Model-based interpretation of hydrogeochemistry and arsenic mobility in a low-enthalpy hydrothermal system

Abstract

Low-enthalpy geothermal systems are widely spread around the world. Their exploitation for geothermal energy requires an accurate knowledge of the system. Detailed hydrogeochemical characterization is of pivotal importance since geothermal systems are often associated to the occurrence of toxic gases and elements which may contaminate any connected water resources. In this study we demonstrate how increased knowledge of a low-enthalpy geothermal system (Cimino-Vico, Central Italy) can be acquired from the analysis and interpretation of major chemistry and 87Sr/86Sr of local spring waters and groundwaters. With a model-based approach, we assess the main processes governing the major ion composition and arsenic (As) mobility in the system. The occurrence of high concentration of arsenic in the groundwater of the study area is a severe problem for the inhabitants that use the resource for domestic purposes. The system's hydrogeology consists of a shallow highly permeable aquifer, composed of alkaline-potassic volcanic rocks and characterized by fresh waters, a semi-confining layer at the base of the fresh water aquifer, and a deeper thermal aquifer in Mesozoic carbonates and Triassic evaporites. Upwelling of hot waters (up to 63 °C) to the shallow aquifer is related to the presence of faults and fractures in the semi-confining layer. The major chemistry of the deep thermal waters was found to be controlled by dedolomitization, while the fresh waters chemistry is governed by the interaction with the volcanic rocks and the mixing with the upwelling CO2-rich thermal waters. The outcomes of geochemical modeling are consistent with a conceptual model positing that arsenic bound to iron-(hydr)oxides becomes mobilized in the shallow, volcanic aquifer when thermal waters ascend into such shallow aquifer, where they promote the desorption of arsenic in favor of bicarbonate.