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May, 2000
Septic Systems and Surface Water Quality
Watersheds across the country are being assessed to determine the "Total Mass Daily Loading" (TMDL) of pollutants potentially entering the water. This is a program mandated by the Federal Government to help watershed managers understand the quantity of pollutants entering waterways, the source of these pollutants, the management or engineering changes needed to reduce these discharges and the overall ability of the watercourse to handle a certain level of pollutants. Typical parameters of concern are pathogenic organisms, nutrients, organic matter and sediment. There are a number of sources of these pollutants including livestock operations, agricultural land and onsite wastewater disposal systems.
Stream sampling can identify the pollutants of concern, but reducing the discharge of a given pollutant requires understanding its major source or sources.
Whether or not septic systems in a given area are major sources of pollutant discharge depends upon the site conditions and the overall condition of the septic systems. Site conditions which might suggest that septic systems be considered in the TMDL process are slowly permeable soils that might lead to surfacing of septic tank effluent from clogged up soil absorption systems and sites with shallow ground water that may discharge to nearby surface waters. In these settings there may be insufficient unsaturated soil between the bottom of soil absorption system trenches and shallow ground water to provide adequate treatment. Sometimes old septic systems are intentionally connected to tile drainage systems, causing almost direct discharge of pollutants to surface waters.
Septic systems can be a source of coliform bacteria, nutrients and oxygen robbing organic matter. In general, organic matter from septic systems will be filtered out in the soil unless there is direct discharge. In some settings, it may be difficult to determine whether coliform bacteria and nutrients come from septic systems, livestock operations, or wildlife.
Determining whether a specific septic system is likely a problem may involve a fairly detailed evaluation of the site and system. Consistent surfacing of effluent will be obvious during the growing season by the color and rate of growth of vegetation, spongy soil conditions or, in extreme cases, dead vegetation, blackened soil surfaces and odor. Systems that are contributing pollutants to shallow ground water or tile systems will be less obvious. Soil borings can be used to determine how close the ground water is to bottom of trenches. A minimum of two feet is necessary for adequate treatment for removal of coliform bacteria. (The state and most counties require four feet of unsaturated soil.)
Determining whether systems are connected to tile lines may require actually sampling of tile flow and testing for the presence of coliform organisms, nutrients, and/or chloride. Dye testing may also be used to identify direct connections.
If you or colleagues you work with are involved in TMDL projects and would like further assistance in identifying septic system effects, please feel free to contact me (loudon@msue.msu.edu) or Patricia Miller (mille906@pilot.msu.edu) for additional information.
Ted Loudon
