Contamination of Water and Soil by Sewage and Water Treatment Sludge

Contamination of drinking water sources by sewage can occur from raw sewage overflow, septic tanks, leaking sewer lines, land application of sludge and partially treated waste water. Sewage itself is a complex mixture and can contain many types of contaminants. The greatest threats posed to water resources arise from contamination by bacteria, nitrates, metals, trace quantities of toxic materials, and salts. Seepage overflow into drinking water sources can cause disease from the ingestion of microorganisms such as E coli, Giardia, Cryptosporidium, Hepatitis A, and helminths.
Raw Sewage Overflow
Storm water systems in urban areas are sometimes combined with sanitary sewer systems en route to sewage treatment plants. Excessive storm water can cause this joint system to overflow. In this event, excess flow will be directed into waterways untreated, resulting in sewage contamination. According to the EPA approximately 20% of the population is served by combined systems (1). Forty-six percent of the population is served by separate systems.Urban runoff is usually collected by a separate storm sewer system and discharged directly into waterways. Combined systems are cheaper, but the potential to harm health is higher. Some systems have diversions to accommodate heavy flow.
Septic Tanks
Septic tanks contaminate 1% of the nations usable aquifers (2). Septic tanks are enclosures that store and process wastes where no sewer system exists, such as in rural areas or on boats. Treatment of waste in septic tanks occurs by bacterial decomposition. The resulting material is called sludge. Large portions of the population are still served by septic systems as opposed to public waste treatment facilities. Contamination of water from septic tanks occurs under various conditions:

Poor placement of septic leachfields can feed partially treated waste water into a drinking water source. Leachfields are part of the septic system for land based tanks and include an area where waste water percolates through soil as part of the treatment process.
Badly constructed percolation systems may allow water to escape without proper treatment.
System failure can result in clogging and overflow to land or surface water.
High density placement of tanks, as in suburban areas, can result in regions containing very high concentrations of waste water. This water may seep to the land surface, run-off into surface water or flow directly into the water table.

There are also site specific environmental factors around the tank and leachfield such as soil properties, water table location, subsurface geology, climate, and vegetation which may affect the quality and quantity of released waste water(1).
For information on maintaining your septic system, link to this article from Rhode Island University, Dept. of Natural Resources (3): Maintaining Your Septic System
Leakage from Sewer Lines
Effluent that leaks from sewer lines is generally untreated raw sewage. It may contain industrial waste chemicals. When leaking sewer lines are located deep underground below the biologically active portion of the soil, the sewage can enter groundwater directly. This can result in the introduction of chlorides, microorganisms, organics, trace metals and other chemicals that may cause disease and foul tastes or odors in drinking water (4).
Sewer leaks can occur from tree root invasion, soil slippage, seismic activity, loss of foundation due to washout, flooding and sewage back up, among other events (1). High pressure systems will push leaks to the soil surface where they can be easily detected by sight or odor. Systematic inspection of sewer lines, exclusion of hazardous waste, and adherence to modern construction and maintenance specifications are necessary preventative measures for protection of groundwater sources from sewer leaks(5).
Land Application of Partially Treated Waste Water and Municipal Sludge
Sludge is the residue of the chemical, biological, and physical treatment of municipal and industrial wastes (1). It can be applied to land as fertilizer or as fill. Land application is an alternative to incineration, which causes air pollution. Sludge usually contains concentrated organic matter, nitrogen, inorganic salts, heavy metals, and bacteria. It is a common practice to use partially treated waste water for fertilization, irrigation, and water supply recharge as an alternative to direct discharge into waterways. Waste water is also commonly stored in wells, holes, trenches, open pits and lagoons. Movement and percolation of waste water through the soil biologically and physically removes biodegradable substances, pathogenic organisms, and inorganic substances (4). The effectiveness of this treatment depends upon:

Processing or turnover time: Waste water must spend a sufficient amount of time on or within the soil to allow for filtration and biological processes to degrade the waste. If sufficient time is not allowed for these treatment processes to bring down contaminant levels before introducing waste water to a water system, contamination will occur.
Excess waste water and high concentrations of contaminants in the waste water: High concentrations of waste can take much longer to treat, especially when the consistency reaches that of a slurry or sludge. On the other hand, irrigation of soil with large quantities of waste water will saturate the soil and overload the biological degradation process. Excess untreated waste water can run off or percolate down to groundwater, causing contamination of drinking water supplies.
Level of biological processing: Lack of appropriate microbial activity can slow the degradation process or provide insufficient treatment. Bacteria which break down wastes without the use of oxygen, known as anaerobic bacteria, are very important in the process of breaking down nitrogen containing substances. Aerobic bacteria, which use oxygen, break down organic waste. Some of the breakdown products include water, carbon dioxide, methane gas, nitrates and other small organic and inorganic substances.
Water movement and abundance of vegetation: water movement and abundant vegetation can greatly enhance the degradation of waste water.

References
1. EPA Handbook, vol. 1: Ground Water and Contamination. September 1990. USEPA, Office of Research and Development, Washington DC.
2. USEPA (1980b). Planning Workshop to Develop Recommendations for a Ground Water Protection Strategy. Appendixes. Washington DC. pp 171.
3. Loomis, George. 1991. Maintaining Your Septic System. University of Rhode Island, Department of Natural Resources Science. URL: http://hermes.ecn.purdue.edu:8001/cgi/convwqtest?fs-88-2.ri.ascii
4. Rail, Chester D. 1989. Groundwater Contamination: Sources, Control, and Preventative Measures. Technomic Publishing Company, Inc., Lancaster,PA. pp.37-40
5. U.S. Environmental Protection Agency. Water Programs, Guidelines Establishing Test Procedures for Analysis of Pollutants. Federal Register, 38(199):28758-28760. Oct. 16, 1973.