Water system analysis and numerical modelling of a coastal aquifer in western Mexico: a case study

Abstract

This study combines a water system analysis of a coastal aquifer in western Mexico that is subject to seawater intrusion, where a hydrogeological characterisation of the area is given, with numerical modelling of the controlling processes and parameters as an instrument to provide a better understanding of the water system. The MOCDENS3D code is used in this study to model density dependent groundwater flow. The hydrodynamic behaviour of the coastal aquifer is similar to an island aquifer, where the freshwater lens “floats” on top of higher salinity water bodies, in this case saline from the sea and hypersaline from the lagoon. The freshwater lens varies in thickness according to the wet and dry seasons. This causes the freshwater lens to be in a delicate equilibrium between recharge and discharge of freshwater. High evaporation rates result in brine concentrations in the lagoon which enter the system in the form of saline fingers that are density driven. The numerical results provide a relatively close approximation to the density distribution that underlies the sand bar where field measurements were carried out. The results are highly sensitive to changes in the precipitation and evapotranspiration rates, with small changes leading to different concentration configurations, particularly in the freshwater lens. Results also demonstrate that a numerical model of this scale can be useful as a conceptual tool to understand the controlling factors that determine groundwater flow. The scenarios of climate change indicate that hydraulic heads will increase due to sea level rise and decrease due to longer periods of drought, and that the volume of fresh groundwater in the freshwater lens is sensitive to both of these scenarios. However, further research is necessary to provide more spatially extensive data that can be used to answer several of the uncertainties evidenced by this model.