Multiple flow dynamics in karst aquifers following recharge events highlighted by dissolved gases – case of the Lez karstic system (South of France)

International audienceKarstic aquifers are important reservoirs for water supply buttheir heterogeneous structure and very different flow velocitiescomplicate their management. Geochemical tracers are variousand give information of water origin and/or residence time.Amongst geochemical tracers, gas tracers are interesting sincetheir solubility depends on physical conditions (temperature,pressure, salinity). They give information about rechargeconditions (Recharge temperature and Excess Air) and physicalprocesses as matrix/conduits flux exchanges.The present work assesses the benefits of gaseous tracers tocharacterise the recharge dynamics in the Lez Karst aquifer,which is a complex system subjected to an active managementfor the water supply and where multiple flow paths mix.Noble gases (Ne, Ar) and N2 were measured during autumnalrecharge events of 4 hydrological cycles at a quasi-daily time-step from 2012 to 2016 at the spring outlet of the Lez system. Inaddition, a high frequency (intra-day time step) monitoring by in-situ mass spectrometry was conducted at the Lez spring duringthe 2019 autumnal recharge event. At the same time, Excess Airwas measured in several boreholes representative of capacitiveand transmissive karst compartments on the Terrieu experimentalsite, located in the main aquifer feeding the spring.A peak of He was observed at the spring one day after theautumnal recharge event, indicating a deep flow contribution bypiston effect. Between three and five days after, the Noble GasTemperature increases of 5°C, associated with an increase of222Rn and of the natural organic matter fluorescence. Thesetracers indicate a contribution of fast infiltrated water from thesurface under warmer recharge conditions compared to the waterflowing at the spring during baseflow. Excess air measuredbefore and after the recharge show opposite trends betweenboreholes intersecting only matrix and those intersectingconduits, highlighting matrix/conduits flux exchanges.These results show the interest of gaseous tracers to discriminate flow origins and recharge dynamic during flood event in the karstic spring both at a local scale (matrix vs conduits) and at the catchment scale (deep flow, main aquifer and fast flow from surficial infiltration