Discharge, Nitrate Load, and Residence Time of Ground Water in the Chesapeake Bay Watershed
By Scott W. Phillips, Michael J. Focazio, and L. Joseph Bachman
Abstract
Ground-water discharge to streams provides a large amount of flow that eventually enters the Chesapeake Bay. Thus, quantifying the discharge, nitrate load, and residence time of ground water in the Chesapeake Bay watershed (fig. 1) assists in developing an understanding of the movement of nutrients from their sources to streams. Some of the nutrients that are applied to the land surface, especially nitrogen, infiltrate into the underlying ground-water system. The nitrate is transported through shallow aquifers and discharges to springs and streams, thereby increasing the nitrate load to streams. If nitrate is assumed to move with the ground water, the residence time of water, which is the average time required for ground water to flow from areas of recharge to areas of discharge, can be used to estimate the rates of chemical transport. The residence time also provides an estimate of the "lag time" between implementation of management actions to reduce nutrient loads and a distinguishable improvement in surface-water quality.
This fact sheet presents an overview of discharge, nitrate load, and residence times of ground water in the Chesapeake Bay watershed based on the findings of two recent U.S. Geological Survey (USGS) reports: Bachman and others, 1998, and Focazio and others, 1998. The technical findings are part of the USGS Place-Based Studies Program (formerly known as the Ecosystem Program), which is studying the response of the Bay ecosystem to changes in nutrient inputs and natural conditions. The findings are being used by the Chesapeake Bay Program (CBP) to better understand the sources and delivery of nutrients, and the management actions to reduce nutrient loads to rivers and streams that drain into the Chesapeake Bay. Nutrients are of special concern because they contribute to eutrophication and low levels of dissolved oxygen in the Bay.
