Hydrothermal gases in a shallow aquifer at Mt. Amiata, Italy: insights from stable isotopes and geochemical modelling^†

† Originally presented at the IAEA International Symposium on Isotope Hydrology: Revisiting Foundations and Exploring Frontiers, 11–15 May 2015, Vienna, Austria.

ABSTRACT

We investigate the interaction between hydrothermal gases and groundwater in a major aquifer exploited for potable supply in the geothermal-volcanic area of Mt. Amiata, Central Italy. Two springs and two wells located on different sides of the volcanic edifice have been repeatedly sampled over the last 11 years. More than 160 chemical analyses and 10 isotopic analyses of total dissolved carbon (δ¹³C – total dissolved inorganic carbon (TDIC) = −15.9 to −7.8 ‰ vs. V-PDB) and sulphate (δ³⁴S-SO₄ = −6.9 to 5.1 ‰ vs. V-CDT) have been processed with geochemical modelling techniques. Best-fitting conditions between analytical data and model outputs have been achieved by numerical optimization, allowing for a quantitative description of gas–water–rock interactions occurring in this aquifer. Numerical calculations support a conceptual model that considers water–rock interactions to occur in the volcanic aquifer after inflow of deep-seated gases (CO_2(g) and H₂S_(g)), and total conversion of H₂S_(g) to SO₄, in the absence of mixing with geothermal waters from reservoirs currently exploited for electricity generation.

KEYWORDS:

• Carbon-13
• sulphur-34
• gas–water–rock interaction
• shallow aquifer
• Mt.Amiata volcano
• Italy

Acknowledgements

The authors thank two anonymous reviewers for their comments.

Disclosure statement

No potential conflict of interest was reported by the authors.

Notes

† Originally presented at the IAEA International Symposium on Isotope Hydrology: Revisiting Foundations and Exploring Frontiers, 11–15 May 2015, Vienna, Austria.

Additional information

Funding

This research was partially supported by a grant to Lisa Pierotti from the Seismic Prevention Service of the Tuscany Region.