National Water-Quality Assessment (NAWQA) Program
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Mixtures of two or more contaminants with concentrations individually greater than a human-health benchmark were not common, occurring in 4 percent of wells. Mixtures of two or more contaminants with concentrations greater than one-tenth of individual benchmarks were found in 73 percent of wells. A concentration of one-tenth of a human-health benchmark was used to indicate contaminants of potential human-health concern when occurring with other contaminants in mixtures, and to identify those mixtures that may warrant additional monitoring and study. Contaminants included major ions, trace elements, nitrate, radon, and organic compounds; microbial contaminants were not included. >, greater than or equal to.