BILLINGS, Mont. (AP) — Conservationists are proposing a citizen's initiative for Montana's 2018 election that would toughen pollution rules for new hardrock mines.

Supporters of the initiative say they filed paperwork with the Secretary of State's office on Wednesday in preparation for a signature gathering effort expected to begin next month.

More than 25,000 valid voter signatures are needed by June 22 for the proposal to be placed on the November ballot.

It would require mining companies to submit plans to clean up their pollution without the need for perpetual treatment of contaminated mine water.

Tainted water that flows out of former hard rock mines is a common source of pollution across the western U.S.

In some cases, government agencies have installed treatment plants for mining pollution at huge taxpayer expense.