EPA Research Partner Support Story: Heavy metal soil contamination from past mining activity
Partners: Oregon Department of Environmental Quality (DEQ)
Challenge: Legacy heavy metal soil contamination from past mining activity
Resource: Prescription for soil amendments to enhance revegetation
Project Period: 2017 – Present
Past mining activities in the United States have left a legacy of soil and mining residues contaminated with heavy metals. Before plant growth can reach levels adequate enough to stabilize the soil, the bioavailability (absorption of the contaminants into living tissue) and acid levels generated by waste rock need to be reduced. EPA’s Superfund program oversees cleanup of these sites. However, with limited resources, the highest priority for remediation goes to the most hazardous sites. In the meantime, many sites receive little or no attention. Often simple steps such as establishment of plant cover can reduce wind and water erosion of contaminated waste materials and the subsequent contamination of downslope receiving waters.
Of special concern is the Formosa Mine Superfund Site in south-central Oregon. This mine has a large area of tailings where it is difficult to establish vegetation. EPA ORD and Region 10 (Pacific Northwest) and the U.S. Department of Agriculture’s Agriculture Research Service partnered to develop new techniques to assist in this cleanup. These techniques use soil amendments that adjust the pH, absorb heavy metals, and provide nutrients essential for plant establishment and growth. By creating a favorable rooting environment with amendments, plants can get established and protect the site from wind and water erosion, preventing the movement of contaminated soil.
“EPA researchers developed a prescription for amending contaminated mine tailings at the Formosa Mine Superfund Site to support the establishment and growth of vegetation where none was growing due to contamination. The research work is providing a site-specific biochar formula suited for the site conditions. Not only is this a significant scientific contribution that will prevent the offsite movement of contaminants at the site, help facilitate the removal of this site from the NPL and support long-term cleanup, but this kind of technology will be useful at numerous other contaminated sites across Oregon.” – Oregon DEQ Hydrogeologist Bryn Thoms
The amendments used at the formosa mine include lime for neutralizing acidity and raising soil pH; biosolids (organic matter recovered from a sewage treatment process and used as fertilizer to supply carbon and nutrients for plant growth); and biochar (a charcoal-like material made by heating waste wood in the absence of oxygen to sorb metals, lower soil bulk density, and improve water infiltration and retention).
Results show that with these amendments the conditions for plant establishment and growth at the Formosa Mine dramatically improved. Douglas fir trees and herbaceous plants grew in amended tailings. Without amendments, the tailings are barren and do not support plants. Tree mortality was observed because surface runoff from unamended tailings overcame the acid neutralizing effectiveness of the lime and soil pH dropped, causing tree mortality. This suggests that all exposed waste rock and tailings need to receive amendments, including sufficient lime to neutralize residual acidity to prevent this from occuring in the future especially in downslope positions. Monitoring is also important as lime and biosolids may need to be reapplied occasionally to support long-term plant health. Other mine sites can use these types of amendment combinations and prescriptions to improve contaminated or degraded soils to establish plants and support their growth. Over time plants will improve soil health and prevent flow of heavy metals downstream from these mine sites.