Complexity of Arsenic Biogeochemistry in Surface Water Systems as Influenced by a Hydrologic Event

The arsenic cycle in oxic, surface water environments is often controlled by oxy-hydroxide minerals through sorption/desorption and precipitation/dissolution reactions. However, there are numerous instances where these minerals are found in low concentrations and/or are minimally reactive with respect to aqueous arsenic species. The presence of other anions may competitively inhibit arsenic sorption to oxy-hydroxide surfaces, thus increasing the bioavailability of arsenic and the potential toxic impacts. Microbe-mediated reactions can further impact arsenic fate and transport through accumulation and biotransformation. Arsenic biotransformation via reduction and/or methylation may result in an increased proportion of thermodynamically unfavorable arsenic species such as arsenite and methylated arsenicals in oxic surface waters. The reduced arsenic species, arsenite, is considered more mobile and toxic than the oxic species, arsenate while methylated arsenicals are often considered less toxic species. The complexity of these biogeochemical characteristics highlights the importance of studying arsenic in surface water environments. Particulate and aqueous phase metals (Fe, Mn, Al) and anions (As, P, S) were measured in surface water samples collected from the outflow creek of an arsenic-contaminated lake at high and low flow rates. Arsenic speciation, quantified via HPLC-ICP-MS, was dominated by methylated arsenicals at concentrations up to 82.7 μg/l. The common oxide-forming elements, Fe, Mn and Al were measured via ICP-AES at concentrations up to 2.4 mg/l, 0.88 mg/l and 3.3 mg/l, respectively. However, arsenic was not associated with the particulate phase mineralogy, being approximately 100% in the aqueous (< 0.2 μm ) phase, indicating high arsenic bioavailability. High alkalinity, phosphorous and sulfur concentrations up to 516 mgHCO3/meq, 2.0mg/l and 50 mg/l, respectively, likely out-competed arsenic for sorption to these oxide mineral surfaces. Geochemical modeling further investigates the impact of these competing anions on arsenic fate and transport.