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Minnesota Rivers and Streams Phosphorus and Algae Goals

epaExcess nutrient loading can adversely impact Minnesota's lakes and rivers. Phosphorus in particular can ultimately lead to nuisance algal blooms in surface waters, a process called eutrophication.

In 2008, Minnesota adopted lake eutrophication standards, which set defined phosphorus, algae, and clarity goals for Minnesota’s lakes. In 2014, as a complement to lake standards, Minnesota adopted river eutrophication standards (RES) to set phosphorus and algae goals for rivers and streams.

The rule includes five parameters:

  • Total phosphorus (TP)
  • Chlorophyll-a (Chl-a)
  • Five –day biochemical oxygen demand (BOD5)
  • Dissolved oxygen flux (DO flux)
  • pH (Minnesota Rule 7050.0222 Subp. 2b.)

When analyzing appropriate wastewater treatment plant TP effluent limits for a river or stream, Minnesota Pollution Control Agency (MPCA) staff use the cause TP and response variables (Chl-a, BOD5, DO flux, pH) for RES to determine if a river or stream is meeting concentrations deemed to be protective of the water body. At a minimum, TP and at least one response variable are needed to make this determination.

For every river section reviewed, MPCA staff also considers a wastewater treatment plant's current discharge rates in addition to the plant's effluent at its permitted potential. Often the permitted load from facilities exceeds current discharge rates. As a result, the determination of new effluent limits is necessary to maintain protection of rivers that are not currently impaired. For more information about MPCA's phosphorus and algae goals, email phosphoruseffluentlimits.pca@state.mn.us.





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