Evolution of selenium concentrations and speciation in groundwater flow systems: Upper Floridan (Florida) and Carrizo Sand (Texas) aquifers

Selenium concentrations and speciation were determined in groundwater samples collected from wells along flow paths in the Upper Floridan (Florida) and the Carrizo Sand (Texas) aquifers. Selenium concentrations in groundwaters from both aquifers are low (i.e., nanomolal concentrations) and vary both in magnitude and species [i.e., Se(VI), Se(IV), Se(-II)] distributions along the flow paths. Redox sensitive parameters (i.e., Fe(III), Fe(II), S(-II) dissolved oxygen, Eh) are consistent with oxic and suboxic conditions in and proximal to the recharge zones where Fe(III) reduction buffers the redox state, followed by anoxic/sulfidic conditions further down-gradient along the flow paths where sulfate reduction predominates. Evolving redox conditions, in conjunction with changes in groundwater pH along the flow paths exert important controls on the distribution of Se species in groundwaters from both aquifers. Within the Upper Floridan aquifer, for example, selenate [Se(VI)] and selenite [Se(IV)] predominate in oxic/suboxic groundwaters from near the recharge zone, whereas beyond 32 km, Se(IV) and organic selenide [i.e., Se(-II)] are the chief dissolved forms. The change in Se speciation near the recharge zone coincides with the apparent initiation of Fe(III) reduction. Further down-gradient along the flow path, where sulfate reduction buffers the redox conditions, Se(-II) becomes locally important. For the Carrizo Sand aquifer, Se(IV) dominates in groundwater from near the recharge zone, where Se(VI) exhibits variable concentrations and Se(-II) is generally low and relatively constant. Redox conditions of groundwater from the recharge zone (i.e., 0 to ∼ 8 km) vary between oxic and suboxic. However, with flow beyond ∼ 8 km, the aquifer becomes confined and dissolved Fe(II) concentrations increase dramatically, suggesting the importance of Fe(III) reduction. In the zone of Fe(III) reduction, Se(IV) is the dominant dissolved Se species, and its coincident rise in concentration with the increasing Fe(II) concentrations suggests Se(IV) release to groundwaters via reductive dissolution of Fe(III) oxides/oxyhydroxides. With flow beyond 41 km, sulfate reduction predominates and pH exceeds 8. These observations suggest that pH induced desorption of Se(VI) from aquifer mineral surface sites may be important, however, mixing of groundwaters from multiple flow paths/line and/or compositional heterogeneity of aquifer sediments may also contribute. Overall, the data are consistent with reductive dissolution of Fe(III) oxides/oxyhydroxides, adsorption/desorption reactions, and mixing of multiple flow lines as exerting important controls on Se concentrations and speciation in groundwater flow systems.