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Scientific Investigations Report 2008–5243

Occurrence of Selected Organic Compounds in Groundwater Used for Public Supply in the Plio-Pleistocene Deposits in East-Central Nebraska and the Dawson and Denver Aquifers near Denver, Colorado, 2002–2004

By Jeffrey B. Bails, Benjamin J. Dietsch, Matthew K. Landon, and Suzanne S. Paschke

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Abstract

The National Water-Quality Assessment Program of the U.S. Geological Survey has an ongoing Source Water-Quality Assessment program designed to characterize the quality of water in aquifers used as a source of drinking-water supply for some of the largest metropolitan areas in the Nation. In addition to the sampling of the source waters, sampling of finished or treated waters was done in the second year of local studies to evaluate if the organic compounds detected in the source waters also were present in the water supplied to the public.

An evaluation of source-water quality used in selected groundwater-supplied public water systems in east-central Nebraska and in the south Denver metropolitan area of Colorado was completed during 2002 through 2004. Fifteen wells in the Plio-Pleistocene alluvial and glacial deposits in east-central Nebraska (the High Plains study) and 12 wells in the Dawson and Denver aquifers, south of Denver (the South Platte study), were sampled during the first year to obtain information on the occurrence and distribution of selected organic chemicals in the source waters. During the second year of the study, two wells in east-central Nebraska were resampled, along with the associated finished water derived from these wells, to determine if organic compounds detected in the source water also were present in the finished water. Selection of the second-phase sampling sites was based on detections of the most-frequently occurring organic compounds from the first-year Source Water-Quality Assessment study results. The second-year sampling also required that finished waters had undergone water-quality treatment processes before being distributed to the public.

Sample results from the first year of sampling groundwater wells in east-central Nebraska show that the most-frequently detected organic compounds were the pesticide atrazine and its degradate, deethylatrazine (DEA, otherwise known as 2-chloro-4-isopropylamino-6-amino-s-triazine or CIAT), which were detected in 9 of the 15 wells (60 percent of the samples). The second most frequently detected organic compound was tetrachloroethylene, detected in 4 of the 15 wells (27 percent of the samples), followed by chloroform, trichloroethylene, and 2-hydroxyatrazine (2-hydroxy-4-isopropylamino-6-ethylamino-s-triazine, or OIET), present in 3 of the 15 wells (20 percent of the samples). The pesticide compounds deisopropylatrazine (2-chloro-6-ethylamino-4-amino-s-triazine, or CEAT), metolachlor, and simazine and the volatile organic compound cis-1,2-dichloroethylene were detected in 2 of the 15 wells, and the compounds diuron and 1,2-dichloroethane were detected in only 1 of the 15 wells during the first-year sampling. Most detections of these compounds were at or near the minimum reporting levels, and none were greater than their regulatory maximum contaminant level.

There were few detections of organic compounds during the first year of sampling groundwater wells in the South Platte study area. The compounds atrazine, deethylatrazine, picloram, tetrachloroethylene, methyl-tert-butyl-ether (MTBE), tris(2-butoxyethyl)phosphate, and bromoform were detected only once in all the samples from the 12 wells. Most detections of these compounds were at or near the minimum reporting levels, and none were greater than their regulatory maximum contaminant level.

Second-year sampling, which included the addition of paired source- and finished-water samples, was completed at two sites in the High Plains study area. Source-water samples from the second-year sampling had detections of atrazine and deethylatrazine; at one site deisopropylatrazine and chloroform also were detected. The finished-water samples, which represent the source water after blending with water from other wells and treatment, indicated a decrease in the concentrations of the pesticides at one site, whereas concentrations remained nearly constant at a second site. The trihalomethanes (THMs or disinfection byproducts), chloroform, bromoform, bromodichloromethane, and dibromochloromethane were detected in all finished-water samples and are a result of the chlorine added during treatment reacting with organics in the source waters. Finished waters at one site also contained concentrations of organic compounds not present in the source-water sample, indicating that other wells may be contributing compounds to the finished waters.

When comparing the source-water data from both studies, the more frequent detection and higher concentrations of pesticides and pesticide degradation byproducts in the High Plains wells are an indication that the agricultural land-use setting near these wells has affected source-water quality to a greater extent than the rangeland setting historically surrounding the Colorado wells. Land use near the Colorado public-supply wells is shifting from rangeland to an urban environment that may eventually affect groundwater quality; however, the potential effects of this transition on water quality currently (2004) are not expressed in the sampled source water.

Results from the High Plains and South Platte studies in Nebraska and Colorado also were compared to similar studies done in Utah and Nevada to provide a larger-scale perspective on the quality of source waters and the effects from other land-use settings. Whereas groundwater-quality data from the Utah and Nevada studies were similar to each other, frequency of detections and concentrations for a majority of the compounds were intermediate to those of the High Plains and South Platte studies. These results indicate that the long history of agricultural land use and the more permeable alluvial deposits in Nebraska are affecting the source-water quality, whereas the potential effects of developing urbanization on source-water quality in the deep Dawson and Denver bedrock aquifers near Denver, Colorado, are not yet apparent.

Posted July 15, 2009

For additional information contact:
Director, U.S. Geological Survey
Colorado Water Science Center
Denver Federal Center
Box 25046, MS 415
Lakewood, CO 80225
(303) 236–4912

Or visit the Colorado Water Science Center Web site at:
http://co.water.usgs.gov

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Suggested citation:

Bails, J.B., Deitsch, B.J., Landon, M.K., Paschke, S.S., 2009, Occurrence of selected organic compounds in groundwater used for public supply in the Plio-Pleistocene deposits of east-central Nebraska and the Dawson and Denver aquifers near Denver, Colorado, 2002–2004: U.S. Geological Survey Scientific Investigations Report 2008–5243, 29 p. with appendixes.



Contents

Foreword

Abstract

Introduction

Description of Study Areas

Study Design and Methods

Regulatory and Non-Regulatory Human-Health Benchmarks

Occurrence of Selected Organic Compounds in Groundwater Used for Public Supply

Comparison to Other Community Water Systems Well Data

Summary

Acknowledgments

References

Appendixes 1–3


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