Contaminant transfer and hydrodispersiveparameters in basaltic lava flows: artificial tracertest and implications for long-term management

Abstract

Abstract The aim of this paper is to evaluate the vulnerabilityafter point source contamination and characterizewater circulations in volcanic flows located in theArgnat basin volcanic system (Chaîne des Puys, FrenchMassif Central) using a tracer test performed by injectinga iodide solution. The analysis of breakthrough curves allowedthe hydrodispersive characteristics of the massivelava flows to be determined. Large Peclet numbers indicateda dominant advective transport. The multimodal featureof breakthrough curves combined with high valuesof mean velocity and low longitudinal dispersion coefficientsindicated thatwater flows in an environment analogousto a fissure system, and only slightly interacts with alow porosity matrix (ne < 1%). Combining this informationwith lava flow stratigraphy provided by several drillingsallowed a conceptual scheme of potential contaminant behaviourto be designed. Although lava flows are vulnerableto point source pollution due to the rapid transfer of waterwithin fractures, the saturated scoriaceous layers locatedbetween massive rocks should suffice to strongly bufferthe transit of pollution through dilution and longer transittimes. This was consistent with the low recovery rate ofthe presented tracer test.