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An important component of the NAWQA Program is the retrospective analysis of available water-quality data. One of the primary concerns in the Western Lake Michigan Drainages study unit is the effect of pesticides on ground-water quality. This fact sheet summarizes 32,064 synthetic-organic pesticide analyses and detections for ground-water samples collected from 4,155 wells in the study unit between 1983 and 1995 (figure 1 and table 1). The term pesticide in this fact sheet refers to both the parent product and its breakdown products, and includes herbicides, insecticides, and fungicides.
Figure 1. Location of the Western Lake Michigan Drainages NAWQA study unit and locations of wells sampled for pesticides.
Table 1. Pesticides analyzed in ground water from sampled wells in the Western Lake Michigan Drainages, 1983-95. (Pesticides in red were detected in at least one well; --, no data)
Ground-water contamination by pesticides is a concern because ground water is used for drinking water by about 50 percent of the nation's population. Until the mid-1970's, it was generally assumed that the soil provided a barrier to pesticide contamination (Barbash and Resek, 1996), and this perception was supported for many years by a lack of detections in sampled ground water. More recently, however, data from more than 300 studies nationwide show that pesticides from every major chemical class have been detected in ground water (Barbash and Resek, 1996). The increase in detections is likely due to improved analytical methods.
Numerous factors influence the susceptibility of ground water to contamination. For example, contamination is more likely in areas of high pesticide use, high ground-water recharge, high soil permeability, unconfined or unconsolidated aquifers, dug or driven wells, and wells with faulty construction.
During the period from 1983 to 1995, the number of wells sampled and the number of different pesticides analyzed for have steadily increased. A total of 117 pesticides have been analyzed in ground-water samples from the study unit, however, most were analyzed only a limited number of times and in a limited area. Only 28 of the 117 pesticides were detected in ground-water samples.
Most of the 28 pesticides detected in ground water in the study unit are herbicides used on corn, beans, cereal grains, and potatoes. Three of the detected pesticides--aldicarb, carbofuran, and terbufos--are insecticides. One of the detected breakdown products, p,p'-DDE, comes from the insecticide DDT which has been banned in the United States since 1972.
A triazine screen was the most common pesticide analysis in the study unit. It was performed on ground-water samples from about 71 percent (2,934 of 4,155) of the wells and accounted for about 12 percent (3,878 of 32,064) of all pesticide analyses. The triazine screen used for these samples uses the principle of enzyme-linked immunosorbent assay (ELISA) to determine the concentration of triazine herbicides such as atrazine, cyanazine, and simazine in a water sample. Results of the ELISA-analyzed samples will be referred to as triazines. Some other common pesticide analyses include alachlor, atrazine, atrazine breakdown products, cyanazine, metolachlor, and simazine. Historically, these have also been some of the most used pesticides in the study unit (Wisconsin Agricultural Statistical Service, 1986 and 1991).
Most of the pesticides listed in table 1 were not detected in any wells. Those detected were found in only a small percentage of the sampled wells and usually at low concentrations. Of the pesticides analyzed in samples from more than 100 wells, atrazine, deethyl atrazine (an atrazine breakdown product), prometon, and triazines were the only pesticides detected in more than 10 percent of sampled wells (table 2).
Table 2. Pesticides detected in water from sampled wells in the Western Lake Michigan Drainages, 1983-95. (na, not applicable; -, no data; µg/L, micrograms per liter; PAL, Preventive Action Limit; ES, Enforcement Standard)
Drinking-water standards for some of the detected pesticides have been set by the U.S. Environmental Protection Agency (EPA) or by the State of Wisconsin, or both. Wisconsin drinking-water Preventive Action Limits (PAL) and Enforcement Standards (ES) are included in table 2 because most of the data from the study unit are from samples collected in Wisconsin. Wisconsin also provides a more extensive list of standards than does the EPA. Atrazine, several atrazine breakdown products, and triazines (using the drinking-water standards for atrazine), were the only pesticides that exceeded a drinking-water standard in more than two percent of sampled wells. Atrazine had the highest percentage of exceedances for the PAL (10 percent) and the ES (2.7 percent), as well as the highest measured concentration [1,000 µg/L (micrograms per liter)] of all the detected pesticides.
Triazine-screen analyses were performed on water samples from much of the study unit and provided semi-quantitative information about where some of the most commonly used pesticides were detected in ground water. Triazines were detected predominately in the southern half of the study unit (fig. 2), where there is a large percentage of agricultural land. Concentrations greater than 0.3 µg/L were detected in many parts of the study unit; however, concentrations greater than 3.0 µg/L were detected primarily in the southwest, where agriculture predominates and surficial deposits are very permeable.
Figure 2. Spatial distribution of triazine concentrations in ground water from sampled wells in the study unit (if a well had multiple triazine analyses, only the highest concentration is shown).
Some of the most commonly used non-triazine pesticides in the study unit are the acetanilide herbicides, which include alachlor and metolachlor. Wells with detectable alachlor and metolachlor are located predominately in the southern half of the study unit, with the highest concentrations generally located in the southwest (fig. 3), similar to the spatial distribution of triazines.
Figure 3. Spatial distribution of (A) alachlor and (B) metolachlor concentrations in ground water from sampled wells in the study unit (if a well had multiple alachlor or metolachlor analyses, only the highest concentration is shown).
Data from the Western Lake Michigan Drainages show similar pesticides detected and percent of samples with detections as the study of near-surface aquifers in the Midwestern United States (Kolpin and others, 1996). Percentages of detections for alachlor, cyanazine, and simazine in the near-surface Midwestern aquifers were within one percent of those for the Western Lake Michigan Drainages. Atrazine detections in the two study areas were within four percent.
Pesticide breakdown products often are found in ground water (Kolpin and others, 1996). Breakdown products from alachlor, atrazine, cyanazine, and dacthal were detected in the near-surface aquifer study ground-water samples more often than the parent pesticides (Kolpin and others, 1996). For example, alachlor was detected in only about 3 percent of water samples from near-surface aquifers, yet alachlor-ESA, a breakdown product of alachlor, was detected in about 46 percent of samples. Additionally, the maximum concentration of alachlor-ESA was nearly twice as high as the highest concentration of alachlor. Alachlor has been one of the most widely used pesticides in the Western Lake Michigan Drainages, and was detected in 2.3 percent of the wells for which it was analyzed, yet alachlor-ESA was rarely analyzed. Some breakdown products are structurally and toxicologically similar to the parent pesticides and they should also be considered important for their environmental occurrence (Kolpin and others 1996). Because samples have rarely been analyzed for breakdown products, pesticide occurrences in ground water have likely been underestimated in the study unit in the past. As more is learned about the toxicity of these compounds and their effects, and as detection methods become more refined, our ability to assess pesticide effects on ground-water quality will improve.
Barbash, J.E. and Resek, E.A., 1996, Pesticides in ground waters-- Distribution, trends, and governing factors: Chelsea, Mich., Ann Arbor Press, 590 p. Hirsch, R.M., Alley, W.M., and Wilber,W.G., 1988, Concepts for a National Water-Quality Assessment Program: U.S. Geological Survey Circular 1021, 42 p. Kolpin, D.W., Thurman, E.M., and Goolsby, D.A., 1996, Occurrence of selected pesticides and their metabolites in near-surface aquifers of the midwestern United States: Environmental Science and Technology, v. 30, no. 1, p. 335-340. Wisconsin Agricultural Statistical Service, 1986, Wisconsin 1985- Pesticide use: Wisconsin Department of Agriculture, Trade, and Consumer Protection, 32 p. ___ 1991, Wisconsin 1991-Pesticide use: Wisconsin Department of Agriculture, Trade, and Consumer Protection, 31 p. Wisconsin Department of Natural Resources, 1992 Wisconsin Water Quality Assessment Report to Congress, 1992: Wisconsin Department of Natural Resources Publ-WR 254-92-REV, 244 p.