Questa molybdenum mine remediation approaches milestone

Environmental remediation at the closed Questa Molybdenum Mine site, which operated for nearly 100 years until it closed permanently in 2014, will reach a milestone this year.

The mine was declared a federal Superfund site in 2011, with cleanup expected to take decades at a cost that the U.S. Environmental Protection Agency estimates will top $1 billion. The site's wastewater treatment plant alone, which came online in 2017, costs $10 million a year to operate.

Milestone

Workers have finished regrading one of the site's nine waste rock piles — several of which tower above the northern landscape next to NM 38 between Questa and Red River — and are preparing to seed ground cover as early as later this year, as well as install seismic equipment to monitor the stability of the manmade formation. The site is regulated by state and federal government agencies due to massive disturbance of the earth over nearly a century of mining.

The "waste rock and tailings are identified as sources of acid rock drainage or tailing seepage that contaminates ground water, surface water and sediment at the site," according to a spokesperson for the New Mexico Environment Department, which oversees "the construction of controls for source containment" at the former mine and tailings sites.

To comply with EPA requirements for preventing erosion, and for revegetation and to better control stormwater and snowmelt runoff, the steep rock piles must be brought to a maximum slope of 3:1 and covered with about 30 tons of cover soil per acre.

As heavy equipment operators installed a mixture of Albuquerque municipal compost and mine site-sourced dirt to the required depth of 3.2 feet across the regraded Capulin rockpile last Thursday (April 6), Don Bush, Chevron's lead engineer at the mine site, looked down from above nearly 80 acres of completed regrading work that includes the neighboring North Goat Hill rock pile.

"We've completely changed the shape," he said, adding that, eventually, "all the rock piles will be graded, covered and seeded," with the "roadside rock piles" next on the company's to-do list.

He pointed to large rocks placed in a channel along one side of the long slope. One of several rockpile drainage features was taking shape. The drainages will direct runoff from different areas of the 3,600-acre mine site to collection points and, eventually, to the site's massive wastewater treatment plant.

"We've started building the main channels for drainage," Bush said. "They're a [water] conveyance that goes down the canyon at the bottom of the rockpile. Currently, we pump it through a borehole that goes from Capulin to Goat Hill, and it goes down Goat Hill into a subsidence zone and it percolates down into the underground mine," from which wells deliver the contaminated water to the wastewater treatment plant.

Water treatment

The treatment facility is housed in what is by far the largest building in Taos County. The plant treats roughly 750 gallons of contaminated water from wells and springs, and around 200 gallons per minute of water that's pumped out of the old underground workings of the mine.

The facility can treat approximately 4 million gallons per day, and "is likely the largest wastewater treatment plant for mine sites in the state of New Mexico," according to a New Mexico Environment Department spokesperson, who noted that the largest wastewater treatment plant of any kind in the state is Albuquerque's Southside Water Reclamation Plant, which has a capacity of 76 million gallons per day.

To fulfill Chevron Mining, Inc.'s responsibility for rehabilitating and preventing further groundwater pollution, the wastewater treatment plant is required to run in perpetuity. Redundancies are built into every stage of the treatment process, which removes metals, suspended solids and ensures the pH level of the treated effluent is neutral before it is discharged into the Red River.

"What's actually produced is cleaner than the water coming upstream from the mine," said Gabriel Herrera, public affairs advisor for Chevron Environmental Management Company, standing alongside the Red River at the discharge point across from the treatment plant. The discharged effluent, which runs clearer than the famously turbid water already descending the Red River toward the Rio Grande, is crystal clear and can make up as much as 30 percent of the river's overall volume.

When the river level is low, the discharge from the water treatment plant can range between 15-31 percent of the overall river volume," Herrera said. "During normal flow conditions, it is between 5-10 percent."

"The New Mexico Environment Department monitors groundwater and surface water quality by means of discharge permits, National Pollutant Discharge Elimination System permit, and through the selected remedy chosen in accordance with the [1980] Comprehensive Environmental Response, Compensation, and Liability Act," known as Superfund, the environment department spokesperson said. "There is an NMED representative onsite three days a week who oversees the construction of controls for source containment of waste rock at the mine site and tailing at the tailing facility," which lies southwest of Questa.

After it's collected from the old underground mine workings and from wells and springs across the site, the contaminated water is sent through a four-stage treatment process to remove metals like aluminum, cadmium, molybdenum and excess iron.

Two 2.3-million-gallon effluent tanks hold the treated water until testing for suspended solids, arsenic, cadmium, copper, lead, molybdenum, zinc and mercury, among other parameters, confirms it meets EPA-mandated standards for release.

A stormwater catch basin near where the demolished mine mill used to stand — directly next to the new treatment facility — feeds water through a separate, less rigorous treatment process. Flows through the secondary treatment train fluctuate; last summer's monsoon meant the stormwater treatment train was working overtime, and snowmelt also adds to the load.

Both treatment trains produce sludge that is concentrated and sent to filter presses — imagine a supersized accordion bellows — that compact the material into dewatered, clay-like plates. The "filter cakes" drop out of the press and through a levered hatch in the floor, falling into the open containers of trucks waiting below.

Although the filter cake represents the solid material removed from the once-contaminated water, the company and the state environment department both characterize it as an inert and pH-neutral substance.

"As part of the water treatment plant pilot project, the filter cake was tested using Toxicity Characteristic Leaching Procedure and Synthetic Precipitation Leaching Procedure, as documented in the Pilot Study Final Report," Herrera said. "The results of the SPLC and TCLP confirm that the filter cake generated from the water treatment process are classified as non-hazardous. Chevron has performed similar tests during operation and the results are consistent with the pilot study. The filter cake is stored in a lined facility and any leachate from the facility is captured and returned to the wastewater treatment plant for treatment."

Nonetheless, the environment department told the Taos News that long-term storage of the material, which is produced at a rate of 4,500 tons per year, "is the primary issue with the WWTP at the former mine site."

Herrera indicated that, over the next few years, a long-term onsite repository area will be identified for the filter cake. Meanwhile, the material is deposited in a temporary pit adjacent to the treatment facility.

Local employment

"My goal is to make sure this is running all the time," said Tito Duran, supervisor of the wastewater treatment plant. Duran worked in the mine mill for 13 years before it closed. When the mine shut down, he began working construction to help build the wastewater treatment plant, and then took a job at the finished facility.

Duran trained on the job and eventually became the facility supervisor for WSP, the company contracted by Chevron to operate the treatment facility. A total of 23 people currently work at the facility, all of them local residents.

They're no longer miners, but the mine still employs hundreds of people to bring the landscape back to as close to original condition as possible. About 300 people were employed at the mine when it closed in 2014; currently, about 270 people are employed in remediation work at the site or in support roles offsite. Herrera said 40 percent of the overall workforce currently is local.

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