Managing Stormwater Ponds for Optimal Function: Characterizing Phosphorus and HAB Export, Plus a New Tool for Prioritizing Maintenance Needs

Abstracts

Fate and Transport of Phosphorus and HABs from Stormwater Ponds - Katie Hembre

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. Both components of this study build upon the knowledge from studies Stantec has completed throughout the Twin Cities metro area over the last decade. If P and HABs are conveyed downstream, this study will inform stormwater management throughout the Twin Cities and likely lead to changes in pond design.

Stormwater Pond Assessment Tool for Evaluating Phosphorus Water Quality - Poornima Natarajan

The proposed work is focused on testing and improving an existing Pond Assessment Tool, which was developed by the authors in a previous project and designed to use readily-available spatial, water quality, and pond data to indicate risk and provide estimates of high water column phosphorus, sediment phosphate release, and anoxia in ponds treating stormwater. A beta version of the Tool was well-received by that project’s Technical Advisory Panel, but further workshops were not feasible under that project. To make the Tool truly useful to stormwater practitioners, we see three research activities as particularly necessary: (1) additional analysis of existing data to develop a pond water balance component in the Assessment Tool; (2) validation of the Tool by testing on ponds with sufficient data and independent estimates of key outputs (pond total phosphorus concentration, anoxia, sediment phosphorus release); and (3) additional workshops to reach a wider audience and solicit feedback to improve the usability and outputs of the Tool, including a framework for using the Tool in prioritizing monitoring and maintenance of ponds. A key output of the proposed work would be a public version of the Pond Assessment Tool; with components for dissolved oxygen status and hydrologic retention of ponds, the Tool has potential applicability to runoff pollutants other than phosphorus as well. We will partner with the city of Minneapolis in all project tasks, making use of their intensive pond data collection efforts, which are relevant to both development and testing of the Tool's components.