pesticides, only nine have established MCLs. Compliance with the Safe Drinking
Water Act is determined by the annual average concentration of a specific contami-
nant in drinking water, based on quarterly sampling. While MCLs do not directly
pertain to concentrations of pesticides in untreated surface waters, they provide
benchmark values for comparisons, and they facilitate perspectives on the significance
of the levels observed in surface waters.^30Pesticides in WellsOn a national scale, fewer than 2 percent of wells sampled in multistate studies
were found to have pesticide concentrations above the established MCL. Due to
repeated detection of various pesticides in U.S. wells, the EPA proposed a State Man-
agement Program that would control or ban those pesticides with the greatest poten-
tial to contaminate groundwater. Five pesticides were initially selected due to the
frequency of their occurrence: alachlor, atrazine, cyanazine, metolachlor, and sima-
zine. According to the EPA, they all have been detected in many states and have the
potential to reach levels that exceed health standards. They are all associated with seri-
ous health effects, including cancer.
The five selected pesticides are herbicides that are used to control broadleaf weeds
and grasses. The EPA estimates that between 200 and 250 million pounds of these
herbicides are applied annually in the United States. Atrazine, simazine, and cyana-
zine are applied to agricultural land before and after planting. Alachlor and metola-
chlor are applied to soil prior to plant growth.^31Historical Study EffortsSeveral large national and multistate studies of pesticides in rivers and streams were
conducted between the late 1950s and the mid-1970s. These and most other studies
during this period focused on organochlorine insecticides, such as DDT and dieldrin;
a few phenoxy acid herbicides, such as 2,4-D; and organophosphorus insecticides,
such as diazinon, all in use at the time. Use of organochlorine insecticides declined
dramatically after the 1960s, while use of organophosphorus and carbamate
insecticides increased. In addition, agricultural use of herbicides increased dramati-
cally, from an estimated 84 million pounds in 1964 to more than 500 million
pounds in 1992.
In response to changes in pesticide use, the number of different types of pesticides
monitored in surface waters from the mid-1970s to the present has increased. The
scale of monitoring studies has changed as well. The national and multistate studies
conducted during the 1960s and 1970s have been largely replaced by state and local
surveys, or by regional studies directed at specific river basins. Recent studies have
been relatively short-term, and their geographic distribution is highly uneven. Iowa,
California, Florida, and the Great Lakes region have been the most frequently studied
areas. The most extensive regional studies have been conducted in the Mississippi202 | Pesticides