Figure 1.4 Atrazine concentrations during April through June 1991 in the White River at Hazelton, IN (a); the Illinois River at Valley City, IL (a); the Platte River at Louisville, NE (a); the Missouri River at Hermann, MO (b); the Ohio River at Grand Chain, IL (b); and the Mississippi River at Thebes, IL, Clinton, IA, and Baton Rouge, LA (c). The maximum contaminant level (MCL) set by the US Environmental Protection Agency for atrazine in drinking water is 3 |jug L '. (After Goolsby, Coupe & Markovchick, 1991.)
American maize belt, Nelson & Jones (1994) noted that a substantial proportion of sampled locations had at least one measurement of atrazine, cyana-zine, or alachlor that was more than four times higher than maximum contaminant or health advisory levels. Most community water systems in the Mississippi River drainage basin are not equipped with technology that can reduce herbicide concentrations to levels lower than government health standards (National Research Council, 1989, p. 101; Goolsby, Coupe & Markovchick, 1991; Nelson & Jones, 1994). Consequently, the American Water Works Association has expressed concern that costly additional treatment systems, such as granular activated charcoal, will have to be installed in many public water systems in the midwestern USA to address violations of the federal Safe Drinking Water Act (Nelson & Jones, 1994).
Because certain herbicides can be harmful to aquatic organisms, "aquatic life guidelines" have been set for several herbicides found in surface water. Canadian standards, which are also used as nonenforceable benchmarks in the USA, are 1 ^g L 1 for metribuzin, 2 ^g L 1 for atrazine and cyanazine, 8 ^g L 1 for metolachlor, and 10 ^g L 1 for simazine (Larson, Gilliom & Capel, 1999). It is clear from the data presented in Figure 1.4 that atrazine concentrations in American rivers can exceed the Canadian aquatic life standard. Aquatic life standards for other herbicides detected in rivers and streams are also often exceeded (Larson, Gilliom & Capel, 1999).
An additional concern is how herbicides affect coastal ecosystems. Goolsby, Battaglin & Thurman (1993) estimated that discharges of atrazine from the Mississippi River into the Gulf of Mexico from April through August were 296 000 kg in 1991,160000 kg in 1992, and 539 000 in 1993 (a flood year). The possible impacts of such discharges on aquatic organisms in the Gulf of Mexico and elsewhere are inadequately understood and require more research.
Herbicides and their degradation products are common contaminants of groundwater in many agricultural regions (Hallberg, 1989; Leistra & Boesten, 1989; National Research Council, 1989, pp. 107-9; United States Geological Survey, 1999). In the USA, groundwater is used for drinking water by nearly half of the total population and by more than 95% of the population in rural areas (National Research Council, 1989, p. 105). Herbicides that have been measured in wells of American agricultural areas at concentrations greater than maximum contaminant or health advisory levels include alachlor, atra-zine, cyanazine, 2,4-D, DCPA, dicamba, dinoseb, metolachlor, metribuzin, and simazine (Hallberg, 1989). In a survey of private wells used for drinking water in Ohio, Indiana, Illinois, West Virginia, and Kentucky, Richards et al. (1996) detected chloroacetamide and triazine herbicides in 9.7% and 4.9% of the 12362 samples tested; maximum contaminant levels for alachlor and atra-zine were exceeded in 1.1% and 0.1% of the samples, respectively. Two large-scale, multistate investigations of herbicides in American wells and springs detected at least one of seven targeted compounds in 35% to 40% of the sites sampled, although maximum contaminant or health advisory levels were exceeded at fewer than 0.1% of the sites (Barbash et al., 1999).
Although concentrations of individual herbicides in American groundwa-ter rarely exceed existing regulatory standards, important concerns remain concerning health risks. Detection of one herbicide in groundwater at an individual site is often accompanied by the detection of others (Barbash et al., 1999), but little is known about the health-related impacts of exposure to multiple herbicides, or to herbicides in combination with nitrates, which are also common water contaminants. Breakdown products of herbicides are generally found in well water more frequently and at higher concentrations than the corresponding parent compounds (Kolpin, Thurman & Goolsby, 1996), but little is known about their possible effects on human health. Health-based standards for breakdown product concentrations in groundwater generally do not exist.
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