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Wastewater management is an important component of watershed protection. Wastewater discharges can affect drinking water supplies, fresh water systems and coastal resources. Wastewater throughout the watershed (as well as the rest of the country) is managed through a variety of on-site systems, clustered (small wastewater treatment facilities), or larger, centralized wastewater treatment plants.
Septic Systems/Wastewater Treatment Facilities: Septic systems are comprised of a septic tank and a leaching facility. The septic tank provides for the settling out of solids and some biological treatment of the wastes. The leaching facility disposes of the liquid wastes into the subsurface environment. If tanks are not properly maintained (pumping once every three years is recommended for single family homes), solids may pass out of the tank and clog the leaching facility. This can cause hydraulic failure of the system resulting in the possible backing up of wastewaters into the building or effluent breaking out onto the land surface. The latter case often offers a direct route of transport to a freshwater resource.
Conventional septic systems are designed to control pathogenic bacteria and are less effective in treating other potential pollutants. Leaching facility effluents contain approximately 40 to 60 mg/l nitrogen and 8-38 mg/l phosphorus (Hall, 1975). Nitrogen compounds generally move through the groundwater system relatively intact, ultimately reaching water bodies. Viruses, being much smaller than bacteria, also move easily through soils at the speed of groundwater. Because their inactivation times in groundwater are approximately 120-200 days, they have been documented to move distances greater than 300 feet in soils. If a septic system is located too close to a waterbody, viruses may reach surface waters. Septic systems sometimes introduce hazardous wastes into the groundwater if the owner uses septic cleaners or improperly disposes of household hazardous wastes.
The cumulative effects of many single family septic systems on nutrient, pathogen, or hazardous waste levels in down-gradient waters can be very significant. These impacts are dependent upon septic system location and density relative to receiving water bodies.
Wastewater treatment facilities are a source of direct discharges to water bodies. In some cases in the southwestern
On-site septic systems typically consist of a septic tank and a leaching or disposal facility (see Figure). Wastewater that flows through the system and reaches the underlying aquifer still contains dissolved materials such as salts, nutrients, solvents, toxic compounds, metals, and soluble pesticides. This wastewater also contains pathogens such as bacteria and viruses and some very small particles of insoluble materials. Water quality concerns associated with the primary constituents of septic system effluent are described below.
Nitrogen: Nitrogen in septic system effluent is present in concentrations well above the 10-mg/L federal drinking water standard. Effluent from a properly operating septic system typically contains 40 milligrams per liter (mg/L) nitrogen (H&W, Inc., 1998). This is four times higher than the Drinking Water Standard. If one is drinking water from a private well that contains nitrogen at or near the 10-mg/L standard, it is possible the water contains approximately 25% recycled wastewater effluent. Elevated levels of nitrate in drinking water can cause serious problems for infants, who could develop methaglobamenia (Blue Baby Syndrome), if the water is used in the preparation of formula or drinking water. It is, at elevated levels, a potential concern for adults as a carcinogen (Weyer, et al., 2001). Nitrate-nitrogen in septic system effluent does not precipitate or sorb to the soil materials through which it travels in groundwater and further attenuation is only caused by mixing and dilution with native groundwater. The effluent travels as a plume as it leaves the septic system, and the mixing only occurs as it either reaches a surface water body (such as the coast) or is withdrawn into a pumping well.
Excessive nitrogen has been found to accelerate eutrophication in some coastal and estuarine waters (Wetzel, 1983). The critical concentration for marine waters can be as low as 0.2 mg/l, depending on the rate of tidal flushing (Nielson, 1981; Buzzards Bay Project, 1991). Excessive nitrogen loading to marine and brackish ecosystems can cause algal blooms, decreased water clarity, and declines in eelgrass beds which are important shellfish and finfish habitat.
Phosphorus: Phosphorus concentrations in typical septic system effluent are far in excess of acceptable levels for surface water bodies and are known to be major causes of algal blooms and eutrophic conditions in ponds, lakes and streams. Unlike nitrogen, phosphorus reacts with, or is sorbed to, sediments and rock particles in the ground and initially does not travel more than a few feet from a septic system leaching facility. However, after years of application, the reactants and sorptive capacity of sand and gravel soils becomes exhausted. Phosphorus will gradually be transported further and further form the septic system, ultimately to a discharge point in a wetland or surface water (such as pond or the ocean).
Bacteria and Viruses: Pathogenic microorganisms (bacteria and viruses) are also found in domestic wastewater. In a properly operating septic system, the effluent does not discharge on land surface and the potential for human contact and infection from bacteria is low. The movement and survival of bacterial contaminants in the groundwater have been studied extensively. It has been shown that most bacteria are attenuated within a short distance of the point where they leave the leaching facility; usually between one to 30 meters in permeable sands and gravel (Canter and Knox, 1986). Bacteria are attenuated by adsorption to solid soil and aquifer materials and by death in slow-moving groundwater. In malfunctioning systems, pathogenic organisms can be discharged to the surface and contaminate soil and surface waters. Research indicates that viruses from septic system effluent can persist over longer periods of time in groundwater than in surface water, and the transport of viruses in groundwater occurs over longer distances and over longer periods of time than does the transport of bacteria (Yates, 1987). Viruses are ultramicroscopic particles, ranging from about 20 to 200 nanometers in diameter of a host cell. Only infected individuals can introduce them to septic system effluent. Unlike bacteria, they do not require nutrients in groundwater to survive and, therefore, can persist in groundwater for longer periods than bacteria.
Attenuation of viruses in groundwater is dependent on time and groundwater temperature. The colder the groundwater, the longer the viruses will survive. The longer the viruses are in the ground, the more they die off, until they eventually become inactive and no longer a health threat. In order to determine how far viruses may travel in the ground, the inactivation rate for virus in groundwater is compared to the rate of groundwater flow. The overall extent of travel will also be influenced by the height of the leaching facility above the water table, as viruses will be attenuated in this zone before they reach the aquifer.
