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Mr. Boryczka Page 3 2. MPCA highly recommends that the field verification be conducted by a qualified geotechnical professional. Has the applicant provided the city with field verification results from a qualified geotechnical professional? If not, the plan does not meet MPCA recommendations because field verification has not been performed. 3. MPCA table 12.BI0.7 states that if the underlying soil infiltration performance allows the facility to drain without anunder-drain in less than 48 hours, no under-drain is necessary and the Bioretention type is an "Infiltration/Recharge Facility". The table also states that if because of the underlying soil infiltration performance, the facility will take more than 48 hours to drain, anunder-drain is needed and the Bioretention type is a "Filtration/ Partial Recharge Facility". The period of inundation is defined as the time from the high water level in the facility to 1-2 inches .above the bottom of the facility. This criterion was established to provide the following: wet-dry cycling between rainfall events; unsuitable mosquito breeding habitat; suitable habitat for vegetation; aerobic conditions; and storage for back-to-back precipitation events. The plan shows an outlet pipe that is 2 feet in elevation above the planned under-drain elevation. This elevation may be too high if the underlying soil has poor infiltration capacity. Also, when infiltration occurs in cold weather, ice can form over the top of the facility, within the soil interstices and in the overflow structure. To avoid this problem to the extent possible, the MPCA highly recommends that the facility be actively managed to keep it dry before it freezes in the late fall. This can be done with under-drainage, but not if the infiltration capacity of the underlying soil is poor. In this plan, water will not drain out of the facility by gravity because the planned under-drain flows into an outlet pipe that is 2 feet above the elevation of the plannedunder-drain. Thus, the City may have to pump out the under-drain when needed so that ice does not damage the facility. 4. MPCA states that a minimum of 3 feet of separation between the bottom of the bioretention facility and seasonally saturated soils is REQUIRED (5 feet is recommended) to maintain the hydraulic capacity of the facility and provide adequate water quality treatment. Aground water mounding analysis is recommended to verify this separation for infiltration designed bioretention facilities. Has the applicant provided the city with information regarding separation from seasonally saturated soils? If not, the plan may not meet the MPCA's requirements for separation distance. 5. MPCA states that pre-treatment is REQUIRED prior to the discharge of stormwater into a bioretention facility, to remove any sediment and fines that may result in clogging of the soils in the basin area. T:\Projects\Gary BoryczkaUtr to Gary 8-27-07.doc Engineering and Environmental Solutions