Using a Coupled Groundwater-Flow and
Nitrate-Balance-Regression Model to
Explain Trends, Forecast Loads,
and Target Future Reclamation
Ward Sanford, USGS, Reston, Virginia
Jason Pope, USGS, Richmond, Virginia
David Selnick, USGS, Reston, Virginia
Objectives:
·
To develop a groundwater flow model that can simulate return-times
to streams (base-flow ages) on the Delmarva Peninsula
·
To explain the spatial and temporal trends in nitrate on the Delmarva
Peninsula using a mass-balance regression equation that includes
the base-flow age distributions obtained from the flow model
·
To use the calibrated equation to forecast total nitrogen loading to
the Bay from the Eastern Shore
·
To forecast changes in future loadings to the bay given different
loading application rates at the land surface
·
To develop maps that will help resource managers target areas that
will respond most efficiently to better management practices
Ground-Water Model of the Delmarva
Peninsula

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
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MODFLOW 2005
500 ft cell resolution
7 Model Layers
4+ million active cells
30-m DEM, LIDAR
300 ft deep
Steady State Flow
MODPATH travel times
Groundwater
Level
Observations
Groundwater
Age
Observations
Age Histograms of two of the Real-Time Watersheds
1
0.9
0.8
Axis Title
0.7
0.6
Chesterville Branch
0.5
Choptank River
0.4
0.3
0.2
0.1
0
1.0
10.0
100.0
Axis Title
1000.0
10000.0
Seven watersheds had
substantial stream nitrate
Data and were used:
1. Morgan Creek
2. Chesterville Branch
3. Choptank River
1
2
3
4
5
4. Marshyhope Creek
5. Nanticoke River
6. Pocomoke River
7. Nassawango Creek
6
7
Nanticoke River near Bridgeville, DE
2.0
5.0
1.8
4.5
1.6
4.0
1.4
1.2
1.0
0.8
0.6
0.4
0.2
Annual Averages
Nitrate as N, in mg/L
Nitrate as N, in mg/L
Choptank River near Greensboro, MD
3.5
3.0
2.5
2.0
1.5
Yearly Average
1.0
Multi-Year Averages
Multi-Year Average
0.5
0.0
1950 1960 1970 1980 1990 2000 2010 2020
0.0
1950 1960 1970 1980 1990 2000 2010 2020
YEAR
YEAR
Morgan Creek near Kennedyville, MD
Choptank River near Greensboro, MD
4.0
10
3.2
2.8
2.4
2.0
1.6
1.2
0.8
Yearly Averages
0.4
Multi-Year Averages
0.0
1950 1960 1970 1980 1990 2000 2010 2020
YEAR
Nitrate as N, in mg/L
Nitrate as N, in mg/L
3.6
1
Low flow
High flow
0
1
10
100
1000
10000
Discharge, in Cubic Feet per Second
Nitrate Mass-Balance Regression Equation
Simulated
surface water
concentration
in stream
=
Concentration
below ag field
X
Non-ag
Term
Soil
Riparian
X
X
Term
Term
Concentration
below ag field
=
Concentration
below ag field
X
Soil
Term
Pre-ag
Term
– RL
Dilution
Denitrification
Simulated
groundwater
concentration
in ag field well
X
– RL
FUE & PUE =Fertilizer and Poultry
Uptake Efficiencies
=
Recharge
X
Rate
Soil
Term
{
Fertilizer
X
Load
= (S/5)m
S = soil drainage
factor
[
]
Riparian
Term
= (AREA)n
1 - FUE
+
Poultry
X
Load
[1 - PUE ]}
AREA is for the watershed
In square miles
Nitrogen Loading from Fertilizer, mg/L
60
Kent
New Castle
50
Sussex
Caroline
Cecil
40
Dorchester
Kent
Queen Annes
Somerset
30
Talbot
Wicomico
Worcester
20
Accomack
average
10
0
1940
1950
1960
1970
1980
Year
1990
2000
2010
Stream and Riparian Denitrification
1.00
Fraction Nitrate Remaining
0.90
For calibrated watershed area
exponent parameter = -0.119
0.80
0.70
0.60
0.50
0.40
0.30
0.20
0.10
0.00
1
4
7
10
13
16
19
22
25
Euivalent distance traveled along the stream, in miles
28
31
Soil Factor (S) in Equation:
1.
2.
3.
4.
5.
6.
7.
Very poorly drained
Somewhat poorly drained
Poorly drained
Moderately well drained
Well drained
Somewhat excessively drained
Excessively drained
Fraction Nitrate Remaining
1.00
0.90
0.80
0.70
0.60
0.50
For calibrated soil drainage
exponent parameter = 0.337
0.40
0.30
0.20
0.10
0.00
1
Very poorly
2
3
Soil Factor, S
4
5
Well
drained
Concentration on Nitrate as N, in mg/L
50
45
Morgan Creek
40
35
Pocomoke River
30
25
20
15
10
5
0
Process Leading to Nitrate Loss or Dilution
Regression Number
Fertilizer Uptake
Efficiency
Manure Uptake
Efficiency
Percent Increase in
Uptake Efficiencies
Riparian and Stream
Denitrificaiton loss
exponent
Soil Deintirficaiton
loss exponent
Number of Parameters
estimated
Sum of Squared
Weighted Residuals
Standard Error of
Regression
1
83%
83%
0
0
0
2
8388
122
2
82%
70%
0
0
0.670
3
5834
85
3
72%
63%
0
-0.152
0
3
3698
54
4
74%
36%
0
-0.146
0.425
4
2540
37
5
67%
40%
24%
-0.114
0.566
5
2181
33
10% upper
parameter value
limit
70%
62%
40%
-0.110
0.8
5
2181
33
10% lower
parameter value
limit
64%
32%
12%
-0.140
0.2
5
2181
33
10% limits as
percent of value
3%
25%
58%
13%
80%
5
2181
33
Nanticoke River near Bridgeville, DE
1.8
4.5
1.6
4.0
Nitrate as N, in mg/L
2.0
5.0
1.4
3.5
1.2
3.0
1.0
2.5
0.8
2.0
0.6
0.4
0.2
Yearly Averages
Multi-Year Averages
Regression Model
0.0
1950 1960 1970 1980 1990 2000 2010
YEAR
1.5
Regression Model
1.0
Yearly Average
0.5
Multi-Year Average
0.0
1940 1950 1960 1970 1980 1990 2000 2010 2020
YEAR
Morgan Creek near Kennedyville, MD
3.6
3.2
Best Fit for Four Parameters
with best-fit changes in
Fertilizer and Uptake Efficiences
Nitrate as N, in mg/L
Nitrate as N, in mg/L
Choptank River near Greensboro, MD
2.8
2.4
2.0
1.6
Yearly Averages
1.2
Multi-Year Averages
0.8
Regression Model
0.4
0.0
1950
1960
1970
1980 1990
YEAR
2000
2010
2020
Stream
Hydrograph
Choptank River near Greensboro, MD
Surface Runoff
Component
HIGH N
FLOW
LTMDFR X 1.4
Long-Term Mean
Daily Flow Rate
(LTMDFR)
Graphical Hydrograph
Separation
Nitrate as N, in mg/L
Groundwater Discharge
Component
10
1
Low N flow
LOW N
FLOW
High N flow
0
1
10
100
1000
10000
Discharge, in Cubic Feet per Second
TIME 
For the Real-time watersheds, the fraction of the water that discharges above
the High/Low N cutoff correlates very well to the fraction of the total streamflow
that is either surface runoff or that which the model calculates as quickly rejected recharge.
Also for the real-time watersheds, the flux-weighted concentration of nitrate in the
high-flow section was consistently equal to about 65% of the low-flow concentration.
These factors combined allow for BOTH a low-flow and high-flow nitrogen flux to be
calculated from all of the other watersheds and for the entire Eastern Shore as a whole.
Model Forecast of Nitrogen Loading to
Chesapeake Bay from the Eastern Shore
24
Total Flow N Loading
Had there been no BMPs
20
High Flow Component
16
Nitrogen Load
to the Bay,
in Millions
of Pounds
per Year
12
8
4
0
1950
1960
1970
1980
1990
2000
YEAR
2010
2020
2030
2040
2050
Model Forecast of Nitrogen Loading to
Chesapeake Bay from the Eastern Shore
20
18
16
14
Nitrogen Load
to the Bay,
12
in Millions
of Pounds
per Year
10
8
6
25% load Reduction
EPA
Targets
50% load reduction
100% load reduction
4
Total Flow N Loading
2
0
1980
1990
2000
2010
2020
YEAR
2030
2040
2050
Targeting of HUC-12 Watersheds
by Average Nitrate Yield
Average nitrate yield to local stream
>5 mg/L
5 mg/L
4 mg/L
3 mg/L
2 mg/L
1 mg/L
<1 mg/L or
outside Bay
watershed
Targeting that Includes Response Time
and Nitrogen Delivered to the Bay
Nitrate
Concentration
Targeting Matrix
< 4 mg/L
5 - 7 mg/L
>7 mg/L
< 7 yrs
7 - 20 yrs
> 20 yrs
Groundwater Return Time
Targeting that Includes Response Time
and Nitrogen Delivered to the Bay
Nitrate
Concentration
Targeting Matrix
< 4 mg/L
5 - 7 mg/L
>7 mg/L
< 7 yrs
7 - 20 yrs
> 20 yrs
Groundwater Return Time
From: USGS NAWQA Cycle 3 Planning Document--unpublished
Summary and Conclusions
Results from a groundwater flow model were coupled to a nitrate-massbalance regression model and calibrated against stream nitrate data.
The calibrated model suggests that nitrogen uptake efficiencies on the
Eastern Shore may be improving over time.
EPA targets for reduced loading of 3 million pounds on the Eastern Shore
per year will not likely be reasonably reached for several decades,
much less by 2020, even with severe cutbacks in fertilizer use
The model can help target areas where reduced nitrogen loadings would
be the most beneficial at reducing total loadings to the Bay.