Molybdenum (Mo) and Arsenic (As) are two redox sensitive ions, often associated with pyrite and organic matter in reducing aquifers. Redox conditions in the aquifer can, however, be temporarily altered through the introduction of oxygen during drilling and well construction. This can potentially lead to false positive As and Mo detections or false negatives when sampling groundwater following drilling.
Here, we present one such scenario, where strongly elevated Mo as well as As concentrations were measured following the installation of irrigation wells in the municipality of Lithia in central Florida, despite the groundwater being generally low in As and Mo (Pichler et al. 2017). In this study, we propose conceptual models, which can explain elevated As and Mo concentrations induced by the drilling process. We evaluate these conceptual models through the application of a reactive transport modelling framework which takes the mineralogical data and the geochemical time series data for the site into account. We explore the processes which can lead to false positive detections of redox sensitive species following the introduction of oxygen into anoxic aquifers due to bore drilling and construction. This includes As and Mo release following pyrite oxidation and organic matter mineralisation triggered by the drilling process. We show the time dependence of elevated concentrations and demonstrate that complexation to neo-formed hydrous ferric oxides as well as dilution due to background flow reduces concentrations over time and allow sampling of representative groundwater samples after a set time period.
Jones, G.W.and Pichler, T., 2007. ES&T, 41(3): 723-730
Pichler, T., et al. ., 2017. Applied Geochemistry, 77: 68-79.