A proposed county parcel tax to bankroll projects aimed at capturing, cleaning and conserving stormwater to prevent it from running downstream and polluting rivers and beaches appeared to be heading for passage Wednesday.

Measure W, dubbed the Safe Clean Water Program—expected to raise $300 million annually—needs the backing of two-thirds of voters to be approved. With all precincts reporting from Tuesday’s election, the measure had 67.5 percent of the vote. Given the likely large number of provisional and mail ballots that still need to be tallied, however, the final result may not be known for weeks.

The measure is expected to bring an estimated $5 million a year to Long Beach.

“Every year a full 100 billion gallons of water runs down our curbs and out into the ocean,” picking up trash and toxins along the way, Supervisor Sheila Kuehl said in July when the Board of Supervisors voted to place the item on the ballot. “We have to get water-wise.”

Kuehl and dozens of environmental advocates said the measure would both increase local water supplies and improve water quality, while prioritizing projects that have added benefits such as creating additional green space and protecting wildlife.

Shelley Luce, president and CEO of the nonprofit Heal the Bay, argued the measure would “save millions of dollars of taxpayer dollars” by putting the county in compliance with the federal Clean Water Act. Opponents have criticized the proposal for adding yet another tax in an already expensive housing market. They also complained the measure does not have a sunset clause, creating a “forever tax.”

In 2013, an effort by then-Supervisor Zev Yaroslavsky to pass a parcel tax was abandoned in the face of strong opposition from business owners, school districts and some cities that said it duplicated their own efforts.

But supporters say Measure W is designed to reflect how heavily individual property owners rely on county flood control systems. It would charge 2.5 cents per square foot of “impermeable areas” such as concrete roofs, driveways and sidewalks that contribute to runoff. Yards and other green space, where stormwater is absorbed rather than running into drainage systems, are not included in the calculation.

The median tax is estimated at $83 per year and interested property owners can calculate their bill at www.safecleanwaterla.org. The site relies on GIS mapping to identify hard-scaped areas versus green space.

Schools would be exempt from the tax.

Supervisor Kathryn Barger cast the lone vote against putting the measure on the ballot, saying she was concerned about the burden on property owners, particularly in light of a potential future parcel tax to fund a structural deficit in the county fire department, and potential upward pressure on rental rates.

Barger said complying with federal laws that carry hundreds of millions of dollars in potential penalties is one thing, but she argued the measure goes above and beyond what is required.

“The proposed initiative far exceeds the requirements of the MS4 permit which governs pollutant levels in local waterways,” Barger said. “The permit is based on assumptions that are antiquated and require updating before we go to the taxpayers for more money.”

The county has been out of compliance for years with federal stormwater requirements, though it has largely avoided fines through good-faith efforts to solve the problem.

Those efforts were not enough for the Natural Resources Defense Council and Santa Monica Baykeeper, which sued the county in a case that went to the U.S. Supreme Court in 2014 and ultimately held the county responsible for cleaning up the Los Angeles and San Gabriel rivers.

Untreated stormwater and urban runoff into the region’s two main waterways have resulted in excessive levels of aluminum, copper, cyanide, zinc and fecal bacteria. Those rivers drain into Santa Monica Bay and the Pacific Ocean, leaving areas near storm drains closed to swimmers and surfers after heavy rains.

If passed, the tax would be allocated 50 percent to watershed agencies for regional water projects, 40 percent to cities for local priority projects and 10 percent to the county flood control district for educational programs and administration.