WEST MONROE, La. (AP) — Drip by drip, West Monroe is running out of drinking water.

Like much of north and central Louisiana, the town of 13,000 gets its water from the Sparta aquifer, an underground reservoir formed thousands of years ago. In all, 16 parishes and over 60,000 people rely on wells drilled into the Sparta.

But West Monroe is on the front line of a looming problem: Sparta’s watery treasure is slowly disappearing, because residents and industry use it up faster than rainwater replenishes it. Officials have known about Sparta’s troubles since at least 2000, when a study confirmed suspicions that parts of the aquifer will dry up unless consumption drops.

‘‘We’re using more water than is coming into the system naturally. We’ve been doing it for decades,’’ said Ben McGee, a Ruston-based hydrologist with the U.S. Geological Survey.

No one knows when the aquifer will stop producing drinkable water, or where it could happen first, McGee said, but towns and cities across the region are looking for alternate water sources.

‘‘It’s a predictible situation: we’re going to run out of water at this rate in this part of the world,’’ Ruston Mayor Dan Hollingsworth said.

West Monroe is the first city with a plan to drastically reduce the amount of water it pulls out of the aquifer. Under the proposal, a new treatment plant would purify the city’s sewer water, for industry to use instead of aquifer water.

Mayor Fred Norris just needs to find $20 million to get it done — before the supply of drinking water disappears.

The Sparta aquifer is something like an underground swimming pool, sprawling for hundreds of miles underneath parts of Louisiana, Texas and Arkansas. Unfortunately for West Monroe, the city sits atop the pool’s shallow end — and its share of the water is disappearing faster than anyone else’s. The city of Ruston, 30 miles west, is in the same situation.

‘‘If you pull the plug, the shallow end empties first,’’ McGee said.

Compounding the problem, West Monroe also sits near an underground saltwater vein. The salty water threatens to contaminate the fresh water used by residents and industry.

Norris and others looked to West Monroe’s biggest consumer of aquifer water: a paper mill run by Marietta, Ga.-based Graphic Packaging International, Inc., one of northeast Louisiana’s largest employers. The plant uses 10 million gallons of Sparta water per day.

Engineer John Stamberg came up with a solution: build a plant that would purify 10 million gallons of sewage per day for use by the paper mill instead of aquifer water.

The company, whose plant makes paper for food packaging, agreed to the plan — but only if the former sewer water is pure enough to meet the Environmental Protection Agency’s standards for safe drinking water. Norris said a pilot program last year met that standard, turning sewage into water that’s cleaner and clearer than Sparta’s.

‘‘We have shown that we can take wastewater, treat it and meet the EPA drinking water standards,’’ the mayor said.

The proposed treatment system, similar to two systems in Africa, would be unique in the U.S., according to Stamberg. It involves taking treated sewage — which is currrently pumped into the Ouachita River — and putting it through three additional stages of filtration, resulting in EPA-approved water for use at the mill.

Now, Norris needs $20 million to get his plant built. He’s hoping the Legislature will supply $7 million this year to get it started.

Other towns and cities are also looking for alternate sources of water.

Ruston officials are considering building a pipeline from Lake D’Arbonne, about 20 miles away. Hollingsworth acknowledged that fishermen at the manmade lake don’t like the idea, but he said that a study finished last year found Ruston could pull 6 million gallons per day out of D’Arbonne without causing its water levels to drop.

Hollingsworth said he’s hoping to get federal money for further studies on the project, which has a pricetag estimated at $75 million.

Ruston has also focused on conservation, and the city’s demand for water has held steady recently, even as its population has grown, he said.

‘‘You have to think about all the ways you use water, how you brush your teeth, how you shave. In the end, you can save millions of gallons, you can really make a difference,’’ state Rep. Hollis Downs of Ruston said. ‘‘Humans can’t live without water, so we’ll find the solution to this problem one way or another.’’