COLLEGE STATION – When Atlantic Richfield Co. was tasked with cleaning up a major superfund site it had purchased in Montana, Dr. Frank Hons, a Texas A&M University professor, got a call to assist the company's consultants, Pioneer Technical Services.
Hons, a soil and crop science professor, spent two years leading a Texas A&M team studying revegetation solutions on land impacted by 100 years of copper mining, mineral processing and smelting in the Anaconda, Mont. area.
The Anaconda Copper Co., a driving force and major employer in the region for 100 years, was purchased by the Atlantic Richfield Co. in 1977. With the purchase came the responsibility for cleaning up more than 3,600 acres of mill tailing settling ponds, Hons said.
"The Anaconda Superfund site is the largest in the U.S., because it includes not just the 3,600 acres of tailings ponds, but thousands of acres surrounding the mineral processing and smelting area," he said. "The Environmental Protection Agency and the state of Montana have been working with Atlantic Richfield to reclaim these areas since the mid-1980s."
Over the years, the smelting and processing of copper ore resulted in millions of tons of waste, Hons said. The vast majority of the mill tailings were captured and stored in settling ponds, but part of these materials escaped to the Clark Fork River. Some of these materials were transported 100 miles downstream and were captured in Milltown Reservoir, where they've remained for up to 100 years.
In 2009, Milltown sediments were dredged and returned to Anaconda, where they were spread over 800 acres of old tailing ponds. The sediments were amended and planted with native plant species with no success, Hons said.
Although Atlantic Richfield has a program for remediating soils in the area, these Milltown sediments represented a unique revegetaion challenge. That's when Dr. Terry Moore, with Atlantic Richfield's Remediation Management team, reached back to his Texas A&M roots and called on Hons.
While the dredged soil is a nice dark color with about 3 percent organic matter, it is high in salt, metals and nitrate concentrations, Hons said. The company tried planting seed on it, but little would grow.
Hons started his project in October 2010. He began lab and greenhouse work to look at the dredged material's chemistry and how it affected plant growth. After reviewing techniques used to successfully revegetate surrounding soils, he then came up with treatments to help render the metal in the Milltown sediments less soluble, making them unavailable for plant uptake and thus less damaging.
"We developed large greenhouse trials to look at both the top and the root growth and uptake of metals by the plants," Hons said. "We were trying to devise methods to make the soil more amenable to plant growth. If the metals dissolve, the plants can take it up and don't thrive. Increasing rooting depth is important because this is a semi-arid environment where water is often limited and deeper rooting is needed for long-term survival."
The solutions they laid out for Atlantic Richfield are a combination of influencing the pH, salinity and organic matter of the soil, he said. Elevating the pH is critical to successful revegetation as it generally reduces the solubility of the heavy metals. Heavy metal uptake is damaging to plants.
Hons' team continued with Atlantic Richfield's practice of beneficially reusing waste products where possible. They chose a cement production byproduct – lime kiln dust – to raise the pH of Milltown sediment. It directly decreases the water solubility of metals or creates new minerals through the process and the metals adhere to the surface of those minerals and are less soluble, he said.
Plowing this byproduct into the soils creates a more oxygenated environment and produced some favorable chemical changes also, Hons said.
The Texas A&M team worked closely with Atlantic Richfield, EPA, the State of Montana and the agencies' scientific experts to make the project a success, he said. The final report to Atlantic Richfield was submitted in February. Hons recommendation was using lime kiln dust, oxygenation and a foot of cover soil mixed with composted manure to enhance plant growth.
Atlantic Richfield plans to start using this treatment this summer to get vegetation growing once again, he said. The EPA and the State of Montana have approved field-scale construction set to begin this summer.
"Later this summer we will see how things are progressing in the reclamation process," Hons said. "But it will be several years before the real sustainability can be assessed."