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U.S. Department of the InteriorU.S. Geological Survey
Importance of Ground-Water Flow and Importance of Ground-Water Flow and Travel Time on Nitrogen Loading from an Travel Time on Nitrogen Loading from an
Agricultural Basin in ConnecticutAgricultural Basin in Connecticut
Importance of Ground-Water Flow and Importance of Ground-Water Flow and Travel Time on Nitrogen Loading from an Travel Time on Nitrogen Loading from an
Agricultural Basin in ConnecticutAgricultural Basin in Connecticut John R. MullaneyU.S. Geological Survey, Connecticut Water Science Center, East Harford, Connecticut [email protected]
In Cooperation with the Connecticut Department of Environmental Protection
John R. MullaneyU.S. Geological Survey, Connecticut Water Science Center, East Harford, Connecticut [email protected]
In Cooperation with the Connecticut Department of Environmental Protection
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Framework and Purpose
Long Island Sound Watershed in Connecticut
TMDL for nitrogen 58.5% reduction by 2015
10 percent reduction in nonpoint source required
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Framework and Purpose (Cont.)
To gain understanding of the contribution of nitrogen from ground-water discharge
Provide information on the effect of ground-water travel time on surface-water quality
Explore scenarios to determine the most effective way to implement BMPs Largest load and shortest ground-water travel time
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Broad Brook Basin on the Connecticut List of Impaired Waters
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Land Use/Land CoverLand Use/Land Cover
42 percent agricultural
13 percent urban
45 percent forest and wetland
42 percent agricultural
13 percent urban
45 percent forest and wetland
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Surficial Geology-Glacial Deposits7.8 in./yr
23.8 in/yrRecharge rate
Sampling site
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Load Estimation
“ESTIMATOR” Developed by Cohn and others for USGS Log-linear regression equation for load estimation
)2cos()2sin()()(
)()()(
652
43
2210
TTTT
QlnQlnCXQln
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Load Estimation-Nitrogen
LOA
D I
N L
BS
/YR
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Base-flow Separation- GW Discharge
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Base-Flow Separation (GW Discharge Averaged 80% of Flow 1993-2004)
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Selection of Base-Flow Samples
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Load Estimation- Nitrate in Base Flow
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Nitrate Load in Base Flow (Averaged 71 % of N Load 1993-2004)
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Wells/Well ClustersNutrients, Age Dating, Dissolved Gases
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Ground-Water Age Dating, Dissolved Gases
Done using 3H, 3H-3He, SF6
Apparent ages ranged from 2.4 to >50 years
Dissolved gas analysis indicated denitrification generally not occurring
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MODFLOW Ground-Water ModelingMODPATH Particle Tracking
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Simulated Particle Tracks
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Simulated Recharge Age of Discharge
X
X
50% pre- 1995
8% pre- 1960
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Simulated Nitrogen Loading to the Water Table
Land Use/Cover(modified from Civco and Hurd)
Estimated nitrate concentration
mg/L
Estimated loading to the water table (2002)
lbs.
Developed 3.0 13,320
Turf and Grass 1.5 440
Other Grasses and Agriculture 7.0 137,000
Forest and Wetland 0.14 1,400
Utility Right of Way and Barren Land
1.0 900
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Future Scenarios Analysis
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Conclusions
Nitrogen load is dominated by ground-water discharge during base flow
Base-flow load of nitrogen is dominated by agricultural land use on glacial stratified deposits
It is important to differentiate the recharge rates to different aquifers in the basin
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Conclusions (Cont.)
Ground-water travel times can be greater than 50 years
Under realistic scenarios for reduction– for a 50-percent reduction in nitrate load to the water table from agricultural and urban land: A 25% reduction in base-flow nitrogen load will occur by
2016 A 45% reduction will occur by 2046
BMPs can be targeted to land uses with shortest travel time and largest nitrate loads
For more info contact: [email protected]