-- The system consists of an anaerobic cell and a concrete diversion dam, both constructed in 1998. Two aerobic cells and a limestone-rock filter were later constructed in 2003.
-- The concrete diversion dam was constructed to control the flow of the McPherson Branch into the constructed wetland and to provide a settlement basin to remove silt from the flow before it enters the wetland.
-- A liner was installed in 1998 on the west bank of the McPherson Branch, 70 meters (m) upstream of the concrete dam to minimize infiltration into, and drainage from, mined waste rock under the roadway parallel to McPherson Branch.
-- The wetland includes a Geosynthetic Clay Liner (GCL) covered by a 0.7 m thick agricultural lime-enriched soil layer; a 0.7 m thick layer of crushed 2.5 centimeter (cm) limestone (minimum 75% Calcium Carbonate [CaCO3]); hay bales; and a 0.15 m layer of spent mushroom compost.
-- The limestone-rock filter and aerobic cells were added to oxygenate the constructed wetland effluent, volatilizate hydrogen sulfides in the effluent, and provide additional settlement for metal precipitates in the effluent.
-- The constructed wetland is 2 acres in size.
-- The average flow of water into the constructed wetland is 291 gallons per minute (gpm) and the average flow out of it is 241 gpm.
-- Heavy Metals: Iron (Fe) (7.0 mg/L), Manganese (Mn) (1.2 mg/L), Copper (Cu) (0.6 mg/L), Zinc (Zn) (1.7 mg/L), Aluminum (Al) (4.2 mg/L).
Regulatory Requirements/Cleanup Goals:
EPA secondary maximum contaminant level (MCL) standards for public water systems:
-- Heavy Metals: Fe (0.3 mg/L), Mn (0.05 mg/L), Cu (1.0 mg/L), Zn (5 mg/L), Al (0.05 - 0.2 mg/L).
After the initial construction of the wetland in 1998, a study was conducted from September 15, 1999 to February 5, 2003 to evaluate the performance of the wetland. The study found that the wetland was reducing the acidity and concentration of most of the metals in the McPherson Branch flow. However, concentration of manganese was not being reduced. The study also found an increase in the hardness of water and a decrease in sulfate concentration. Later in 2003, two additional aerobic cells and a limestone-rock filter bed were installed to help decrease manganese concentrations.
As of 2006, the effluent concentrations of heavy metals are:
-- Al at 0.055 mg/L
-- Fe at 0.133 mg/L
-- Mn at 0.294 mg/L
-- Cu at 0.017 mg/L
-- Zn at 0.197 mg/L
With the exception of manganese, all metal concentrations have been reduced to below the EPA MCL standards.
-- The construction cost of the anaerobic wetland in 1998 was approximately $1 million. This included the initial removal of waste material and the construction of the anaerobic cell.
-- In 2003, the cost of adding the two additional aerobic cells to the wetland was approximately $300,000. This included the cost for the installation of the two cells, the cost for adding a rock filter, and the restoration of a segment of habitat on McPherson Branch downstream of the anaerobic wetland.
The Copper Basin Mining District is located in Polk County, Tennessee and Fannin County, Georgia. Copper and sulfur mining and processing occurred at the site from 1843 until 1987, with sulfuric acid production continuing until 2000. As a result of mining activities, an area of more than 35 square miles, including the Davis Mill Creek Watershed, the North Potato Creek Watershed, and sections of the Ocoee River, had become contaminated.
The site is currently being investigated and remediated through a collaborative three-party effort that was formalized by a Memorandum of Understanding (MOU), dated January 11, 2001. The three parties overseeing remediation of the site are: the EPA, the Tennessee Department of Environment and Conservation, and OXY USA (a subsidiary of Occidental Petroleum Corporation). Glenn Springs Holdings, Inc. (GSHI), also a subsidiary of Occidental Petroleum Corporation, is conducting the remedial work at the site.
The constructed wetland was installed by GSHI on the McPherson Branch near its convergence with Burra Burra Creek within the North Potato Creek Watershed. The two-acre wetland was constructed on a highly eroded watershed, near the location of a former ore roast yard. In 1998 the initial anaerobic cell of the wetland was installed on the McPherson Branch. The construction cost of the wetland and removal of waste from the area was approximately $1 million.
After construction of the wetland, a study was initiated in September 1999 to monitor the performance of the system. The study ended in February 2003 and found that the wetland had succeeded in reducing the acidity and concentration of most of the metal contamination in the McPherson Branch. The only metal that was not reduced to below the EPA MCL was manganese.
To help reduce the concentrations of manganese, two additional aerobic cells were added to the wetland system. In addition, a rock filter was constructed to provide oxygenation, volatilization of hydrogen sulfide, and settlement for metal precipitates. These additions to the wetland were conducted in 2003 at a cost of $300,000. This also includes the cost for the restoration of a segment of the stream downriver from the wetland.
The average volume of influent into the constructed wetland system is 291 gpm. Iron, copper, zinc, and aluminum concentrations have been reduced by an order of magnitude. In addition, acidity has been reduced with the pH of treated water increasing from 3.82 to 6.50.