Wallis, I. and T. Pichler.
Science of the Total Environment 631-632:723-732(2018)

Following installation of 20 monitoring wells in close geographic proximity in central Florida, opposing concentration trends for As and Mo were observed. In the first yr after well installation, As and Mo concentrations increased in some wells by a factor of 2 and in others decreased by a factor of up to 100. Given this relatively short period of time, a natural change in groundwater composition of such magnitude was not expected, leaving well installation itself as the likely cause for the observed concentration changes. Generally, As and Mo were present in the aquifer matrix in either pyrite or organic matter, both of which are susceptible to dissolution if redox conditions change due to the addition of oxygen. Thus, introduction of an oxidant into an anoxic aquifer through use of an oxygen-saturated drilling fluid served as the conceptual model for the trends where concentrations decreased with time. Mixing between drilling fluid and groundwater (i.e., dilution) was used as the conceptual model for scenarios where increasing trends were observed. Conceptual models were successfully tested through formulation and application of data-driven reactive transport models using the USGS code MODFLOW in conjunction with the reactive multicomponent transport code PHT3D.