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| Publications—Scientific Investigations Report |
By John R. Mullaney
U.S. Geological Survey Scientific Investigations Report 2006-5278
AVAILABLE ONLINE ONLY
The body of the report is available in PDF Format ( 1,809 KB)
Nutrient loads from ground-water discharge were studied in Broad Brook Basin, a 15.8-square mile basin in north-central Connecticut, dominated by agricultural activity. Loads were calculated, along with the travel times of ground water from recharge to discharge areas, to estimate the time required for the effects of Best Management Practices (BMPs) to be observed. Most concentrations of nitrogen and phosphorus in Broad Brook exceeded U.S. Environmental Protection Agency Ecoregion
XIV nutrient criteria for streams. During the study period (1993–2004), annual loads of nitrogen from Broad Brook Basin ranged from 117,000 to 270,000 pounds (lb), and yields were about 10 times larger than those from forested basins in
Connecticut.
Ground-water discharge from the aquifer to the streams (base flow) during the study period was estimated with hydrograph separation and accounted for 82 percent of the total runoff from the basin. Nitrate nitrogen in base flow averaged
71 percent of the annual load of total nitrogen discharged from the basin, indicating that the largest source of nitrogen was likely from ground-water discharge. Annual loads of total phosphorus
from the basin ranged from 2,330 to 14,400 lb, and yields were about five times higher than those from forested basins in Connecticut. Dissolved phosphorus averaged about
71 percent of the total phosphorus load, and ground-water discharge
accounted for only as much as 40 percent of the annual load of dissolved phosphorus; therefore, phosphorus loads are dominated by stormwater-runoff events.
Ground-water samples collected from 11 wells in the basin contained elevated concentrations of nitrite plus nitrate nitrogen.
Dissolved gas analyses indicated that little denitrification was occurring in the aquifer. Apparent ages of the ground-water samples ranged from greater than 2 to more than 50 years based on sulfur hexafluoride, tritium, and tritium/helium-3 analyses. A three-dimensional ground-water-flow model was used in conjunction with a particle-tracking program to determine advective travel times to streams from all subareas in the basin. The model simulations indicated that about half the discharge to Broad Brook and its tributaries was recharged more than
10 years ago, and that about 8 percent of the discharge was recharged prior to 1960.
The effects of changes in nitrate nitrogen loading at the water table were evaluated by applying new loading rates from urban and agricultural land and the simulated advective ground-water travel times. Five scenarios were tested: reducing estimated
nitrate nitrogen concentrations in recharging ground water under urban and agricultural land areas to concentrations in forested areas; reducing estimated nitrate nitrogen concentrations
under urban and agricultural land areas to the U.S. Environmental
Protection Agency recommended nutrient criteria for streams; and reducing estimated nitrate nitrogen concentrations under urban and agricultural land areas by 50 percent, 10 percent,
and 5 percent. Under the first two scenarios, the base-flow load of nitrate nitrogen could be reduced by 25 percent in slightly more than 5 years, although the reduction required by these scenarios is likely unrealistic. A 25-percent reduction in base-flow load of nitrate nitrogen could be achieved in about
10 years under the third scenario (where concentrations from urban and agricultural areas were reduced by 50 percent). Under this scenario, a 46-percent reduction could be achieved in about 60 years. The scenarios indicate that in this basin, and in other similar basins in Connecticut underlain by extensive glacial stratified deposits, there can be a substantial time lag between activities at the land surface and effects on the quality of water discharged to streams from ground water. This finding may be important in the expectations for water-quality improvements from land-use changes or BMPs.
Abstract
Introduction
Purpose and Scope
Previous Investigations
Description of Study Area
Methods of Data Collection and Analysis
Stream Base-Flow Analysis
Water-Quality Samples
Nutrient Load Estimation
Ground-Water Age Dating
Ground-Water-Flow Simulation
Ground-Water Discharge from Broad Brook Basin
Ground-Water Quality
Ground-Water Age
Nutrient Loads from Broad Brook Basin
Nitrogen
Phosphorus
Base-Flow Loads
Simulation of Future Nitrogen Loads from Ground Water
Recharge Rates, Nitrate Concentrations, and Loads
Current Loads from Ground Water to Broad Brook and Age of Ground-Water Discharge
Scenarios of Future Nitrate Nitrogen Loads from Ground-Water Discharge
Summary and Conclusions
References
Appendix 1. Development of Ground-Water-Flow Model for Broad Brook Basin
Description of Flow Model and Model Assumptions
Boundary Conditions
Ground-Water Recharge
Aquifer Properties
Internal Sources and Sinks of Water
Model Calibration and Hydrologic Budget
Estimation of Ground-Water Travel Time
References
Appendix 2. Load Estimates from ESTIMATOR
| 3. | Estimated concentrations in precipitation of sulfur hexafluoride in North America and tritium at Ellington, Connecticut, 1954–2002. |
| 4. | Estimated concentrations of tritium in precipitation at Ellington, Connecticut, decayed to 2004. |
| 5. | Base flow and overland runoff at U.S. Geological Survey streamflow-gaging station 01184490, Broad Brook at Broad Brook, Connecticut, water years 1993–2004. |
| 6. | Average monthly base flow and overland runoff at U.S. Geological Survey streamflow- gaging station 01184490, Broad Brook at Broad Brook, Connecticut, water years 1993–2004. |
| 7. | Relation between concentrations of sulfur hexafluoride and tritium and mixing models for younger (2001) and older pre-tracer water in Broad Brook Basin, north-central Connecticut. |
| 8. | Flow-duration curve and discharge conditions during sampling at Broad Brook at Broad Brook, Connecticut, water years 1993–2004. |
| 9. | Nitrogen loads at U.S. Geological Survey station 01184490, Broad Brook at Broad Brook, Connecticut, for total nitrogen, nitrate nitrogen, and total ammonia plus organic nitrogen. |
| 10. | Concentrations of total nitrogen by month at U.S. Geological Survey station 01184490, Broad Brook at Broad Brook, Connecticut, water years 1993–2004. |
| 11. | Phosphorus loads at U.S. Geological Survey station 01184490, Broad Brook at Broad Brook, Connecticut, for (A) total phosphorus, and (B) dissolved hosphorus. |
| 12. | Base-flow loads at U.S. Geological Survey station 01184490, Broad Brook at Broad Brook, Connecticut, for (A) nitrate nitrogen and (B) dissolved phosphorus. |
| 13. | Base-flow loads of nitrate nitrogen and total nitrogen loads from surface runoff at U.S. Geological Survey station 01184490, Broad Brook at Broad Brook, Connecticut. |
| 14. | Base-flow loads of total nitrogen from selected subbasins, September 18-20, 2002, Broad Brook Basin, north-central Connecticut. |
| 15. | Base-flow loads of nitrite plus nitrate nitrogen from selected subbasins, September 18-20, 2002, Broad Brook Basin, north-central Connecticut. |
| 16. | Base-flow loads of total phosphorus from selected subbasins, September 18-20, 2002, Broad Brook Basin, north-central Connecticut. |
| 17. | Simulated cumulative curve of percent of ground-water discharge in year 2006 from Broad Brook Basin by simulated year/age of recharge. |
| 18. | Simulated future base-flow load of nitrate nitrogen from Broad Brook Basin, north-central Connecticut, under five scenarios for reductions in concentration of nitrate in recharging ground water. |
| 1-1. | Map showing extent of finite-difference ground-water-flow model grid, boundary conditions, and zone arrays for recharge and hydraulic conductivity, Broad Brook Basin, north-central Connecticut. |
| 1-2. | Generalized west-to east-cross section along row 38 in the finite-difference ground-water- flow model, showing model layers and hydraulic-conductivity zones, Broad Brook Basin, north-central Connecticut. |
| 1-3. | Map showing location of measurements of ground-water levels and streamflow, Broad Brook Basin, north-central Connecticut. |
| 1-4. | Graph showing simulated and measured hydraulic head, Broad Brook Basin, north-central Connecticut. |
| 1-5. | Graph showing simulated and measured streamflow, Broad Brook Basin, north-central Connecticut. |
| 1-1. | Values of hydraulic conductivity used in finite-difference ground-water-flow model, Broad Brook Basin, north-central Connecticut. |
| 1-2. | Parameter estimates of hydraulic conductivity and recharge from nonlinear regression, finite-difference ground-water-flow model, Broad Brook Basin, north-central Connecticut. |
| 1-3. | Water levels in October 2003 and average water levels at selected U.S.Geological Survey network wells with long-term measurement records, Broad Brook Basin, north-central Connecticut. |
| 1-4. | Summary of error statistics and comparison of observed and simulated ground-water levels, Broad Brook Basin, north-central, Connecticut, April 16-20, 2002. |
| 1-5. | Model-calculated hydrologic budget for steady-state conditions. |
| 1-6. | Simulated and apparent ages at selected monitoring wells, Broad Brook Basin, north-central Connecticut. |
| 2-1. | Load estimates from ESTIMATOR Log-Linear Regression for U.S. Geological Survey station 01184490, Broad Brook at Broad Brook, Connecticut. |
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For further information, contact:
Virginia deLima, Director
Connecticut Water Science Center
101 Pitkin St.
East Hartford, CT 06108
Phone: (860) 291-6741
Email: dc_ct@usgs.govWeb site: http://ct.water.usgs.gov
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Persistent URL: https://pubs.water.usgs.gov/sir20065278 Page Contact Information: USGS Publishing Network Last modified: Tuesday, February 13 2007, 02:48:47 AM |
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