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 definition of a “wild rice water” has been revised. Since Nov. 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. MPCA evaluated the effect of sulfide on wild rice and used statistical analysis to identify a tolerable sulfide concentration to be able to develop a sulfate standard.
Refinements to the proposal: Based on feedback, MPCA used a new approach 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.
Refinements to the proposal: The equation has been adjusted using a new statistical approach that has a lower error rate than the approach used in the initial proposal.
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.
Go to the MPCA website: www.pca. state.mn.us to see the full draft technical support document. The public will have an opportunity to formally comment on the proposed standard during a rulemaking process in 2017.
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