Occurrence of arsenite in surface and groundwater associated with a perennial stream located in Western Nebraska, USA

Dissolved arsenic typically results from chemical weathering of arsenic rich sediments and is most often found in oxidized forms in surface water. The mobility of arsenic is controlled by its valence state and also by its association with iron oxides minerals, the forms of which are both influenced by abiotic and biotic processes in aqueous environment. In this study, speciation methods were used to measure and confirm the presence of reduced arsenic species in the surface water of Frenchman creek, a gaining stream that crosses the Colorado- Nebraska border. Selective extraction analysis of aquifer and stream bed sediments shows that the bulk of the arsenic occurs with labile iron-rich oxy(hydroxide) minerals. Total dissolved arsenic in surface and groundwater ranged from ~3–18 μg L–1, and reduced arsenic species comprise about 41% of the total dissolved arsenic (16.0 μg L–1) in Frenchman creek. Leachable arsenic in the aquifer sediment samples ranged up to 1553 μg kg–1, while samples from Frenchman creek bed sediments contained 4218 μg kg–1. Dynamic surface and groundwater interaction sustains arsenite in iron-rich surface headwaters, and the implied toxicity of reduced arsenic in this hydrogeological setting, which can be important in surface water environments around the globe

Sediment Associated Arsenic in Stream and Aquifer Sediments

Arsenic (As) is an element commonly associated with aquifer sediment, with a global concentration 1.7 mg/kg, and can be subsequently released into groundwater. In recent times, increased anthropogenic inputs have contributed to geochemical changes in the subsurface that affect the mobilization of arsenic into subsurface water resources. Concentrations above the EPA’s maximum contaminant level (MCL) of 10 µg/L have recently been reported in many areas across the United States, including Nebraska. The health risk of prolonged exposure to high arsenic concentrations are skin ailments, like hyper pigmentation and keratosis, and cancer. Furthermore, many rural communities, especially in western Nebraska, lack the resources to treat their drinking water supplies. Recent studies have indicated As concentrations above the MCL in Wauneta, NE, a small community in western Nebraska. Wauneta is quite concurrent with other well-studied As contaminated regions but exhibits some important hydrological and geological differences including the presence of oxidizing groundwater, carbonate-rich sediments, and the influence of Frenchman Creek, which flows through the town. Sediment geochemical analyses revealed two sources of As: 1) As derived from Frenchman Creek and 2.) As sourced from the aquifer sediments. Sequential extraction experiments indicate that sediments contain as much as 4.22 mg/kg As and have the potential to contaminate the groundwater up to 99.75 µg/L. Speciation experiments also exhibit high, easily mobilized, sediment associated arsenic concentrations in Frenchman Creek, providing potential for the impact of surface-groundwater interactions on the quality of Wauneta’s drinking water. This is a significant health risk to Wauneta and others using this aquifer for drinking water