Impact of the unsaturated zone in the hydrochemical and hydrodynamical functioning of karst aquifer: example of the Tlemcen Mountains, Northwest Algeria

Abstract

The aim of this work is to understand the role of unsaturated zone on the functioning of karst aquifers. The easy infiltration of water into the system explains the expansion of karstification through the saturated zone. The unsaturated zone is largely involved in the flow on the overall karstic system. For this purpose, we use the environmental tracing, namely the calco-carbonic equilibrium elements (CO2, CO32−, HCO3−, Ca2+, Mg2+), TOC, and isotopic elements (14C, 13C, and 3H) interesting tracers of residence time. Result obtained from TOC and Mg2+ enabled us to classify the behavior of systems to fissured type with a low average and restricted fluctuation of TOC concentrations and relatively high Mg2+ (inertial system). The second functioning type is characterized by a very high average and important variation of TOC with low contents of Mg2+ (perched system with fast infiltration). The values of 3H and 14C allow to distinguish three categories of water (old, mixture actual-old, and actual) with the predominance of actual waters. The comparison of the measured δ13C with those calculated shows that the majority of the karstic aquifers of Tlemcen Mountains are open systems with respect to the soil biogenic CO2. The piston flow in the unsaturated zone puts in charge the drains and stops drainage in the lower part of the system. The direct participation of infiltration is relatively low for all systems.