A study has been undertaken to determine the time required for the effects of nitrogen-reducing best management practices (BMPs) implemented at the land surface to reach the Chesapeake Bay via groundwater transport to streams. To accomplish this, a nitrogen mass-balance regression (NMBR) model was developed and applied to seven watersheds on the Delmarva Peninsula. The model included the distribution of groundwater return times obtained from a regional groundwater-flow (GWF) model, the history of nitrogen application at the land surface over the last century, and parameters that account for denitrification. The model was (1) able to reproduce nitrate concentrations in streams and wells over time, including a recent decline in the rate at which concentrations have been increasing, and (2) used to forecast future nitrogen delivery from the Delmarva Peninsula to the Bay given different scenarios of nitrogen load reduction to the water table. The relatively deep porous aquifers of the Delmarva yield longer groundwater return times than those reported earlier for western parts of the Bay watershed. Accordingly, several decades will be required to see the full effects of current and future BMPs. The magnitude of this time lag is critical information for Chesapeake Bay watershed managers and stakeholders.

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This study by the U.S. Geological Survey, prepared in cooperation with the Virginia Department of Environmental Quality, quantifies the components of the hydrologic cycle across the Commonwealth of Virginia. Long-term, mean fluxes were calculated for precipitation, surface runoff, infiltration, total evapotranspiration (ET), riparian ET, recharge, base flow (or groundwater discharge) and net total outflow. Fluxes of these components were first estimated on a number of real-time-gaged watersheds across Virginia. Specific conductance was used to distinguish and separate surface runoff from base flow. Specific-conductance data were collected every 15 minutes at 75 real-time gages for approximately 18 months between March 2007 and August 2008. Precipitation was estimated for 1971-2000 using PRISM climate data. Precipitation and temperature from the PRISM data were used to develop a regression-based relation to estimate total ET. The proportion of watershed precipitation that becomes surface runoff was related to physiographic province and rock type in a runoff regression equation. Component flux estimates from the watersheds were transferred to flux estimates for counties and independent cities using the ET and runoff regression equations. Only 48 of the 75 watersheds yielded sufficient data, and data from these 48 were used in the final runoff regression equation. The base-flow proportion for the 48 watersheds averaged 72 percent using specific conductance, a value that was substantially higher than the 61 percent average calculated using a graphical-separation technique (the USGS program PART). Final results for the study are presented as component flux estimates for all counties and independent cities in Virginia.

For more information on the USGS-the Federal source for science about the Earth, its natural and living resources, natural hazards, and the environment, visit http://www.usgs.gov or call 1-888-ASK-USGS.For an overview of USGS information products, including maps, imagery, and publications, visit http://www.usgs.gov/pubprodTo order this and other USGS information products, visit http://store.usgs.gov Any use of trade, firm, or product names is for descriptive purposes only and does not imply endorsement by the U.S. Government.Although this information product, for the most part, is in the public domain, it also may contain copyrighted materials as noted in the text. Permission to reproduce copyrighted items must be secured from the copyright owner. AbstractEstimating future loadings of nitrogen to the Chesapeake Bay requires knowledge about the groundwater flow system and the traveltime of water and chemicals between recharge at the water table and the discharge to streams and directly to the bay. The Delmarva Peninsula has a relatively large proportion of its land devoted to agriculture and a large associated nitrogen load in groundwater that has the potential to enter the bay in discharging groundwater. To better understand the shallow aquifer system with respect to this loading and the traveltime to the bay, the U.S. Geological Survey constructed a steadystate groundwater flow model for the region. The model is based on estimates of recharge calculated using recently developed regression equations for evapotranspiration and surface runoff. The hydrogeologic framework incorporated into the model includes unconfined surficial aquifer sediments, as well as subcropping confined aquifers and confining beds down to 300 feet below land surface. The model was calibrated using 48 water-level measurements and 24 tracer-based ages from wells located across the peninsula. The resulting steady-state flow solution was used to estimate ages of water in the shallow aquifer system through the peninsula and the distribution and magnitude of groundwater traveltime from recharge at the water table to discharge in surface-water bodies (referred to as return time). Return times vary but are typically less than 10 years near local streams and greater than 100 years near the stream divides. The model can be used to calculate nitrate transport parameters in various local watersheds and predict future trends in nitrate loadings to Chesapeake Bay for different future nitrogen application scenarios.