Research Guides Restoration of Mine Tailings Sites

As a natural resources company, Doe Run is committed to restoring various operational sites at the end of their productivity. Restoration of mine tailings sites is one example. Through our research partnership with Missouri University of Science and Technology (Missouri S&T) we are exploring better ways to revegetate these sites.

Tailings are the granular, left-over rock material that remains after the desired minerals are liberated from mined ore and recovered during the milling process. Doe Run's tailings are predominately dolomite and may retain trace amounts of minerals. From Doe Run's mills, tailings travel through pipes in a water-based slurry and are deposited in permitted tailings impoundments on our property. Water from these impoundments is separated from the tailings and treated prior to discharge through permitted outfalls. The remaining fine rock material does not retain moisture in the upper layers where vegetation would grow and contains few nutrients. When tailings impoundments are no longer in use, one closure option is to grow vegetation on the dry tailings. Soil amendments are necessary to encourage and sustain vegetation.

Over the past several years, Doe Run has funded nearly $200,000 in laboratory research at Missouri S&T on revegetating mine tailings sites. This research will inform our work to return portions of our tailings impoundments to a natural habitat benefiting pollinators and wildlife. In 2023, a multi-year research campaign on soil establishment and vegetative growth on mine tailings sites was brought to the field for testing.

"We have been fortunate to work with Missouri S&T's Dr. Joel Burken, endowed Mathes Chair of Environmental Engineering, and Dr. Mariam Al-Lami for several years to identify strategies to revegetate mine tailings," said Genevieve Sutton, remediation business manager at Doe Run. "Previously, we attempted revegetation with miscanthus grass. This new research considered a variety of native plant species and various soil amendments, providing a better chance for success in the field."

The Research

The research led by Dr. Al-Lami and Dr. Burken covered three major areas:

• Screening native and prairie plants for successful revegetation of mine tailings and tracking the plant's performance (health) over an extended period.
• Analyzing the role of carbon-rich residues (biochar, woodchips, sawdust and compost) in enhancing plant growth.
• Evaluating the role of arbuscular mycorrhizae fungi symbiosis in sustaining vegetation and stimulating soil-forming processes.

In order to analyze the plants efficiently and in a non-destructive manner, researchers used a novel approach by designing and building an imaging chamber, which expedited photography and review of the plants. The plants were analyzed over a 16-week period for their physical form and external structure.

Next, the plants were harvested for further analysis, including biomass and nutrient and metal uptake. Significant metal uptake in plants can be undesirable, particularly in areas where the plants may be consumed by animals. In this case, the plants were analyzed for lead, copper and zinc uptake against the Domestic Animal Toxicity Limits (DATL) for cattle. Researchers found that image-based measurements of the plants' characteristics correlated with the same characteristic measurements conducted through physical analysis that required plants to be destroyed. This finding supported using non-destructive imaging as a robust, rapid, high-throughput and cost-effective tool for plant screening purposes.

The Process

Screening experiments were completed on 34 plant species, including trees, grasses and leafy plants known as forbs and legumes. Four plants were selected for further investigation: one grass Adropogon gerardii, known as Big bluestem; and three forbs and legumes species were selected: Desmanthus illinoensis, known as Illinois bundleflower; Chamaecrista fasciculate, an annual legume known as Showy partridge pea; and Amorpha fruticose, known as Desert false indigo. These four species were evaluated over a period of six growing seasons, in seven combinations evaluating the addition of biosolids, plus a variety of carbon-rich materials.

Results demonstrated that three combinations of biosolids with carbon-rich materials supported the best prolonged growth: biosolids plus sawdust; biosolids plus biochar; and biosolids plus compost.

The investigation also analyzed, in a separate experiment, the role of native arbuscular mycorrhizae fungi (AMF) in enhancing the growth of prairie species in amended tailings. AMF, which colonizes on the roots of plants, forms a symbiotic relationship with plants, improving the host plant's uptake of water and nutrients, as well as helping to reduce stressors to the plant, including metals in soils. In turn, AMF receives up to 20% of the plant-fixed carbon and ingests plant photosynthetic products and lipids necessary for their own lifecycles.

The results of adding AMF were analyzed over five years. AMF improved the vitality of the prairie vegetation when applied with biosolids and biochar. These improvements were significantly more apparent in subsequent years, suggesting that AMF not only benefits the plant but also stimulates soil development, which provides long-term benefits to the ecosystem.

"This research has far-reaching implications, not only for Doe Run but also for hard rock mine tailings sites generally. We know that tailings can provide a challenging environment for most prairie species. However, with the proper selection of amendment strategies, we can significantly improve the success rate. The findings could provide guidance for an economical and ecologically beneficial remediation solution for active and inactive tailings sites across the U.S."

In 2023, armed with data and recommendations from this research, Doe Run created 18 plots near the Sweetwater Mine tailings site and worked with a contractor to amend the soils in a manner consistent with the research before planting the recommended species. Plant response monitoring and further analysis will take place in 2024.

Back to Sustainability Report