Trace elements are generally present in small concentrations in natural water systems.
Their occurrence in ground and surface water can be due to natural sources such as dissolution of naturally occurring minerals containing trace elements in the soil zone or the aquifer material or to human activities such as mining, application of pesticides, burning of fossil fuels, smelting of ores, and improper disposal of industrial wastes. The EPA has established primary and secondary MCLs, HAs, and issued guidelines for the establishment of aquatic-life criteria as they apply to trace elements. The States of Colorado, New Mexico, and Texas have also established individual aquatic-life criteria for selected trace elements that apply to specific river reaches within the Study Unit. The relatively low number of detections and small concentrations of most trace elements indicate that they are influenced more by natural processes and less by land use. Two trace elements that consistently exceeded SMCLs were manganese and iron, which can stain plumbing fixtures or impart a metallic taste to the water.