Agriculture Reference
In-Depth Information
A.
PROTECTING GROUNDWATER
Groundwater is water located beneath the earth's surface. Many people
think that groundwater occurs in vast underground lakes, rivers, or streams.
Usually, however, it is located in rock and soil. It moves very slowly through
irregular spaces within otherwise solid rock or seeps between particles of sand,
clay, and gravel. An exception is in limestone areas, where ground water may
flow through large underground channels or caverns. Surface water may move
several feet in a second or a minute. Ground water may move only a few feet
in a month or a year. If the ground water is capable of providing significant
quantities of water to a well or spring, it is called an aquifer.
Pesticide con-
tamination of aquifers is very troubling, because these are sources of drinking,
washing, and irrigation water.
1.
Sources of Groundwater
Groundwater is recharged (replaced) primarily from rain or snow that en-
ters the soil. However, some water from lakes and streams and from irrigation
also becomes ground water. Water that is above the ground can move in three
ways—it can evaporate into the air; it can move across the surface, as in a
stream or river; or it can move downward from the surface. Some of the water
that moves downward is absorbed by plants and other organisms. Another
portion of the downward-moving water is held in the upper layers of the soil.
The rest moves down through the root zone and the relatively dry soil zone
until it reaches a zone saturated with water. This saturated zone is the upper-
most layer of groundwater and is called the water table. The water table is the
"dividing line" between the groundwater and the unsaturated rock or soil above
it (see
Figure 9.4
).
2.
Pesticide Contamination of Groundwater
When water that is moving downward from the surface contains pesti-
cides—or comes into contact with them as it moves—the pesticides may be
carried along with the water until they eventually reach the groundwater. Five
major factors determine whether a pesticide will reach groundwater:
•
the practices followed by pesticide users
•
the presence or absence of water on the surface of the site where the
pesticides are released
•
the chemical characteristics of the pesticides
•
the type of soil in the site where the pesticides are released
