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, probably 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) semi-consolidated 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, Fe, 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, Cu, Pb, Mn, Mo, Ni, Se, Ag, Sr, Tl, U, and Zn. Mn occurred most often at concentrations greater than its human-health benchmark (12 percent), followed by As (7.0 percent), Sr (4.3 percent), U (4.0 percent), B (1.9 percent), and Mo (1.5 percent). Rn occurred at concentrations greater than the U.S. Environmental Protection Agency (USEPA) proposed maximum contaminant level of 300 pCi/L in more than 65 percent of water samples, and concentrations of Rn in 2.7 percent of samples were greater than the USEPA-proposed alternate maximum contaminant level of 4,000 pCi/L. There were more exceedences of HHBs in the dry-region groundwater than in the humid-region groundwater. Groundwater pH and redox conditions were significant factors for the occurrence of many trace elements. Low pH (less than 7) was a significant factor in the occurrence of many cationic metals, such as Al, Fe, Mn, and Ni; these metals, as well as Cu, Pb, and Zn, adsorb more strongly to aquifer materials as pH increases. Anoxic conditions often were related to the increased occurrence of many oxyanion-forming elements, such as As, Cr, and Mo, whereas oxic conditions often were related to higher Se occurrence. Groundwater redox and pH effects were evident for As, Cr, Mo, and Se. Based on all samples, As occurrence generally increased as geochemical conditions became increasingly anoxic for groundwater with pH less than 7, but was consistently high for samples with pH greater than 7. A similar pattern was evident for Mo and, to a lesser extent, Cr. For groundwater in aquifers in glacial unconsolidated sand and gravel, however, As occurrence increased as groundwater became increasingly anoxic and as pH increased, indicating that redox is an important process throughout the range of pH. Al, Cu, Pb, and Zn occurred more often in low-pH groundwater and, except for Al, in oxic conditions. In general, older waters (mostly pre-1953) had more occurrences of trace elements, greater pHs, were from deeper wells, and had lower concentrations of dissolved oxygen than younger waters (defined as waters containing a fraction younger than 1953). Most oxyanion-forming trace elements occurred more frequently in old groundwater. However, although U occurrence was greater in older water collected from dry-region aquifers, U occurred more often in young water in humid-region aquifers. This difference may be related to old, humid-region groundwater having lower concentrations of dissolved oxygen, a condition which can inhibit U mobility. Overall, As co-occurred primarily with silica (SiO2) and Mo in water with slightly high pH. About 12 percent of water samples without SiO2 or Mo had concentrations of As greater than or equal to 1 ?g/L. This number increased to about 45 percent if either Mo or silica were present and to about 85 percent if both Mo and SiO2 were present. From a subset of water samples selected from 2,714 wells for co-occurrence analysis, samples from 572 wells (21.1 percent) had HHB exceedences of one or more trace elements. Of the 2,714 samples, 62 (2.3 percent) had two or more trace elements that exceeded HHBs. Fifty of the 62 (80 percent) were from unconsolidated sand and gravel aquifers and involved at least As and Mn, Mn and U, or As and U.