Saltwater intrusion of the Costa de Hermosillo aquifer, Sonora, Mexico: A numerical simulation

Abstract

Inversion of gravity data enabled us to obtain the basement topography of the Costa de Hermosillo aquifer. The depth to the basement ranges from 300 to 3500 m, and has a complex character. Between Hermosillo and Kino Bay, the basement constitutes a subcircular structural high. Around this structural high basement is characterized by a series of horsts and grabens. Several grabens are interconnected, allowing the flow of seawater inland. The horsts act as barriers to the seawater flow. The pattern of total dissolved solids in groundwater supports this interpretation. Gravity data also suggest the existence of tectonic lineaments correlating with the southward prolongation of the Cerro Prieto fault, and with the Craton margin lineament. From electrical resistivity distribution the seawater front extends inland more than 25 km and is more than 200 m deep. Based on borehole information and on the electrical resistivity distribution we inferred the stratigraphy of the aquifer. Geophysical, geological and geohydrological information were used to derive a geological model for the Costa de Hermosillo aquifer, which was subsequently used in a 2-D numerical simulation of salt water intrusion. A non-dimensional formulation based in the equations of Darcy, of continuity (for the water and for the salt, respectively) and in a constitutive relation (fluid density and salt concentration), was used to simulate the seawater intrusion. The resulting equations were solved, through a finite difference scheme, for the parametrized geologic model along selected profiles.
We find an advective pattern where no barrier exists to the seawater flow. However, those sections separated by intrusives or horsts show convective cells. The aquifer flow is complicated by the changing nature of the sedimentary infill and the overexploitation of the aquifer in the zone. Assuming the present rate of withdrawal the steady-state simulated reproduces salt concentration agrees with the observed values.