MINNEAPOLIS — Critics of proposed copper-nickel mining in northern Minnesota said Wednesday they have gathered petition signatures from all 87 Minnesota counties.

Officials with Mining Truth said 12,604 people have signed the petition since the summer push began July 8.

The petition asks Democratic Gov. Mark Dayton to require “yes” answers to four questions: Will Minnesota’s water stay safe and clean? Are there safeguards in place from when things go wrong? Will the company leave the site clean and maintenance-free? Will Minnesota’s taxpayers be protected?

A stretch of northeastern Minnesota contains large, untapped reserves of copper, nickel and precious metals, but they’re bound up in sulfur compounds that can produce sulfuric acid when exposed to air and water.

Officials developing the proposed PolyMet and Twin Metals mines say the minerals can be mined responsibly, but the Mining Truth coalition says similar mines elsewhere have left behind serious pollution and huge cleanup costs. A long-awaited revised environmental review of PolyMet will be released Nov. 22

“Our lakes and rivers are a big part of what it means to be a Minnesotan,” Scott Strand, executive director of the Minnesota Center for Environmental Advocacy, said in a statement. “People across the state want Governor Dayton to put our water first when considering sulfide mining projects like PolyMet and Twin Metals.”

Dayton has been feeling conflicting political pressures from his environmentalist allies on one side and his labor and Iron Range supporters who want the jobs a new era of mining would create. Dayton has said he’s pro-environment and pro-jobs, and there needs to be a balance.