Improving the Nutrient Retention of Stormwater Ponds

Abstract Submission: Stormwater retention ponds exist primarily to reduce peak rainfall runoff rates and to protect natural water bodies from pollutants such as sediment and phosphorus in stormwater runoff from developed land. Ponds store runoff and settle solids along with associated pollutants to the bottom of the pond, reducing their release to receiving water bodies. However, recent studies indicate that some stormwater ponds are not working as intended, and are phosphorus sources. Extensive field measurements of stormwater ponds in Minnesota indicate that older ponds are stratified, creating a bottom region of low dissolved oxygen water with internal phosphorus loading. To better understand the pond features that lead to stratification and internal loading, and to determine how to best remediate that loading, we are using a 1-D numerical pond model to characterize how pond geometry, wind sheltering, chloride loading, and floating leaf macrophytes affect vertical mixing, oxygen dynamics, and phosphorus retention. The model will be used to assess a series of potential pond treatments or retrofits to improve phosphorus retention, such as dredging, aeration, and alum treatments. The results from this study can be used to help watershed managers decide how to best maintain existing ponds and to inform the location and design of new ponds to optimize phosphorus retention. We will present modeling results for pond stratification and phosphorus dynamics for a range of pond sizes, depths and loading characteristics.

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