Dual porosity mechanism for transient groundwater and gas anomalies induced by external forcing

The geochemical response of groundwater to external forcing is studied in a tunnel hosted in fractured gneiss near and above an artificial lake having large water level variations. Dripping waters have been monitored in two zones characterized by highly different flow rates representing different contributions of water from matrix porosity and fractures. Transient increases of sulfate and magnesium concentrations, observed in both zones, are not related to meteorology but associated with radon bursts into the tunnel air. This indicates that transient enhancement of conductance, with discharges of saline water and radon from the matrix porosity to the fractures, is a mechanism able to produce both groundwater and gas anomalies, in response to hydrogeological or mechanical processes such as increases in pore pressure or changes in crack geometry. Besides its applications to environmental issues, the proposed discharge mechanism may be relevant for the understanding of geochemical precursors of earthquakes, as an alternative to aquifer mixing.