Experimental study of infiltration through subsidence-reactivated surface discontinuities

The main objective of this work is to investigate the influence of surface geological discontinuities on the hydraulic properties of the subsoil, studying an area affected by a fault (surface scarp) and a fracture (horizontal surface opening), which are separated by about 18 meters each other, forming part of a discontinuity system located within the Aguascalientes Valley (VA), Mexico. Both discontinuities are currently active due to the process of subsidence that the VA has been experiencing for more than 45 years, this emphasizes the relevance of this study since in most cases the studies are focused on regional inactive faults. In addition, studying some of the hydraulic properties of active discontinuities in the VA is significant because this valley currently has a large number of such structures with similar geologic conditions. As part of the methodology, 33-meter profiles of infiltration rates and hydraulic conductivity were obtained by applying permeability tests in the field, as well as 38-meter Electrical Resistance Tomography (ERT) profiles, in order to obtain and analyze the general characteristics of the subsoil and discontinuities. Subsequently, four ERT tests were applied in synchrony with punctual permeability test. The first ones in profiles of 14–17 meters, and the second ones in four points located at distances of under one meter from the discontinuities. These synchronized tests made it possible to visualize, in ERT profiles, the flow path of water injected into the subsurface through the permeameter, as well as the influence of the discontinuities on the hydraulic parameters mentioned above. The results show that the hydraulic parameters obtained in the areas close to the discontinuities, particularly the fault, are higher than those in the areas more distant from both discontinuities, probably due to their constant mechanical activity, being these increases of up to two orders of magnitude, obtaining maximum values of 2.08E-03 m/s for the infiltration velocity and 5.22E-05 m/s of hydraulic conductivity. On the other hand, observations from the simultaneous ERT-permeability tests suggest that discontinuities, particularly the fault, directly influence the path of infiltrated water, allowing the water to tend to move towards the displacement plane.