12/19/2001 Env Bd Packet - Laserfiche WebLink

Home Search

minutes to reach the first layer of the ground water system. An area that is extremely low in sensitivity to ground water pollution surrounds most of the rest of the island. This area is labeled as taking from years to a century for contaminants to reach the first level of the groundwater. The Study sited above includes flow systems, aquifer capacity, ground water chemistry, and near - surface deposits and bedrock considerations. Barr Engineering (Exhibit 9) establishes the need for site - sensitivity analysis, to prevent ground water contamination. Any kind of infiltration system must consider the above study considerations the Barr report states. Furthermore, Barr states that the seasonal high water table must be far enough below the bottom of the infiltration basin (at least 3 feet) to allow the structure to function hydraulically and to allow trapping and treatment of pollutants by the soil (MPCA 2000). Both proposed ponds on the island are typical storm water conveyance, non- attenuated water holding devices. Both ponds are excavations into organic soils (Exhibit 6), below the normal water levels of the lake. Again, the western pond is in an extremely sensitive, ground -water area. Recommendations based on Barr Engineering BMPS suggest neither pond should be built. Swale: The proposed infiltration swale on the North side of the island appears to be set in organic soils. This swale design is also contradicted in the Barr BMP Manual. Alternatives might include using the two proposed lot areas on the eastern side of the island for an infiltration /stormwater management area. Lot reduction may be necessary to accommodate some of the concerns of surface water runoff. The infiltration swale on the north side would have little benefit in terms of infiltration, and would not meet criteria for suggested BMS for such a swale. Existing vegetation would be lost as well in the process. Another possible alternative might be to move the swale idea to the North side of the proposed road and design a filter system that would move surface water to an infiltration area on the East (where ground water pollution is less of a concern). Removing two lots on the West Side of the island would allow for a suitable area that could be used for infiltration and surface water treatment. The site would need to be reestablished with native vegetation. In addition, the lots themselves should be designed with on -lot infiltration as suggested in Metropolitan Council /Barr Engineering Co BMP manual for Surface Water Management (Exhibit 10). One other alternative would be to cluster houses in one area of the island and use the rest of the island for surface water management. Trees: The Grading, Drainage, Erosion Control and Tree Preservation Plan shows a "Tree Preservation Limit" line for Pheasant Hills Preserve 12th Addition. I have indicated to the developer that we need an inventory of the trees on the island to develop such a plan. Our Tree Ordinance makes the inventory mandatory in proposed soil disturbance areas. The entire island is a proposed soil disturbance area. Only the proposed house pad locations are shown on the maps. Missing are driveways, surface water treatment areas, backfill issues on landscaping, etc. Lot 5 for example would have 5.7 feet of basement excavation. Lot 5 would be the eastern most lot. Lot 1, would have 3.2 feet