Reactive transport of arsenic through basaltic porous media

Abstract

This thesis studied the volcanic and geothermal source of arsenic (As) and its fate in shallow ground waters and upon entering the ocean by means of experimental and field measurements combined with geochemical modeling. Arsenic enters the atmosphere and hydrosphere from degassing magmas and during volcanic eruptions. The November 2004 eruption within the Vatnajökull Glacier, Iceland, provided an opportunity to study elemental fluxes from volcanic eruptions into the environment. According to geochemical modeling, lowering of pH due to magma gases during the eruption led to rapid tephra dissolution with corresponding change in flood water chemistry. Geochemical modeling of floodwater/seawater mixing indicated localised decrease in dissolved arsenic and sulphur due to adsorption on the suspended floodwater materials. As the floodwater was diluted the As desorbed and limited effect of the floodwater was predicted after thousand fold dilution. Laboratory experiments were carried out to generate and validate sorption coefficients for arsenite and arsenate in contact with basaltic glass at pH 3 to 10. The mobility of arsenite decreased with increasing pH. The opposite was true for arsenate, being nearly immobile at pH 3 to being highly mobile at pH 10. A 1D reactive transport model constrained by a long time series of field measurements of chemical composition of geothermal effluent fluids from a power plant was constructed. Thioarsenic species were the dominant form of dissolved As in the waters exiting the power plant but converted to some extent to arsenite and arsenate before feeding into a basaltic lava field. Chloride, moved through the basaltic lava field (4100 m) in less than 10 yrs but arsenate was retarded considerably due to surface reactions and has entered a groundwater well 850 m down the flow path in accordance to prediction by the 1D model, which further predicted a complete breakthrough of arsenate in the year 2100 while arsenite will be retained for about 1000 yrs.