Besides the effect on flow, land use directly affects water quality in many other ways. To understand these effects, it helps to understand the difference between point source pollution and nonpoint source pollution.
Point source pollution refers to contaminants that enter the water directly, usually through a pipe. The specific location where the pollutant enters a stream can be identified, because it is usually at the end of a pipe. Examples of point source pollution are sewage treatment plants (which treat wastewater but still release a regulated amount of pollution in their discharge) and industrial sources. Point sources are easier to regulate than nonpoint sources and were the original target of the 1972 Clean Water Act that has dramatically improved the quality of many U.S. rivers and streams. Although point sources were formerly the worst culprits in impairing water quality, most point sources have greatly reduced the pollution they discharge as a requirement of the permits they must obtain.
Nonpoint source pollution, also known as “polluted runoff,” is different. The exact location where this type of pollution enters a stream cannot be identified because it comes from entire landscape areas: anywhere that rain falls and carries pollutants as it runs off. Your driveway and the road near your house may be sources of pollution if spilled oil, leaves, or other contaminants flow from them to a stream. Agricultural areas, because they occupy so much of the Indiana landscape, are important sources of pollution when rainfall carries sediment, nutrients, or chemicals to streams. Urban areas also are the source of important, but sometimes different, nonpoint source pollutants. Nonpoint source pollution is currently the major water quality problem in the U.S. – and nonpoint source pollution is directly related to land use. Common nonpoint source pollutants in agricultural areas are sediment, pathogens, nutrients, and pesticides. Common nonpoint source pollutants in urban areas are sediment, pathogens, nutrients, oxygen-demanding substances, heavy metals, oil and other petroleum products, and road salt.
Sediment is the largest pollutant in Indiana by volume. It affects aquatic life, shortens reservoir life, and complicates water treatment. Its sources are cropland erosion, construction sites, washoff from streets and other impervious areas, and streambank erosion. Streambank erosion in particular is increased by the added runoff due to development.
Pathogens include E. coli (a bacteria used to indicate the presence of fecal waste) and other viruses, bacteria, and protozoa. The source of most pathogens is fecal material from any warm-blooded animal. In agricultural areas, sources include wildlife, livestock manure, and malfunctioning septic systems. In urban areas the major sources are pet wastes, wildlife that may be present in high numbers (such as birds), septic systems in unsewered areas, and sewage treatment plant discharges (which are considered a point source). A particularly significant source is the outfall from combined sewers, where raw sewage in combination with urban runoff is allowed to bypass the treatment plant during storms. Although combined sewer overflows are an urban source, they are rarely a concern related to current development, because new areas have separate sanitary and storm sewers.
Nutrients of concern are primarily nitrogen and phosphorus. High concentrations of nitrate in drinking water are toxic to infants and may be harmful to pregnant women. Nitrate in the Mississippi River is one cause of hypoxia in the Gulf of Mexico. Hypoxia is a zone of low oxygen where fish cannot live. Phosphorus leads to overproduction of algae that clog lakes and reservoirs. Sources of nutrients in agricultural areas include fertilizer, livestock manure, and septic systems. Sources of nutrients in urban areas are fertilizer used on lawns, gardens, and golf courses; pet waste runoff; and discharge from sewage treatment plants or industry.
Pesticides can be a concern in drinking water supplies that use surface water. Pesticide concentrations in most Indiana streams in agricultural areas rise above drinking water standards after application in the spring, but these elevated concentrations do not typically last long enough to be a violation of drinking water standards. Sources of pesticides are simpler to identify than sources of pathogens or nutrients. They are limited to pesticide application, either in agricultural or urban areas. Studies by the US Geological Survey in the White River Basin found that concentrations of primarily agricultural pesticides such as atrazine are much higher than concentrations of any pesticide used primarily in urban areas. However, concentrations of certain pesticides, such as diazinon, an insecticide for lawns and gardens, were higher in urban areas.
Oxygen-demanding substances consist of organic matter that depletes dissolved oxygen when decomposed by microorganisms. Dissolved oxygen is critical to maintaining water quality and aquatic life. Studies have shown that urban runoff with high concentrations of decaying organic matter (such as leaves, grass clippings, and other organic debris) can severely depress dissolved oxygen levels after storm events.
Metals include lead, copper, cadmium, zinc, mercury, and chromium. They can accumulate in fish tissues and affect sensitive animal and plant species. One of the major causes of fish consumption advisories in Indiana is mercury. Sources of metals are automobiles (copper is lost from brake pads, for example), industrial activities, illicit sewage connections, and atmospheric deposition (for example, mercury that is released into the air from combustion and then falls to earth in rainfall at another location).
Oil and other petroleum products degrade the appearance of water surfaces, impair fish habitats, and may be toxic to sensitive species. Sources are oil leaks; auto emissions coming off parking lots, roads, and driveway; and improper disposal of waste oil. Concentrations of petroleum-based hydrocarbons are often high enough to cause mortalities in aquatic organisms.
Road salt increases levels of sodium and chlorides in surface and ground water. Snow runoff produces high salt/chlorine concentrations at the bottom of ponds and lakes. Not only does this condition prove toxic to certain organisms, but it also prevents crucial vertical spring mixing.