Journal of Student Research 2018

Urban Photsphorus Runoff and Loading to Half Moon Lake, Wisconsin Urban Phosphorus Runoff and Loading to Half Moon Lake, Wisconsin

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Mai Lia Vang Senior, Environmental Science Program

William F. James Faculty Advisor

Abstract

Half Moon Lake, a shallow oxbow, is located in the heart of Eau Claire, Wisconsin. Since the lake loses water volume from seepage, shallow groundwater from the Chippewa River is pumped in to maintain pool elevation. Other hydrological inputs to the lake include urban runoff from storm sewers draining ~ 15 subwatersheds, with a total area of 2.3 km 2 , that surround the lake. While internal phosphorus (P) loading dominates the P budget, there is concern that P inputs from urban runoff could stimulate algal growth after aluminum sulfate treatment to control internal P loading. Best management practices (BMPs) have been implemented throughout the city to increase nutrient infiltration and reduce urban P loading to the lake. The objectives of this research were to examine flow, P concentrations and P loading from one of the storm sewer inputs in 2017 for comparison with a long-term data base, collected pre-1999 and from 2011-2017, to evaluate the potential impacts of BMPs on urban loading to the lake. Storm samplers (ISCO 6700) and area-velocity probes (ISCO 750) were deployed to capture flow (5-min intervals) and samples (15-min intervals) for total P and soluble P analysis between May and September 2017. Collection methodology for previous years followed the same protocol. Overall, numerous large and small precipitation events resulted in runoff to the lake and flow-weighted concentrations averaged ~ 0.10 mg/L total P and ~0.018 mg/L soluble reactive P between 2012 and 2017. These mean summer concentrations were lower than those estimated in 1999 (0.15 mg TP/L and 0.05 mg SRP/L), suggesting that implementation of BMPs post-1999 have led to reduced P loading from this subwatershed.

Introduction

Cultural eutrophication is the accelerated process of excess loading of nutrients, usually phosphorus (P) and nitrogen, to aquatic ecosystems (Carpenter et al. 1998, Cooke et al. 2005). Although eutrophication occurs