Water
pollutionOrganic wastes
Organic wastes, including domestic sewage, comprising of degraded plant and animal matter and other biological products in a broken-down form. They contain carbohydrates, fats, nitrogenous and phosphorus compounds. Limited amounts of organic wastes can be broken down in the aquatic environment without harmful effects. If too much organic material is released into a water body, the oxygen is used up more quickly than it can diffuse into the water from the atmosphere. The resulting drop in oxygen levels may then be sufficient to kill fish and other aquatic organisms. If all the oxygen in the water is used up, unpleasant odours can result.
Nutrients and Biostimulants
The breakdown of organic wastes by bacterial action releases inorganic compounds of nitrogen and phosphorus. These compounds are also major ingredients of agricultural fertilisers and may be washed into rivers and lakes from adjacent farmlands. Increased inputs of these substances to water bodies stimulate the growth of aquatic plants. Excessive growths of aquatic weeds and algae may choke lakes and streams and lead to dramatic daily fluctuations in dissolved oxygen levels.
During the day the photosynthetic production of oxygen by the plants more than compensates for that used up in respiration. However, during the night the combined oxygen consumption by water plants and animals may deplete it to levels which are dangerous to fish and other organisms. Eventually, massive die-offs of these plants may occur with even more serious reductions in oxygen levels. Examples have occurred at Lakes Colac and Burrumbeet in western Victoria.
Suspended solids
This term refers to all the particulate matter present in the water. This material may adversely affect fish and other aquatic organisms by:
- reducing the amount of light in the water available for plant growth and thereby reducing the supply of food for other organisms;
- clogging and damaging sensitive tissues such as the gills of fish; and
- suffocating organisms which live on or in the bed of lakes and streams by
forming thick deposits when this suspended material settles out.
The main source of this pollutant is soil erosion, particularly from farm lands and road and building construction in towns. Pollution of streams by suspended material is a major problem in Victoria as the soils in many areas are particularly susceptible to erosion and subsequent suspension in the water, eg. the Yarra River.
Salinity
Salinity refers to the amount of mineral salts dissolved in waters. The salinity of water is important in determining its suitability for many uses. Freshwater organisms can only tolerate limited increases in salinity. Certain crops are particularly sensitive to increased salinity in irrigation waters. Water with high salinity is unsuitable for drinking by both humans and animals.
One of the major salinity problems in Australia is associated with irrigation in semi-arid regions, eg. north-western Victoria. Irrigation in these areas causes the salts leached from the soils to build up in groundwaters. Eventually, the groundwater table rises bringing saline water to the surface and making soils unsuitable for cultivation. This saline groundwater may also find its way into lakes and streams. The salinity problem in the lower Murray River is attributable in part to the adverse effects of irrigation practices.
Toxicants
This class of pollutants is particularly diverse and covers pollutants from a wide range of sources. Toxicants are substances which in sufficient amounts, poison living organisms or damage their life processes in some other way. It is useful to distinguish between those toxicants which persist in the environment for long periods and those which break down into non-toxic or less dangerous substances.
Persistent toxicants
These include the heavy metals such as mercury, cadmium, chromium, zinc, copper, lead and nickel. Small quantities of these materials are continually entering the waters of rivers and oceans through the weathering of rocks and soils. Fish and other aquatic organisms seem to be adapted to these background trace concentrations. These same metals however, occur in relatively high concentrations in the waste products of various industries (eg. electroplating works, battery manufacturing, mineral processing and steel and alloy works). Heavy metals are not only toxic to aquatic life but can accumulate in the tissues of organisms at levels which are hazardous when eaten.
Widespread awareness to these dangers was aroused by the Minimata incident. People in a Japanese fishing village suffered nervous disorders and many died from eating fish contaminated with mercury over a long period of time. The mercury was traced back to an industrial discharge.
Pesticides
Pesticides form another class of persistent toxicants. Not all pesticides are persistent but many of the commonly-used compounds are. These include the chlorinated hydrocarbons such as DDT and chlordane; the chlorinated napthalines such as dieldrin, aldrin and endrin, and the organophosphates such as parathion and malathion. Small amounts of these compounds are washed into streams from crops and soils sprayed with them.
Since pesticides are used to kill nuisance organisms such as insects and weeds it is not surprising that many pesticides are highly toxic to aquatic organisms. As with heavy metals, many persistent pesticides accumulate in living tissues and are concentrated through the food chain. Animals at the top of the food chains such as fish-eating birds have been found to have high levels of these compounds in their tissues. While these levels may not be directly lethal, they may endanger the animal's survival in other ways. DDT has been associated with egg-shell thinning in numerous birds. In severe cases this thinning can result in eggs breaking before they can hatch.
Oils and waste products of the petroleum industry can also persist in the environment for long periods. They contain water-soluble compounds such as benzene and toluene which are highly toxic to fish and other aquatic life. Oil spills in ports and at sea are particularly hazardous to marine life. Not only are fish and other organisms affected by toxicants which dissolve in the water, but marine birds and animals, such as seals and dolphins, may be poisoned and fouled in the surface oil slick. In the past, many of the chemicals used to clean-up oil spills were equally as hazardous to marine life. Now physical techniques that remove floating oil are being used more to counter this.
Non-persistent toxicants
These toxicants include ammonia, most surfactants (substances found in detergents), chlorine, sulphides, cyanides and many organic chemicals. They may be quite harmful to fish and aquatic life but in time are broken down or transformed into relatively harmless substances. Ammonia and surfactants are present in sewage effluents. Chlorine may also be present in sewage effluents since it is commonly used to disinfect these wastes prior to discharge to the environment.
Pathogens
Sewage and wastes from animals and animal processing industries can contain very high numbers of bacteria and viruses. Some of these organisms can cause illnesses, including hepatitis and gastroenteritis. It is important to prevent these organisms entering waters used for drinking and swimming. Chlorination is one common treatment method for removing these organisms from effluents. Swimming pools are chlorinated for this purpose. However chlorine might produce chlorinated organics. Another common method is to retain the effluents in lagoons for long periods (greater than 30 days) to permit the organisms to die off naturally.
Heat
Thermal power stations and numerous industries use large amounts of water for in-plant cooling processes. These cooling waters absorb great quantities of heat which can cause increased temperatures in waters to which they are discharged. High temperatures are lethal to fish and other aquatic organisms and lesser temperature increases may impair the growth and reproduction of sensitive organisms. Elevated water temperatures stimulate the growth of nuisance plants and algae. This and other effects can lead to decreased levels of dissolved oxygen which can threaten other aquatic life.
Source: http://www.epa.vic.gov.au/Water/