A STORY OF SUCCESSFUL SITE CLOSURE IN THE CAROLINA SLATE BELT

The Carolina Slate Belt has offered significant opportunity for mining gold and silver. Slate Belt exploration, driven by high metals prices, is ongoing. However, mining ventures often encounter negative public perception stemming from the industry’s legacy of abandoning mines, which today have become EPA Superfund sites. Two such cases are the Barite Hill Gold Mine in McCormick County and the Brewer Mine in Lancaster County, SC. In contrast, two examples of responsible gold mining and reclamation in the Slate Belt are the Haile Mine in Lancaster County and the Rio Tinto Kennecott Ridgeway Mine in Fairfield County, SC.

The open pit Ridgeway mine operated from 1988 to 1999 and underwent closure activities from 1996-2001. Closure included installation of a clay cover on the tailings facility, revegetation, wetlands construction, and development of two open pits into fresh water lakes. From 2000 to 2004, a research effort monitored the developing pit lakes. The lakes were sampled weekly for temperature, pH, conductivity, dissolved oxygen, phytoplankton and zooplankton, and light penetration. Thermistor strings and meteorological stations allowed for assessment of physical forcing factors while discrete chemistry samples were collected regularly.

The data showed one lake to be circumneutral by 2000. The lake underwent several filling events from rainfall and draining of a decant pond and completely mixed after the first year. In the second year, the lake developed a dense bottom layer as a result of a phytoplankton bloom, bacterial respiration of the algal biomass in the bottom of the lake, and subsequent sediment flux of reduced iron and manganese. That lake has not mixed since 2001. The lake developed a large oxygen-rich mixolimnion (upper lake layer) and a small, anoxic monimolimnion (lower lake layer). Once developed, this oxic/anoxic interface was stable against wind induced mixing and was significant in conveying, trapping, and removing metals that are usually a concern after gold mining. The second lake followed a similar remediation trajectory two years later. Pit geometry, bathymetry, in-situ lime titrations, controlled algal growth, and monitoring were the keys to reclamation of these lakes.