Abstract

Estimating 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 steady-state 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.

First posted December 7, 2012