1.
INTRODUCTION
The Horsley Witten Group (HW) in collaboration with the Massachusetts Wetlands Restoration
Program (MWRP) and the Coonamessett River Restoration Working Group (CRRWG) has
completed a representative hydrologic evaluation of the Coonamessett River area in Falmouth,
Massachusetts. This hydrologic assessment is one of the initial tasks to help make future
planning-level decisions about potential wetland restoration along the river. The tasks
undertaken in this hydrologic assessment included background data collection and mapping, field
installation of new hydrologic monitoring points, field hydrologic data collection, water table
mapping, and water budget determination.
2.
BACKGROUND DATA COLLECTION
Due to the location of the Coonamessett River downgradient from active groundwater
remediation activities associated with the Massachusetts Military Reservation (MMR), a
significant number of monitoring wells and staff gages were already present in the Coonamessett
River watershed, particularly in its northern reaches. Information about these pre-existing data
points is maintained primarily by the Air Force Center for Environmental Excellence (AFCEE)
and the United States Geological Survey (USGS). Both agencies supplied important information
concerning applicable data points (coordinates, elevations, and previously gathered data).
After receiving the available background information, HW mapped all of the potential preexisting data points and began a program to field verify their locations. The majority of the
potential points were identified in the field and their mapped locations were verified or corrected,
as appropriate. Subsequently, all of the available pre-existing monitoring wells were
characterized as either water table wells (open to the aquifer at a depth of less than 50 feet below
the water table), intermediate wells (open to the aquifer at a depth of between 50 and 100 feet
below the water table), or deep wells (open to the aquifer at a depth of greater than 100 feet
below the water table) in order to identify each well’s utility for different monitoring purposes.
Figure 1 illustrates all of the monitoring locations used in this study. Appendix A details the
characteristics of each of the monitoring locations used in this study
3.
INSTALLATION OF NEW MONITORING POINTS
Once all of the pre-existing data points had been accurately mapped and identified, HW selected
key locations for the installation of new monitoring wells and staff gages in order to improve the
resolution of the hydrologic data collection network in the Coonamessett River area. The
majority of these points were installed in the southern portion of the study area and along the
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western and eastern fringe of the watershed in order to improve the spatial distribution of
monitoring points and to better define the locations of the western and eastern groundwater
divides to the river. Two of these points, however, were installed to evaluate the quantity of
water Flax Pond adds or subtracts from the Coonamessett River.
A total of 12 additional monitoring points were installed as part of this study. Four of the new
points were installed by a truck mounted drill rig as water table monitoring wells in locations
where the depth to water was approximately 30 to 40 feet. These four deeper monitoring wells
were installed by Desmond Well Drilling of Orleans, Massachusetts on June 3rd and 4th of 2004.
Two of the other points were installed by hand as water table monitoring wells on June 8th in
locations where the depth to water was less than 10 feet. The remaining six locations were
installed on June 9th as surface water gaging stations along the Coonamessett River, in tributary
streams, and in nearby ponds. Figure 1 illustrates all of the monitoring locations used in this
study. Appendix A details the characteristics of each of the monitoring locations used in this
study.
Boring logs and construction details for the four deeper new monitoring wells are contained in
Appendix B. The hand-driven shallow wells are either 1 ¼ or 2 inches in diameter and consist of
3 feet of stainless steel screen attached to a sufficient length of galvanized steel riser to stick up
above the ground surface (3-6 feet of riser). These shallow wells are capped with either a
compression cap or a screw cap and the excavated space around the wells were backfilled with
native materials. Each surface water gaging station consists of a USGS-style staff gage attached
to a 1-inch by 4-inch piece of lumber secured to a galvanized pipe securely driven into the
bottom sediments. Nearby benchmarks were selected for each gaging station so that the
reference elevation of each gage could be easily updated in case a gage is disturbed in the future.
All new monitoring points and benchmarks were surveyed on June 16th and 17th for horizontal
and vertical location by Nord-East Survey, Inc, a surveying firm from Bedford, Massachusetts.
Appendix C lists the survey data for all of the HW-installed monitoring points and nearby
benchmarks.
4.
HYDROLOGIC DATA COLLECTION
On June 16, 2004, HW conducted a synoptic (measurements taken over a wide area at a specific
time) round of water level and river flow measurements throughout the Coonamessett River area.
Approximately 105 groundwater elevation measurements and 19 surface water elevation
measurements were made, along with 16 measurements of river flow. Figure 1 illustrates all of
the measurement points utilized during the synoptic round. Appendix A contains the
characteristics of all of the hydrologic data points, their horizontal and vertical coordinates, and
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their water level measurements obtained during the synoptic data collection round. Appendix D
contains the measured surface water flow data.
One difficulty related to the synoptic round of field data collection is worthy of further
discussion. Unfortunately, and unreported to HW, the AFCEE groundwater treatment facility for
the FS-28 plume was shut down for carbon replacement for approximately 24 hours prior to our
synoptic round of data collection. The system came back online shortly after our data collection
activities commenced. AFCEE had been informed beforehand of our intention to conduct this
synoptic round of hydrologic measurements.
In order to compensate for atypical groundwater conditions in the vicinity of the FS-28 extraction
well on June 16th, during the synoptic round, at the time the treatment facility was offline, a
second round of limited measurements was conducted on July 2nd. During this round,
groundwater levels were measured at eight locations proximal to the FS-28 extraction well and
one control location distal from the extraction well. The two rounds of data from the control
well were used to correct for ambient changes in groundwater elevation between the two
measurement rounds. Corrected groundwater elevations from those wells proximal to the
extraction well were then inserted back into the original June 16th data set in order to simulate
extraction conditions in the vicinity of the extraction well at a time (June 16th) when the
treatment facility was offline. This second round of July 2d data and the data correction activities
are shown in Appendix E. The two rounds of measurement data showed that prevailing
groundwater elevations in the vicinity of the FS-28 treatment facility were different by less than a
foot between conditions of treatment plant operation and shutdown. This small amount of
difference in a localized area, had little effect on the overall, 2-foot contour interval, water table
map of the Coonamessett River area.
5.
WATER TABLE MAPPING
The groundwater and surface water elevation data collected on June 16, 2004 were used to create
a water table map of the Coonamessett river area. Because the second round of water elevation
data (July 2nd, 2004) exhibited minimal differences from the initial June 16th round, as discussed
above, the initial June 16th data were used unaltered for the water table mapping. In locations
with clusters of wells screened at various depths, the elevation from the shallow, water table well
was utilized because that data was most appropriate for mapping near-surface water table
conditions. In areas with a less dense distribution of wells, groundwater elevations from deeper
wells were used if no data were available from shallow wells. The full set of water elevation data
were entered into the contour-mapping program “Surfer” (Golden Software, 1997) to create the
original rough water table contours. In order to make the computer-simulated contours as
realistic as possible, ponds of known elevation were surrounded with additional data points to
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simulate the geographic shape of the flat water table surface in these areas. The initial computergenerated contours were then overlain atop the GIS map of the Coonamessett River watershed
and the contours were altered slightly using basic hydrologic judgment (e.g. anomalous data
points were deleted and erratic contours were smoothed around surface water features). The
resulting water table map is shown in Figure 1.
As can be seen in Figure 1, groundwater flows from high elevations around Coonamessett Pond
to lower elevations at the southern end of the study area. Groundwater flows perpendicular to the
mapped elevation contours. The red line on the map encloses the groundwater contributing area
to the Coonamessett River. Groundwater inside the contributing area will likely flow to and
discharge to the river, while groundwater outside of the contributing area will not.
6.
SURFACE WATER FLOW ASSESSMENT
On June 16, 2004, at the same time as the synoptic round of water level measurements, HW
measured the volumetric flow rate at 16 locations along the Coonamessett River, and in its major
tributaries and ditributaries. As discussed above regarding atypical water level measurements,
atypical surface water flow conditions were also observed on June 16th due to the unanticipated
shutdown of the FS-28 treatment facility. Under normal operating conditions, the treatment
facility discharges treated groundwater to the river and that discharge constitutes a significant
percentage of the river’s flow in its upper reaches. The fact that the treatment plant was off-line
at the start of the June 16th measurement round, and then came back on line during the course of
measurements, means that some flow measurements included the plant discharge while others
did not. In order to compensate for this discrepancy, alterations to some of the field
measurements were made when the data was processed. Specifically, those flow measurements
which were likely to have not included the plant discharge were increased by the quantity
normally contributed by upstream plant discharge. Those flow measurements that were likely to
have included the plant discharge were left unaltered from their field-measured value. For
several stations in the central reach of the area, it was uncertain if plant discharge had been
included in the field measurement. These stations also had physical limitations that made
accurate flow measurements difficult and these stations were, therefore, removed from the flow
analyses. Appendix D summarizes all of the flow measurements and the corrections made to
adjust for treatment plant discharge anomalies. Figure 2 illustrates the spatial distribution of
adjusted flows throughout the study area.
As can be seen in Figure 2, flow increases in the Coonamessett River on its downstream journey
(from 0.32 cubic feet per second at the Coonamessett Pond spillway to 11.48 cubic feet per
second at the southern end of the study area) as is typical of gaining streams. Gaining streams
receive groundwater infux along their length and therefore gather flow in a downstream
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direction. Flow is also added to the river from the Broad River tributary (0.93 cfs) and the Flax
Pond tributary ( 1.17 cfs). A component of Flax Pond’s contribution to the river (0.89 cfs)
consists of water previously diverted out of the Coonamessett main stem at Pond 14.
7.
WATER BUDGET DETERMINATION
HW used the data collected in earlier phases of the project to create a representative, annual
water budget of the Coonamessett River. The water budget is a planning-level representation of
the various inputs and outputs to the system focused on explaining the components that control
river flow at the southern boundary of the study area, 69SG2009S ( Figure 1). Two endpoint
evaluations of the representative water budget are shown in Table 1. The first endpoint
evaluation is for current conditions and the second is for a hypothetical condition with no
cranberry bog operations. The two endpoints were chosen because they represent the most and
least intensive water-use scenarios likely for the river, respectively.
The different components of the water budget were determined in different ways. The majority
of the components were estimated by measuring the surface areas of various land cover types
(upland, wetlands, ponds, cranberry bogs) and multiplying those areas by the precipitationderived, long-term-average, areal recharge rate for each land cover. This calculation produces a
volumetric quantity of annual water available for groundwater recharge and, eventually, riverine
baseflow. The groundwater recharge rate for upland areas on the western cape is approximately
26 inches per year ( Masterson and Walter, 2000). Ponds tend to have an approximately equal
recharge rate as upland area because the greater evaporation from pond surfaces is balanced by
the lesser amount of plant transpiration, no loss of overland runoff, and some addition of
overland runoff from surrounding areas. Wetlands in this type of hydrologic setting tend to be
areas of zero recharge because they are generally acting as groundwater discharge locations and
because of the high rates of evaporation and transpiration. Managed cranberry bogs tend to have
a negative groundwater recharge rate because irrigation practices mean that the bogs will actually
use more water each year than precipitation alone can supply. A recharge rate of -17 inches per
year was used in this analysis as described in the USGS study of the Plymouth-Carver Aquifer
(Hansen and Lapham, 1992).
Surface water inflow and outflow to the system was directly measured as part of the synoptic
round on June 16, 2004 at the Coonamessett Pond spillway (69SG0057-04) and at the southern
end of the study area (69SG2009S), respectively (Figure 2). Groundwater input to the system
from Coonamessett Pond and other upgradient sources was calculated by subtraction after all of
the other components had been accounted. In other words, the difference between the amount of
water measured to have flowed out of the system and all of the known or estimated inputs to the
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system constitutes the amount that would have to have been added through groundwater inflow
from Coonamessett Pond and upgradient sources.
The different components of the representative water budget, described above, contain estimates
derived from long-term average data (e.g. precipitation-based, areal recharge rates) as well as
point data collected at a particular time that represents a snapshot of time-specific hydrologic
conditions (e.g. measured surface water flows and measured water levels). Although not ideal,
this combination of long-term average data with snapshot data is suitable for this type of
planning-level evaluation and representative water budget determination. Review of water level
data from nearby USGS Index wells indicates that hydrologic conditions at the time of this
study’s field measurements on June 16, 2004 were perhaps slightly drier than long-term average
conditions but not anomalous. If all of the data utilized were truly representative of long-term
average conditions than there would likely be slightly more groundwater and surface water inputs
and outputs to the system than presented in this representative water budget.
The representative hydrologic budget reveals several items of note. First, the water balance for
the Coonamessett River is dominated by groundwater influence. Approximately 28% of the
water input to the system is estimated to be added by local groundwater recharge south of
Coonamessett Pond, and approximately 69% is estimated to be added through more distant,
upgradient groundwater recharge. Only 4% of the water is estimated to be added as surface
water. Second, the vast majority of the water added to the system is estimated to originate as
groundwater recharge at or upgradient from Coonamessett Pond. This is not a surprise as the
groundwater contributing area to Coonamessett Pond, and further downgradient to the
Coonamessett River, extends many miles upgradient to the peak of the groundwater mound for
the Western Cape Aquifer in the Town of Sandwich. Groundwater recharged from this large
upgradient area will either discharge to Coonamessett Pond , where some will subsequently
recharge back to the aquifer, travel deep below the pond and discharge to the Coonamessett River
downgradient from the pond, or travel still deeper through the aquifer and discharge
downgradient from the study area. Third, the two endpoint alternatives assessed showed a
relatively minor estimated difference in total water budget going from current conditions to a
complete cessation of active bog maintenance. Approximately 4 million cubic feet of water per
year are estimated to be available for additional riverine flow if active cranberry bog management
were ceased. This increase in flow represents only approximately 1% of the total annual water
input to the system. Again, this is consistent with the estimated groundwater dominance of the
Coonamessett River’s annual water budget.
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8.
SUMMARY
This representative hydrologic assessment has improved the hydrologic toolset available to
evaluate riverine restoration options for the Coonamessett area in a number of important ways:

A database of available hydrologic monitoring points and their relevant characteristics has
been compiled for the Coonamessett River area, and those points have been mapped to
facilitate the collection of additional data in the future.

A snapshot round of groundwater level and surface water level measurements, accompanied
by surface water flow measurements, was collected that is more comprehensive than was
previously available for the entirety of the Coonamessett River area.

A water table map of the Coonamessett River area was created at a 2-foot contour interval
that is more refined than previously available for the entirety of the study area.

An assessment of surface water flow rates throughout the Coonamessett River watershed was
compiled and mapped that is more comprehensive than was previously available for the
entirety of the study area.

A representative water budget for the Coonamessett River groundwater contributing area was
estimated that provides a planning-level tool for evaluating the relevant importance of
various hydrologic components.
The following significant hydrologic observations are helpful for understanding the big-picture
hydrology of the Coonamessett River and considering how to evaluate potential riverine
restoration options:

The vast majority of the water supplying the Coonamessett River is derived from
groundwater that originated as precipitation-based recharge, and most of that groundwater
input originates upgradient from and at Coonamessett Pond.

Discharge from the AFCEE FS-28 groundwater treatment facility constitutes a significant
portion of the river’s flow in its upper reaches but a less significant component in its lower
reaches.

The current condition of wide-spread cranberry cultivation along the river is probably the
most water intensive of any likely riverine management option and, since a significant
amount of water (almost 12 cubic feet per second) is available to leave the system under
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current conditions (at the time of measurement), then most restoration options under
consideration would be hydrologically feasible and require a similar or lesser amount of
water to maintain.
It should be noted that the data collected during this study were intended to enable this specific
planning-level hydrologic assessment. Use of these data for other, potentially more specific
applications, should be undertaken with care to ensure that the analyses undertaken are
appropriate for the data available.
9.
REFERENCES
Golden Software, Inc. 1997. SURFER for Windows, v.6.0, computer software, Golden, CO.
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