Botkin Environmental Science Earth as Living Planet 8th txtbk

19.9 Groundwater Pollution 413
Some pollutants are heavier than water and sink or
move downward through groundwater. Examples of
sinkers include some particulates and cleaning solvents.
Pollutants that sink may become concentrated deep in
groundwater aquifers.
The method used to treat or eliminate a water pollutant
must take into account the physical and chemical properties
of the pollutant and how these interact with surface
water or groundwater. For example, the extraction
well for removing gasoline from a groundwater resource
(Figure 19.13) takes advantage of the fact that gasoline
floats on water.
Because cleanup or treatment of water pollutants in
groundwater is very expensive, and because undetected
or untreated pollutants may cause environmental damage,
the emphasis should be on preventing pollutants
from entering groundwater in the first place.
Groundwater pollution differs in several ways from
surface-water pollution. Groundwater often lacks oxygen,
a situation that kills aerobic types of microorganisms
(which require oxygen-rich environments) but may provide
a happy home for anaerobic varieties (which live in
oxygen-deficient environments). The breakdown of pollutants
that occurs in the soil and in material a meter or so
below the surface does not occur readily in groundwater.
Furthermore, the channels through which groundwater
moves are often very small and variable. Thus, the rate of
movement is low in most cases, and the opportunity for
dispersion and dilution of pollutants is limited.
Long Island, New York
Another example—that of Long Island, New York—
illustrates several groundwater pollution problems and
how they affect people’s water supply. Two counties on
Long Island, New York (Nassau and Suffolk), with a
population of several million people, depend entirely on
groundwater. Two major problems with the groundwater
in Nassau County are intrusion of saltwater and shallow-aquifer
contamination. 27 Saltwater intrusion, where
subsurface salty water migrates to wells being pumped,
is a problem in many coastal areas of the world. The general
movement of groundwater under natural conditions
for Nassau County is illustrated in Figure 19.14. Salty
groundwater is restricted from migrating inland by the
large wedge of freshwater moving beneath the island.
Long Island Sound
Long Island
Nassau County
Atlantic Ocean
1 Seepage to streams
2 Subsurface outflow
3 Evapotranspiration
4 Spring flow
East
General movement of
fresh groundwater
West
4
Water
Long Island Sound
table 3
3
1
Atlantic Ocean
2
2
Fresh groundwater
2
2
Salty
groundwater
FIGURE 19.14 The general
movement of fresh groundwater
for Nassau County, Long Island.
(Source: G.L. Foxworth, Nassau
County, Long Island, New York,
“Water Problems in Humid County,”
in G.D. Robinson and A.M. Spieke,
eds., Nature to Be Commanded,
U.S. Geological Survey Professional
Paper 950, 1978, pp. 55–68.)