Using a Coupled Groundwater-Flow and Nitrate-Balance-Regression Model to

Explain Trends, Forecast Loads,and Target Future Reclamation

Ward Sanford, USGS, Reston, VirginiaJason Pope, USGS, Richmond, VirginiaDavid 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

MODFLOW 2005500 ft cell resolution7 Model Layers4+ million active cells30-m DEM, LIDAR300 ft deepSteady State FlowMODPATH travel times

GroundwaterLevel

Observations

GroundwaterAge

Observations

1.0 10.0 100.0 1000.0 10000.00

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1

Chesterville BranchChoptank River

Axis Title

Axis

Titl

e

Age Histograms of two of the Real-Time Watersheds

1 2

34

5

6

7

Seven watersheds hadsubstantial stream nitrateData and were used:

1. Morgan Creek

2. Chesterville Branch

3. Choptank River

4. Marshyhope Creek

5. Nanticoke River

6. Pocomoke River

7. Nassawango Creek

1950 1960 1970 1980 1990 2000 2010 20200.00.20.40.60.81.01.21.41.61.82.0

Choptank River near Greensboro, MD

Annual Averages

Multi-Year Averages

YEAR

Nitra

te a

s N,

in m

g/L

1950 1960 1970 1980 1990 2000 2010 20200.0

0.4

0.8

1.2

1.6

2.0

2.4

2.8

3.2

3.6

4.0Morgan Creek near Kennedyville, MD

Yearly AveragesMulti-Year Averages

YEAR

Nitra

te a

s N,

in m

g/L

1 10 100 1000 100000

1

10Choptank River near Greensboro, MD

Discharge, in Cubic Feet per Second

Nitra

te a

s N,

in m

g/L

Low flow High flow

1950 1960 1970 1980 1990 2000 2010 20200.00.51.01.52.02.53.03.54.04.55.0

Nanticoke River near Bridgeville, DE

Yearly Average

Multi-Year Average

YEAR

Nitra

te a

s N,

in m

g/L

Nitrate Mass-Balance Regression Equation

Simulatedsurface waterconcentration

in stream

=

Concentrationbelow ag field

X SoilTerm

RiparianTerm

Non-agTerm

Pre-agTermX X X

Denitrification Dilution

Simulatedgroundwaterconcentrationin ag field well

=

Concentrationbelow ag field

X SoilTerm

Concentrationbelow ag field = Fertilizer

Load [ XPoultryLoad

RechargeRate ]X X{ [] }1 - 1 -FUE PUE+

FUE & PUE =Fertilizer and PoultryUptake Efficiencies

SoilTerm = (S/5)m

– RL

– RL

RiparianTerm = (AREA)n

S = soil drainagefactor

AREA is for the watershedIn square miles

0

10

20

30

40

50

60

1940 1950 1960 1970 1980 1990 2000 2010

Nitr

ogen

Loa

ding

from

Fer

tiliz

er, m

g/L

Year

Kent

New Castle

Sussex

Caroline

Cecil

Dorchester

Kent

Queen Annes

Somerset

Talbot

Wicomico

Worcester

Accomack

average

1 4 7 10 13 16 19 22 25 28 310.00

0.10

0.20

0.30

0.40

0.50

0.60

0.70

0.80

0.90

1.00

Stream and Riparian Denitrification

Euivalent distance traveled along the stream, in miles

Frac

tion

Nitra

te R

emai

ning

For calibrated watershed areaexponent parameter = -0.119

Soil Factor (S) in Equation:

1. Very poorly drained2. Somewhat poorly drained3. Poorly drained4. Moderately well drained5. Well drained6. Somewhat excessively drained7. Excessively drained

1 2 3 4 50.00

0.10

0.20

0.30

0.40

0.50

0.60

0.70

0.80

0.90

1.00

Soil Factor, S

Frac

tion

Nitra

te R

emai

ning

For calibrated soil drainageexponent parameter = 0.337

Very poorlydrained

Welldrained

Fertiliz

er in Rec

harge

After C

rop Uptake

After S

oil Den

itrific

ation

After n

on-ag di

lution

After p

re-ag

dilutio

n

After r

iparian

denitri

ficati

on

observe

d0

5

10

15

20

25

30

35

40

45

50

Morgan Creek

Pocomoke River

Process Leading to Nitrate Loss or Dilution

Con

cent

ratio

n on

Nitr

ate

as N

, in

mg/

L

Regression Number

Fertilizer

Uptake

Efficiency

Manure Uptake Efficiency

Percent Increase in

Uptake Efficiencies

Riparian and Stream

Denitrificait

on loss exponent

Soil Deintirficaiton loss

exponent

Number of

Parameters

estimated

Sum of Squared

Weighted Residual

s

Standard Error of

Regressi

on

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

limit70% 62% 40% -0.110 0.8 5 2181 33

10% lower parameter value

limit64% 32% 12% -0.140 0.2 5 2181 33

10% limits as percent of value 3% 25% 58% 13% 80% 5 2181 33

1950 1960 1970 1980 1990 2000 20100.00.20.40.60.81.01.21.41.61.82.0

Choptank River near Greensboro, MD

Yearly AveragesMulti-Year AveragesRegression Model

YEAR

Nitra

te a

s N,

in m

g/L

Best Fit for Four Parameterswith best-fit changes inFertilizer and Uptake Efficiences

1950 1960 1970 1980 1990 2000 2010 20200.0

0.4

0.8

1.2

1.6

2.0

2.4

2.8

3.2

3.6Morgan Creek near Kennedyville, MD

Yearly AveragesMulti-Year AveragesRegression Model

YEAR

Nitra

te a

s N,

in m

g/L

1940 1950 1960 1970 1980 1990 2000 2010 20200.0

0.5

1.0

1.5

2.0

2.53.0

3.5

4.0

4.5

5.0Nanticoke River near Bridgeville, DE

Regression ModelYearly AverageMulti-Year Average

YEAR

Nitra

te a

s N,

in m

g/L

1 10 100 1000 100000

1

10Choptank River near Greensboro, MD

Discharge, in Cubic Feet per Second

Nitra

te a

s N,

in m

g/L

Low N flow High N flowLong-Term MeanDaily Flow Rate

(LTMDFR)

LTMDFR X 1.4

StreamHydrograph

Groundwater DischargeComponent

Surface RunoffComponent

LOW NFLOW

HIGH NFLOW

TIME

FLO

W R

ATE

Graphical HydrographSeparation

For the Real-time watersheds, the fraction of the water that discharges abovethe High/Low N cutoff correlates very well to the fraction of the total streamflowthat 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 thehigh-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.

1950 1960 1970 1980 1990 2000 2010 2020 2030 2040 20500

4

8

12

16

20

24

Model Forecast of Nitrogen Loading toChesapeake Bay from the Eastern Shore

Total Flow N Loading

Had there been no BMPs

High Flow Component

YEAR

Nitrogen Loadto the Bay,in Millionsof Poundsper Year

1980 1990 2000 2010 2020 2030 2040 20500

2

4

6

8

10

12

14

16

18

20

Model Forecast of Nitrogen Loading toChesapeake Bay from the Eastern Shore

25% load Reduction

50% load reduction

100% load reduction

Total Flow N Loading

YEAR

Nitrogen Loadto the Bay,in Millionsof Pounds

per Year

EPATargets

Targeting of HUC-12 Watershedsby 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 oroutside Baywatershed

Targeting Matrix

Targeting that Includes Response Timeand Nitrogen Delivered to the Bay

Groundwater Return Time

< 7 yrs 7 - 20 yrs > 20 yrs

NitrateConcentration

< 4 mg/L

5 - 7 mg/L

>7 mg/L

Targeting Matrix

Targeting that Includes Response Timeand Nitrogen Delivered to the Bay

Groundwater Return Time

< 7 yrs 7 - 20 yrs > 20 yrs

NitrateConcentration

< 4 mg/L

5 - 7 mg/L

>7 mg/L

From: USGS NAWQA Cycle 3 Planning Document--unpublished

Summary and Conclusions

Results from a groundwater flow model were coupled to a nitrate-mass-balance 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.