Assessment and prediction of heavy metal fate and transport in the Boone Aquifer, Ottawa County, Oklahoma

The Tar Creek Superfund Site (TCSS) is located in Ottawa County, Oklahoma. Historic mining activities at the site resulted in the accumulation of large piles of crushed mining waste and open underground mine workings. Groundwater quality within mine workings is severely degraded due to the oxidation of sulfidic minerals present in mine voids and mining waste. Contaminated groundwater is in direct contact with the surrounding rocks of the Mississippian-aged Boone Group, which host the Boone aquifer. The Boone aquifer crops out and supplies water to local springs in the eastern portion of Ottawa County. A preliminary study of spring water and watercress showed that metals were accumulating in these receptors. This was a major concern for local residents who subsist on spring water and associated plants as natural resources. Pb isotope ratio analysis of sediment samples from springs cropping out of the Boone Group showed that accumulated Pb was sourced from the TCSS. Existing groundwater flow models of the area did not predict that contaminants would be transported in the direction of these springs. However, geophysical and geological investigations revealed that the most probable mechanism by which Pb was transported in the subsurface was through the extensive, open fracture network present in the Mississippian rocks of northeastern Oklahoma. The fracture network was mapped and incorporated into a groundwater model of the region. Fracture permeability (which has not been incorporated into prior groundwater models) was shown to greatly enhance groundwater velocity in the model area as well as influence the direction of groundwater flow. Specifically, contaminant particle tracking simulations showed that metals released from the TCSS could reach spring discharge points in as little as ten to twenty years. These results have implications for the success of the ongoing mitigation strategy at the TCSS where mine wastes are being injected into mine voids. Additions of new, chemically active sources of contamination to the Boone aquifer pose a significant and undocumented risk to nearby surface water systems.