Since the 1980s there has been increasing recognition that herbicides, applied in the course of normal farming practices, have contaminated surface and ground water in many agricultural regions (Barbash et al., 1999; Larson, Gilliom & Capel, 1999; United States Geological Survey, 1999). Among the herbicides detected most frequently in drinking-water sources, there are a number of compounds classified as probable (e.g., acetochlor), likely (e.g., alachlor), and possible (e.g., atrazine, cyanazine, metolachlor, and simazine) carcinogens (United States Environmental Protection Agency, 1999). Several herbicides contaminating drinking-water sources are also under scrutiny as possible disrupters of human immune, endocrine, and reproductive systems (see section "Acute and chronic effects of herbicides on human health" below). The effects of low-level exposure to herbicides are poorly understood, but there is considerable popular and regulatory concern over contamination of drinking-water sources.
Herbicide contamination of the Mississippi River drainage basin has been particularly well documented (United States Geological Survey, 1999). The 12 states that drain to the Mississippi River contain about 65% of the harvested cropland in the USA, and fields of maize, soybean, sorghum, rice, wheat, and cotton are dominant features of the region's landscape (United States Department of Agriculture, 1999b). The Mississippi River basin receives the majority of herbicides applied in the USA; during the late 1980s, more than 125 000 metric tons of herbicide active ingredients were applied annually to cropland in the watershed (Gianessi & Puffer, 1991; Goolsby, Battaglin & Thurman, 1993).
About 18 million people rely on the Mississippi River and its tributaries as their primary source of drinking water (Goolsby, Coupe & Markovchick, 1991). Public water systems serving that population are required to take at least four samples each year to measure concentrations of pollutants, including certain herbicides, for which the US Environmental Protection Agency (1996) has set legally enforceable safety standards called maximum contaminant levels. A public water system is out of compliance with the federal Safe Drinking Water Act of 1986 if the yearly average concentration of a pollutant exceeds its maximum contaminant level, or if a pollutant's concentration in any one quarterly sample is more than four times higher than its maximum contaminant level.
For several herbicides currently lacking legally enforceable standards, the US Environmental Protection Agency (1996) has specified health advisory levels, which are maximum chemical concentrations that may be consumed in drinking water over an average human lifetime with minimal risk that they will cause "adverse non-carcinogenic effects." Health advisory levels can eventually become enforceable standards. Both maximum contaminant and health advisory levels have been established only for individual compounds; standards have not been set for mixtures of herbicides and other chemicals, including metabolites of herbicides (Goolsby, Battaglin & Thurman, 1993).
After application to cropland in the midwestern USA, herbicides not degraded or bound to soil are detected in surface water in pulses corresponding to late spring and summer rainfall (Thurman et al., 1991). In 1991, the US Geological Survey detected atrazine, which is widely used for weed control in maize and sorghum, in each of 146 water samples collected at eight locations throughout the Mississippi River basin (Goolsby, Coupe & Markovchick, 1991). More than 75% of the samples also contained other herbicides used in maize, soybean, and sorghum production: alachlor, metolachlor, cyanazine, and simazine. Between April and July 1991, atrazine concentrations exceeded the US Environmental Protection Agency's maximum contaminant level of 3 ^g L 1 for 6 to 9 weeks at sites in the Illinois, Mississippi, Missouri, Platte, and White Rivers (Figure 1.4). In those same rivers, cyanazine concentrations exceeded the US Environmental Protection Agency's health advisory level of 1 ^g L 1 for 7 to 14 weeks. Alachlor concentrations exceeded the agency's maximum contaminant level of 2 ^g L 1 for 1 to 3 weeks in the Illinois, Platte, and White Rivers.
In a review of data from 12 studies of herbicide concentrations in finished tap water and raw drinking-water sources (rivers and reservoirs) in the
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