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Riverstone SouthEAWDraft December 2020 <br />ii.Stormwater -Describe the quantityand quality of stormwater runoff at the site prior to and post <br />construction. Include the routes and receiving water bodies for runoff from the site (major <br />downstream water bodies as well as the immediate receiving waters). Discuss any environmental <br />effects from stormwater discharges. Describe stormwater pollution prevention plans including <br />temporary and permanent runoff controls and potential BMP site locations to manage or treat <br />stormwater runoff. Identify specific erosion control, sedimentation controlor stabilization measures <br />to address soil limitations during and after project construction. <br />Pre-Construction Site Runoff <br />Soils in the project area are quite sandy and most precipitation likely infiltrates into the ground, <br />resulting in little surface runoff. Surface runoff from the project area under existing conditions likely <br />containssome sediment,pesticides,fertilizers, and other nutrients.Existing runoff drains overland <br />from the north end of the site, south toward the Mississippi River. Although the areacontains highly <br />permeable soils, some surface runoff from the site may reach the Mississippi River during runoff <br />events. <br />Post-Construction Site Runoff <br />Compliance with LRRWMO, City of Ramsey, and NPDES requirements will minimize and mitigate <br />potential adverse effects on receiving waters. Project development will change the land usefrom <br />agriculturaland woodlandto residentialwith streets and parks. This land use change is expected to <br />have mixed effects on runoff water volume and quality. Post-development runoff water quality will <br />be typical of suburban developments, and will likely be slightly degraded by pollutants carried in <br />runoff from streets, roofs, and driveways.Residential development may reduce concentrationsof <br />agriculturalchemicals, but may increase the loading of suspended solidsduring construction. Other <br />typical components of impervious surface runoff,such as hydrocarbons and heavy metals,may also <br />increase. <br />Residential project construction will add about25.62acres of impervious surface, consisting of <br />streets, homes, and driveways. The increased impervious surface area is expected to increase runoff <br />volume and urban pollutants locally during significant storm events.However,stormwater rate and <br />volume controls that comply with City of Ramseyand LRRWMOrequirementswill limit increases <br />in runoff volume and associated pollutant transport. Much of the increased runoff and pollution is <br />expected to be associated with large infrequent storm events.The creation of stormwater ponds and <br />infiltration basins is expected to mitigate potential adverse effects from the increasedimpervious <br />surfacearea. <br />The project will include approximately 4.82acres of stormwater and infiltration basinsin <br />compliance City of RamseyandLRRWMOrequirements(Figure 3, Appendix A).Potential <br />adverse effects of runoff volume and quality will be mitigated by the construction of stormwater <br />basins designed to manage peak runoff rates, runoff volume, and water quality. <br />Impervious surface runoff from storm events will be retained in twostormwater ponds and two <br />infiltration basins. These basins will include a 3.5- <br />depth of about 26 feet. This deep pond will not have an outlet, but will be designed to serve as the <br />14 <br /> <br />