| Abstract: |
The Chesapeake Bay has been suffering from degraded water quality for a number of decades. One large cause of the problem has been increased delivery of nitrogen to the bay by groundwater and surface water. The nitrogen originates at the land surface throughout the watershed from a variety of locations that include point sources of wastewater discharge and diffuse sources of fertilizer and wastes from agriculture and suburbia. To date much of the efforts to simulate the loading of nitrogen to the bay have focused around the HSPF watershed model. Studies, however, indicate that much of the nitrate being delivered to the bay is via groundwater, and the associated groundwater lag time is not accounted for by the HSPF model. A major effort is now underway at the USGS to simulate the groundwater transport of nitrate to the bay. These simulations will cover several large sections of the watershed, will be used to estimate the magnitude and distribution of groundwater lag times, and will help land managers target areas to adjust nitrogen loading.
Four MODFLOW models of regions of the bay watershed are being constructed. The first of these covers the Maryland and Delaware sections of the Delmarva Peninsula. The Delmarva model is of the shallow unconfined aquifer (<300 ft deep) and has been calibrated for steady-state conditions. The grid has a 500-ft horizontal resolution and the entire model has more than 6 million cells. The model was calibrated using 48 groundwater levels and 24 ages. Processing GIS-based datasets comprised a substantial amount of the effort in this modeling project. Digital elevation models (including LIDAR), land cover, percent impervious surface, percent soil clay content, temperature and precipitation were all used, with the latter four being used to estimate evapotranspiration and recharge.
MODPATH was used on the MODFLOW results to estimate the distribution of groundwater ages that contribute to several smaller watersheds within the peninsula. The distribution of ages is being used to help forecast changes in nitrate levels in the streams for various best management practice scenarios. The history of fertilizer and poultry sales across the peninsula has been used to recreate temporal input functions of the nitrogen loading. A nitrate transport-and-fate, mass-balance model was calibrated using 728 nitrate values from wells and 7 temporal nitrate datasets from streams. The delivery of nitrate by submarine groundwater discharge to the bay is also being quantified.
Initial results indicate that 75 and 85 percent of nitrate is lost (by plant uptake and denitrification) above the water table from fertilizer and manure, respectively. Also, denitrification occurs during groundwater transport with an average first order rate loss constant of 0.033 per year. Finally, 10 to 20 percent of the remaining nitrate is lost in the stream-riparian corridor. The median age of water reaching the streams in the various smaller watersheds ranges between 10 and 40 years. Simulated forecasts of future nitrate levels in the streams indicate that levels should take years to decades to respond substantially to changes in loadings at the land surface.
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