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• ,1,O�PV AND (o44.�Ir •■t- Division of Public Health � Environmental Health Section DANE COUNTY Director—Susan Crowley Division Administrator—Gareth R.Johnson DEPARTMENT OF HUMAN SERVICES KATHLEEN M.FAIL{ DANE COUNTY EXECUTIVE JOHN DAIHL 6013 WINNEQUAI I ROAD MADISON WI 53716 July 23, 1997 re: Lot Sizes in Subdivisions Served by Private Sewage Systems Dear Mr. Dahl, This will acknowledge your letter dated July 17, 1997 requesting information about minimum lot sizes in subdivisions being served by private sewage systems. I did not intend my comments at a subcommittee of the zoning committee last fall to be used as a basis for determining these minimum lot sizes. There are a number of considerations that need to be factored into that type of decision. The two most important considerations I believe should be taken into account are the soil type and depth of the water table used for drinking water wells. Soils that are very sandy, such as in the central sands region and along major river systems should require larger lot sizes. Sandy soils have rapid permeability and have very low ion exchange capabilities. Both of these conditions impair the soils' ability to purify sewage effluent. Therefore it makes sense from a public health standpoint to require larger lot sizes in order to provide greater distance between the sources of effluent and allow more soil to treat the applied sewage. On the other hand, soils that have a significant fraction of silt and/or clay have better ion exchange and water retention characteristics. These soils naturally impede the flow of water(effluent) due to the tremendous amount of surface area of the soil particles. The surface area also contributes significantly to the reactivity of the soil, and thus the increased purification capabilities. These are the types of soils found in much of Dane County, including the area of your proposed subdivision. Shallow groundwater tables are generally more susceptible to contamination from surface activities than deep bedrock water tables. The shallow water tables used for drinking water are also generally found in the central sands region of Wisconsin and along rivers or lakes. The typical well there is a driven point, which has a maximum operating depth of about 40 II.. The water table used for drinking water in most of Dane County is from aquifers found at depths up to 300 ft. The typical well here is a drilled and cased well. Records for the Rolling Wheels area indicates well depths of 150 -270 ft. The primary factor impacting groundwater in our area which can be traced to private sewage systems is nitrate. Estimates from the University of Wisconsin Extension indicate that agricultural activities are far and away the greatest source of nitrates in groundwater in Dane County and in 1202 Northport Drive, Madison, Wisconsin 53704 • (608) 242-6515 • FAX(608) 242-6256 Ci