Leaching
Some pesticides move through the soil and leach into groundwater. Several factors
influence pesticide leaching. A pesticide that is easily dissolved in water moves with
the water as it seeps through the soil. Soil structure and texture influence the rate and
depth of pesticide leaching. Sandy and gravel soils have poor adsorption characteris-
tics and allow water and pesticides to leach through quickly. A heavy clay soil does
not allow for rapid leaching. Adsorption influences pesticide leaching because pesti-
cides that are strongly attached to soil particles leach less. Leaching of pesticides from
treated areas, mixing and rinsing sites, waste-disposal areas, and manufacturing facili-
ties is a major groundwater concern.
Most of our activities change the quality of the underground water sources. Polluted
water typically enters an aquifer in recharge water originating at the land’s surface. Pol-
lution can also be injected directly into an aquifer, for example, by back-siphoning
directly into a well.^37
Groundwater Protection
Groundwater is an important and still relatively untapped natural resource in the
United States. Groundwater accounts for nearly all of our freshwater reserves, but
only 20 percent of total water consumption and 50 percent of drinking-water use.
However, groundwater contamination is rapidly becoming a serious and ubiquitous
environmental concern. While groundwater contamination may result from minerali-
zation or other natural processes, it is usually attributed to waste-disposal practices
and industrial and agricultural activities.
Management of groundwater aquifers to satisfy drinking-water standards is a for-
midable task. It is difficult to monitor the movement of groundwater, and there are
substantial time lags between emissions and detection of chemical residues. Once the
aquifer is contaminated, residues may remain in the groundwater for long periods,
and it is technically difficult and costly to treat the aquifer. At present, the best reme-
dial actions are filtration at the wellhead, which can be extremely costly if contamina-
tion is widespread, or the use of substitute drinking-water sources.
Generally speaking, the best strategy for protecting groundwater supplies is to con-
trol source emissions. In most cases, the elimination of source emissions will eventu-
ally mitigate groundwater pollution. However, such drastic measures are often not
necessary; it may be possible to meet groundwater quality standards without eliminat-
ing emissions. The problem policy makers face is establishing the relationship
between on-site emissions and groundwater contaminant concentrations. Lacking this
knowledge, state or local governments may lean toward a complete ban on a particu-
lar chemical. For example, when the pesticide aldicarb was detected in wells on Long
Island, New York, it was subsequently banned from use by farmers. Assuming the
drinking-water standard for aldicarb adequately protects consumers, it might be asked
whether a ‘‘safe’’ application rate could have been established for this pesticide.^38
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