Lindsey, B.D., Johnson, T.D., and Belitz, Kenneth, 2018, Data from Decadal Change in Groundwater Quality Web Site, 1988-2012: U.S. Geological Survey data release, https://doi.org/10.5066/F7PC30WD.
Data were collected as a part of the NAWQA Land use studies or Major aquifer studies. Initially data were collected to characterize the quality of shallow and medium depths of groundwater in the nation. Subsequent samples were collected to evaluate changes in concentrations over a decadal time span. Mapping criteria and benchmarks: The NAWQA Project has developed an interactive mapping tool that displays decadal changes in concentrations. From among the more than 300 constituents sampled during the two decades, 24 constituents were prioritized for analysis for decadal change based on the following criteria: (1) they exceeded a human health benchmark in at least 1 percent of the wells used as a source of drinking-water public supply wells (Toccalino and Hopple, 2010) or domestic supply wells (Desimone, 2009), (2) they exceeded a U.S. Environmental Protection Agency (EPA) secondary maximum contaminant level (SMCL) in at least 1 percent of the wells used as a source of drinking water, (3) they were among the five most frequently detected volatile organic compounds (VOCs) in the Nation (Zogorski and others, 2006), or (4) they were among the five most frequently detected pesticides in the Nation (Gilliom and others, 2006). Other constituents were added to this list based on regional importance. Radium, radon, and gross alpha (α) activity met the criteria for analysis, but do not have sufficient data for analysis; thus, they are not included in the mapping tool. Benchmark used to prioritize constituents were the following: EPA maximum contaminant levels (MCLs) (U.S. Environmental Protection Agency, 2012), Health-Based Screening Levels (HBSLs) (Toccalino and others, 2014b), and nonregulatory SMCLs (U.S. Environmental Protection Agency, 2012). Of these, only MCLs are enforceable (regulatory) drinking-water standards; all but SMCLs are human-health benchmarks. The following is a list of the 24 constituents selected for analysis and the reason that they were selected: Criteria for mapping constituents 1) Exceeded human-health benchmark in more than 1 percent of domestic- or public supply wells: arsenic, boron, fluoride, manganese, molybdenum, nitrate (measured as nitrite plus nitrate), strontium, tetrachloroethene, uranium, and dieldrin. 2) Exceeded SMCL in more than 1 percent of domestic- or public-supply well: chloride, iron, sulfate, total dissolved solids. 3) Constituent of special or regional interest: orthophosphate 4) The five most frequently detected pesticide compounds in groundwater in the nation: atrazine, deethylatrazine, simazine, metolachlor and prometon. 5) The five most frequently detected volatile organic compounds in groundwater in the nation. chloroform, methyl tert -butyl ether, tetrachloroethene (also exceeded health benchmark), toluene, trichloroethene.