The Determination and Estimation of Arsenic and Uranium in Private Wells throughout the United States

Abstract:

Approximately 45 million Americans rely on private wells or small systems for their domestic water supply¹. With the exception of a few states (e.g., WA and NJ), private wells or systems serving fewer than 15 connections are not required by the Safe Drinking Water Act to regularly monitor water quality^1. This is a public health concern as a lack of monitoring/information can lead to prolonged exposure to levels of contaminants that pose health risks such as arsenic and uranium. Based on data from the United States Geologic Survey’s National Water Information System (NWIS), arsenic and uranium exceeded their respective maximum contaminant levels (MCL), set by the Environmental Protection Agency, in 11% and 4% of the wells tested, respectively. As monitoring is not required, but the presence of contamination is possible, it is important to be able to estimate the likelihood of an unmonitored well to be contaminated with arsenic or uranium. A national model was developed using NWIS data from ~260,000 wells across the United States and PMPE data (Precipitation minus evapotranspiration). CART analysis was used to determine the likelihood of a well to have arsenic based on geochemical and hydrometerological parameters. PMPE was the most important determiner of arsenic mobility, followed by pH and pe. Of the two, pH was primary in driest environments, and dissolved iron (proxy for pe) was primary in wetter environments. Uranium analysis on CART is still pending. It is expected that PMPE will also be the primary determiner of uranium mobility followed by pe for all environments. Using this information, the national model can predict the likelihood of a well to have arsenic or uranium based on location and other geochemical parameters previously measured. To estimate arsenic and uranium in wells that have never been monitored requires use of geospatial statistical tools like kriging to fill in the areas where no information is known. In these areas it is impossible to have certainty of contamination, however it aids decision makers in guiding areas of future testing and monitoring.

1. Kenny JF, Barber NL, Hutson SS, Linsey KS, Lovelace JK, Maupin MA. Estimated use of water in the United States in 2005: US Geological Survey Reston, VA; 2009.