NANOCRYSTALLINE IRON AND MANGANESE OXIDE DEPOSITS IN PAUTLER CAVE: IMPLICATIONS FOR HEAVY METAL TRANSPORT AND SEQUESTRATION IN A SHALLOW KARST AQUIFER

Nanocrystalline iron and manganese oxide minerals play a critical role in controlling the geochemistry of trace elements and heavy metals. These minerals are common in many environments, including cave and karst systems. However, little is understood about the biogeochemical processes controlling their dissolution, transport, and precipitation in cave and karst systems or the mechanisms by which they sequester trace elements. Recent surveys of Pautler Cave in southwest Illinois have identified the presence of numerous Fe and Mn oxide deposits. XRD and SEM-EDS analyses indicate that these materials consist primarily of ferrihydrite [Fe₁₀O₁₄(OH)₂•xH₂O] and birnessite [(Na,Ca,Ba)Mn₇O₁₄•2.8H₂O], respectively. Structural and compositional properties of Fe and Mn oxides within the cave differ spatially and temporally and appear to correlate with the minerals’ depositional environments. These natural deposits contain substantial concentrations of Cr, Co, Ni, Cu, Zn, As, Ba, and Pb, as determined by XRF. The speciation of As, Zn, and Co in select samples of these deposits has been characterized by XAFS spectroscopy. These observations suggest that iron and manganese oxide minerals may provide previously unrecognized limitations on the transport of trace elements and contaminants in karst aquifers by sequestering these elements through adsorption and/or coprecipitation.