into aquifers. In some parts of the Southwest, sewage effluent may constitute the major portion of streamflow (Stanford, 1997), making the management of such streams problematic (see Box 3.4). In most parts of the country, the effluent is discharged into reservoirs and rivers that serve as drinking water sources for people living downstream. The effluent may also be used for irrigation and industrial purposes.
In non-seaward urban and rural areas, wastewater is treated on-site septic systems. Increasingly stringent public health codes and inspections are reducing the ground water and surface water pollution caused by these systems. However, in many places, septic systems are still placed too close to the ground water table, leading to ground water pollution, or in soils that are too thin, allowing the effluent to move along bedrock into springs and streams.
Industrial wastes (heat, chemical, infectious agents, and radiation) are treated prior to release into the waters of the United States. Some industries pretreat their wastewater and then release it into the wastewater treatment system. Other industries treat the waste themselves to National Pollution Discharge Elimination System (NPDES) permit standards prior to releasing it into streams and rivers. The 1995 Toxic Release Inventory Report (TRI) shows that 630,000 tons of toxins were released into the waters of the United States and 120,000 tons transferred to wastewater treatment plants (USEPA, 1997).
Air emissions from industrial sources carry many pollutants. These airborne pollutants fall directly into the rivers and lakes ("dry fall") or are collected by precipitation and brought to earth. Sulfur oxides from burning of coals have been implicated in acid precipitation, which has heavily affected both terrestrial and aquatic ecosystems. Many streams of the Northeast, where poorly buffered acidic soils predominate, have been rendered sterile. Air currents have carried pollutants, including DDT, dioxins, and other carcinogens, into all parts of the globe, so that there are no places that do not have measurable concentrations of these and other toxic chemicals. The TRI data for 1995 show a release of approximately 631,000 tons of toxic chemicals into the air (USEPA, 1997). Some of these chemicals eventually get into the water, although many are retained in the soils.
Feedlots are important point sources of pollution from agriculture. They may range from isolated barnlots for small herds to very large feeding areas where thousands of animals are kept in relatively small areas. The runoff from feedlots is toxic, high in biochemical oxygen demand (BOD), looks and smells bad, and carries a high load of nitrogen and phosphorus. Confined feeding operations must now treat their wastewater, but problems remain.
Mining can create both sediment and chemical pollution. Environmental laws have greatly curtailed impacts from present-day mining, but past mining activity has left a harsh legacy. In areas of high relief, spoil from mining often was sufficient to aggrade streams and floodplains, while clearing of forests for ore smelting caused stream erosion elsewhere (Graf, 1979). Perhaps the most dramatic example of mining impacts was from hydraulic mining for gold in