Pesticides pollution of groundwater in the humid United States

Abstract

Since 1980, many studies have begun to look at the occurrence of pesticides in groundwater; these range from controlled field/plot research studies, to simple monitoring of public water supplies to assess the occurrence of pesticides. The compounds most commonly detected are: (1) mobile and/or volatile soil fumigants and nematicides, used on vegetable or specialty crops; (2) commonly used herbicides from the humid corn-belt regions. The fumigants-nematicides have been the most widely looked for compounds. The herbicides commonly detected are also some of the most widely used of all pesticides, but pesticide properties clearly affect how commonly they are detected. In nearly all corn-belt areas, the herbicide atrazine is the most commonly detected compound. A general overview shows that a total of 39 pesticides have been detected in groundwater from 34 states or provinces. This includes occurrences related to commercial point sources (but not manufacturing sites, or detections exclusively referred to commercial sources) as well as nonpoint sources. From states with appropriate studies, there appears, within the context of pesticide properties, some general proportionality between pesticide use and detections in groundwater; this is also confounded by climatic differences. Kansas studies suggest that 8–10% of private, rural drinking-water supply wells, and 10% of susceptible public water-supply wells may exhibit pesticide contamination. Surveys in Minnesota and Iowa, with more intensive pesticide usage suggest that 20–30% of the more susceptible public water-supply wells, and 30–60% of private wells may exhibit pesticide residues. In contrast however, Illinois, a very high use state, has detected few pesticides in public water-supply wells (except where related to a commercial point source); why such differences occur among adjacent states is not clear. Pesticides are leaching through the soil and into groundwater far more commonly than the preconceptions of a decade ago would have predicted. Point sources may be widespread but are not the sole cause; it is also clear that many pesticides are leaching to groundwater from routine, nonpoint source use. Controlled plot studies show the intermittent, often rapid delivery of many pesticides to shallow groundwaters. The preferential flow of water and solutes through soil and rock materials clearly contributes to the delivery of pesticides to groundwater. The occurrence of preferential flow also implies that notions about the ‘relative susceptibility’ of various soils to pesticide leaching may not be accurate. Some studies show pesticide leaching more commonly in finer-textured soils than coarser, sandy textured soils. Generally, the concentrations of pesticides in groundwater are low, in the 0.1–5.0 μg l −1 range. Even at these concentrations there are concerns for long-term, chronic exposure to a large segment of the public through drinking-water supplies. While concerns with groundwater merit attention, the concentrations of pesticides in drinking water derived from surface waters will typically be greater. Current policy discussions often fail to understand the difference between groundwater protection and drinking-water protection programs and may not provide adequate protection for private rural groundwater supplies. Research and policies to resolve the problems of agricultural impacts on the environment will require a new focus on integrated farm-management systems, that enhance efficiency and reduce off-site impacts.