The aquatic geochemistry of arsenic in volcanic groundwaters from southern Italy
Introduction
The diffuse occurrence of As in natural waters, its relevant geochemical mobility in aqueous systems and its toxicity to human beings when consumed in enhanced doses, make this element one of the most problematic challenges of present water research (National Research Council, 1999). Contrary to other toxic species, As pollution in groundwater is mostly natural in origin. Most As-related drinking water problems covered up to now are associated with aquifers hosted in recent alluvial formations, where As accumulation develops due to either reductive dissolution of As-rich sediments or As leaching and transport by alkaline oxidising groundwaters circulating in arid regions (Smedley et al., 2000, Smedley and Kinniburgh, 2002).
Volcanic degassing also represents an important natural source of As to shallow aqueous systems. Arsenic is a minor but recurrent constituent of volcanic gases (Symonds et al., 1987, Symonds et al., 1992, Zreda-Gostynska et al., 1997) and geothermal fluids (Ballantyne and Moore, 1988). It is also widely found in epithermal sulphide ore deposits, either as a major constituent (arsenopyrite, orpiment, realgar) or as a minor element in pyrite (Heinrich and Eadington, 1986, Ballantyne and Moore, 1988). As a result of the interaction with deep-rising fluids or leaching of ore deposits, groundwaters circulating in active volcanic areas may contain appreciable amounts of As (Stauffer and Thompson, 1984, Welch et al., 1988, Yokoyama et al., 1993, Nimick et al., 1998, Aiuppa et al., 2001).
Studying the concentrations and the aqueous mobility of As in volcanic groundwaters is of interest both for theoretical and practical reasons. First, volcanic aquifers are characterised by extreme heterogeneity of major ion composition, TDS, pH and redox potential, due to the sometimes exceptionally aggressive chemical environment engendered by the dissolution of reactive magma-derived volatiles and the consequent intense rock leaching. Therefore, the study of volcanic aquifers provides insights into the As cycling along various stages of the hydrological cycle, from the infiltration of oxygenated meteoric waters in permeable volcanic layers to the interaction with rocks and ascending volcanic volatiles in a reduced and acid environment to the eventual discharge to the surface. During the flow path, As may become enriched in groundwater by rock leaching or depleted by precipitation, ion exchange or surface complexation. Generally, As is present in solution at concentrations lower than those required to achieve saturation of authigenic solid phases. In shallow environments, As can be adsorbed onto the surface of Fe oxy-hydroxides (Wilkie and Hering, 1996, and references quoted therein) or can form weathering products such as scorodite (FeAsO4.2H2O) (Dove and Rimstidt, 1985).
On the other hand, it should be considered that volcanic aquifers often constitute an important water resource for people living close by. For instance, the Etnean aquifer represents the main water supply for drinking and agricultural purposes for about 106 people living in eastern Sicily, Italy.
This study aims to highlight the main geochemical processes that control the abundance and fate of As during gas–water–rock interaction processes occurring in volcanic aquifers from southern Italy (Fig. 1), one of the most active and studied volcanic areas in the world. Chemical thermodynamic modelling is used based on the available literature data on the composition of Italian volcanic aquifers, in order to get insights into the mineral–solution equilibria controlling As distribution. The authors also make use of new data to compare the theoretical equilibrium speciation state of As with experimentally determined As(III) to As(V) relative abundances, in order to better assess the reliability of equilibrium computation.
Section snippets
Methods
This paper is based on a review of the published data on As content in groundwater from Etna, Vesuvius, Vulcano Island, Stromboli and the Phlegrean Fields (Table 1). The data are mostly part of a hydrogeochemical survey of volcanic areas undertaken by Dipartimento Chimica e Fisica della Terra (CFTA) and Istituto Nazionale Geofisica e Vulcanologia, Palermo. Fifty-five water samples are from the Somma-Vesuvius volcanic complex (Federico, 1999), 53 from Etna (Aiuppa et al., 2000a) and 24 from
Hydrogeochemical background
All the studied sites are volcanically active and have experienced eruptions in historical times. Etna and Stromboli currently show a persistent open-conduit state of activity, and the last effusive phases occurred in 2001 and 1985, respectively. The remaining volcanoes, Vesuvius, Vulcano and Phlegrean Fields, are presently characterised by intense hydrothermal activity, although they have been active in the recent past (Vesuvius in 1944, Vulcano Island in 1888–1990 and Phlegrean Fields in
The abundance of As in groundwaters
The analysis of available literature data, summarised in both Table 1 and the cumulative frequency distribution diagram of Fig. 3, indicates that As concentrations in groundwaters from the areas in this study are characterised by a wide variability, with a 0.1–6940 μg/l range for the whole data set. Even within each study area, the cumulative distribution curves span over 2–4 log-cycles on the x-scale, and the presence of several inflection points on the curves is a clear indication of the
General discussion
The wide range of As contents in the studied volcanic aquifers requires the involvement of different mineral–solution interactions, which may include precipitation–dissolution and adsorption–desorption processes. In the choice of the most relevant gas–water–rock interaction process, the main constraints to be fit are the observed correlations of As with groundwater temperature, Cl− and S contents (Fig. 4). The dependence of As concentration on groundwater redox Fe and iron content (Fig. 4, Fig.
Conclusions
This study summarises the As data obtained from volcanic groundwater in southern Italy and outlines the controls on As mobility and speciation.
High As concentrations (up to about 7000 μg/l) are characteristic of groundwaters of Vulcano Island and the Phlegrean Fields. These waters are high-T Cl-rich waters. Temperature exerts a major control on As mobilisation enhancing mineral solubility during water–rock interaction. Arsenic–SO4 positive correlation for these waters suggests the control of
Acknowledgements
The authors wish to thank the staff of British Geological Survey for providing As determinations and INGV-PA for major ion determinations. Thanks are due to R. Cidu for editorial handling and to Y. Kharaka and J. Cleverley for useful comments and suggestions.
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2021, Science of the Total EnvironmentCitation Excerpt :Thus, their impact is predictably limited, consistent with Somma et al.'s (2021) PCA results. The Agnano underground waters originate from NaCl brines with high As concentrations (up to 6900 μg·l−1, Aiuppa et al., 2003). The presence of arsenic is due to prolonged water–rock interactions at reservoir T, fO2 and fH2S conditions, and buffering action of an arsenopyrite + pyrite + pyrrhotite rock assemblage (Aiuppa et al., 2003; Barbieri et al., 2018).