CONSERVATIVE TRANSPORT OF OXYANION-FORMING TRACE ELEMENTS IN AN OXIC GROUNDWATER FLOW SYSTEM

Groundwater samples collected along a flow path in a shallow, fractured tuffaceous aquifer from the Oasis Valley – Beatty Wash region of southern Nevada, USA, were analyzed for oxyanion-forming trace elements including arsenic (As), antimony (Sb), selenium (Se), molybdenum (Mo), and tungsten (W). We also analyzed major solutes, silica, dissolved oxygen, iron, and manganese concentrations along with pH, temperature, and conductivity of the groundwater samples. Arsenic concentrations range from ~70 nmol/kg up to 316 nmol/kg, and generally exhibit increasing concentrations with flow down-gradient along the flow path. Concentrations of Sb, W, and to a lesser extent, Mo, also increase with flow down-gradient in the aquifer, albeit, at lower concentrations levels (e.g., mean ± SD for Sb, W, and Mo are 2.3 ± 0.9 nmol/kg, 7.4 ± 3.7 nmol/kg, and 101 ± 19 nmol/kg, respectively). On the other hand, Se concentrations, which range between ~4 and 11 nmol/kg, generally decrease in groundwaters with flow down-gradient in the flow systems. Inverse modeling of groundwater chemistry evolution from the lower reaches of the flow path using PHREEQC indicate that the groundwater composition is consistent with mixing of nearly equal proportions of groundwater from upper reaches of Oasis Valley and Beatty Wash groundwater, along with dissolution of volcanic glass, potassium feldspar, and gypsum, followed by calcite precipitation, and formation of secondary zeolites (analcime), clay minerals (Ca-montmorillonite), and cristobalite. The geochemical modeling indicates that the concentrations of As and the other oxyanion-forming trace elements are controlled by dissolution of volcanic glass, water-rock interaction with mineralized zones within the aquifer (i.e., sulfide oxidation), desorption from aquifer surface sites, and mixing of Oasis Valley and Beatty Wash groundwaters.