NATURAL AND HUMAN INFLUENCES ON GROUND-WATER QUALITY AT LOCAL AND REGIONAL SCALES IN THE SURFICIAL AQUIFER IN THE NORTH ATLANTIC COASTAL PLAIN, USA

Ground-water quality in the surficial unconfined aquifer in the North Atlantic Coastal Plain (New York through North Carolina) is affected by hydrogeologic, geochemical, and other natural conditions, as well as human influences. The aquifer provides drinking water in many parts of the Coastal Plain and the majority of flow in local streams. Unconsolidated sediments composing the aquifer are generally permeable in many areas, however, and unconfined ground water in these areas is vulnerable to human influences. Water quality in the aquifer was evaluated at the regional scale in several hundred wells in a variety of settings, and along local flowpaths in agricultural areas in four selected hydrogeologic settings. Insights gained from local studies are useful for interpreting and understanding broader patterns and variability observed at the regional scale. Regionally, observed nitrate concentrations generally exceeded estimated natural levels, and selected pesticide compounds (atrazine, metolachlor, simazine, and desethylatrazine, an atrazine degradate) were occasionally detected, though rarely at concentrations exceeding 0.1 micrograms per liter. Nitrate greater than 0.4 mg/L (milligrams per liter) (as nitrogen) may be derived from a variety of sources, although concentrations greater than 3 mg/L are more likely to occur in agricultural areas. Nitrate is also more likely in oxic ground water and in relatively well-drained areas; local studies demonstrate that dissolved oxygen may persist for decades along flowpaths in such settings, but may be consumed relatively quickly (within 15 years) in poorly drained areas with abundant organic matter. Nitrate concentrations may be quite variable locally over short vertical and horizontal distances in the aquifer, due to variable redox conditions along flowpaths and, in oxic areas, to differences in nitrogen applications over time. Atrazine, desethylatrazine, and metolachlor are also more likely to be present in agricultural areas, particularly in relatively well-drained areas with low organic matter, where ground water is commonly relatively oxic and dilute. Pesticide variability along flowpaths in such settings is similarly related to temporal trends in applications and agricultural practices.