AUGUSTA – It will be difficult to meet the deadline for collecting enough signatures to force a referendum on an initiative aimed at taxing bottled-water companies in Maine, an environmental activist said.

“We need you to help with the water campaign immediately,” Jonathan Carter told attendees at the Maine Green Independent Party’s weekend convention.

A group called H2O for Maine must obtain 50,519 signatures by Sept. 28. So far, volunteers have gathered about 35,000, Carter said.

The referendum question would ask voters whether the state should assess a tax on drinking water sold in containers. It would hit Nestle Waters, Poland Spring’s parent company, along with smaller bottled water companies in Maine.

Nestle Waters operates bottling plants in Poland Spring and Hollis and is exploring options to build a pumping station and a bottling facility in Franklin County.

Carter, a former gubernatorial candidate, supports a push by H2O for ME for a tax of 3 cents per 20 ounces of water. The group also wants a committee to be formed to monitor Nestle’s water extraction from Maine and to create a trust with the tax revenue.

Carter compared large, privately owned water companies to oil cartels that draw natural resources from the ground to earn profits. “Water is our next environmental crisis,” he said.

Michigan adopted a moratorium last May on water withdrawals. Spokesman Jane Lazgin of Nestle Waters, based in Greenwich, Conn., has drawn a distinction between Maine and Michigan, saying that Maine already has taken steps to regulate groundwater.

But Jim Wilfong of H2O for ME believes there are legal pitfalls to negotiating with bottled-water companies in the absence of a state water-extraction policy, as the state did for Poland Spring, which withdraws water from Range Pond State Park in Androscoggin County.

The Baldacci administration, recognizing the complexities of water extraction issues, has sought to review and clarify groundwater management regulations.