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Contaminants in Water

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					 11.2                                      Contaminants in Water
                                           Water is a very effective solvent; it can dissolve many substances.
                                           Land plants obtain most of their nutrients by absorbing nutrient-rich
                                           water through their roots. Aquatic plants obtain oxygen and carbon
                                           dioxide from water. Animals also benefit from drinking water that
                                           contains dissolved minerals. However, toxic substances can dissolve
                                           in water, too. Unwanted substances that dissolve in water are called
contaminant: an undesirable                contaminants. Taking in water that contains contaminants can be
substance in a mixture                     hazardous to both plants and animals.

                                           Sources of Chemical Contaminants
                                           There are many places where contaminants may enter the water
                                           cycle. Contaminants in the air can dissolve into precipitation that
                                           falls to Earth. Surface water and groundwater dissolve many chemical
                                           substances as they flow to the ocean. These can include salt and toxic
                                           chemicals (toxins). For example, surface water flowing across the
                                           Canadian prairies dissolves a great deal of salt. Water also dissolves
                                           minerals from soil and rock. Farmers in southern Ontario often add
fertilizer: a substance used to provide    fertilizers to their fields to increase crop production. Fertilizers are
nutrients to plants, usually for the       substances that contain nutrients that help plants grow. Farmers
purpose of increasing crop production
                                           and gardeners may use herbicides to kill weeds, or pesticides to kill
herbicide: a chemical substance used       unwanted insects (Figure 1). Surface water and groundwater dissolve
to kill or slow the growth of certain
                                           these substances, which contaminate the water. Acid rain also affects
plants; often used to kill weeds that
compete with crop plants                   water sources. Road, rail, and boat accidents can spill unwanted
                                           chemicals into the water supply.
pesticide: a chemical substance used
to kill animal pests; often used to kill
insects that damage crop plants




                                           Figure 1 Pesticides and herbicides are sometimes sprayed on crops from the air.

                                           Salt
                                           We use road salt, gravel, and sand in the winter to prevent ice from
                                           forming on roads (Figure 2). Melting snow and rain wash road salt
                                           into ditches and streams. Road maintenance crews must store and use
                                           road salt carefully to prevent it from scattering into nearby forested
                                           areas or washing into surface water. Municipalities are studying
Figure 2 Roads should be salted to         and testing alternatives to scattering solid road salt. Spraying a
prevent ice buildup only when needed.      concentrated salt solution is one alternative.
292 Chapter 11 • Water Quality                                                                                               NEL
Minerals
Surface water and groundwater both travel through various kinds of
rock. Minerals from rock dissolve in the water. Limestone in southern
Ontario is a soft rock that slowly dissolves in water, adding calcium
carbonate to the water. The harder rock of the Canadian Shield is
much less soluble and contributes few, if any, minerals to water.
Fertilizers
Surface runoff can dissolve fertilizers in farm fields and carry them
into streams and lakes. Fertilizers used on lawns and gardens can
also enter surface water and groundwater. Fertilizers encourage the
growth of tiny water plants called “algae,” just as they promote plant
growth on land (Figure 3). When algae die, they decompose. The             Figure 3 A sudden growth of algae
decomposition process uses oxygen, which reduces the amount of             in water is called an “algal bloom.”
                                                                           Algal blooms indicate that the water
oxygen in the water. This can harm some fish populations because fish
                                                                           contains high concentrations of
also need oxygen to survive.                                               nutrients, often from fertilizers.
Herbicides and Pesticides
Herbicides and pesticides are toxins intended to kill unwanted
plants and animals. Not all of the toxins reach the target organisms.
Excess herbicides and pesticides can dissolve into groundwater and
surface water that may carry them to nearby waterways. Herbicides
and pesticides can remain toxic for up to five years before they are
naturally converted into less harmful substances.
                                                                             LINKING TO LITERACY
   A popular pesticide used in the twentieth century was DDT.
It was very effective at killing flies, mosquitoes, and potato beetles.      Cause and Effect
                                                                             The story of DDT describes a
DDT also had an undesirable effect: it caused birds to lay eggs with
                                                                             cause and effect text pattern.
very thin shells. The shells broke before the chicks hatched, and bird       Take a moment to study this
populations suddenly dropped. The government was persuaded to                situation by
outlaw the use of DDT in Canada. Since then, bird populations have           • identifying the cause
recovered (Figure 4). It is encouraging to know that we can reverse          • locating information about
some of the negative effects that we cause in the environment.                  the effect
                                                                             • reflecting on people’s
                                                                                reactions and the long-term
                                                                                impact on the environment
                                                                                 Work with a partner to
                                                                             discuss your findings.




                                                                           Figure 4 Since DDT was outlawed
                                                                           in the 1970s, the whooping crane has
                                                                           returned to its Ontario habitat.
NEL                                                                       11.2 Contaminants in Water 293
                                                                      Acid Rain
                                                                      Industries and vehicles that burn fossil fuels
                                                                      release many chemicals into the air. Some of
                                                                      these chemicals react with water vapour in the
                                                                      atmosphere to form acid rain. Acid rain can
                                                                      fall hundreds of kilometres away from where
                                                                      the chemicals are released, damaging land and
                                                                      aquatic ecosystems. Maple forests in Quebec and
                                                                      northeastern Ontario were damaged by acid rain in
                                                                      the twentieth century (Figure 5). Many organisms
Figure 5 Acid rain damage has cost the Canadian forestry              cannot live in water that is affected by acid rain.
industry hundreds of millions of dollars.

                                         Other Toxic Chemicals
                                         The transportation industry is a major polluter of freshwater sources.
                                         Ships on the Great Lakes release pollutants when they empty ballast
                                         (stored water) or dump garbage and sewage. This is now illegal, but is
                                         difficult to enforce and still happens regularly. Accidental collisions of
                                         ships carrying chemicals or other harmful substances often result in
                                         spills. Train derailments and transport truck collisions can also release
                                         chemicals that can contaminate both surface water and groundwater.
                                            Table 1 summarizes how some types of pollution get into our water.
Table 1 Chemical Contaminants in Fresh Water

 Pollution type       Examples            Sources                                       Harmful effects

 salt                 • naturally         • soil that contains salt                     • kills crops or reduces yields
                        occurring salt    • stored road salt                            • contaminates freshwater supplies,
                      • road salt         • salt applied to roads                         making them undrinkable

 minerals             • carbonate         • rock and soil                               • none
                                          • chemicals applied to farm fields

 nutrients            • fertilizers       • runoff from agricultural lands and urban    • stimulate growth of algae, which then
                                            areas                                         decomposes, using up oxygen and
                                          • industrial discharge                          harming aquatic life

 toxic chemicals      • PCBs              •    industrial sites                         • mild immune suppression
                      • pesticides        •    automobiles                              • acute poisoning
                                          •    farms and gardens                        • reproductive failure
                                          •    municipal wastewater
                                          •    chemical spills

 acid                 • sulfuric acid     • acid rain from electric power generation,   • harms aquatic organisms
                      • nitric acid         industrial stack, and auto/truck            • moves heavy metals from soils into
                                            emissions                                     water bodies
                                          • chemicals from mining                       • damages trees

 heavy metals         •   lead            • industries                                  • accumulates in the tissues of fish and
                      •   cadmium         • mines                                         shellfish
                      •   zinc            • landfill sites                               • toxic to animals and humans who eat
                      •   mercury                                                         them



294 Chapter 11 • Water Quality                                                                                                    NEL
Sources of Biological Contaminants
Sources of water are sometimes vulnerable to biological contamination
and must be monitored closely. Surface runoff, for example, often
mixes with farm manure in low-lying pastures. Manure
can contaminate nearby water sources, such as aquifers,
with dangerous bacteria. This was the case in 2000, when a
deadly strain of E. coli contaminated the local water supply of
Walkerton, Ontario, killing seven people.
   Drinking water containing parasites can also cause illnesses.
Parasites are organisms that get their nourishment by living
inside, or attaching to, a host. All animals, including humans,
can be affected by parasites. Some parasites are unicellular
organisms too tiny for us to see; others are large enough to
be seen easily (Figure 6). Parasites often enter water sources
                                                                    Figure 6 Worms like these can grow inside the
from feces excreted by infected animals. Runoff carries the         bodies of mammals, including humans.
contaminated feces into nearby bodies of water.
   Giardiasis is a common parasitic infection in Ontario. It is caused
by a parasite called Giardia, which infects mammals. Giardiasis is
                                                                               To learn more about diseases
contracted by drinking untreated water. The infection causes nausea,           transmitted in water,
cramps, or diarrhea.                                                            Go to Nelson Science

   Animal feces also add organic matter into the water system. Organic
matter can smell bad as it decomposes, and it can harm aquatic
animals by reducing the amount of oxygen in the water (Table 2).
Table 2 Biological Contaminants in Fresh Water

 Pollution type       Examples             Sources                                        Harmful effects

 micro-organisms      • Salmonella         • domestic sewage                              • spreads infectious diseases through
                      • E. coli            • cattle and other livestock                     contaminated drinking water
                      • Giardia            • natural sources

 organic matter       • waste from         • industrial wastewater                        • uses up oxygen from the water as it
                        cattle             • farm sewage                                    decomposes, suffocating aquatic life


 Unit Task            Consider how you can use this information about contaminants as
 you plan to address your water issue for the Unit Task.


      C
      CHECK YOUR LEARNING
  1. Name two parts of the water cycle in which contaminants         3. Choose two chemical contaminants that could be found in a
     can be introduced.                                                 freshwater source. For each one,
  2. (a) What are the two main categories of contaminants in            (a) identify a source of the contaminant
         water?                                                         (b) describe how the contaminant got into the water
     (b) Why are they a problem?                                     4. Briefly describe how a biological contaminant was able to get
                                                                        into groundwater wells at Walkerton.




NEL                                                                                            11.2 Contaminants in Water 295

				
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