Perchlorate reconnaissance sampling in streams and groundwater in the Central and Southwestern U.S.


Perchlorate Occurrence in Ground Water in the North Central United States – Do Agricultural Practices Contribute Perchlorate to Shallow Aquifers

Stephen J. Kalkhoff, U.S. Geological Survey, Iowa City, IA.  
Sarah J. Stetson, Colorado School of Mines, Golden, CO.
Richard B. Wanty, U.S. Geological Survey, Denver, CO.
Gregory L. Linder, U.S. Geological Survey, Brooks, OR.

Presented at the NGWA Conference on MTBE and Perchlorate: Assessment, Remediation, and Public Policy; May 26-27, 2005,  San Francisco, California

An ongoing investigation to study the occurrence of perchlorate in ground water is being conducted by the U.S. Geological Survey in selected agricultural areas in the north-central United States from eastern Iowa through Nebraska and to central Colorado to look for a relationship between the presence of perchlorate in shallow ground waters and historical and current agricultural practices. Most documented occurrences of perchlorate to date have been related to military and chemical-manufacturing operations, as perchlorate is a primary ingredient of rocket fuels. However, a number of studies have documented the presence of perchlorate in nitrate-bearing minerals from Chile and potash-bearing minerals from the United States and Canada that have been or are still being used as fertilizers in the study area. Land use in the study area ranges from intensive row-crop agriculture in Iowa to irrigated row-crop agriculture in Nebraska to dry land farming in Colorado.

More than 150 wells screened in shallow aquifers were sampled from June through August 2004. Wells were pumped to remove stagnant water before sampling. Samples were filtered through a 0.45 µm pore capsule filter to remove suspended solids and stored chilled until analysis.

Samples were analyzed using techniques that provide about an order of magnitude lower detection levels than U.S. Environmental Protection Agency Method 314. The reporting limit for perchlorate in this study is 0.2 µg/L.  Analysis was by ion chromatography using a Dionex DX-500, 2 mm (microbore) ion chromatography system with AG16/AS16 columns, a flow rate of 0.25mL/min; and 35 mM KOH eluent and a 1000 µL injection loop. Samples were run through pre-treatment cartridges to remove sulfate, halogens, and carbonate. Quality assurance samples that included field blanks and replicates and laboratory standards and replicates were run to ensure accurate results.

About 40 percent of sampled wells contained water with detectable concentrations (greater than 0.2 µg/L) of perchlorate. Detected concentrations were low; all but one sample had concentrations that were less than 4.0 µg/L. Perchlorate detections were found throughout the study area, however, most wells containing water with perchlorate concentrations greater than 1 µg/L were found from western Iowa, through Nebraska, and into Colorado. The wide spatial distribution of perchlorate detections in ground water suggests that fertilizer use is one possible source of perchlorate, but other sources also may contribute perchlorate to shallow ground water in the north central United States. Further analyses will examine potential perchlorate sources in more detail.