Environmental Engineering Reference
In-Depth Information
in the agricultural areas where pesticides are most often used, as about 95%
of that population relies upon groundwater for drinking water. Before the
mid-1970s, the common thought was that soil acted as a protective filter,
one that stopped pesticides from reaching groundwater. Studies have now
shown that this is not the case. Pesticides can reach water-bearing aquifers
below ground from applications onto crop fields, seepage of contaminated
surface water, accidental spills and leaks, improper disposal, and even
through injection of waste material into wells.
Pesticides are mostly modern chemicals. Many hundreds of these com-
pounds are used, and extensive tests and studies of their effect on humans
have not been completed. This leads us to ask, “Just how concerned should
we be about their presence in our drinking water?” Certainly, considering
pesticides to be potentially dangerous and handling them with appropriate
care would be wise. We can say they pose a potential danger if they are con-
sumed in large quantities, but as any experienced scientist knows we cannot
draw factual conclusions unless scientific tests have been done. Some pesti-
cides have had a designated maximum contaminant level (MCL) in drinking
water set by the USEPA, but many have not. Another serious point to con-
sider is the potential effect of combining more than one pesticide in drinking
water, which might be different than the effects of each individual pesticide
alone. This is another situation where we do not have sufficient scientific
data to draw reliable conclusions—in other words, we don't know what we
do n't k now.
Saltwater intrusion
In many coastal cities and towns as well as in island locations, the intrusion
of salty seawater presents a serious water quality problem. Because freshwa-
ter is lighter than saltwater (the specific gravity of seawater is about 1.025), it
will usually float above a layer of saltwater. When an aquifer in a coastal area
is pumped, the original equilibrium is disturbed and saltwater replaces the
freshwater (Viessman and Hammer, 1998). The problem is compounded by
increasing population, urbanization, and industrialization, which increases
the use of groundwater supplies. In such areas, while groundwater is heav-
ily drawn upon, the quantity of natural groundwater recharge is decreased
because of the construction of roads, tarmac, and parking lots, which pre-
vent rainwater from infiltrating, reducing the groundwater table elevation.
In coastal areas, the natural interface between the fresh groundwater flow-
ing from upland areas and the saline water from the sea is constantly under
attack by human activities. Because seawater is approximately 2.5 times more
dense than freshwater, a high-pressure head of seawater occurs (in relation
to freshwater), which results in a significant rise in the seawater boundary.
Potable water wells close to this rise in sea level may have to be abandoned
because of saltwater intrusion.
