Water Quality in Selected Carbonate Aquifers of the United States, 1993-2005

The U.S. Geological Survey National Water-Quality Assessment Program collected 1,048 samples from wells and springs in 12 carbonate-aquifer systems across the United States during 1993-2005. The systems sampled are the Basin and Range, Biscayne, Castle Hayne, Edwards-Trinity, Floridan, Mississippian, Ordovician, Ozark Plateaus, Piedmont and Blue Ridge, Prairie du Chien of the Cambrian-Ordovician system, Silurian-Devonian /Upper Carbonate, and Valley and Ridge. Samples were analyzed for major ions, nutrients, 47 pesticides, and 54 volatile organic compounds (VOCs) at most sites. A single sample from each site was used to characterize water quality, trends were not assessed. Water quality in the 12 carbonate-aquifer systems was highly variable. Samples from 54 wells (5 percent) exceeded the U.S. Environmental Protection Agency Maximum Contaminant Level of 10 mg/L for nitrate in drinking water. Nitrate concentrations were affected by land use, extent of aquifer confinement, ground-water age, oxidation-reduction state, and sinkhole density. The most frequently occurring pesticide compounds were four herbicides—atrazine, simazine, metolachlor, and prometon—and deethylatrazine, a degradate of atrazine. At least 1 of the 47 pesticides analyzed was reported as present in about half of the samples from the 1,033 sites where pesticide data were collected. Samples from unconfined aquifers where land use was agricultural or urban accounted for the majority (81 percent) of detections of pesticides. Unconfined aquifers overlain by urban land had the most detections of VOCs. Land use patterns and ground-water characteristics such as extent of confinement or ground-water age, help explain why some systems have higher levels of contamination than others. These findings can be used by water-resource managers and planners to better understand and help manage water quality in these carbonate-aquifer systems.