Trace metal modeling of groundwater–gas–rock interactions in a volcanic aquifer: Mount Vesuvius, Southern Italy

Abstract

We report a detailed study of trace metals in groundwaters from the Somma-Vesuvius volcanic complex and present a model of the chemical processes that control the fate of these components during gas–water–rock interactions. Trace metal concentrations in Vesuvian groundwaters range from 0.01 to 0.1 μg/l for ultra-trace elements (Sb, Cs, Co, Cd, and Pb) up to 0.1–10 mg/l for minor elements (Fe and Sr), leading to water–rock ratios from ∼0.5 to 10−9 when normalized to trace element concentrations in the host rocks. Our results indicate non-isochemical dissolution of local volcanic rocks by groundwaters, during which mobile trace elements (As, Se, Mo, V, Li) are enriched and elements such as Al, Pb, Co, and Mn are depleted in the aqueous phase compared to the pristine composition of unleached rocks. Speciation computation and mineral–solution equilibria provide insights into the processes controlling the abundance and mobility of both major and trace elements in the fluids and allow quantitative modeling of gas–water–rock interactions. This latter was done using a forward reaction path model based on the principle of irreversible reactions involving minerals and aqueous solutions (Helgeson, H.C., 1968. Evaluation of irreversible reactions in geochemical processes involving minerals and aqueous solutions: I. Thermodynamic relations. Geochim. Cosmochim. Acta, 32, 853–877), and incorporating transition-state theory to account for rates of mineral dissolution reactions (Aagaard, P., Helgeson, H.C., 1982. Thermodynamic and kinetic constraints on reaction rates among minerals and aqueous solutions, 1. Theoretical considerations. Amer. J. Sci., 282, 237–285). The EQ3NR/6 software package (Wolery, T.J., 1994. EQ3NR, Letter report: EQ3/6 version 8.0. Differences from version 7. UCRL_ID_129749, Lawrence Livermore National Laboratory, Livermore, California) was used to simulate the reaction paths and the aqueous concentrations of trace elements with increasing extent of rock weathering. Fairly good matching between the modeled and analytical groundwater compositions supports the validity of our approach and provides reliable information on the main sources and sinks of trace metals during gas–water–rock interactions in the volcanic aquifer of Vesuvius.