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Siting Criteria for Wisconsin Mounds to Avoid Failure

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E. Jerry Tyler
Small Scale Waste Management Project
Professor of Soil Science and Director of the Small Scale Waste Management Project (SSWMP)
University of Wisconsin-Madison,
1525 Observatory Drive,
Madison WI 53706,
(806) 262-0853
FAX (608) 265-2595

Small Scale Waste Management Project (SSWMP),
1525 Observatory Drive,
Madison WI 53706,
(608) 265-6595
FAX (608) 265-2595
Web Page

A Wisconsin Mound System is a soil based onsite wastewater treatment and disposal system. It is a sand filter with the final discharge to the surface of the natural soil from the base of the mound. A select sand placed directly on a prepared soil surface supports an infiltration area. Domestic septic tank effluent is distributed over the sand with a pressure distribution network. The wastewater infiltrates and percolates through the sand to the soil. Water enters the soil and moves vertically or horizontally away form the mound dispersing the wastewater and completing treatment mechanisms.

The most common failures in Wisconsin Mound systems are at the aggregate sand interface in the mound and therefore are not soil related. These failures are caused by the quality and quantity of wastewater or the sand fill. Sand fill selection is seldom included as part of the site evaluation procedure but could logically be included. Less common are failures associated with the soil.

Soil conditions must allow for the tillage of the soil and placement of the sand, a path to transmit the wastewater away from the mound and soil conditions conducive to the final treatment of the wastewater. Construction procedures can easily destroy soil characteristics important to the transmission and treatment of wastewater because soils at sites intended for mound placement are often thin and fragile. Removal of surface soil exposing less desirable subsoil and decreasing depth should be avoided. Failures due to compaction and puddling are often seen as leakage at the contact between the soil and the sand fill. Fragile soil conditions should be identified during site evaluation and guidance about the moisture conditions at the same time of construction provided.

The rate water will enter the soil is usually estimated. The rate wastewater will continue to move away from the mound is usually not given much consideration during the site evaluation process but is critical particularly when the width of the mound increases. Failure at the sand to soil contact will occur should the soil ability to accept and transmit wastewater be exceeded.

Failure to treat wastewater passing through the soil and not coming to the ground surface is seldom considered except for research. Treatment failure may be the most important failure. The risk of treatment failure can be estimated based on site evaluation information.

Soil texture, structure, and consistence of each soil horizon complemented with soil color is used to define soil characteristics related to ease of construction and the transmission and treatment of wastewater within the soil for successful Wisconsin Mound wastewater treatment and disposal systems.