Home > Research Highlights > Phosphate Mine Waste Rock Remediation
Phosphate Mine Waste Rock Remediation
Phosphate mine waste rock in southeast Idaho contains potentially toxic levels of selenium and other trace elements such as vanadium, chromium, molybdenum, nickel, and cadmium. Dump sites have millions of cubic yards of mine waste from which selenium is leaching and being taken up by plants. The sites pose risk to grazing animals (there have been incidents of horse and sheep mortality), fish and waterfowl, and potentially human health for subsistence users. Selenium levels in creeks have been as high as 900 ppb, far exceeding the cold water biota standard of 5 ppb and the drinking water Maximum Contaminant Level of 50 ppb. Water drains from beneath the North Maybe canyon East Mill Dump into East Mill Creek, which consequently has a fish consumption advisory for children due to high selenium. Yellowstone cutthroat and other species are known to reside in the lower reaches of East Mill Creek and it is possible that cutthroats spawn on this tributary to the Blackfoot River. In fall 2003, two environmental groups filed a Notice of Intent to sue two mining companies and the Forest Service under the Clean Water Act due to selenium water quality standard exceedances in discharges to surface water at North Maybe canyon, South Maybe Canyon, and Smokey canyon.
Several remediation measures have been developed and are being evaluated to reduce exposure risk to grazing animals from potentially toxic levels of Se in waste rock dumps. A new bioavailable Se soil test was shown to identify Phosphoria Formation soils and waste sedimentary rock such as chert and limestone that can be safely used as capping materials over seleniferous waste rock to limit plant uptake of Se. High and low Se accumulating plant species have been identified to produce reclamation seed mixtures to minimize Se uptake. Selenium levels in plants grown in waste rock can be reduced to below the National Research Council recommended guideline of 5 mg/kg total Se in forages to protect grazing animals with as few as 3 to 5 repeat harvests and removal of above ground plant biomass. Plant levels remain below the 5 mg/kg guideline in subsequent harvests because available Se levels in the waste rock appear to be permanently reduced. Because repeat mechanical harvesting would be impractical on many waste rock dumps with steep slopes, a novel use of repeat prescribed burning as a means of permanently reducing available Se levels in waste rock dumps is being tested. A burn plan was prepared, the first test burn was done in the fall of 2004, and the second burn was done in the fall of 2005. Repeat burning must be done for several years (3 to 5) to achieve Se reduction goals and plant re-growth and Se uptake must be monitored for several years to document Se removal goals. Sulfur (gypsum or elemental) mixed with waste rock at a rate of 5 metric tons per hectare will suppress Se uptake by at least 65 % for substantial periods of time as measured by repeat harvests. This alternative treatment measure is undergoing further testing in field plots established in 2004 and sampled in 2005.
|
Rocky Mountain Research Station
Congressional Notebook
