Pollutants and How They Are "Purified"

There are a variety of pollutants that enter our waterways that can harm humans and other organisms both directly and indirectly. Removing such contaminants is often very expensive or not technically feasible. Moderate amounts of many of these materials can be filtered and removed naturally before they reach public and private water supplies and natural areas.

Major pollutants include:

Nutrients such as nitrogen and phosphorus are pervasive water pollutants. These nutrients are essential for life but in excess can cause water-quality problems such as algal blooms. Algal blooms, tremendous proliferation of algae that occur when a body of water contains very large amounts of nitrogen and phosphorus, can lead to the loss of oxygen from the water body (hypoxia). This occurs when large numbers of algae die without being eaten. These algae sink to the bottom and begin to decompose, consuming oxygen in the process. If the water is not well-mixed, there is no way to replace the oxygen consumed by decomposition. Decreasing levels of oxygen in the water can, in turn, lead to fish mortality and growth of other pathogens. Excess nitrogen and phosphorus enter our waterways through non-point source runoff from manure, fertilizers, and septic tanks. Nutrients are removed in the natural purification process through direct uptake by plants, uptake by algae, bacteria, insects and fish, soil absorption, and denitrification (13). Riparian plant communities are essential for filtering nutrients from farmlands.

Pesticides and Herbicides
Pesticides and Herbicides can kill aquatic organisms directly and can cause developmental abnormalities and disease in animals and humans. Pesticides and herbicides are very expensive to remove if they enter the drinking water supply (13). These pollutants can enter rivers and other systems via runoff from road margins, agricultural areas, golf courses, and other human dominated areas. Thousands of species of bacteria make a living by degrading specific kinds of organic chemicals. Many of these bacteria are important in breaking down pesticides and herbicides that enter aquatic water supplies.

Pathogens such as viruses and harmful microorganisms get into aquatic systems through animal wastes and sanitary systems. Wetlands are particularly important in the removal of pathogens because of their prolonged retention of water and penetration of ultaviolet light in shallow waters. The process by which pathogens are removed or transformed within a wetland is less well-understood than the process of removal of other pollutants. We do know, however, that, if the system is overloaded with pollutants, it may harbor pathogens.

Salt, Sand, Petrochemicals, and Heavy Metals
Salt, Sand, Petrochemicals, and Heavy Metals such as mercury, enter from roads and other urban areas and can disrupt aquatic systems, harm aquatic organisms, and make water unsafe for consumption. Wetlands remove 20 to 60% of metals in runoff (15). Heavy metals are processed through chemical oxidation or reduction, leading to precipitation, adsorption by plants or soil, or filtration and sedimentation (2).


Excessive Sedimentation
Excessive Sedimentation can result as soil is eroded and washed down into waterways or blown in from exposed earth. Construction, road building, logging, and agricultural activities can trigger this process. Large quantities of sediment can reduce the ability of waterways to control floods by exhausting the system's capacity to store the extra sediments that are brought along in floods. Excess sedimentation can increase stream turbidity and directly harm fish and other aquatic organisms. It can also bring in other pollutants such as fertilizers and pesticides. Wetlands can trap 80 to 90% of sediment from runoff (8).

Thermal Pollution
Thermal Pollution can occur when shade trees are removed from along waterways or when water is heated as it runs over hot impervious surfaces such as roads and parking lots. If the temperature exceeds the range that aquatic organisms can tolerate, they may become stressed and/or die. Water holds less oxygen when it is warm. As a result, less oxygen is available to aquatic organisms and hypoxia may occur. Thermal pollution can also result from warm water entering waterways with industrial and energy generation effluents.