SILVERTON — A surge of heavy metals-laced waste leaking from the Gold King and possibly other mines in southwestern Colorado raised the acidity of Animas River headwaters to the strength of black coffee, according to early testing results.

EPA officials on Saturday morning also said they are bracing for another surge of acid discharge from mines above Silverton due to recent rainfall that could increase pressure. Such a surge could dislodge contaminants along the banks.

And La Plata County emergency management director Butch Knowlton was preparing to launch a domestic well-testing operation. A well owner near Durango, drawing from alluvial sediments along the Animas, reported his water was coming up yellow, Knowlton said.

PHOTOS: Animas River flooded with contaminated mine water

The Animas River flowing through Durango (population 17,000) remained mustard-yellow Saturday, and yellow-orange contaminants continue to trickle out of old mine portals about 60 miles to the north above Silverton. EPA contractors are hastily building plastic-lined retention ponds and setting up pumps to try to catch concentrated bright orange contaminants before they reach streams and rivers.

The disaster began last Wednesday when EPA workers at the Gold King triggered a sudden spill of an estimated 1 million gallons of acid orange-yellow wastewater that had built up inside the Gold King — a deluge that reportedly wiped out a vehicle and apparently knocked out culverts as it tore down into Cement Creek, which flows into the Animas River.

The Animas serves as a main source of water for Durango and the sole source of water for communities in New Mexico including Farmington.

EPA and state officials said they’d been investigating four mines leaking contaminants in the area and failed to grasp the severity of the situation. These are among tens of thousands of old mines across Colorado and the West that have leaked acid wastewater at varying rates for decades into headwaters. The overall cleanup has been complicated, stalled for lack of funding and political and legal issues, and remains far from complete.

EPA officials on Saturday revealed data from intitial water sampling indicating pH levels reached 3.74 above Silverton in Cement Creek and 4.8 below Silverton in the Animas. Pure water has a pH of 7; the lower the pH level, the higher the acidity. The EPA results were limited and officials said Ph levels before the spill were between 6 and 7.

An acidity at pH 3.74 is comparable to Dr. Pepper or orange juice, according to a table EPA information officers released. A pH of 4.8 compares roughly with pH 4.5 for black coffee.

Federal crews will be sampling water from the mine portal south across mountains to the Colorado-New Mexico border and are awaiting results, EPA officials said. They said samples also are being taken in New Mexico as the snout of the yellow-orange plume moves toward the San Juan River, which flows into the Colorado River.

Significant dilution is expected to occur as the acid contaminants — including zinc, cadmium, arsenic, aluminum, copper and calcium — move downriver.

Meanwhile in Durango, federal, county and city officials were gathering in an emergency response meeting.

“It has been raining. We are anticipating another possible big release,” EPA spokeswoman Libby Faulk said.

One immediate focus at the meeting — an interagency session closed to a reporter — was addressing reports of discoloration in wells along the Animas above and below Durango. Ranchers, growers and homeowners rely on the wells, which are bored into alluvial sediments along the glacial-cut river valley.

La Plata county responders are particularly concerned about potentially rapid movement of contaminants through gravel into wells, Knowlton said. “We are worried about those.”

The river remains closed. County health officials said the health risk remains high and awaited more testing results. Local river rafting companies closed due to the pollution.

“It doesn’t look very healthy,” said Michael Nowara, 25, of Australia, visiting with a group that were unable to raft as planned.

“Considering that was going to be the main attraction in town, yeah,” he said. “It is a bit disappointing.”

Bruce Finley: 303-954-1700, bfinley@denverpost.com or twitter.com/finleybruce