Water Quality Benefits of a MN Floodwater Storage Impoundment

Abstract

Nutrient and sediment pollution in Lake Winnipeg and its watershed, the Red River Basin, MN, are degrading water quality and impairing aquatic health, fishability, swimmability and recreational potential. It is necessary to capture and store the pollutants phosphorus (P), nitrogen (N), and total suspended solids (TSS) on the landscape, to improve water quality and protect valuable water resources in the region. Floodwater storage impoundments have the potential to effectively capture and store nutrient pollutants and suspended sediments, and consequently improve downstream water quality. There are already several dozen similar impoundments in the state, and plans to build approximately 200 in total. The water quality benefits of a floodwater storage impoundment in the Red River Basin were tested through various methods in this study. Nutrient budgets were built for the impoundment in 2014, 2015, and 2016. Load and concentration reductions were calculated for water entering and leaving the system, for nitrogen, phosphorus, and total suspended solids. In 2016, nitrogen and phosphorus reductions of 73% and 66%, respectively, were achieved. A hypothetical load reduction calculation was also modeled to determine the effects of impoundment water release speed on pollutant capture. The soil phosphorus storage potential of the impoundment was determined through a laboratory sorption experiment. Soils at the site were analyzed for their linear adsorption coefficient (K) and equilibrium P concentration at zero-sorption (EPC0). Analysis compared soils under various land uses, including: cropped, planted with native vegetation, and flooded. Results suggest that all soils within the impoundment outperform soils at the exterior of the structure regarding phosphorus storage and buffering potential. Variation in soil-phosphorus sorption properties between sites with different vegetation types will advise cropping and planting plans to optimize water quality benefits. Results of this research are intended to advise management of the study site, similar impoundments, and constructed wetlands for water quality treatment.