Temporal Trends of Selected Agricultural Chemicals in Iowa's Groundwater, 1982-95: Are Things Getting Better?

Dana W. Kolpin, Debra Sneck-Fahrer, George R. Hallberg, and Robert D. Libra

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INTRODUCTION

Iowa has some of the most productive farmland in the world. To maximize crop yields, agricultural chemicals (pesticides and nitrogen fertilizers) have become an integral part of farming in Iowa, where some of the most intensive applications of these chemicals in the United States occurs (Battaglin and Goolsby, 1995). Intensive chemical use has made Iowa a focus for research on the occurrence of agricultural chemicals in groundwater (e.g., Detroy et al., 1988; Glanville et al., 1995; Hallberg, 1989a; Hallberg, 1989b; Kross et al., 1990; Libra et al., 1987), documenting that alachlor, atrazine, cyanazine, and metolachlor have historically been the most frequently detected pesticides in Iowa's groundwater. This research has increased public awareness on the occurrence of agricultural chemicals in groundwater. A continuing public concern has been whether the severity of groundwater contamination from agricultural chemicals has changed over time (is water quality getting better or worse). Studies that have obtained long-term groundwater chemistry data have documented that temporal changes in chemical use affect concentrations of agricultural chemicals in groundwater (Hallberg, 1987; Hallberg, 1989b; Hallberg, et. al., 1989; Libra et al., 1991; Rowden et al., 1995). Few studies to date, however, have exhibited the consistency and longevity required to adequately address this issue on a regional scale (Barbash and Resek, 1996). One of the few Iowa studies that could address the temporal issue on a statewide level documented a steady increase in nitrate concentration from 1949-79 in groundwater from shallow municipal wells (<30 m deep) sampled in Iowa (McDonald and Splinter, 1982).

A monitoring program in Iowa that has provided long term water-chemistry data is the Iowa Groundwater Monitoring (IGWM) Program, a joint effort by the University of Iowa Hygienic Laboratory (UHL); the Iowa Department of Natural Resources, Geological Survey Bureau; and the U.S. Geological Survey (USGS) (Detroy, 1985; Detroy, et al., 1988). Since 1982, 2,041 nitrate samples from 1,024 municipal wells and 1,375 pesticide samples from 730 municipal wells have been collected for the IGWM. Several features of the IGWM make it appropriate for an examination of temporal trends of agricultural chemicals in Iowa's groundwater. First, only water from municipal wells has been sampled for this study. Thus, the question of using water-chemistry results from differing well types is not an issue. Second, the sampling protocol has not changed during this study -- eliminating the possibility of potential temporal changes in chemical concentrations being caused by changes in how the water samples were collected. Third, the same laboratory has been used to analyze all water samples. The use of multiple analyzing laboratories could have introduced variability in chemical concentrations based on differing analytical accuracy and precision. Fourth, the analytical method (and the corresponding analytical reporting limits) have remained unchanged for the duration of this project. Changes in the method of analysis or the analytical reporting limits would have to be taken into account for a temporal analysis of any data set. Research has shown an inverse relation between analytical reporting limits and the frequency of pesticide detection (Kolpin et al., 1995).

This paper describes the results of an investigation of the temporal variability in the concentrations of nitrite plus nitrate as nitrogen (N) (hereafter referred to as "nitrate"), alachlor, atrazine, cyanazine, and metolachlor in Iowa's groundwater from 1982-95 using data collected for the IGWM. These compounds represent the most frequently detected agricultural chemicals in groundwater for the state (Detroy et al., 1988; Hallberg, 1989a).

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