Fate and Transport of Phosphorus and HABs from Stormwater Ponds- Understanding Risk of Internal Phosphorus Load and Harmful Algal Bloom Export from Stormwater Ponds to Downstream Waterbodies

There are two parts of this study to determine pollutant export from stormwater ponds 1) phosphorus (P) and 2) cyanobacteria and cyanotoxins (HABs). High phosphorus concentrations can cause eutrophication in lakes. A traditional paradigm of watershed management has been that stormwater ponds capture and retain P, thereby serving as important practices for reducing P loads to downstream waterbodies. However, recent studies have documented the prevalence of internal P loading from stormwater pond sediments. Stormwater ponds are designed to capture P, thus high in-pond P concentrations from internal P loading is primarily problematic if the pond is exporting P downstream. This project aims to quantify export of total P and orthophosphate from different types of stormwater ponds (levels 1-3 from the Minnesota Stormwater Manual). A better understanding of which type of pond exports more will inform pond retrofit opportunities and potentially new pond design guidance to improve performance.

HABs are a human and ecological health risk. Therefore, the second part of this study is to determine if the HABs that originate within a stormwater pond can be exported to downstream waterbodies, and if cyanobacteria and cyanotoxins can survive/persist after being discharged from the pond outlet and traveling through the stormwater conveyance systems.

Project Outcomes:

  • Fill the knowledge gap as to which poses the biggest risk to downstream waterbodies P and HAB export.
  • Determine if there are trends to inform pond function through pond maintenance, pond retrofits, outlet design, pond design level (from the Minnesota stormwater manual) to minimize P and HAB load.
  • Determine if there are trends which suggest storm sewer/outlet designs to impede viable P/HAB export.