs N/acre/yr)
180
Average fertilizer application rate in Tulare/Salinas (t N/acre/yr)
0.07
Average fertilizer application rate in Tulare/Salinas (lbs N/acre/yr)
145

Sewer (67 t N/yr): Viers et al. (2012) relied upon national data that municipal sewer system leakage
rates range from 1% to 25% of the total sewage generated. Total sewage generated can be estimated
by the average N excretion rate per capita (13.3 g N/capita/day) and the population treated by local
sewer system. The Watsonville Wastewater Treatment Plant treats an estimated 55,000 people,
yielding a potential high end estimate of annual N leaching rate to groundwater of 67 t N/yr, assuming
a 25% loss rate.

Septic (66 t N/yr): Viers et al. (2012) use national data that assumes septic systems retain 15% of the
total N delivered. There are nearly 4500 septic systems located in the PVGB and assuming an average
household of 3.6 persons (US Census City of Watsonville 2010), an estimated 16,100 people are on
septic in the area, yielding an estimated 66 t N/yr leakage rate to groundwater.

WWTP (80 t N/yr): Viers et al. (2012) noted that localized N inputs from waste water treatment plants
can be significant as a result of infiltration via percolation ponds. In order to provide a simple order of
magnitude comparison to other sources, the WWTP in Watsonville treats an average of
6.6 million GPD and Viers et al. (2012) estimates an average N concentration of WWTP percolation
water in the Tulare and Salinas Basins to be 16 mg/L. Even if 50% of the treated water at the WWTP
were infiltrated the total annual N load to groundwater would be 80 t N/yr. However it must be noted
that the Watsonville WWTP pipes secondary treated effluent in an outfall located 2 miles offshore in
the Monterey Bay, making this likely an over estimate. In addition, the water recycling facility has
reduced the volume of secondary effluent discharged to Monterey by approximately 2,500 AFY, and
has the potential of reducing discharges by up to 4,000 AFY.

Surface water recharge (746 t N/yr): Primary nitrate inputs to surface waters within the basin are
either generated from agricultural land use practices or inherited from the upper Pajaro River
Watershed. A simple surface water recharge nitrate loading estimate is based on the USGS average
annual streamflow infiltration (SI;Table 3.1) and an average surface nitrate concentration of 30 mg/L
based on the PVWMA surface water monitoring dataset (Section 4.2.2) yields an estimated 746 t N/yr
via surface water recharge. This estimate ranges from 250 to 2500 t/yr if average stream nitrate
concentration range from 10 to 100 mg/L, respectively. Given the significant spatial and temporal
variations in surface water nitrate concentrations and stream bed vertical conductivity, the locations
where stream recharge are of greatest concern are indicated in Figure . While recharge is identified as
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the 2nd highest source of nitrogen to the PVGB, the source of nitrogen to surface waters within the
basin are predominately agriculture with farms located within the riparian buffer (Figure 5.17). In the
instance of inherited nitrate loads from the upper Pajaro River Watershed, local N management
strategies will have little relevance, and thus coordination and collaboration to reduce N sources with
IRWM partners is a PVWMA priority.
Based on the above estimates, approximately 2,700 t/yr of N leached to the PVGB on an average annual basis.
The priority source is direct infiltration via agricultural land uses, followed by streamflow infiltration. The
stream infiltration risk varies spatially and temporally (Figure 5.16), but the likely primary sources of elevated
nitrate in steams are local agricultural practices and/or inherited poor water quality transported into the
PVWMA area from the upper watershed. While the accuracy of these estimates could be debated, the relative
significance of agricultural practices on N loading to groundwater identified in the PVGB is consistent with the
Viers et al (2012) finding that 96% of all N leached to groundwater is derived from agricultural sources.
Table 5.15. Summary of annual N leaching to groundwater estimates for PVGB priority sources.
N Source
Average annual N load
to groundwater (t/yr)
Agricultural land use
Septic systems
67
Sewer systems
66
WWTP
80
Streamflow infiltration
746
Total PVGB
5.4
1,742
2,700 t/yr
KEY LOADING ANALYSIS FINDINGS
A series of risk maps have been created based on best available local data, research and expected relative risk
to groundwater for each priority source of both salts and nutrients. These maps provide specific locations
where the greatest potential risks of salt and nutrient loading to groundwater exist throughout the PVGB.
These maps provide guidance to focus subsequent steps of the PVWMA SNMP, specifically objective
development and identification of management strategies necessary to achieve stated objectives. Consistent
with other regional, state and national findings, agricultural land practices in the Pajaro Valley are the dominant
source of salts and nutrients to the local groundwater basin.

The priority controllable sources of salt loading to local groundwater are seawater intrusion and
irrigation of agricultural lands.
o Seawater Intrusion: PVWMA estimates 11,100 acres of land possesses seawater contaminated
groundwater as a result of years of aquifer overdraft to support irrigation, and to a lesser
extent, municipal water supply (Figure 5.9).
o Irrigation of agricultural lands: Irrigation salt accumulation risk increases on low permeability
soils and when irrigation water TDS content exceeds 1000 mg/L. Currently, 9,000 acres of land
are within high risk of salt leaching to groundwater a result of current irrigation practices
(Figure 5.10).
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PVWMA Salt and Nutrient Management Plan (SNMP) Development
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
Streamflow infiltration of elevated TDS waters delivered from the upper Pajaro River watershed are
high risk along Pajaro River reaches just downstream of the PVWMA eastern boundary.

An agricultural fertilizer risk analysis integrated local and regional data on crop water usage rates, crop
fertilizer usage rates, and soil characteristics to create a relative agricultural N loading risk map for the
PVGB. The result is a spatial distribution of the agricultural lands within the PVGB that are currently
considered relatively high risk. These locations should be priorities for implementation of land specific
strategies that will reduce the amount of N available to leach into the local groundwater basin.

Septic systems are only effective at retaining 15% of the total N generated. Locations of high density
septic systems will have higher overall N leaching to groundwater. High density septic systems located
on permeable soils in Corralitos, Freedom and Las Lomas (Figure 5.15) where density exceed 400
septic units/ sq mile are the priority locations for septic system management strategies to maximize
effectiveness and minimize leaks and failures.

The priority controllable sources of nitrate loading to local groundwater are associated with
agricultural land uses. A mass balance comparison of the average annual nitrate loads to the PVGB
suggests agricultural land is the primary source of N (Table ). The second greatest source of nitrate to
groundwater is estimated to be streamflow infiltration, but the primary sources of elevated nitrate to
surfaces waters are expected to be agricultural practices along riparian corridors within the PVGB and
inherited poor water quality water from the upper Pajaro River Watershed. The results of the PVGB
mass balance source comparison is similar to the findings by Viers et al. (2012) that suggested 96% of
the annual nitrate loaded to groundwater is derived from agricultural practices.
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6.0 REFERENCES
6.1
LITERATURE CITED
Cahn, Michael et al. 2011. Strawberry water use on the Central Coast. UC Cooperative Extension Monterey
County Crop Notes July – Aug. 2011. Salinas, CA
Cahn, Michael. 2012. Unpublished data for water usage by crops grown on the Central Coast of California. Data
from trials conducted 2006 – 2012.
Flint, L.E., and Flint, A.L. 2012. Simulation of climate change in San Francisco Bay Basins, California: Case studies
in the Russian River Valley and Santa Cruz Mountains: U.S. Geological Survey Scientific Investigations Report
2012–5132, 55 p.
Hart, J. et al. 2006a. Nutrient Management Guide – Cane berries. EM 8903-E. Oregon State University
Extension Service, Corvallis, OR
Hart, J. et al. 2006b. Nutrient Management Guide –Strawberries, Western Oregon. FG14. Oregon State
University Extension Service, Corvallis, OR
Helsel, D.R., 2005. Nondetects and Data Analysis. Wiley. p. 268.
Le Strange, Michelle et.al. 2010. Broccoli Production in California. UCANR publication 7211. UC Cooperative
Extension. Oakland, CA.
Mahler, R.L. and D.L. Barney. 2000. Northern Idaho Fertilizer Guide – Blueberries, Raspberries, and
Strawberries. CIS 815. University of Idaho Cooperative Extension. Moscow, ID.
Monterey County Water Resources Agency (MCWRA). 2010. 2010 Ground Water Summary Report. Figure 5.3.
2010 reported acre-feet/acre by crop type and hydrologic subarea.
Munster, Hecht, Lockwood – Groundwater Resources Association GRACast Presentation, 2010.
Nolan, B.T., Hitt, K.J., and Ruddy, B.C. 2002. Probability of nitrate contamination of recently recharged
groundwaters in the conterminous United States. Environmental Science & Technology, 36 (10), 2138-2145.
Pajaro Valley Water Management Agency (PVWMA), 2012. 2012 Basin Management Plan Update. Board Review
Draft. November 2012.
Pettygrove, Stuart et.al. 2003. Nutrient Management in Cool Season Vegetables. UCANR publication 8098. UC
Cooperative Extension. Oakland, CA.
Platts, B. 2011. Summary of Nitrate Surveys for PVWMA, 2009 – 2011. Unpublished data. PVWMA, Watsonville,
CA
Prichard, Terry L. et al. 1989. Orchard water use and soil characteristics. California Agriculture, July – August
1989. Vol. 43: 23 – 25. Oakland, CA
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PVWMA Salt and Nutrient Management Plan (SNMP) Development
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Regional Water Quality Control Board 2010. http://www.waterboards.ca.gov/gama/docs/coc_salinity.pdf
Regional Water Quality Control Board 2011. Water Quality Control Plan for the Central Coast Basin. June 2011.
Righetti, T. et al. 1998. Nutrient Management Guide – Apples. EM 8712. Oregon State University Extension
Service, Corvallis, OR
Smith, Richard et.al. 2008. Artichoke Production in California. UCANR publication 7221. UC Cooperative
Extension. Oakland, CA.
Turini, Tom et.al. 2010. Iceberg Lettuce Production in California. UCANR publication 7215. UC Cooperative
Extension. Oakland, CA.
U.S. Environmental Protection Agency (US EPA). 1977. The report to Congress—Waste disposal practices and
their effects on groundwater. EPA Publication No. 570977002. Washington, DC: Office of Water Supply, Office
of Solid Waste Management Programs.
US EPA 2009. EPA National Primary Drinking Water Regulations: US Environmental Protection Agency 2009. EPA 816-F-090004, May 2009.
USDA. National Agricultural Statistics Service (NASS). 2011a. Agricultural Chemical Use Fruit Crops 2011.
www.nass.usda.gov/surveys/Guide_to_NASS_Surveys/Chemical_use
USDA. National Agricultural Statistics Service (NASS). 2011b. Agricultural Chemical Use Vegetable Crops 2010.
www.nass.usda.gov/surveys/Guide_to_NASS_Surveys/Chemical_use
Viers, J.H., Liptzin, D., Rosenstock, T.S., Jensen, V.B., Hollander, A.D., McNally, A., King, A.M., Kourakos, G.,
Lopez, E.M., De LaMora, N., Fryjoff-Hung, A., Dzurella, K.N., Canada, H.E., Laybourne, S., McKenney, C., Darby,
J., Quinn, J.F. & Harter, T. (2012) Nitrogen Sources and Loading to Groundwater. Technical Report 2 in:
Addressing Nitrate in California’s Drinking Water with a Focus on Tulare Lake Basin and Salinas Valley
Groundwater. Report for the State Water Resources Control Board Report to the Legislature. Center for
Watershed Sciences, University of California, Davis. http://groundwaternitrate.ucdavis.edu/files/139110.pdf
6.2
DATASETS USED
City of Watsonville Water Quality Database (2013) – Groundwater concentrations of TDS, Cl, and nitrate-NO3.
PVWMA 2011 and summarized in table – Pajaro Valley Groundwater Basin Land Use and Estimated Irrigation
Volumes.
PVWMA Water Quality Database (2013) – Surface Water and Groundwater concentrations of TDS, Cl, and
nitrate-NO3.
Natural Resources Conservation Service. 2009. Soil Survey Geographic (SSURGO) database for Santa Cruz and
Monterey Counties, California: US Department of Agriculture, Natural Resources Conservation Service, digital
data, accessed June, 2012 at URL http://soildatamart.nrcs.usda.gov.
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State Water Resources Control Board 2011. Sanitary Sewer Reduction Program – sanitary sewer overflow
incident map. URL http://www.waterboards.ca.gov/water_issues/programs/sso/sso_map/sso_pub.shtml
US Census Bureau 2010. American Factfinder Population Data for City of Watsonville, Santa Cruz County,
California: US Census Bureau, data tables, URL http://factfinder2.census.gov/faces/nav/jsf/pages/index.xhtml
Draft Sections 1-5 Stakeholder Review: March 2013