SOIL GEOCHEMISTRY AT THE NEW CORNELIA MINE, AJO, ARIZONA

The New Cornelia open pit copper mine in Ajo, Arizona, has been inactive since 1987. Copper extraction began with Native Americans and the mine continued expanding as an open pit mine until it became inactive.

The current area surrounding the mine, tailings, and holding pond is BLM land and accessed by public roads. These public lands are often used for camping, grazing cattle and recreational activities. Water-soluble heavy metal contamination of soils would thus be of public health concern.

Geochemical analyses were conducted on samples of these soils to determine the mine’s impact on the surrounding environment via potentially water-soluble metals such as As, Cd, Pb, and Cu. Twenty-seven sites in the surrounding landscape were sampled at each visible soil horizon for a total of 49 samples. Vegetation growing at the sites was also evaluated for evidence of potential toxicity; the samples were analyzed by ICP.

Some metals showed extremely high concentrations at some locations and negligible amounts at other sites. Maximum As was 264.74 ppb when the EPA recommends no more than 10 ppb in drinking water, whereas minimum As was 19.78 ppb, which is still elevated for drinking water, but is less than a control sample which showed 31.83 ppb. Samples that contained outwash from the tailings pile, were taken in close proximity to train tracks, that were obtained near chemical staining on nearby tailings, or collected where visible copper carbonates occurred in the soil were found to have the highest concentrations of various metals. Other sites without obvious signs were found to have metal concentrations that were equally high. Plants around the mine appeared to be relatively unaffected by even high levels of contaminants in the soil.

A control sample collected adjacent to the boundary of the Cabeza Prieta National Wildlife Refuge, ensured that at least one sample served as a baseline that should have had more typical concentrations background of metals. The control sample was found to have much lower concentrations of the metals of concern than the majority of the sites around the mine, though it was also determined than many contaminants released by mining activity are also common in soils not directly adjacent to the mine. Further study will be necessary to determine the overall prevalence of these metals in undisturbed regional soils.