How we contaminate groundwater
Any addition of
undesirable substances to groundwater caused by human activities is
considered to be contamination. It has often been assumed that
contaminants left on or under the ground will stay there. This has been
shown to be wishful thinking. Groundwater often spreads the effects of
dumps and spills far beyond the site of the original contamination.
Groundwater contamination is extremely difficult, and sometimes
impossible, to clean up.
Groundwater
contaminants come from two categories of sources:
point sources
and distributed, or
non-point sources.
Landfills, leaking gasoline storage tanks, leaking septic tanks, and
accidental spills are examples of point sources. Infiltration from farm
land treated with pesticides and fertilizers is an example of a
non-point source.
Among the more
significant point sources are municipal landfills and industrial waste
disposal sites. When either of these occur in or near sand and gravel
aquifers, the potential for widespread contamination is the greatest.
In
Ville Mercier, Quebec, for example, the disposal of industrial
wastes into lagoons in an old gravel pit over many years rendered the
water supplies of thousands of residents in the region unusable. Water
had to be pumped from a well 10 kilometers away to replace the area's
supply.
Other point sources
are individually less significant, but they occur in large numbers all
across the country. Some of these dangerous and widespread sources of
contamination are septic tanks, leaks and spills of petroleum products
and of dense industrial organic liquids.
Septic systems are
designed so that some of the sewage is degraded in the tank and some is
degraded and absorbed by the surrounding sand and subsoil. Contaminants
that may enter groundwater from septic systems include bacteria,
viruses, detergents, and household cleaners. These can create serious
contamination problems. Despite the fact that septic tanks and cesspools
are known sources of contaminants, they are poorly monitored and very
little studied.
Contamination can
render groundwater unsuitable for use. Although the overall extent of
the problem across Canada is unknown, many individual cases of
contamination have been investigated such as Ville
Mercier in Quebec; the highway de-icing salt problem in Nova
Scotia; industrial effluents in Elmira, Ontario; various pesticides in
the Prairie provinces; industrial contamination in Vancouver, British
Columbia; and so on. In many cases, contamination is recognized only
after groundwater users have been exposed to potential health risks. The
cost of cleaning up contaminated water supplies is usually extremely
high.
Contamination problems
are increasing in Canada primarily because of the large and growing
number of toxic compounds used in industry and agriculture. In rural
Canada, scientists suspect that many household wells are contaminated by
substances from such common sources as septic systems, underground
tanks, used motor oil, road salt, fertilizer, pesticides, and livestock
wastes. Scientists also predict that in the next few decades more
contaminated aquifers will be discovered, new contaminants will be
identified, and more contaminated groundwater will be discharged into
wetlands, streams and lakes.
Once an aquifer is
contaminated, it may be unusable for decades. The residence time, as
noted earlier, can be anywhere from two weeks or 10 000 years.
Furthermore, the
effects of groundwater contamination do not end with the loss of
well-water supplies. Several studies have documented the migration of
contaminants from disposal or spill sites to nearby lakes and rivers as
this groundwater passes through the hydrologic cycle, but the processes
are not as yet well understood. In Canada, pollution of surface water by
groundwater is probably at least as serious as the contamination of
groundwater supplies. Preventing contamination in the first place is by
far the most practical solution to the problem. This can be accomplished
by the adoption of effective groundwater management practices by
governments, industries and all Canadians. Although progress is being
made in this direction, efforts are hampered by a serious shortage of
groundwater experts and a general lack of knowledge about how
groundwater behaves.
 Groundwater
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