Estimating Contributions of Nitrate and Herbicides From Groundwater to Non-Tidal Coastal Streams In the Mid-Atlantic USA

Restoration of surface waters requires a thorough understanding of contaminant sources, which is often complicated where groundwater represents a significant vector for contaminant transport. Poor water quality has been well documented in Chesapeake Bay, Pamlico Sound, and other Mid-Atlantic estuaries, and groundwater discharge provides a significant portion of nitrogen and herbicide flux to contributing Coastal Plain tributaries. We used a probabilistic approach to estimate the regional flux of nitrate and selected commonly-used herbicides (alachlor, atrazine, metolachlor, and selected degradates) from groundwater to non-tidal headwater streams of the Northern Atlantic Coastal Plain (NACP, North Carolina through New Jersey) on the basis of water-quality data collected from 174 such streams during base-flow conditions in the late winter and spring of 2000. Estimated daily base-flow flux during the study period exceeded 21,000 kilograms of nitrate and 25 kilograms of selected herbicide compounds. Base-flow flux of alachlor and metolachlor is dominated by ethanesulfonic and oxanillic acid degradates. Flux of parent compounds is less than 3 percent of the total flux of parent plus degradates, which suggests that previous estimates of total flux of herbicide compounds in surface waters based only on measurement of parent compounds may be very misleading. Base-flow flux of nitrate and herbicide compounds as a percentage of estimated applications generally varies predictably with variations in hydrogeologic settings, being typically highest in well-drained areas of the Middle Coastal Plain and lowest in poorly-drained areas such as the Coastal Lowlands. The combination of abundant non-point sources and well-drained, aerobic soils and shallow aquifer sediments contributes to particularly large base-flow flux from the Eastern Shore of the Chesapeake Bay Watershed, which includes only 9 percent of the total non-tidal NACP, but generates more than half of base-flow nitrate, alachlor, atrazine, and metolachlor flux from that area.