Vanadium geochemistry along groundwater flow paths in contrasting aquifers of the United States: Carrizo Sand (Texas) and Oasis Valley (Nevada) aquifers

Abstract

Vanadium (V) concentrations were measured along the flow paths in two contrasting aquifer systems to study how changing solution composition and redox conditions affect V cycling and transport in groundwater flow systems. The Oasis Valley groundwater system in Nevada is an unconfined system where groundwaters generally remain oxic along the flow path, whereas the Carrizo Sand aquifer is a deep, confined coastal plain aquifer in Texas that contains well-defined Fe(III) and SO42− reduction zones along the flow path. Speciation modeling predicts that in groundwaters from the Oasis Valley, V chiefly occurs as the vanadate oxyanion (e.g., H2VO4−, HVO42−), which is mobile along the entire flow path because of vanadate's solubility in oxic and alkaline waters. Furthermore, the data indicate that V concentrations increase in groundwaters along the flow path suggesting that mineral dissolution and/or desorption reactions progressively contribute V to groundwaters of the Oasis Valley system. Geochemical modeling is consistent with dissolution of mafic phenocrysts that are common in the local tuffaceous aquifer rocks as a probable source of V to Oasis Valley groundwaters. Within the Carrizo Sand aquifer, groundwater V concentrations decrease with flow beyond the recharge zone and into the confined portion of the aquifer. Thereafter, total V concentrations remain low and relatively constant along the remainder of the studied flow path, exhibiting no systematic variations across the Fe(III) and SO42− reduction zones. Groundwaters from the Carrizo Sand aquifer are generally reducing, and as such, geochemical modeling suggests that both V(V) and V(IV) species occur in Carrizo Sand groundwaters, and furthermore, that the proportion of V(IV) species increases concurrent with the decrease in total V concentrations along the flow path. Results from sequential extraction experiments on Carrizo Sand aquifer sediment samples suggest that V adsorption onto Fe(III)/Mn(IV) oxides/oxyhydroxides acts to decrease the aqueous V concentrations in Carrizo Sand groundwater. Furthermore, geochemical modeling is consistent with complexation of V(IV) with dissolved organic matter (DOM) within the Carrizo Sand aquifer groundwaters, which, owing to the low DOM concentrations, appears to buffer V at low, but relatively constant concentrations within the confined and reducing Carrizo Sand groundwaters. These results support previous reports of V(IV)–DOM binding and imply that aquifer material maintains an essential role in not only determining the availability of V to enter aqueous solution but also in influencing partitioning between solid and aqueous phases.