Types of Groundwater PollutionUnder certain conditions, contaminants including soil nutrients, wastes, and chemi-
cals can migrate to groundwater sources. Pesticides applied directly to a site may be
moved downward with rain or irrigation water. This method of contamination is called
non-point source pollution. When pesticides enter a well directly from spillage or
back-siphonage and enter the groundwater directly, it is called point source pollution.
Twenty-two pesticides have been detected in U.S. wells, and up to eighty are esti-
mated to have the potential for movement to groundwater under favorable condi-
tions. More than half of the states have reported some pesticide contamination of
groundwater.
Because agricultural runoff is a diffuse source of pollution, it is hard to control. It
is the number-one category of non-point source pollution in rivers and lakes. Non-
point source is most simply defined in contrast to point source pollution, which
comes from a specific place such as a pipe or smokestack. Contaminant concentra-
tions arising from industrial or other point sources can easily be measured at the
‘‘end of the pipe.’’ Non-point source pollution is difficult to assess because the source
is spread over a large area, as in agricultural or mining regions.
Because groundwater moves slowly, contaminants do not spread quickly. After pes-
ticides reach groundwater, they may continue to break down, but at a much slower
rate because light, heat, and oxygen are less available. Thus, they can remain under-
ground in slow-moving columns for an indefinite period. When groundwater
becomes contaminated, the polluted water may eventually appear in the surface water
of streams, rivers, and lakes.
Although some organophosphorus compounds are highly toxic to humans, they
generally break down rapidly in the environment and rarely have been found in
groundwater. Another group that replaced the chlorinated hydrocarbons are carba-
mate pesticides, including aldicarb, carbofuran, and oxamyl. These compounds tend
to be soluble in water and weakly adsorbed to soil. Consequently, if not degraded in
the upper soil layers, they have a tendency to migrate to groundwater. The most sig-
nificant occurrences of groundwater contamination have been by carbamate pesti-
cides. Aldicarb has been detected in more than 2,000 wells on Long Island as well as
in twelve other states, including Maine and New Jersey. As awareness has grown of
the potential for pesticides to leach to groundwater, attention has focused on ways of
changing registration and monitoring requirements to prevent such contamination
from occurring in the future. Intensive studies have also been carried out in an
attempt to determine what levels of pesticides are acceptable in water supplies.^39Unacceptable RisksThe Safe Drinking Water Act charges the EPA to protect public health by estab-
lishing allowable levels of contaminants in drinking water. These include a Maximum
Contaminant Level Goal (MCLG) and an MCL for each regulated contaminant.206 | Pesticides