Weed management: a need for ecological approaches - Ecological Management of Agricultural Weeds - page 12

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, includ-
ing certain herbicides, for which the US Environmental Protection Agency
(1996) has set legally enforceable safety standards called maximum contami-
nant 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 even-
tually 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 correspond-
ing 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