The distribution of arsenic (III) and arsenic (V) in geothermal waters: Examples from the Massif Central of France, the Island of Dominica in the Leeward Islands of the Caribbean, the Valles Caldera of New Mexico, U.S.A., and southwest Bulgaria

Abstract

Data for As (III), As (V) and total As for various types of geothermal spring water are given. The different species of As were separated at the time of sampling after filtration, acidification and dilution where necessary, using an anion-exchange method. Total As, As (III) and As (V) were determined by flameless atomic absorption spectrometry. The total As content ranges from 5 to 1800 μg l −1. Samples from Dominica show evidence of different sulfur-arsenic oxidation kinetics, As (III) being the major component (85–99%) in fluids from deep levels of the geothermal reservoir and also (99%) in oxidized waters of the acid sulfate type. The bicarbonate spring water of the Valles Caldera, New Mexico, contains more As (III) than As (V), but there As (V) is the major component of the acid sulfate springs. As (III) proves to be the predominant form (60–100%) in the warm alkaline waters from southwest Bulgaria. A selection of CO 2-rich spring waters from the Central Massif, France (Cézallier) is analysed. Two groups are distinguished on the basis of major-element chemistry and total As content. Oxidized forms are mainly encountered in the first group, including a well sample containing 0–5% As (III). The As (III) content of the second group ranges from 10% to 70%. These preliminary results indicate that the fluids directly related to hydrothermal systems contain mainly As (III), whereas acid sulfate springs have a variable As (III) content. Bicarbonate waters are generally enriched in oxidized As. In the absence of springs or sinks within a geothermal region, the total As concentration may display conservative properties, but both As (III) and As (V) may be reactive during mixing and cooling.