Mean wet deposition of chloride, 1994–2003, and locations of salt mining in the glacial aquifer system study area, northern United States. John R. Mullaney, David L. Lorenz, and Alan D. Arntson, USGS

Groundwater-quality data from a sampling of 1,329 wells in 19 states were analyzed. Chloride concentrations were greater than the secondary maximum contaminant level established by the U.S. Environmental Protection Agency of 250 milligrams per liter in 2.5 percent of samples from 797 shallow monitoring wells and in 1.7 percent of samples from 532 drinking-water supply wells. Water samples from shallow monitoring wells in urban areas had the largest concentration of chloride, followed by water samples from agricultural and forested areas (medians of 46, 12, and 2.9 milligrams per liter, respectively).

U.S. Geological Survey, Chloride in Groundwater and Surface Water in Areas Underlain by the Glacial Aquifer System, Northern United States [pdf], Scientific Investigations Report 2009–5086, Figure 3.