Trace elements and radon in groundwater across the United States, 1992-2003

Abstract

Trace-element concentrations in groundwater were evaluated for samples collected between 1992 and 2003 from aquifers across the United States as part of the U.S. Geological Survey National Water-Quality Assessment Program. This study describes the first comprehensive analysis of those data by assessing occurrence (concentrations above analytical reporting levels) and by comparing concentrations to human-health benchmarks (HHBs). Data from 5,183 monitoring and drinking-water wells representing more than 40 principal and other aquifers in humid and dry regions and in various land-use settings were used in the analysis. Trace elements measured include aluminum (Al), antimony (Sb), arsenic (As), barium (Ba), beryllium (Be), boron (B), cadmium (Cd), chromium (Cr), cobalt (Co), copper (Cu), iron (Fe), lead (Pb), lithium (Li), manganese (Mn), molybdenum (Mo), nickel (Ni), selenium (Se), silver (Ag), strontium (Sr), thallium (Tl), uranium (U), vanadium (V), and zinc (Zn). Radon (Rn) gas also was measured and is included in the data analysis. Climate influenced the occurrence and distribution of trace elements in groundwater whereby more trace elements occurred and were found at greater concentrations in wells in drier regions of the United States than in humid regions. In particular, the concentrations of As, Ba, B, Cr, Cu, Mo, Ni, Se, Sr, U, V, and Zn were greater in the drier regions, where processes such as chemical evolution, ion complexation, evaporative concentration, and redox (oxidation-reduction) controls act to varying degrees to mobilize these elements. Al, Co, Fe, Pb, and Mn concentrations in groundwater were greater in humid regions of the United States than in dry regions, partly in response to lower groundwater pH and (or) more frequent anoxic conditions. In groundwater from humid regions, concentrations of Cu, Pb, Rn, and Zn were significantly greater in drinking-water wells than in monitoring wells. Samples from drinking-water wells in dry regions had greater concentrations of As, Ba, Pb, Li, Sr, V, and Zn, than samples from monitoring wells. In humid regions, however, concentrations of most trace elements were greater in monitoring wells than in drinking-water wells; the exceptions were Cu, Pb, Zn, and Rn. Cu, Pb, and Zn are common trace elements in pumps and pipes used in the construction of drinking-water wells, and contamination from these sources may have contributed to their concentrations. Al, Sb, Ba, B, Cr, Co, Fe, Mn, Mo, Ni, Se, Sr, and U concentrations were all greater in monitoring wells than in drinking-water wells in humid regions. Groundwater from wells in agricultural settings had greater concentrations of As, Mo, and U than groundwater from wells in urban settings, possibly owing to greater pH in the agricultural wells. Significantly greater concentrations of B, Cr, Se, Ag, Sr, and V also were found in agricultural wells in dry regions. Groundwater from dry-region urban wells had greater concentrations of Co, Fe, Pb, Li, Mn, and specific conductance than groundwater from agricultural wells. The geologic composition of aquifers and aquifer geochemistry are among the major factors affecting trace-element occurrence. Trace-element concentrations in groundwater were characterized in aquifers from eight major groups based on geologic material, including (1) unconsolidated sand and gravel; (2) glacial unconsolidated sand and gravel; (3) semiconsolidated sand; (4) sandstone; (5) sandstone and carbonate rock; (6) carbonate rock; (7) basaltic and other volcanic rock; and (8) crystalline rock. The majority of groundwater samples and the largest percentages of exceedences of HHBs were in the glacial and nonglacial unconsolidated sand and gravel aquifers; in these aquifers, As, Mn, and U are the most common trace elements exceeding HHBs. Overall, 19 percent of wells (962 of 5,097) exceeded an HHB for at least one trace element. The trace elements with HHBs included in this summary were Sb, As, Ba, Be, B, Cd, Cr,