Abstract
The circulation of groundwater is influenced by several factors, including permeability changes due to the dynamics of the stress field acting along tectonic discontinuities. Open faults can act as preferential pathways for the escape of crustal and subcrustal gases, such as CO2, towards the surface, allowing their chemical interaction with meteoric fluids. Here, we present the first map of dissolved CO2 partial pressure in the groundwater of Sicily, Italy. Based on the chemical analysis of 557 samples of groundwater, which were collected in wells and springs, we calculated the related CO2 partial pressure (PCO2) using PHREEQC software. The spatial distribution of the calculated PCO2 values highlights a general homogeneity at the regional scale, interrupted by positive anomalies linked to the main seismogenic or volcanic areas. Faults work as preferential escape pathways for deep CO2. The subsequent CO2–water–rock interaction determines the chemical composition of circulating water. As a consequence, groundwater composition can be successfully used to identify anomalous degassing areas.
Highlights
The chemical composition of groundwater is the result of a series of complex interaction processes occurring between natural waters, rocks, and gases, being strictly influenced by the atmosphere and biosphere
Data used in this work refer to physicochemical parameters and concentrations of dissolved ions taken from different literature sources, namely, the “Piano Tutela Acque Regione Siciliana” ([10,49,50], integrated with previously unreleased data for western Sicily), for a total of 618 groundwater samples
Due to its high solubility in water [64], CO2 dissolves in water and favors water–rock interactions
Summary
The chemical composition of groundwater is the result of a series of complex interaction processes occurring between natural waters, rocks, and gases, being strictly influenced by the atmosphere and biosphere. Previous studies addressed the modeling of reactions governing silica phases and aqueous solutions [2,3,4,5,6], or carbonate aquifer systems, focusing on the interactions between minerals and aqueous species, their effects on water composition, and residence times [7,8,9]. Primary minerals are dissolved and/or altered during the chemical interaction with groundwater and dissolved gases. Several processes contribute to CO2 dissolution in groundwater. It can be generated from biodegradation of organic matter in the soil. CO2 is the major gas species, after water vapor, released by volcano-magmatic processes, because it is the first gas to be exsolved from a magmatic melt due to its low solubility [11]
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