Processes releasing arsenic to groundwater in the Caldes de Malavella geothermal area, NE Spain.
Arsenic concentrations exceeding the World Health Organization drinking water guideline (10 μg/L) have been measured in thermal and non-thermal groundwaters from the Caldes de Malavella geothermal area (La Selva graben, NE Spain). The CO(2)-rich Na-HCO(3) thermal waters (up to 60 °C at the spring) have elevated arsenic concentrations ([As(T)] from 50 to 80 μg/L). The non-thermal waters are of Ca-Na-HCO(3)-Cl type and have [As(T)] between <1 and 200 μg/L, defining a hot-spot distribution. The present-day contribution of As from CO(2)-rich thermal waters to non-thermal aquifers is very limited, as shown by the concentration of geothermal tracers such as Li and B. Redox-controlling processes appear to govern the mobility of As in the non-thermal waters. Arsenate is clearly predominant in most oxidizing groundwaters (>85% of As(V) over total As), whereas reducing, high-As groundwater reaches up to 100% in arsenite. The reductive dissolution of Fe(III) oxyhydroxides and the coupled release and reduction of adsorbed As explain the elevated dissolved arsenite (up to 190 μg/L) and Fe (up to 14 mg/L) content in the more reducing non-thermal groundwater. Conversely, the high levels of nitrate (up to 136 mg/L) ensure an oxidizing environment in most non-thermal groundwaters ([As(T)] between <1 and 60 μg/L). Under these conditions, Fe(III) oxyhydroxides are stable and As release to groundwater is not related to their dissolution. Instead, dissolved arsenate concentrations up to 60 μg/L are explained by a competition for sorption sites with other species, mainly bicarbonate and silicic acid, while arsenate desorption due to pH increase is not considered a major process.
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Angels
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Departament de Cristal·lografia, Mineralogia i Dipòsits Minerals, Facultat de Geologia, Universitat de Barcelona, c. Martí i Franquès s/n, 08028 Barcelona, Spain. angels.pique@amphos21.com
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Canals
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D001151:Arsenic / Q000032:analysis*; D001639:Bicarbonates / Q000737:chemistry; D002244:Carbon / Q000032:analysis; D002851:Chromatography, High Pressure Liquid; D005618:Fresh Water / Q000737:chemistry*; D045482:Hot Springs / Q000737:chemistry*; D058085:Iron Compounds / Q000032:analysis; D013058:Mass Spectrometry; D009566:Nitrates / Q000032:analysis; D010084:Oxidation-Reduction; D012824:Silicic Acid / Q000737:chemistry; D013030:Spain; D013696:Temperature; D014874:Water Pollutants, Chemical / Q000032:analysis*
D001639:Bicarbonates; D058085:Iron Compounds; D009566:Nitrates; D014874:Water Pollutants, Chemical; D012824:Silicic Acid; D002244:Carbon; D001151:Arsenic