09-11-1989 - Agenda Packet City Council - regular meeting

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3- It'. POT1:XTI.M. OF SOUS FOR ON-S[TE WASTE TREATMENT The potential of the soils mapped in the area for on -site treatment of septic tank effluent will be discussed. According to the recommenL:ed "Ordinance and Code Regula- ting individual Selvage Disposal Systems" published by the Minnesota Dcj)artmcnt of Health in 1971 "No Soil absorption system shall be installed in an area where the watertable is at any time less than 6 1/2 feet below ground level or J feet below the bottom of the drain field trench". This line of thinking has changed somewhat over the past 6 years to reflect research results conducted in various areas of the county. The proposed Minnesota Pollution Control Agency WPC-10 "Individual Sewage Treatment Systems Standards" states only that "The bottom of trenches and beds shall be at least three feet above the watertable or bedrock" and allows for installation of additional innovative systems other than the standard subsurface trench system to meet this requirement An example of the innovative systems is the mound system. In the mound system the seepage bed or trench is elevated by the use of sand fill to provide the 3 foot separation dis- tance between the bottom of the seepage trench and a barrier layer such as the watertable or bedrock. Concern about this minimum separation distance is due to the fact that it takes approximately 3 feet of unsaturated soil material to properly treat the septic tank effluent. Once the se tic tank effluent reaches a watertable treatment of the ettluent ceases. This results in the presence of i pol- luted water body near the surface. Additional inputs of ef- fluent and/or the backing up of the effluent into the house. This creates not only a nftisance to the home owner, but can also pose a health hazard if the surrounding area is well populatcu. The best indicator available for determing the maximum height of a seasonal watertable which often, occurs in the soils of Hennepin County is the presence of soil mottles. Mottling is a distinctive coloration of grey with red and yellow splot- ches in the subsoil. An%- investigation of a proposed soil absorption (treatment) system site should include a deter- mination of the depth at which soil mottles occur. With this background the potential of the orihinal soils mapped in this development area for treatment of septic tank effluent will L.! discussed.