MINNEAPOLIS — The Minnesota Pollution Control Agency unveiled a new site-specific approach Tuesday for protecting waters where wild rice grows against sulfate pollution that’s meant to replace a 1973 state law that largely went unenforced until recently.

The dispute over whether the old standard is obsolete set up a politically charged clash between the state’s iron mining industry and Iron Range legislators on one side versus American Indians who consider wild rice a sacred food source and environmentalists who support them.

Minnesota’s current law sets the sulfate limit at a flat 10 milligrams per liter for all lakes and streams that support wild rice stands. The MPCA’s new plan, rather than relying on a single limit, proposes a complicated mathematical formula for calculating allowable sulfate levels for individual wild rice waters, based on how much iron and organic carbon are in the sediments where the plants are rooted.

The MPCA plans to begin a formal rulemaking process late this summer or early fall to change the standard, a process that could take two years from then. The agency also compiled a draft list of about 1,300 waters that would be subject to the standard, and said others could be added later.