Pollution of Ground
Water
Ground water in its
natural state tends to be relatively free of contaminants in most areas.
Because it is a widely used source" of drinking water, pollution of ground
water can be a very serious problem.
Pesticides and herbicides (such as DDT and 2, 4-D)
applied to agricultural crops (figure 11.14A)
can find their way into ground water when rain or irrigation water leaches the
poisons downward into the soil. Fertilizers are also a concern. Nitrate,
one of the most widely used fertilizers, is harmful in even small quantities in
drinking water.
Rain can also leach
pollutants from city dumps into ground-water supplies (figure 11.14B). Consider for a moment some
of the things you threw away last year. A partially empty aerosol can of ant
poison? The can will rust through in the dump, releasing the poison into the
ground and into the saturated zone below. A broken thermometer? The toxic
mercury may eventually find its way to the ground-water supply. A half-used can
of oven cleaner? The dried out remains of a can of lead base paint? Heavy
metals such as mercury, lead, chromium, copper, and cadmium, together with
household chemicals and poisons, can all be concentrated in ground- water
supplies beneath dumps.
Liquid and solid wastes
from septic tanks, sewage plants, and animal feedlots and slaughterhouses may
contain bacteria, viruses, and parasites that can contaminate
ground water (figure 11.14 C). Liquid wastes
from industries (figure
11.14D) and military bases can be
highly toxic, containing high concentrations of heavy metals and compounds such
as cyanide and PCBs (polychlorinated biphenyls), which are widely used
in industry. A degreaser called TCE (trichloroethylene) has been
increasingly found to pollute both surface and underground water in numerous
regions. Toxic liquid wastes are often held in surface ponds or pumped down
deep disposal wells. If ponds leak, ground water can become polluted. Deep
wells may be safe for liquid waste disposal if they are deep enough, but
contamination of drinking water supplies and even surface water has resulted in
some localities from improper design of the disposal wells.
Acid mine drainage from coal and metal mines can contaminate both surface
and ground water. It is usually caused by sulfuric acid formed by the oxidation
of sulfur in pyrite and other sulfide minerals when they are exposed to air by
mining activity. Fish and plants are often killed by the acid waters draining
from long abandoned mines.
Radioactive waste is both an existing and a very serious potential
source of ground-water pollution. The shallow burial of low-level solid
and liquid radioactive wastes from the nuclear power industry has caused
contamination of ground water, particularly as liquid waste containers leak
into the saturated zone and as the seasonal rise and fall of the water table at
some sites periodically covers the waste with ground water. The search for a
permanent disposal site for solid, high-level radioactive waste (now
stored temporarily on the surface) is a major national concern for the United
States. The permanent site will be deep underground and must be isolated from
ground-water circulation for thou- sands of years. Salt beds, shale, glassy
tuffs, and crystalline rock deep beneath the surface have all been studied,
particularly in arid regions where the water table is hundreds of feet below
the land surface. The likely site for disposal of high-level waste, primarily
spent fuel from nuclear reactors is Yucca Mountain, Nevada, 180 km (110 miles)
northwest of Las Vegas. The site would be deep underground in volcanic tuff
well above the current (or predicted future) water table, and in a region of
very low rainfall. The U .S. Congress, under intense political pressure from
other candidate states who did not want the site, essentially chose Nevada in
late 1988 by eliminating the funding for the study of all alternative sites, but
the final decision regarding the safety of Yucca Mountain will not be made
until after much additional study. Even if the site is deemed safe, it could
not open before the year 2010. It could be delayed much later than this: in
1992, a 5.6-magnitude aftershock of the Landers, California, earthquake
occurred 19 kilometers (12 miles) from the proposed disposal site. The quake
caused $1 million damage to a U.S. Energy Department office building near the
site, and may indicate that the region is too seismically active for the site
to be built here at all.
Not all ground-water
pollutants form plumes within the saturated zone as shown in figure 11.15.
Gasoline, which leaks from gas station storage tanks at tens of
thousands of U.S. locations, is less dense than water, and floats upon the
water table (figure
11.16). Some liquids are heavier than water such as TCE and sink to the
bottom of the saturated zone, perhaps traveling in unpredicted directions upon
the surface of an impermeable layer (figure 11.16).
Determining the extent and flow direction of ground-water pollution is a
lengthy process requiring the drilling of tens, or even hundreds, of costly wells
for each pollution site.
Not all sources of
ground-water pollution are man- made. Naturally occurring minerals within
rock and soil may contain elements such as arsenic, selenium, mercury, and
other toxic metals. Circulating ground water can leach these elements out of
the minerals and raise their concentrations to harmful levels within the water.
Not all spring water is safe to drink. Like a "bad waterhole"
depicted in a Western movie, some springs contain such high levels of toxic elements
that the water can sicken or kill humans and animals that drink it. Many desert
springs contain such high concentrations of sodium chloride or other salts that
their water is undrinkable.
Source: Physical
Geology, by Plummer, McGeary, and Carlson, 8th Edition (1999),
WCB/McGraw Hill. Excerpts from Chapter 11
