Variable contributions of mixing end-members during small scale sprinkling experiments in partially weathered black marls

Rainfall is a well-know factor of hillslope instability. It influences the soil water content, allowing a rise in pore water pressures and decreasing the effective stresses in the soil. To simulate the infiltration processes a simplified approach consists in considering the soil as a homogeneous medium, so that water flow can be described by the Richard’s equation for both saturated and unsaturated conditions. Unfortunately, several field observations have proven that landslide materials were highly heterogeneous. Preferential flow paths such as fissures, cracks, macropores and structural voids were shown to influence the water infiltration and the transport capacity through the unsaturated zone. The purpose of this work was to identify groundwater dynamics and active reservoirs interactions (matrix flow and preferential flow) within heterogeneous and fissured black marl material, using small-scale (1m2) infiltration experiments located on the Super-Sauze mudslide (French South Alps). Artificial rainfalls enriched in conservative tracer (Br- and Cl-) were applied over a period of 2 days with a mean rainfall intensity of about 44.7 mm/h. Matrix-water interactions have been analysed using End Member Mixing Analyses (EMMA) with Br-, Cl- and SO4 2- contents. Results confirm a complex and spatially different behaviour with the presence of preferential and/or soil matrix flow paths, cases of piston flow and chemical interaction with matrix (dissolution of mineralogical phases containing sulphate)