Use of trace elements to distinguish flows from different types of carbonated rocks

Chemistry of groundwater is heavily affected by water-rocks interactions. Major elements in groundwater provide valuable indications on main aquifer rock types but requires strong lithological contrasts. Carbonate rocks differ because of deposit conditions and diagenesis and it bears significant implications for groundwater flow. It is established that trace elements in carbonates depend on deposit environment and diagenesis (Gholami Zadeh et al., 2019; Hood et al., 2018; Li et al., 2020). In waters from carbonate aquifers, variation of trace elements contents is observed at different spatial and temporal scales (Dematteis, 1995; Morgantini et al., 2010; Schürch et al., 2004). While some of this variability is due to recharge characteristics, in the absence of anthropic contamination the main process affecting trace elements contents is water-rock interactions. This leads to the potential for trace elements to be a natural tracer of limestone rock types.

This work aims to develop a natural tracer of limestone rock types that could distinguish the fingerprints of the different sub-reservoirs of carbonate aquifers.

To this purpose, we studied the Fontaine de Vaucluse karst system (southern France). It has a large recharge area (~ 1160km²; Ollivier, 2020) composed of Upper Barremian limestones of inner/outer platform, slope and basinal environnements (Masse and Fenerci-Masse, 2011; Léonide et al., 2012). We analysed major and trace elements of a set of 107 rock samples representative of the complete lithologie, facies and rock type properties of these carbonates. We also performed a one-year sampling campaign on 17 flow points: 12 in unsaturated zone from the LSBB gallery (lsbb.cnrs.fr), 3 springs, 1 stream losses, and at the main outlet of the karst system. In addition to trace elements we also monitored major elements, stables isotopes, TOC and fluorescence of natural organic matter.

A hierarchical clustering was performed with trace element contents of rock samples. It splits data into four groups according to facies description (inner/outer platforms, slope, basin). In water, first results show temporal and spatial variation of trace elements of Fontaine de Vaucluse system.

These promising results hint that trace element seem to be relevant natural tracers of the path taken by water in different limestones.