06-09-1980 Planning Packet - Laserfiche WebLink

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m UNIVERSITY OF MINNESOTA AGRICULTURAL EXTENSION SERVICE Department of Agricultural Engineering 201 Agricultural Engineering Building 1390 Eckles Avenue St. Paul, Minnesota 55108 May 8. 1980 I’A'.'ii ^ C\TC OF. 030«a Mr. Mike Gaffron City Sewage Treatment Inspector City of Orono Box 66 Crystal Bay, Minnesota 55323 SUBJECT: Rick Hawkinson Lot Dear Mike: #556 On Monday, May 5, 1980, we made a site investigation of the south portion of a lot owned by Mr. Hawkinson. The purpose of the site investigation was to evaluate the suitability of the soil for an on-site sewage treatment system. The Hennepin County soil survey indicated that the soil was an Erin or Kilkenny loam. The soil actually occurring on the site determined by our own borings is a Lerdal loam. This soil is closely associated with Erin and Kilkenny soils; but with a periodic perched watertable. About 12 inches of a loam topsoil grades into a clay loam and then into a clay with mottling beginning at the 14- to 16-inch depth. The mottling is indicative of sea sonally saturated soil conditions, although we did not observe saturated soil in the boring holes which we made. However, the spring of 1980 has been unusually dry and thus it is not unusual that we failed to observe water in any of the boring holes. During a wet spring such as the one which was experienced in*1979, saturated soil conditions would very likely exist at the depth which mottling begins. At the present time most sanitary ordinances, including the standards adopted by the Minnesota Pollution Control Agency, call for a separation distance of three feet between the bottom of the trench rock and evidence of saturated soil conditions. We have discussed on various occasions whether this sepa ration distance is actually needed on clay soils. With clay soils the problem is not that of inadequate treatment but rather one of hydraulics. In other words, can the soil function to accept sewage when the bottom of the trench is closer than three feet to saturated soil? We do not know of any research which has established the hydraulic performance of a clay soil when saturated soil conditions are present this close to the bottom of the trench. We would be very interested in observing the performance of a shallow trench system installed in such soil conditions. Based on our site investigation we will make an estimate of the hydraulic performance of such trenches. However, it must be fully understood by the city of Orono and Mr. Hawkinson that we can provide no assurance that the trenches will perform in a manner that will treat and accept the hydraulic loading. If we knew exactly how the trenches UNiVtHSlIY Of MINNESOTA. U S DEPARTMENT OF AGRICULTURE. AND MINNESOTA COUNTIES COOPERATING