RESPEC is completing a bacteria Total Maximum Daily Load (TMDL) assessment on four reaches of the Big Sioux River from Dell Rapids to Brandon, South Dakota. The project area is unique in that it encompasses a mixture of rich agricultural land as well as the densely populated area within and surrounding the city of Sioux Falls, which is a Phase I National Pollutant Discharge Elimination System (NPDES) community.
The assessment is being completed in three phases: planning, monitoring, and TMDL development. During the planning phase, available water-quality and water-quantity data, along with relevant background watershed data, were compiled to gain a basic understanding of the impairment, waterbodies, and watershed characteristics affecting impairment. The subsequent monitoring phase involves implementing an adaptive monitoring plan that provides additional historical data as identified in Phase I. The final phase of the project will require calibrating and validating a watershed model application, using the data gathered in the first two phases, to develop the TMDL document and identify best management practice (BMP) scenarios to bring the Big Sioux River into compliance with water-quality standards.
The challenge of the project is to separate the impact the city of Sioux Falls is having on the overall bacteria loading from that originating outside the city limits. Furthermore, quantification of the relative loadings from each of the land uses within the city (industrial, commercial, residential) and below existing BMPs need to be assessed to understand the effectiveness of the city’s current stormwater treatment capability and properly prioritize future BMP implementation. Implementing BMPs within an NPDES community is different than those for a nonpoint source-driven load in that adoption is regulated rather than voluntary, making the accurate quantification of the water-quality impacts and associated BMP recommendations imperative.