Multiple flow dynamics in karst aquifers following recharge events highlighted by dissolved gases – case of the Lez karstic system (South of France)
Résumé
Karstic aquifers are important reservoirs for water supply but
their heterogeneous structure and very different flow velocities
complicate their management. Geochemical tracers are various
and give information of water origin and/or residence time.
Amongst geochemical tracers, gas tracers are interesting since
their solubility depends on physical conditions (temperature,
pressure, salinity). They give information about recharge
conditions (Recharge temperature and Excess Air) and physical
processes as matrix/conduits flux exchanges.
The present work assesses the benefits of gaseous tracers to
characterise the recharge dynamics in the Lez Karst aquifer,
which is a complex system subjected to an active management
for the water supply and where multiple flow paths mix.
Noble gases (Ne, Ar) and N2 were measured during autumnal
recharge events of 4 hydrological cycles at a quasi-daily time-
step from 2012 to 2016 at the spring outlet of the Lez system. In
addition, a high frequency (intra-day time step) monitoring by in-
situ mass spectrometry was conducted at the Lez spring during
the 2019 autumnal recharge event. At the same time, Excess Air
was measured in several boreholes representative of capacitive
and transmissive karst compartments on the Terrieu experimental
site, located in the main aquifer feeding the spring.
A peak of He was observed at the spring one day after the
autumnal recharge event, indicating a deep flow contribution by
piston effect. Between three and five days after, the Noble Gas
Temperature increases of 5°C, associated with an increase of
222Rn and of the natural organic matter fluorescence. These
tracers indicate a contribution of fast infiltrated water from the
surface under warmer recharge conditions compared to the water
flowing at the spring during baseflow. Excess air measured
before and after the recharge show opposite trends between
boreholes intersecting only matrix and those intersecting
conduits, 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)