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18. W:llter Quality-Surface Water Runoff a. Compare the quantity and quality of site runoff before and after the project. Describe methods to be used to manage and/or treat runoff. Quantity-Because of the large increase in impervious surface as a result of the proposed development, runoff from the site will increase significantly. For the 10-year, 24-hour rain event, site runoff volume is expected to be approximately three times the present amount. However, the treatment pond system will be designed to provide sufficient storage capacity so that peak outflow rates to the storm sewer from the site during the 10-year and 100-year storm events will not surpass existing lO-year flow rates. Quality-The water quality of the site runoff is expected to change as a result of the proposed development. Presently, the site is comprised mainly of woods, grass and brushlands, and lawns. Site runoff would be expected to be relatively low in suspended sediments, organics, and metals. Nutrient (phosphorus and nitrogen) content of the runoff may be relatively high, depending on the fertilizer use patterns in the residential portion of the site. The 30 acres of impervious surface of the proposed development are expected to increase the quantity of suspended solids, metals, and organic compounds in the runoff. Nutrient contributions of the site may decrease, because of the decrease in lawn area. Methods-Treatment of stormwater runoff from the site will be accomplished by means of a two-cell pond system. The first of the two cells will remove grit and large sediment particles from the runoff. The second (created wetland) cell will provide an opportunity for the smaller particles, and many of the attached pollutants, to be removed from the stormwater. Routing the outflow from the second pond through the adjacent wetland area will further treat the runoff before it is discharged to the City of Roseville's storm sewer system. b. Identify the route(s) and receiving water bodies for runoff from the site. Estimate the impact of the runoff on the quality of the receiving waters. (If the runoff may affect a lake consult "EA W Guidelines" about whether a nutrient budget analysis is needed.) The water from all roof drains and from all road and parking lot catch basins will be directed to a storm sewer system that will drain to the treatment pond system being constructed as part of the project. The overflow from the treatment ponds will flow to an existing City of Roseville storm sewer system on the north side of the Highway 36 frontage road. That storm sewer drains to a pond (Fulham Pond), which drains to a ditch that runs north to New Brighton and eventually to Rice Creek. The water quality of the runoff from the proposed development is discussed in part 18a., above. The treatment pond system will be designed in accordance with the guidelines of the National Urban Runoff Program (NURP), and can be expected to remove the majority of the suspended sediments and nutrients. Because the site presently has no water treatment facilities, the proposed development may actually improve the quality of the site runoff, and therefore have no detrimental effect on the receiving waters. 2362468\48185-1f.r~ 9