Distribution and mobility of geogenic molybdenum and arsenic in a limestone aquifer matrix

To investigate the potential of Mo and As as possible geogenic contaminants, three sediment cores were examined to evaluate their mineralogical association, distribution and mobility. The cores were described and analyzed for total organic carbon (TOC), Ca, Mg, Si, Al, P, Sr, As, Mo, Fe, and S content. Except in the uppermost segment, limestone was the main lithology with the occasional presence of dolomite and clay. That change in lithology was also observed in the bulk chemical composition, where Ca, Mg and Sr concentrations increased with depth, while Si, Al and P concentrations decreased with depth. Minor minerals included pyrite (FeS2), powellite (CaMoO4) and ferrihydrite. The minimum, maximum, median and standard deviations for all analyzed elements, including As and Mo were comparable for all three cores. Molybdenum and As, however, varied significantly with depth and median As and Mo values were above their respective crustal averages of approximately 1.1 mg/kg and 1.5 mg/kg. The median values for As were 1.9 mg/kg in core DEP-1, 3.3 mg/kg in DEP-2 and 1 mg/kg in DEP-5. The median values for Mo were 2.3 mg/kg in core DEP-1, 2.5 mg/kg in DEP-2 and 2.5 mg/kg in DEP-5. Maximum concentrations for As were 101.9 mg/kg, 47.5 mg/kg and 56.2 mg/kg in cores DEP-1, DEP-2 and DEP-5, respectively. Maximum concentrations for Mo were 880 mg/kg, 123 mg/kg and 225 mg/kg in cores DEP-1, DEP-2 and DEP-5, respectively. Electron microprobe analyses of individual minerals revealed variable concentrations of As ranging from approximately 300 to 9000 mg/kg, in pyrite and up to 17,600 mg/kg in powellite (CaMoO4). The Mo concentration in pyrite was consistently below the detection limit of approximately 100 mg/kg. In powellite the Mo concentration was up to 42 wt%.

A subset of 10 samples from different stratigraphic sections and with different As and Mo concentrations was further investigated to assess As and Mo mobility under changing physicochemical conditions. Leaching the aquifer matrix with a 1 M NaOAc solution at a pH of 8.1 removed more than 70% Mo in 8 of the 10 samples. The maximum value was 97%. In contrast to Mo, As was mobilized to a lesser degree. In 8 of the samples less than 30% were removed and the maximum was only 50%. Molybdenum, which seemed to be loosely bound to mineral and organic matter surfaces thus could easily be removed from the aquifer matrix, while As on the other hand should be much less mobile, because it occurred either tightly adsorbed by hydrous ferric oxide or as an impurity in pyrite. Thus, it is advisable to include Mo in the analytical program whenever elevated As concentrations are encountered in groundwater.