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The sight and smell of grossly polluted waterways provided some of the original
impetus to the environmental movement in the 1970s. Nearly a century before
that, the dangers of polluted water to human health drove what became known
as the "sanitary revolution" in Europe and the United States, emphasizing clean
water supplies and sewer systems in cities. Today, despite progress in cleaning
up waterways in some areas, water pollution remains a serious global problem,
with impacts on the health of freshwater ecosystems and the human communities
that rely on them for water supply.
The Changing Pollution Profile
Water pollution spans a wide range of chemical, physical, and microbial factors,
but over the years the balance of major pollutants has shifted markedly in most
industrialized countries. (See Figure 1 for a summary of major pollution sources
and their effects.) One hundred years ago, the main water contamination problems
were fecal and organic pollution from untreated human waste and the byproducts
of early industries. Through improved treatment and disposal, most industrialized
countries have greatly reduced the effects of these pollutants, with consequent
improvements in water quality. Pollution laws and pollution control technologies
have succeeded especially well in cutting emissions from concentrated "point
sources" like factories and sewage treatment plants. For example, from 1972
to 1992 the amount of sewage treated at wastewater treatment plants in the United
States increased by 30 percent, yet the organic pollution (measured as the Biological
Oxygen Demand) from these plants dropped 36 percent (CEQ 1995:229).
Unfortunately, a new suite of contaminants from intensive agriculture and development
activities in watersheds has kept the cleanup from being complete. In general,
national water clean-up programs have not been effective in reducing "nonpoint"
pollutants such as nutrients, sediments, and toxics that come in runoff from
agriculture, urban and suburban stormwater, mining, and oil and gas operations
(NRC 1992:47; EEA 1999:178).
Meanwhile, in most developing countries, the problems of traditional pollution
sources like sewage and new pollutants like pesticides have combined to heavily
degrade water quality, particularly near urban industrial centers and intensive
agricultural areas. (Shiklomanov 1997:28; UNEP/GEMS 1995:6). An estimated 90
percent of wastewater in developing countries is still discharged directly to
rivers and streams without any waste processing treatment (WMO 1997:11). (See
Figure 1.)
Nutrient Pollution: The New Danger
The level of nutrients such as nitrates and phosphorous in freshwater
ecosystems is a problem worldwide (Shiklomanov 1997:34-36). In most
cases, the major cause of these contaminants is the increased use
of manure and manufactured fertilizer in global agriculture. In the
United States, for example, agriculture is the single greatest source
of pollution degrading the quality of surface waters like rivers and
lakes, with croplands alone accounting for nearly 40 percent of the
nitrogen pollution and 30 percent of the phosphorous (Faeth 2000:6-7).
(See Figure 2.)
Natural waters have very low concentrations of nitrates (a soluble
from of nitrogen) and phosphorous, but nutrient levels increase with
runoff from farm lands as well as from urban and industrial wastewater.
Dissolved nutrients act as fertilizers, stimulating algal blooms and
the eutrophication of many inland waters. This can rob the water column
of dissolved oxygen, kill aquatic organisms, and degrade water quality.
Dissolved nitrates in drinking water can also harm human health.
Data on nutrient trends in global waters are spotty and only give
the most generalized picture of current conditions. The relevant water
data from the UN's Global Environment Monitoring System (GEMS), for
example, only cover 1976-1990. Of these globally monitored watersheds,
the highest nutrient concentrations come from sampling stations in
Europe. Nitrate concentrations are higher in watersheds that have
been intensively used and modified by human activity, such as the
Weser, Seine, Rhine, Elbe, and Senegal. High levels are also found
in such watersheds in China, South Africa, and the Nile and Mississippi
basins (UNEP/GEMS 1995:33-35).
In South America, nitrate concentrations in the monitored watersheds
are relatively low and follow human land use. The highest nitrate
concentrations are found in the Uruguay watershed, where some of the
most intensive agriculture on the continent is found. Nitrate concentrations
are also greater in the Magdalena watershed of Colombia than in the
less densely populated watersheds of the Amazon basin (UNEP/GEMS 1995:33-35).
The nitrate concentrations in South America correspond to lower fertilizer
application rates, compared to Europe.
More detailed and recent data available in Europe show distinct regional
trends in the concentrations of nitrates and phosphorous in rivers.
Nitrate loadings are highest in areas with intensive livestock and
crop production, especially in the northern parts of western Europe.
Nitrate concentrations are lowest in Finland, Norway, and Sweden.
Overall nitrate concentrations in monitored European rivers have not
changed significantly since 1980, despite lower nitrogen fertilizer
application rates since the 1990s (EEA 1998:194-197; EEA 1999:176-177).
Similar regional patterns are also evident in phosphorous trends.
Rivers in Finland, Norway, and Sweden have the lowest phosphorous
concentrations, whereas areas from southern England across central
and western Europe show the highest levels (EEA 1999:174). In general,
phosphorous concentrations have decreased significantly since 1985,
mostly due to improvements in wastewater treatment and the reduced
use of phosphorous in detergents. However, phosphorus levels remain
a problem in most regions of Europe (EEA 1999:174). Despite some positive
trends, the overall state of many European rivers with respect to
nutrient concentrations remains poor (EEA 1998:194-196).
Figure 3 shows water quality data for the United States for the 1980s.
For the 1980-89 period, nitrate concentrations remained relatively
stable, with most monitoring stations showing no discernable trend.
This probably reflects the fact that nitrogen fertilizer use in the
United States leveled off after steady increases in the 1970s. Fertilizer
application rates increased for the period 1974-1981, and nitrate
concentrations increased as well during that period. Average nitrate
concentrations were greater in agricultural and urban areas than in
forested areas (Smith et al. 1994:122).
Trends in phosphorous concentrations in the United States showed
greater improvement, with five times more states showing downward
trends than upward trends. Decreases were more likely to be found
in the East, Midwest, and the Great Lakes regions, while the majority
of increases occurred in the Southeast (Smith et al. 1994:124).
The decreased concentrations of phosphorous in streams and rivers
in the United States is attributable to reduced phosphorous in laundry
detergents and improved controls in wastewater treatment plants.
The increased number of sewage treatment plants has also reduced
the amount of nitrogen in the form of ammonium, which is toxic to
fish. However, the sewage treatment process converts ammonium to
nitrates that are still released into waterways. Thus, the greater
number of sewage treatment facilities has not necessarily decreased
the total amount of nitrogen flowing into waterways (Mueller and
Helsel 1996).
Groundwater Contamination
Surface waters like streams and lakes are not the only water sources
that suffer from pollution. Groundwater aquifers, which are critical
sources of both drinking water and irrigation water, are also affected.
The major causes of groundwater pollution are leaching of pollutants
from agriculture, industry, and untreated sewage, as well as saltwater
intrusion caused by overpumping.
Once pollutants enter a groundwater aquifer, the environmental damage
can be severe and long lasting, partly because of the very long
time needed to flush pollutants out of the aquifer (UNEP 1996:14).
Because groundwater is primarily used for drinking water, pollution
from untreated sewage, intensive agriculture, solid waste disposal,
and industry can cause serious human health problems (Shiklomanov
1997:42).
Global data on the quality of groundwater resources are lacking.
Even where available, data usually are not comparable because of
the different measures and standards used, which vary by country
(Shiklomanov 1997:42; Scheidleder et al. 1999:11; S. Foster, personal
communication, 2000). However, there is evidence that groundwater
contamination from fertilizers, pesticides, industrial effluents,
sewage, and hydrocarbons is occurring in many parts of the world.
As with surface waters, nitrate pollution is one of groundwater's
most serious threats. In general, the risk of nitrate pollution
for groundwater supplies is directly related to the amount of fertilizers
or other nitrogen inputs to the land, as well as the permeability
of the soil. For example, half the groundwater samples in a heavily
fertilized region of northern China contain nitrate levels above
the safe limit for drinking water (Zhang et al. 1996:224). In the
United States, where groundwater supplies drinking water for more
than half the population, a preliminary analysis of nitrate contamination
found that high nitrate concentrations are widespread in shallow
groundwater aquifers in agricultural areas (USGS 1999:41). Groundwater
pollution in Europe is similarly widespread (Scheidleder et al.1999).
Conclusion
Surface water quality has improved in most developed countries during
the past 20 years, but nitrate and pesticide contamination remain
persistent problems. Data on water quality in other regions of the
world are sparse, but water quality appears to be degraded in almost
all regions with intensive agriculture and rapid urbanization. Unfortunately,
little information is available to evaluate the extent to which
chemical contamination has impaired the health of freshwater ecosystems.
However, incidents of algal blooms and eutrophication are widespread
in freshwater systems all over the world—an indicator that
these systems are profoundly affected by water pollution. In addition,
the massive loss of wetlands at a global level has greatly impaired
the capacity of freshwater systems to filter and purify water. Groundwater
quality suffers from many of the same pollution problems as surface
waters and faces the additional challenge of being very difficult
to restore once the underlying aquifer is contaminated. |
