Agenda - Council - 04/13/2021 - Laserfiche WebLink

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M1MINNESOTA POLLUTION CONTROL AGENCY 520 Lafayette Road North I St. Paul, Minnesota 55155-4194 I 651-296-6300 800-657-3864 I Use your preferred relay service I info.pca@state.mn.us I Equal Opportunity Employer March 8, 2021 Tim Gladhill Community Development Director City of Ramsey 7550 Sunwood Drive NW Ramsey, MN 55303 Re: Riverstone South Environmental Assessment Worksheet Dear Tim Gladhill: Thank you for the opportunity to review and comment on the Environmental Assessment Worksheet (EAW) for the Riverstone South project (Project) in the city of Ramsey, Anoka County, Minnesota. The Project consists of a new residential development. Regarding matters for which the Minnesota Pollution Control Agency (MPCA) has regulatory responsibility or other interests, the MPCA staff has the following comments for your consideration. Water Resources (Item 11) • Since the Project will disturb 50 acres or more and discharges to the Mississippi River, which has construction -related impairments and is also classified as a Scenic and Recreational and Restricted water, the Stormwater Pollution Prevention Plan (SWPPP) for the Project will require submittal to the MPCA prior to obtaining National Pollutant Discharge Elimination System/State Disposal System General Construction Stormwater permit (CSW Permit) coverage. • The EAW does not discuss additional erosion and sediment control best management practices (BMPs) required during construction for special waters identified as restricted. The Project will need to implement parts 23.9, 23.10 and 23.11 of the CSW Permit and include the applicable requirements in the SWPPP. Please direct questions regarding CSW Permit requirements to Roberta Getman at 507-206-2629 or Roberta.Getman@state.mn.us. • The primary stormwater treatment for the Project appears to be the proposed ponds, including one pond referred to as a 26 foot deep "lake" to be constructed on the site. The Project proposer needs to be aware that the lake will not meet MPCA stormwater treatment system requirements. A pond constructed to treat stormwater can be no more than 10 feet deep per CSW Permit requirements. Any deeper and the stormwater pond will stratify and the bottom of the pond will lose all oxygen and become anoxic. Any phosphorus that is attached to the sediments that have settled out and are now on the bottom of the pond will be released and discharged during the next rain event. Also, a constructed stormwater pond built in an upland area would be considered a treatment facility and can't be classified as a lake. The lake cannot serve as both a lake and a stormwater treatment pond. • The CSW Permit requires that 1 inch of the water quality volume is treated via a volume reduction method, such as infiltration, unless site conditions prohibit per one of the reasons listed in the CSW Permit. A wet sediment pond is not a volume reduction practice. The EAW does include plans for infiltration basins, which should be the primary method of treating stormwater from the new impervious surfaces. Ponds that meet CSW Permit requirements can act as pre-treatment practices and extend the life of the infiltration basins. It is important to note that the infiltration "bench" shown on the Project plan may not meet the requirements of the CSW Permit if good soils are present and standard infiltration practices can be constructed. The MPCA recommends the Project