PERCHLORATE OCCURRENCE IN SHALLOW GROUND WATER IN THE NORTH CENTRAL UNITED STATES

The occurrence of perchlorate in shallow ground water was investigated in agricultural areas in north-central United States during the summer and fall of 2004. Previous studies have documented the presence of perchlorate in potash-bearing minerals from the United States and Canada that are currently being used as fertilizers. The purpose of this project was to determine if perchlorate is leaching to shallow aquifers in north-central United States where land use ranges from intensive row-crop agriculture in Iowa to irrigated row-crop agriculture in Nebraska to dry land farming in Colorado.

Perchlorate samples were collected as part of several ongoing U.S. Geological Survey monitoring programs. More than 150 wells screened in shallow unconsolidated aquifers were sampled from June through September 2004. Use of these monitoring sites provided additional water chemistry, geologic, climatologic and land use data needed to assist in the understanding of perchlorate occurrence.

Samples were analyzed for perchlorate using ion chromatography techniques developed by researchers at Texas Tech University that provided about an order of magnitude lower detection levels than US. Environmental Protection Agency method 314. The method reporting limit using chromatography techniques was 0.4 μg/L.

A substantial number of wells, about 40 percent, contained water with detectable concentrations of perchlorate. Perchlorate detections were found across the study area, however, most wells containing water with perchlorate concentration greater than 1 μg/L were found from western Iowa, through Nebraska, and into Colorado. Although there was a significant positive correlation between perchlorate concentrations and concentrations of two constituents (nitrate and chloride) commonly attributed to agriculture, additional work is needed to fully understand the sources of perchlorate in ground water in north-central United States.