Pollution can originate from a variety of sources. That coming from an identifiable source is termed point source pollution, while nonpoint source pollution comes from a diffuse source and may be more difficult to identify. A point source often originates from a pipe whereas a nonpoint source may be the result of land use practices throughout the watershed.
Pollutants resulting from the decomposition of living organisms, plant or animal, and their by-products: grass clippings, leaves, human sewage, and pet wastes originate from organic sources. Too much organic matter in a system causes an increase in the biochemical oxygen demand (BOD), decreasing the amount of oxygen available. Breakdown of organic matter also leads to the release of nutrients, particularly nitrogen and phosphorus.
Inorganic pollution consists of suspended and dissolved solids; silts, salts, and other minerals carried into the system from streets or exposed soil. Common sources of inorganic pollutants include runoff from construction sites, plowed fields, and roads that have been salted. Inorganics lead to and increase in turbidity and warmer water temperatures.
Often by-products of industrial processes, toxic pollutants may be deadly or interfere with an organism's biological processes, such as reproduction. Examples of toxics include heavy metals, such as cadmium, mercury, chromium, iron, and lead and organic and synthetic chemical compounds, such as PCBs, PBBs, and DDT. Toxins attach themselves to soil particles and are washed into the waterway causing an overall decline in the number and diversity of aquatic organisms. Benthic macroinvertebrate communities are often good indicators of the presence of toxins.
"Waste heat" often results from using water to cool industrial or power generation processes, and then returning the water at a much higher temperature into receiving water bodies. Warm water itself does not constitute a pollutant; however, it becomes a pollutant when it affects aquatic life. Smaller streams are more vulnerable to thermal pollution than are larger systems. A rise in temperature results in a decrease in the ability of the water to hold oxygen. An artificial environment is created with some organisms becoming dependent on warmer temperatures for their survival. Life cycles may be altered, and the food chain may be affected. For example, insects may complete metamorphasis early, affecting birds and other species dependent on them for food.
