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

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.