Sequential indicator simulation for a three-dimensional distribution of hydrofacies in a volcano-sedimentary aquifer in Mexico City

The Mexico City aquifer is a complex mix of alluvial deposits and volcanic rocks overlapped by an aquitard composed of lacustrine deposits. To characterize this heterogeneous hydrogeologic system, a three-dimensional model of the distribution of hydrofacies is constructed using borehole lithological records. The analysis is based on 111 borehole logs with an average depth of 300 m, in an area of 234 km2, providing a nominal scale of resolution of 2.1 km in the plane and 2-m resolution in the vertical direction. These records were discretized to generate a georeferenced dataset of 13,518 points associated with a lithological category; nine lithological categories were observed. These categories were subsequently grouped into four hydrofacies: A and B, grouping low-permeability lithological categories (lacustrine and volcano-sedimentary materials, respectively); and C and D, grouping high-permeability lithological categories (volcanic rocks and alluvial deposits, respectively). The database was analyzed in terms of proportion of hydrofacies at depth, distribution of layer thickness, and behavior of experimental horizontal and vertical variograms. The experimental variograms of each hydrofacies were fitted to exponential models via minimization of cross-validation errors. Three-dimensional models of probability of occurrence of each hydrofacies and the combined distribution of hydrofacies were then constructed via ensemble averaging of 1,000 realizations obtained by sequential indicator simulation. The potential use of this model for water management, modeling land subsidence, and groundwater pollution is discussed.