9.1 SR 10-18-2021 - Laserfiche WebLink

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Over Excavation (aka soil correction) at Rolling Hills’ Existing Soil Treatment Area Dan Wheeler – UM Soil Scientist Knowns: 1. The native and natural soils across the entire mowed grass area are drumlin-like in their characteristics. This means that they naturally perch water (have saturated soil conditions) seasonally within 12” of the soil surface. Underlying these unsuitable wet soil conditions is many feet of densely compacted glacial deposit of varying textures including clay and clay loams as verified during earlier fall 2020 soil pits dug and described. These soils do not allow for water movement, which is why there is saturation evident in the overlying soils. These very similar soils were observed in their more natural state on the north side of the Rolling Hills property just north of the trail off of Cleveland Avenue. 2. The soils across this entire mowed grass area have been compromised by previous activities on this site. a. There are severely compacted soils from agricultural and/or construction activities. b. There are areas closer to Jarvis Street that have the presence of fill in wetland areas as verified during earlier fall 2020 soil pits dug and described. c. Existing wastewater does not infiltrate/move vertically into and through these soils and has likely plugged any voids with solids. 3. Clearly, the status of the soil resource in the entire mowed area is problematic. Solutions: 1. Removal and correction of the problem soil. a. This effort will remove some compacted and/or clayey soil. b. Any replacement soil would have to be able to withstand placement with minimal compaction – MN Rule Chapter 7080 specifications limit this to clean sand. c. This effort will not remove or change the fact that water is moving in the landscape. Digging a large hole in relatively impermeable materials and filling it full of different material creates a bathtub in the landscape. Water will not be able to move into the surrounding unexcavated soils so will fill the new area with water. In addition, the filled wetlands are still wet as was evidenced by gley soil conditions (permanent saturated conditions) during our earlier fall 2020 soil pit in this area and will add to the water in this area. 2. Underdrains to lower and/or drain water (in addition to soil removal/correction). a. MPCA has considered any artificial drainage within the area of a wastewater soil treatment area to require a surface discharge permit known as a National Pollution Discharge Elimination (NPDES). This moves the project into another permitting group with the MPCA and would require additional design, monitoring, reporting, etc. along with larger permitting fees. This is considered a tall hurdle by many and is one way the MPCA discourages such a solution (see attached MPCA factsheet). b. In addition, saturated soil elevation does not necessarily lower with an underdrain. Underdrains provide an easier outlet for the water. Additional work would be required to verify. A drainage maintenance agreement would also be required. c. The elevation of the periodically saturated soil will remain the same on the site for the design of a new system. There will not be any credit given to any fill material that is at or below the original soil surface as the surrounding soils remain and possess this limiting soil condition. This means mounds would still be required to be constructed similar to earlier discussions, layouts, etc. 3. Find a natural soil condition that is favorable for treatment and acceptance of wastewater. a. Allows for the use of established 7080/7081 design practices. b. Provides a greater certainty of long-term performance and life span. c. There will be a simplified permitting process