A Procedure for Selection of Stormwater Treatment Practices in Established Areas

A recently calibrated urban runoff model, the Source Loading and Management Model (SLAMM), is used to compare the cost-effectiveness of using source area and regional stormwater treatment practices. The demonstration is done for the totally urbanized Lake Wingra watershed in Madison, Wisconsin. The goal is to retrofit practices that are able to reduce the annual total suspended solids load by 40%. Model results indicate the parking lots and streets are the most important sources of total suspended solids. Practices evaluated for the parking lots include the Delaware Perimeter Sand Filter, Stormceptor, Multi-Chamber Treatment Tank, bioretention, porous pavement, and infiltration trenches. Individually they reduced the solids load to Lake Wingra by 7 to 19%. High efficiency street sweeping is projected to reduce the annual solids load by 17%. Since the reduction in total suspended solids is expected to reduce the loads of total phosphorus and total recoverable zinc, the model results for these two pollutants is also presented. The parking lots and streets are also important sources of total phosphorus and zinc, but each pollutant has one additional important source. Lawns are an important source of total phosphorus and roofs are an important source of zinc. Fewer practice are evaluated for total phosphorus and total recoverable zinc, because test results for some of the practices do not include these two pollutants. Nine combinations of the source area practices are able to achieve the 40% reduction goal. For example, a 42% reduction in solids load to Lake Wingra is estimated for the combination of high efficiency street sweeping on all the streets and Delaware Perimeter Sand Filters on all the parking lots. Three combinations of the source area practices achieve a 20 to 30 percent reduction in total phosphorus loads and a 30 to 40 percent reduction in total recoverable zinc loads. Alternatively, the 40% reduction is achieved by using regional detention ponds with a total of 20 acres of permanent pool area. The same regional practices achieve a 21 percent reduction in total phosphorus and a 31 percent reduction in total recoverable zinc. Many of the combinations of source area practices are more cost-effective than the regional practice. Assuming a lifespan of 20 years the annual cost of the source area practices ranges from about $573,000 to $1,504,000, while the range for the detention ponds is $963,000 to $1,840,000. The least expensive combination of source area practices would only increase the annual stormwater utility bill for the Madison taxpayers by about $6, while the most likely detention pond alternative will increase the utility bills by about $18. Cities should consider retrofitting source area practices as a cost-effective way to meet reduction goals for total suspended solids.