The contaminated floodplain groundwater is a major concern for human health. Contaminants include manganese, nitrate, selenium, sulfate, and uranium. As well, ammonium and strontium are of concern to the ecology. The San Juan River adjacent to the floodplain is at lower risk owing to the dilution effect of the high-volume river flow. Constituents of concern in the terrace ground water are ammonium, manganese, nitrate, selenium, strontium, sulfate, and uranium.