Science of Water : Concepts and Applications

Water Pollution 213
chloride, organics, heavy metals, nitrate, and other contaminants, has little diffi culty reaching the
groundwater in such disposal sites. In the United States, literally thousands of inactive or abandoned
dumps like this exist.
AGRICULTURE
Fertilizers and pesticides are the two most signifi cant groundwater contaminants that result from agricultural
activities. The impact of agricultural practices wherein fertilizers and pesticides are normally
used is dependent upon local soil conditions. If, for example, the soil is sandy, nitrates from fertilizers
are easily carried through the porous soil into the groundwater, contaminating private wells.
Pesticide contamination of groundwater is a subject of national importance because groundwater
is used for drinking water by about 50% of the nation’s population. This especially concerns
people living 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 fi lter, 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 fi elds, 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 compounds are used, and
extensive tests and studies of their effect on humans have not been completed. That leads us to ask,
“Just how concerned we should be about their presence in our drinking water?” Certainly, treating
pesticides as potentially dangerous, and thus handling them with care would be wise. We can say
they pose a potential danger if they are consumed in large quantities, but as any experienced scientist
knows, you cannot draw factual conclusions unless scientifi c tests have been done. Some pesticides
have had a designated MCL in drinking water set by the USEPA, but many have not. Another
serious point to consider 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 don’t have suffi cient scientifi c data to draw reliable conclusions—in other words,
again, we don’t know what we don’t know.
SALTWATER INTRUSION
In many coastal cites and towns as well as in island locations, the intrusion of salty seawater presents
a serious water-quality problem. Because fresh water is lighter than saltwater (the specifi c gravity of
seawater is about 1.025), it will usually fl oat above a layer of saltwater. When an aquifer in a coastal
area is pumped, the original equilibrium is disturbed and saltwater replaces the fresh water (Viessman
and Hammer, 1998). The problem is compounded by increasing population, urbanization, and
industrialization, which increase use of groundwater supplies. In such areas, while groundwater is
heavily drawn upon, the quantity of natural groundwater recharge is decreased because of the construction
of roads, tarmac, and parking lots, which prevent rainwater from infi ltrating, decreasing
the groundwater table elevation.
In coastal areas, the natural interface between the fresh groundwater fl owing from upland areas
and the saline water from the sea is constantly under attack by human activities. Since 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 signifi cant rise in the seawater boundary. Potable water
wells close to this rise in sea level may have to be abandoned because of saltwater intrusion.
OTHER SOURCES OF GROUNDWATER CONTAMINATION
To this point, we have discussed only a few of the many sources of groundwater contamination.
For example, we have not discussed mining and petroleum activities that lead to contamination
of groundwater or contamination caused by activities in urban areas. Both of these are important