Isotopic tracing of groundwater recharge variability

Alexandra Mattei – 2019

Due to modern increase in anthropogenic and climatic pressures on freshwater resources, groundwater resources are being more and more solicited. The long-term management of these resources is one of the major challenges facing our societies. Understanding the renewal of groundwater resources, i.e recharge, is necessary to ensure a sustainable water resources management. In order to identify the processes, seasonality and variability of recharge, this PhD project aims at understanding the mechanisms of water transport within the unsaturated zone using water stable isotopes. A new analytical protocol for the measurement of the isotopic signature of pore water by laser spectroscopy was developed. This protocol improves the accuracy of the results while reducing the necessary analysis time. It was thus possible, at the scale of the soil matrix, to demonstrate the effect of the particle size on the measured isotopic signature. Since this effect may be greater than the seasonal variability of the observed signal in precipitation, these results underline the need to take granulometric analysis into account when interpreting isotopic composition profiles. A numerical calculation code taking into account the evolution of stable isotopes of water, from precipitation to groundwater, including isotopic fractionation linked to evaporation, was developed in the framework of this project to estimate the recharge. It is based on the use, on a 1D column, of the Metis hydrogeological model developed at MINES ParisTech. Stable water isotopes were shown to be powerful tools for constraining, by inverse modeling, the values of hydrodynamic parameters needed to describe and predict the transport of water and solutes in the unsaturated zone. A realistic estimate of the recharge is thus possible from a single depth profile of both pore water isotopic composition and water content. The approach was validated on two study sites located in Quebec. Also, this work showed that the addition of a temporal monitoring, even discontinuous, of the isotopic composition of the pore water at a single depth, provides, for the inversion, as much information as a continuous monitoring of the water content at different depths and is helpful to improve the daily evolution of the recharge. The development of new methods for sampling the isotopic composition of water within the unsaturated zone was undertaken for this purpose and tested on the Evian impluvium in order to specify the evolution of the recharge as a function of elevation. Finally, the study of a small watershed located in southern Quebec highlighted the interest of using stable isotopes of the water molecule to characterize the variability of water transport mechanisms within the vadose zone in soil models on a larger scale than that of the soil profile.