The concentrations of As in surface- and up to 90 degreesC ground waters in a tholeiite flood basalt area in N-Iceland lie in the range < 0.03-10 mug/kg. With few exceptions surface waters contain < 0.5 mug/kg As whereas ground waters generally contain > 0.5 mug/kg As. The As content of ground waters increases on the whole with rising temperature. Arsenic is highly mobile in the basalt-water environment of the study area. An insignificant fraction of the As dissolved from the rock is taken up into secondary minerals. Arsenic is less mobile than B but considerably more mobile than Na which has the highest mobility among the major aqueous components. A significant fraction of the As in the basalt occurs in an easily soluble form. The As hosted in the primary minerals is expected to be concentrated in the titano-magnetite. This mineral is stable in contact with both surface- and ground waters and does not, therefore, supply As to the water, explaining the difference in mobility between As and B. Aqueous As concentrations are a reflection of water/rock ratios, i.e. how much rock a given quantity of water has dissolved. This ratio increases with increasing temperature and increasing residence time of the water in contact with the rock. The distribution of As species has been calculated on the assumption of equilibrium at the redox potential retrieved from measurement of aqueous Fe(II) and Fe(III) concentrations. These calculations indicate that pentavalent As is stable in surface waters and in ground waters with an in situ pH of < 10 and would occur mostly as H2AsO4- and HAsO4-2. In higher pH ground waters the concentrations of the arsenite species H2AsO3- is significant at equilibrium, up to 65% of the total dissolved As. (C) 2003 Elsevier Science Ltd. All rights reserved.
