ARSENIC IN NEBRASKA’S GROUNDWATER AND PUBLIC WATER SUPPLIES

The U.S. Environmental Protection Agency (EPA) recently lowered the maximum contaminant level (MCL) for arsenic from 50µg/L to 10 µg/L. EPA has estimated that of the 74,000 public water systems (PWS) regulated by this MCL, approximately 4,000 PWS will have to make changes to comply with it. According to the Nebraska Health and Human Services System, 81 PWS will have arsenic concentrations above the 10 µg/L MCL. Data from the U.S. Geological Survey (n=534) and the National Uranium Resource Evaluation program (n=3,396) clearly document distinct spatial and geologic variability of arsenic in Nebraska's ground water on a regional scale. The highest arsenic concentrations are found in the Nebraska Panhandle and the western Sand Hills. Arsenic concentrations greater than 15 µg/L are most common in these regions. Average arsenic concentrations generally decrease to the east, where the concentrations are commonly below 5 µg/L. This variability reflects differences in the geology, ground water flow systems, and geochemical environments throughout Nebraska. These factors, along with the history of the aquifers, preclude the making of accurate a priori predictions of arsenic concentrations at given sites. However, the assessment of existing information can improve the likelihood of finding low-arsenic water sources for PWS.

Arsenic behavior in 10 rural Nebraska PWS was investigated from October 2002 to December 2003. Data from 20 wells, two per PWS, indicate that arsenic concentrations are relatively constant over monthly to yearly time scales. The near-absence of variability in arsenic concentrations in individual wells indicates that the collection of one sample per year will adequately characterize the arsenic concentrations to which the population drinking this water will be exposed. In other parts of the United States and the world, arsenic concentrations can vary significantly on weekly to monthly time scales. Therefore, water supply managers need to assess the extent to arsenic varies within individual wells with their PWS in order to make informed decisions.