Understanding solids loading in Minnesota stormwater

We are missing the phosphorus in stormwater. Traditional stormwater samples will often miss or intentionally discard large organic solids, also known as gross solids. These gross solids, consisting of eroded soil, tree leaves, tree seeds, grass clippings and other organic or inorganic material, are a major problem for stormwater management and are often discarded in sample processing due to the size and water content, or when small and granular they are mistaken for mineral sediments. Large solids clog stormwater conveyances and quickly fill stormwater ponds requiring expensive clean out. These solids also convey large amounts of nutrients such as nitrogen (N) and phosphorus (P), which may mean that true nutrient loadings to surface waters may be considerable larger than measured by conventional monitoring programs. 

This study will greatly improve characterization of stormwater solids, particularly with respect to mass loadings and nutrient composition. We propose to collect complete samples of stormwater gross solids from watersheds with varying tree canopy, then partition and analyze each sample for size fractions, settling rates, density, and nutrient composition. We also propose to develop improved pollutant input parameters for the widely used P8 model, thus improving the design of stormwater BMPs and watershed analysis. Finally, we propose to develop a simple model to evaluate tradeoffs between street sweeping and catch basin cleanout with respect to environmental goals and cost. 

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