NATURAL OXIDATION OF PYRITE IN UNDISTURBED PENNSYLVANIAN COAL-BEARING BEDROCK

The oxidation of pyrite in rocks is a natural chemical weathering process. However, the diffusion rate of oxygen into undisturbed bedrock is thought to be extremely slow. How then are acidic, iron-rich springs and groundwater able to form in undisturbed areas. Environmental (RCRA) studies are being conducted at a site in the midwestern U.S. where up to 180 feet of Pennsylvanian coal-bearing strata are present at the earth's surface, the residual soil layer is generally less than 10 feet thick, and no coal mining or coal exploration has taken place in the area. The topography is hilly and ridges are covered with forest. Anthropogenic activities are conducted on the top of the flat ridgetop areas, but the amount of physical disturbances is minimal. Hundreds of monitoring wells have been installed in Pennsylvanian bedrock at the facility, ranging in depth from 15 feet below ground surface (ft bgs) up to approximately 155 ft bgs. A significant percentage of these wells contain low-pH groundwater (pH range = 2.66 to 5.00 S.U.) and high concentrations of dissolved metals. The wells with the lowest pH values tend to be the ones drilled and screened in the uppermost bedrock. The pH and the metal concentrations detected in groundwater have been investigated to determine if there are any observable spatial patterns. The elapsed time since well installation, the topographic location of the well and well screen relative to the ridge, the number of times the well was sampled, and the position of the groundwater elevation relative to the well screens have each been evaluated as possible factors affecting the pH and metal concentrations in the wells. It is likely that much or all of the pyrite oxidation that resulted in low pH values was due to natural oxygen diffusion and advection into the ridges. The rates of diffusion and advection may be affected by the degree of bedrock fracturing that controls ingress of oxygen. Another factor which is being evaluated is the ingress of oxygen through the well bore itself. By penetrating the rock with a boring, a new pathway is created whereby oxygen can enter the subsurface. The act of installing a monitoring well may in fact have a large impact on the groundwater system being evaluated.