Mill Creek Bacteria and Pearl Lake Nutrients: TMDL Project

Mill Creek is a tributary to the Sauk River, located in the southeastern portion of the Sauk River watershed in central Minnesota. Mill Creek is impaired for Escherichia coli bacteria. Pearl Lake is a shallow, eutrophic lake approximately 750 acres in size, with a maximum depth of 18.2 feet and is impaired for Nutrient/Eutrophication Biological Indicators. Mill Creek flows through Pearl Lake.

The MPCA has determined that Mill Creek from its headwaters to the Sauk River (reach ID 07010202-537) is impaired and does not meet Minnesota water quality standards for pathogen indicator bacteria (E. coli). This reach was placed on the 303(d) list in 2006 because monitoring data indicate that E. coli levels typically exceed the monthly geometric mean standard of 126 E. coli organisms per 100mL. E. coli bacteria is used in water quality monitoring as an indicator organism to identify water that is contaminated with human or animal waste and the accompanying disease-causing organisms. Bacterial abundance in excess of the water quality standards can pose a health risk to swimmers and bathers and can limit other recreational uses.

Pearl Lake was assessed for nutrient impairment in 2007, and was listed as impaired in 2008. At that time, both total phosphorus and chlorophyll a exceeded the full support thresholds.

Map and location

Mill Creek starts southwest of Pearl Lake and flows into the Sauk River at Rockville. The two impairments listed below included in this project can be viewed on the map.

  • Mill Creek Escherichia coli impairment
  • Pearl Lake Nutrient/Eutrophication Biological Indicators

For a map of this impairment, see the Google map below or the Impaired Waters Viewer.

TMDL report and implementation plan

The Total Maximum Daily Load (TMDL) report for Mill Creek and Pearl Lake was approved by the US EPA in 2012. The Sauk River Watershed District is leading the TMDL Implementation Plan, which involves a variety of stakeholders, including the District, local agencies, local communities, and individual property owners and managers.

Pearl Lake. Based on the 2008 water quality conditions, in order to meet the overall load capacity of the lake needed to achieve water quality standards, a 25% decrease in phosphorus load will be required. To achieve this load reduction a combination of external and internal phosphorus load reductions will be needed.

Targeted phosphorus reductions in the watershed:

  • 20% reduction of phosphorus load from the watershed, including full compliance for all Subsurface Sewage Treatment Systems (SSTS) adjacent to Pearl Lake.
  • 31% reduction of internal phosphorus loading from lake sediments and Curly leaf pondweed.

Mill Creek. This analysis indicates that riparian pastures, surface applied manure, and feedlots without runoff controls are likely the primary sources of E. coli contamination. Overall E. coli load reductions of between 59% and 93% are required in order to meet water quality standards, depending on the flow conditions.




Information contact

Anna Bosch
Watershed project manager
7678 College Road Suite 105
Baxter, MN 56425