Accumulation, Nutrient Content, and Settling Rates of Coarse Organic Solids in Minnesota Stormwater

Abstract

Urban stormwater monitoring and modeling helps identify pollutant sources andguides management efforts. Current stormwater monitoring practices measures soluble material and fine solids but screens out gross solids with a strainer at the end of water sampling lines. The goals of this study were to estimate yearly gross solid mass loadings and measure the nutrient content of gross solid samples by collecting solids at 13 watersheds in Minneapolis and St. Paul, Minnesota. Collection was achieved by installing mesh bag samplers at the inlet of stormwater control measures at each of the 13 watersheds and collecting samples roughly every two weeks from April to December 2021. Total mass and moisture content was recorded for every sample. Laboratory methods for organic fraction and total phosphorus were conducted on a set of the gross solid samples. Trends in gross solids accumulation, organic content and total phosphorus were tied to watershed characteristics and the time of year. Mass accumulation of gross solids was highest in the spring and fall. Organic fraction and total phosphorus content of gross solids were driven by the phenology of the trees inside the watershed boundaries. The results presented here will improve current stormwater management practices by revealing accumulation rates and nutrient content of gross solids in small watersheds. The settling rate of a particle is an integral parameter in stormwater modeling anddesign. The fate of stormwater particles determines if they contribute to nutrient loading in a watershed. Prediction of settling rates for inorganic particles is generally well-researched and understood. Organic particles tend to vary widely in their physical properties and there are currently no set standards or empirical equations for estimating the settling rate of organic particles. This paper presents data from tree leaves and seeds settling rate experiments to better understand how organic particles settle in the context of settling rate equations such as the one developed by Ferguson and Church. Analysis of the collected data showed that the second of the two shape factors (C2) used in the Ferguson and Church Equation fell within a normal range in the case of tree leaves and changed with the maximum Feret diameter in the case of tree seeds. By averaging the solved C2 by particle type and species, there was a relatively strong correlation between the observed settling rates and the settling rates predicted by the Ferguson and Church Equation (R2 = 0.69). With these results, stormwater modelers and designers are equipped with a better understanding of how to represent common organic particles in terms of settling rates. Additional research on a wider variety of organic particle types and species would expand on the limited dataset presented here.