Overexploitation of groundwater resources in the faulted basin of Querétaro, Mexico: A 3D deformation and stress analysis

The City of Querétaro is located on a continental basin filled since the Oligocene with lacustrine and alluvial sediments, pyroclastic deposits, and interbedded fractured basalts. The graben structure of the basin was formed by two major North-South trending normal faults, among which the thicknesses of the filling materials vary many tens of meters in close distances. Hence, important differences of hydraulic and mechanical properties characterize the various geologic units. Groundwater has been strongly withdrawn over the last three decades in the study area, with a decline of the piezometric level exceeding 100 m. Because of the high variability of the geologic deposits, the piezometric decrease and consequently the effective stress increase are characterized by a large spatial variability. Piezometric variations are also due to faults that strongly impact on groundwater flow dynamics. The deformation and effective stress variability has caused large differential subsidence causing ground fracturing that has damaged the urban infrastructure of the City of Querétaro. This complex geological setting has been properly accounted into a three-dimensional (3D) flow and geomechanical modeling approach to quantify the displacement, deformation, and stress fields caused by water withdrawal. The static geologic model was accurately defined using geological logs from extraction wells, field mapping of faults, fractures, and the integration of major structures reported in previous geophysical works. The model has been calibrated using observed groundwater levels and land settlement records. The simulations have spanned the period from 1970 to 2011. The modeling results highlight that the areas where large differential subsidence and horizontal displacements developed correspond to the portions of the city where ground fractures are observed. Normal and shear components of the stress field changes accumulate along the discontinuity surfaces at depth, providing evidence that large piezometric declines can be a key factor triggering the fault reactivation. The spatial relationship between major withdrawals, discontinuities of the geologic structure, and accumulation of large stress and strain fields clearly emerged from the outcomes of the 3D geomechanical model.