A much-anticipated decision on sulfate pollution in Minnesota's lakes and rivers has been postponed by the state Pollution Control Agency, which said it needs more time to study variables in wild rice production and water quality before proposing any new rules.

Wednesday's announcement was the second delay for the sensitive decision, which has pitted environmentalists and Minnesota Indian tribes against the state's mining industry and some Iron Range legislators. The agency had planned to release its recommendation in late February, but abruptly canceled the announcement without explanation.

The decision delays the agency's widely anticipated recommendation for a "sulfate standard," designed chiefly to protect the state's treasured wild rice crop, and raises the possibility that standards ultimately could vary from place to place.

"There's a lot more analysis needed," Commissioner John Linc Stine said Wednesday. "Perhaps higher levels of sulfate can be allowed. It's potentially variable from place to place."

Sulfate is a type of mineral salt discharged by mines and other industry in northeast Minnesota that can harm rice stands.

Stine said it would be late this year or sometime in 2015 before the agency recommends water quality standards to protect the state's most famous plant. The U.S. Environmental Protection Agency would have to approve the standards.

Nancy Schuldt, water quality expert with the Fond du Lac Band of Lake Superior Chippewa, said she applauds the expanded scientific observation, which will include a peer review of existing data.

A crucial part of the process will be classifying which waters will be protected, she said.

Tony Kwilas, director of environmental policy for the Minnesota Chamber of Commerce, said industry will seek to be represented in the upcoming peer review. In the end, he said, the standards will govern permitting of industrial discharges. "It has statewide implications for our members," Kwilas said.

According to the agency report released Wednesday, scientific reviews so far have shown that wild rice can withstand higher levels of sulfate in the water depending on the proportion of dissolved iron in the sediment. Although sulfate is not directly toxic to wild rice, it can break down into sulfide, which limits the plants' ability to grow. The presence of iron can strongly limit the production of sulfide.

The agency said more study is needed to explore the possibility of a sulfide standard for sediment to protect wild rice.

Naturally occurring sulfate is not common in northeast Minnesota, where most of the state's wild rice grows. The primary source of sulfate is leachate from piles of waste rock and tailings ponds from a century of iron and taconite mining on the Range. Other sources, including municipal wastewater treatment plants, contribute as well.

In 1973, the Pollution Control Agency adopted a federally approved sulfate limit of 10 parts per million in water used for the production of wild rice. But the standard wasn't enforced, in part because no one was sure which of Minnesota's rivers, lakes and wetlands qualified as wild rice waters. In 2011, the Legislature directed the agency to re-evaluate the standard.

"There are many questions and considerations yet to be discussed about protecting the health of our state's wild rice," Stine said Wednesday.