ENVIRONMENTAL PETROGRAPHY SURVEY OF SELECTED UPSTATE NEW YORK BEDROCK FORMATIONS AND APPLICATIONS IN CHEMICAL HYDROGEOLOGY

Environmental petrography involves characterizing the mineralogy, chemistry, texture, structure, and porosity of geological materials for environmental investigation purposes using standard petrographic evaluation techniques. Environmental petrographic examination can assist drinking water source assessment efforts and environmental investigation and remediation efforts for contaminated environmental media including soil, sediment, bedrock, and groundwater. Several studies in the published literature have documented the occurrence of several reactive iron minerals that can facilitate the abiotic natural attenuation (ANA) of chlorinated organic compounds in environmental media under certain geochemical conditions. A recent study by others has also documented the ANA of hexavalent chromium via reduction reactions. Petrographic evaluation and geochemical modeling can be used to characterize reactive iron minerals and local geochemical conditions as part of an evaluation of the potential of a site’s geology to facilitate the in-situ degradation of these contaminates. Reactive iron minerals identified in the literature to date include a small group of iron sulfide, iron oxyhydroxide, phyllosilicate, iron phosphate, or iron carbonate minerals. Samples of selected bedrock formations were collected and petrographically evaluated using polarized light microscopy, x-ray powder diffraction, and scanning electron microscopy/energy dispersive spectroscopy. Elemental mapping was performed on two samples via electron probe microanalysis using wavelength dispersive spectroscopy to further evaluate and illustrate elemental and associated mineral distributions. Our findings document the occurrence of previously identified reactive iron minerals and importantly also document the occurrence of additional minerals that are also likely to function as reactive iron minerals. A chemical hydrogeology case study involving an extensive plume of chlorinated organic compounds in bedrock groundwater is used to illustrate an application of petrographic evaluation and geochemical modeling, including the development and refinement of a hydrogeologic conceptual site model which illustrates upgradient, source area, and downgradient geochemical conditions and trends.