MPCA refines its proposed standard for protecting wild rice from excess sulfate

Contact: Mary Connor, 651-757-2629

St. Paul, Minn. — A new technical support document (TSD) released by the Minnesota Pollution Control Agency (MPCA) details how the agency has refined its draft proposal to protect wild rice in response to more than 600 comments. The agency released its initial Draft Proposal for Protecting Wild Rice from Excess Sulfate in March 2015, and requested comments on the proposal in October 2015.

In response to comments and questions received, the MPCA re-analyzed its wild-rice study data using different statistical approaches. The agency has revised its conclusions in four areas:

Categorizing wild rice waters

2015 proposal:
The proposal released last year includes a draft list of wild rice waters where the standard would apply, along with a process to add waters to the list over time. The planned definition for a “wild rice water” is a water that:

Contains a self-perpetuating population of wild rice — now or any time since November 28, 1975 — with at least  8,000 wild rice stems over the surface of a lake or wetland, or a minimum of 800 wild rice stems over a river-mile reach.

Refinements to the proposal:

The definition of a “wild rice water” has been revised:
Since November 28, 1975, a lake, stream or wetland has had a documented history of wild rice harvest or contained a natural bed of wild rice of at least:

  • 0.25 acres, with a stem density of at least 8 stems per square meter OR
  • 0.5 acres, with a stem density of at least 4 stems per square meter

The correct sulfide level

2015 proposal:
Sulfate in lake or stream water can diffuse into the sediment where wild rice grows and turn into sulfide, which can be toxic to wild rice at high concentrations. The MPCA evaluated the effect of sulfide on wild rice and used statistical analysis (EC10 developed through regression) to identify a tolerable sulfide concentration to be able to develop a sulfate standard.

Refinements to the proposal:

Based on feedback, the MPCA used a new approach (graphing the study data) and has revised the concentration of sulfide in the sediment that wild rice can tolerate, detailed in the MPCA draft technical support document.

Calculating appropriate sulfate levels

2015 proposal:
Iron and organic carbon in the sediment affect the rate at which sulfate in the water is converted to sulfide in the sediment. Iron in sediment binds to sulfide and neutralizes it, making it non-toxic to wild rice. Organic carbon, on the other hand, causes more sulfide to be produced. Sulfate is converted to sulfide differently in each body of water, depending on the concentrations of iron and organic carbon in the sediment. The MPCA proposed an equation that accounts for iron and organic carbon levels. Plugging a site’s iron and carbon concentrations into the equation would identify a sulfate concentration that allows for wild rice growth and regeneration.

Refinements to the proposal:

The equation has been adjusted using a new statistical approach (multiple binary logistic regression) that has a lower error rate than the approach used in the initial proposal (structural equation modeling).

Implementing the standard

2015 proposal:
In order to implement the standard in Minnesota lakes and streams, the MPCA will collect sediment samples in wild rice waters, measure the iron and organic carbon concentrations, and plug them into the equation to find a protective sulfate level. 

Refinements to the proposal:

Based on a 2015 survey of wild rice bed sediments in six bodies of water, the MPCA is now proposing that five composite sediment samples consisting of five cores each be collected in wild rice beds to more accurately represent iron and carbon levels. The sulfate standard derived from these samples would apply as a 12-month average.

See the full draft TSD on the MPCA's Protecting wild rice waters webpage. The public will have an opportunity to formally comment on the proposed standard during a rulemaking process in 2017. Feedback on the information being released today is appreciated by Sept. 6, 2016; send it to