TRACE ELEMENTS IN GROUNDWATER IN THE UNITED STATES
Overall, 19 percent of wells exceeded a human-health benchmark (HHB) for at least one TE. Mn exceeded its HHB in more samples (12%) than other TEs, followed by As (7.0%), Sr (4.3%), U (4.0%), B (1.9%), and Mo (1.5%). Rn exceeded its proposed HHBs (300 and 4,000 picocuries per liter) in 65% and 2.7% of samples, respectively.
The geochemistry of groundwater was a significant controlling factor for many TEs. Slightly acidic pH favored the occurrence of many cationic metals, such as Al, Fe, Mn, and Ni; whereas slightly alkaline pH and anoxic conditions favored occurrence of oxyanions such as As and Mo. Arsenic occurrence generally increased as geochemical conditions became increasingly anoxic for groundwater with pH < 7, but was consistently high for samples with pH > 7. In general, older waters had more oxyanion-forming TEs, greater pHs, lower dissolved oxygen, and were from deeper wells than younger waters.
Large-scale climatic patterns were correlated with the occurrence and concentrations of TEs in groundwater, with more exceedences of HHBs in groundwater samples from dry areas than in those from humid areas. Specifically, concentrations of As, Ba, B, Cr, Cu, Mo, Ni, Se, Sr, U, V, and Zn were greater in the drier regions. Chemical evolution, ion complexation, evaporative concentration, and oxidation-reduction reactions act to varying degrees to mobilize these TEs. Al, Co, Fe, Pb, and Mn concentrations in groundwater were greater in humid regions of the United States, partly due to lower groundwater pH and or more frequent anoxic conditions.
Regional-scale patterns in TE occurrence were due, in part, to contrasts in the composition and geochemistry of aquifers. Characterization of TE concentrations in eight major groups based on geologic material showed that 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 TEs that most commonly exceed HHBs.