Thesis defense of Madeleine Nicolas

Impact of heterogeneity on natural and managed aquifer recharge in weathered fractured crystalline rock

Abstract

The factors governing the intensity and distribution of natural and artificial recharge in weathered and fractured crystalline aquifers are poorly understood. However, it is the defining characteristics of this type of rock—these rocks are very heterogeneous—that make the estimation of fluxes and the hydraulic properties controlling them difficult. In this presentation we will introduce the numerical and experimental work carried out to deepen our understanding of the dynamics of groundwater flows in these heterogeneous underground environments at several scales. 

The first line of research focuses on natural recharge processes at the watershed scale. Diffuse recharge was modelled with a simple physical infiltration model and compared to previous estimates of total recharge. Our results highlight the strong dependence of recharge on rainfall and irrigation, and the importance of focused recharge. The factors responsible for the spatial distribution of recharge are also explained. The second axis is based on the monitoring of the filling of an artificial recharge basin at a highly monitored and well-equipped site. These observations were interpreted with analytical and numerical models to improve our knowledge of flow dynamics in fractured crystalline rocks at the medium scale. These models illustrated the existence of preferential horizontal flows, but also of a lateral compartmentalization that hinders the propagation of recharge inputs.