Geochemistry of Tungsten In Groundwater Flow Systems

Groundwater samples were collected along groundwater flow paths within the Carrizo Sand aquifer in Texas and the Aquia aquifer in Maryland for analysis of dissolved tungsten (W) concentrations. In addition, at each sampling site we measured the pH, specific conductance, temperature, alkalinity, dissolved oxygen (DO), oxidation-reduction potential (Eh), iron speciation ([Fe (II); Fe(III)]), and dissolved sulfide [S(-II)] concentrations for these groundwaters. Tungsten concentrations in the Carrizo Sand aquifer range from 3.64 to 1297 pmol/kg with the lowest concentrations reported from the recharge area. Tungsten concentrations progressively increase along the flow path, reaching the highest levels in sulfidic groundwater roughly 50 – 60 km downgradient. Tungsten is strongly correlated with S(-II) concentrations and pH in Carrizo groundwaters (r = 0.95 and 0.78, respectively). Conversely, although W concentrations alternate between 14.31 and 184.41 pmol/kg (mean = 80.41 pmol/kg) in Aquia groundwaters, there are no systematic trends in W concentrations with other measured parameters along the flow path. We suggest that the increase in W concentrations in Carrizo groundwaters reflects, in part, pH-related desorption, which has been shown to be substantial for pH > 8. Furthermore, owing to the similar chemical properties of W and Mo, which form thiomolybdates in sulfidic waters, we postulate that formation of thiotungstate complexes may partially explain the elevated W concentrations in the sulfidic waters of the Carrizo aquifer. The substantially lower W concentrations in Aquia aquifer groundwaters may instead reflect the suboxic nature of these groundwaters that have not undergone appreciable sulfate reduction. The evolution of W concentrations in the Aquia aquifer is consistent with conservative behavior in these generally oxic to suboxic groundwaters. Therefore, our data indicate that pH related adsorption/desorption reactions are the key factors controlling W concentrations in oxic, and sub-oxic waters, whereas formation of thiothungstate complexes are likely important in sulfidic/anoxic waters.