Controls on tungsten concentrations in groundwater flow systems: The role of adsorption, aquifer sediment Fe(III) oxide/oxyhydroxide content, and thiotungstate formation

Groundwater samples were collected along flow paths within the Carrizo Sand aquifer (southeastern Texas) and the Aquia aquifer (coastal Maryland) for analysis of dissolved tungsten (W) concentrations, along with the major solutes, pH, and measures of in situ redox conditions [e.g., dissolved Fe(II), Fe(III), and S(-II) concentrations]. In addition, sediment samples were collected from both aquifers to evaluate the solid-phase speciation of W. Tungsten concentrations in the Carrizo Sand aquifer range from 3.64 to 1297pmolkg⁻¹ (mean±SD=248±440pmolkg⁻¹), with the lowest concentrations reported from the recharge area. Tungsten concentrations progressively increase down gradient along the flow path within Carrizo Sand aquifer groundwaters, reaching the highest levels in sulfidic groundwater roughly 50–60km from the recharge zone. Tungsten is strongly correlated with pH and dissolved sulfide [i.e., S(-II)] concentrations in Carrizo groundwaters (r=0.78 and 0.95, respectively). In Aquia aquifer groundwaters W concentrations range between 14.3 and 184.4pmolkg⁻¹ (mean±SD=63.9±59pmolkg⁻¹), and exhibit no, or at best, weak relationships with other geochemical parameters measured along the flow path (e.g., r=0.4 and 0.08 for W vs. pH and S(-II), respectively). The data suggest that the increase in W concentrations in Carrizo groundwaters reflects, in part, pH-related adsorption/desorption, which has been shown to be substantial for groundwaters and lake waters with pH>8. Owing to the similar chemical properties of W and Mo, which form thiomolybdates in sulfidic waters, the formation of thiotungstate complexes may also be important in the sulfidic waters of the Carrizo aquifer. The substantially lower W concentrations in Aquia aquifer groundwaters reflect the fact that the maximum groundwater pH never exceeds 8.4 (mean±SD pH=7.97±0.23), dissolved low S(-II) concentrations remain low (mean±SD S(-II)=0.29±0.16μmolkg⁻¹), and that Aquia aquifer sediments have substantially higher Fe(III) oxide/oxyhydroxide contents compared to the Carrizo Sand aquifer. Our data indicate that pH-related adsorption/desorption reactions and the Fe(III) oxide/oxyhydroxide content of aquifer sediments are key controls affecting W concentrations in oxic, suboxic, and anoxic groundwaters, and, further, that the formation of thiotungstate species may also be important in some anoxic (i.e., sulfidic) waters.