Just 20 days ago, Dunkard Creek, which meanders lazily back and forth across the border of Pennsylvania and West Virginia, was one of the most ecologically diverse streams in both states, containing freshwater mussels, mudpuppy salamanders and a host of fish species from minnows to 3-foot-long muskies.

Generations of families picnicked along its sycamore-lined banks and swam in its warm water. Fishermen plied its green, slow-moving pools with lures and bait in hopes of catching lunker bass.

But today, the 38-mile creek is all but dead, its 161 species of fish, mussels, salamanders, crayfish and aquatic insects killed by mysterious pollutants coming from sources state and federal agencies have yet to pinpoint despite aggressive field work.

“We’ve just been decimated down here. Everything is being killed almost from the headwaters of the creek to where it flows into the Monongahela River,” said Betty Wiley, president of the Dunkard Creek Watershed Association. “It’s such a tragedy for the creek. An ecosystem has been destroyed.”

And fish continue to die as the initial mass of pollution moves down the creek, which flows into the Monongahela just down river from Point Marion, Fayette County, and as additional pollution is discharged from its mysterious source.

Environmental agencies are treating the creek as a crime scene. Longtime environmental and fisheries officials say the fish kill, which preliminary counts have put at more than 10,000, is one of the worst they’ve seen.

“A lot of supposition and science needs to be pieced together, but this is bad,” said John Arway, chief of the Pennsylvania Fish and Boat Commission’s environmental services division. “The fish that couldn’t escape up side tributaries were killed.”

The Pennsylvania Department of Environmental Protection on Friday said more than 30 miles of the stream have been damaged by the discharge. It has killed 18 species of fish and at least 16 species of freshwater mussels, including the salamander mussel and the snuffbox mussel — both candidates for federal listings as endangered species.

“DEP will continue to monitor water quality so that when the responsible party is determined by West Virginia and [the U.S. Environmental Protection Agency] we are positioned to take appropriate enforcement action,” said Ronald Schwartz, DEP acting regional director.

“This is the worst fish kill I’ve experienced in 21 years in West Virginia,” said Paul Ziemkiewicz, director of the National Research Center for Coal and Energy’s Water Research Institute at West Virginia University.

Environmental agencies in West Virginia and Pennsylvania, the Pennsylvania Fish and Boat Commission, the West Virginia Department of Natural Resources and the EPA each have had inspectors on the creek in recent weeks, testing water samples, collecting dead fish and observing discharges into the water.

An early and continuing focus of the investigation has been discharges from a mine water treatment facility located at Consol Energy’s Blacksville No. 2 mine in West Virginia.

But state and federal investigators are confounded because chemical analysis shows the creek water at the treatment facility site contains extremely high total dissolved solids, or TDS, and chlorides — properties found in wastewater from Marcellus Shale gas well drilling operations but not mine water. Total dissolved solids may include metals, salts and other elements.

Marcellus Shale well drilling water contains about 100 chemicals added to reduce friction, eliminate algae growth and perform other functions when water is pumped underground under pressure to fracture the shale and release natural gas.

Up to 4 million gallons are used for each Marcellus Shale well. Disposal of wastewater from the wells has caused problems throughout Pennsylvania, including TDS readings that exceeded federal safe drinking water standards in the Monongahela River last winter and this year.

On Thursday, investigators found dead fish for the first time about a mile and a half up the creek above the treatment plant discharge.

“Our hypothesis was that it’s coming out of the Blacksville No. 2 mine, but the finding of dead fish upstream from the Blacksville discharge indicates the sole cause cannot be Blacksville,” said West Virginia DEP spokeswoman Kathy Cosco.

The state agencies now are looking at the possibility that someone has illegally dumped drilling wastewater into the creek to avoid the expense of complying with laws governing its disposal. The water must be treated in Pennsylvania or injected deep underground in West Virginia.

The West Virginia DEP on Friday sent a helicopter to fly over the creek to look for unauthorized discharges and places where tanker trucks could pull up and dump drilling wastewater.

“The elevated levels of TDS and chlorides in the creek indicates oil and gas drilling wastewater,” Ms. Cosco said. “We are following up on every lead that people give us. If they saw a truck pull up to the creek and put a hose in, we want to know about it. We want the name on the truck, a license plate number, anything we can use to identify it.”

Unlike Pennsylvania, the West Virginia DEP doesn’t permit water or sewage treatment facilities in the state to accept or discharge Marcellus well wastewater, Ms. Cosco said.

Consol spokesman Tom Hoffman said the company’s facility does not accept or treat gas well drilling wastewater. The company’s field teams also are trying to figure out what’s happening.

“Neither they nor we have been able to sort out what’s going on,” he said. “It’s confounding because we’re seeing fish kills in the vicinity of the treatment plant where you might expect Blacksville No. 2 is at fault, but also further downstream than you would expect was our fault, and recently upstream from the Blacksville 2 discharge.

“So Blacksville is a possible contributor, but it’s not clear if it’s the lone cause.”

Mr. Hoffman said the mining company, at the suggestion of the West Virginia DEP, agreed on Thursday to shut down plant operations to assess the effect on the creek.

Water samples taken from the creek at the Blacksville mine treatment facility show extremely high levels of total dissolved solids, in the 25,000- to 35,000-milligrams-per-liter range, or about the same as in seawater. The federal safe drinking water standard is 500 milligrams per liter.

The fish started turning belly up on Sept. 1. By Sept. 4, dead fish were lining the deep pool below the Lower Brave Dam near the Greene County town of Brave.

“It’s disgusting to see that much life wiped out,” said Ed Presley, who owns property along the creek at the Lower Brave Dam.”To see the quality and beauty of that stream and then to see what happened to it, well, it really tears at you. I’m not really a tree-hugger but to see natural things destroyed and wasted like this, it’s just dead wrong.”

“We’re very concerned about this going on and this clearly is not an easy thing to find the source of,” said EPA spokeswoman Bonnie Smith. “There are a lot of factors, … but this is a tough one.”

Don Hopey can be reached at dhopey@post-gazette.com or 412-263-1983.