A field and reactive transport model study of arsenic in a basaltic rock aquifer

The use of geothermal energy as a source for electricity and district heating has increased over recent decades. Dissolved As can be an important constituent of the geothermal fluids brought to the Earth’s surface. Here the field application of laboratory measured adsorption coefficients of aqueous As species on basaltic glass surfaces is discussed. The mobility of As species in the basaltic aquifer in the Nesjavellir geothermal system, Iceland was modelled by the one-dimensional (1D) reactive transport model PHREEQC ver. 2, constrained by a long time series of field measurements with the chemical composition of geothermal effluent fluids, pH, Eh and, occasionally, Fe- and As-dissolved species measurements. Di-, tri- and tetrathioarsenic species ( As ( OH ) S 2 2 - , AsS3H2−, AsS 3 3 - and As ( SH ) 4 - ) were the dominant form of dissolved As in geothermal waters exiting the power plant (2.556 μM total As) but converted to some extent to arsenite (H3AsO3) and arsenate HAsO 4 2 - oxyanions coinciding with rapid oxidation of S 2 - to S 2 O 3 2 - and finally to SO 4 2 - during surface runoff before feeding into a basaltic lava field with a total As concentration of 0.882 μM following dilution with other surface waters. inuous 25-a data set monitoring groundwater chemistry along a cross section of warm springs on the Lake Thingvallavatn shoreline allowed calibration of the 1D model. Furthermore, a series of ground water wells located in the basaltic lava field, provided access along the line of flow of the geothermal effluent waters towards the lake. The conservative ion Cl− moved through the basaltic lava field (4100 m) in less than10 a but As was retarded considerably due to surface reactions and has entered a groundwater well 850 m down the flow path as arsenate in accordance to the prediction of the 1D model. The 1D model predicted a complete breakthrough of arsenate in the year 2100. In a reduced system arsenite should be retained for about 1 ka.