Contribution of $$ {\text{P}}_{{{\text{CO}}_{ 2} {\text{eq}}}} $$ and 13CTDIC Evaluation to the Identification of CO2 Sources in Volcanic Groundwater Systems: Influence of Hydrometeorological Conditions and Lava Flow Morphologies—Application to the Argnat Basin (Chaîne des Puys, Massif Central, France)

Abstract

Mineralization of groundwater in volcanic aquifers is partly acquired through silicates weathering. This alteration depends on the dissolution of atmospheric, biogenic, or mantellic gaseous CO2 whose contributions may depend on substratum geology, surface features, and lava flow hydrological functionings. Investigations of $$ {\text{P}}_{{{\text{CO}}_{ 2} {\text{eq}}}} $$ and δ13CTDIC (total dissolved inorganic carbon) on various spatiotemporal scales in the unsaturated and saturated zones of volcanic flows of the Argnat basin (French Massif Central) have been carried out to identify the carbon sources in the system. Mantellic sources are related to faults promoting CO2 uplift from the mantle to the saturated zone. The contribution of this source is counterbalanced by infiltration of water through the unsaturated zone, accompanied by dissolution of soil CO2 or even atmospheric CO2 during cold periods. Monitoring and modeling of δ13CTDIC in the unsaturated zone shows that both $$ {\text{P}}_{{{\text{CO}}_{ 2} {\text{eq}}}} $$ and δ13CTDIC are controlled by air temperature which influences soil respiration and soil-atmosphere CO2 exchanges. The internal geometry of volcanic lava flows controls water patterns from the unsaturated zone to saturated zone and thus may explain δ13C heterogeneity in the saturated zone at the basin scale.