Watershed Science Improves Environmental and Regulatory Outcomes: A Case Study with Phosphorus in the Midwest

The City of Rochester operates a 24-MGD wastewater treatment plant (WWTP) that discharges to the South Fork of the Zumbro River in southeastern Minnesota. Although the WWTP has removed phosphorus for decades, preliminary modeling suggested the waste load allocation would need to be cut by more than 50% to achieve lake and river eutrophication standards. The city worked with the Minnesota Pollution Control Agency (MPCA) and other partners to evaluate the underlying watershed science and determine what was necessary to protect designated uses. The project involved three major components:
- development of a site-specific phosphorus criterion for Lake Zumbro;
- recalibration and adjustment of the watershed model to improve simulation of phosphorus sources and transport; and
- development of equitable point and nonpoint source implementation scenarios. The outcome was a modest reduction from the current waste load allocation, which will require that the city maintain its excellent long-term performance with phosphorus removal. However, this represents a much more favorable and affordable regulatory outcome than initially proposed and will give the city much-needed flexibility in the nature and timing of future plant improvements.