THE COMPLICATIONS OF MONITORING TREATMENT BMPs

The use of best management practices (BMPs) in an urban watershed can provide pollutant load reductions at a relatively low cost. BMPs can range from being management operations (such as street sweeping or reducing the amount of pesticides used on urban lawns) to structural treatment options (the four most common alternatives being detention/retention ponds, swales, filter/buffer strips and constructed wetlands). Evaluation of BMP performance requires adequate monitoring to report “true” values of their treatment capabilities. This paper focuses on the complications associated with monitoring structural BMPs and provides an overview of the drawbacks of existing monitoring systems, limitations of monitoring devices and methodologies, inadequacies of commonly used monitoring parameters and their impacts on the performance and design of treatment BMPs.One of the complications of existing monitoring systems is that we measure standard parameters instead of those pertinent to pollutant removals. For example, total suspended solids is frequently measured to indicated the solids loading and removal in a retention pond. But a more important measurement could be total settleable solids, since only those particles that will settle out in the retention pond are pertinent to its effectiveness. Further, determination of particle size and settling velocity distribution of the solids is important because a significant portion of pollutants present in stormwater are adsorbed on the finer fraction of the suspended solids. A significant portion will not settle out in a retention pond, so no matter how large the pond is designed, reasonable pollutant removals cannot be expected. However, particle size distribution estimates are not only time consuming but also expensive, so they are rarely done in routine monitoring of BMPs.Proper monitoring is absolutely necessary if we are to best evaluate the effectiveness of BMPs and their impact in improving water quality in urban watersheds.