Preface: Saltwater and freshwater interactions in coastal aquifers

Abstract

The study of coastal aquifers, i.e., aquifers that have a hydraulic connection with a saline surface-water body, forms a somewhat separate discipline in groundwater science. This type of aquifer is typically characterized by variations of groundwater salinity in space and time, which warrants special treatment of groundwater flow, groundwater chemistry and water resource management problems. The interest in the study of this type of aquifer arose both from its strategic importance for the water supply of highly populated coastal areas and its susceptibility to salinization problems. Saltwater intrusion, i.e., the displacement of fresh groundwater by groundwater with a higher salinity, has become an accepted scientific term in the research field. Coastal aquifers and the interactions between fresh and saline groundwaters are the topic of this theme issue of Hydrogeology Journal. We decided to give this issue a twofold objective. On one hand, it is meant to present a series of papers that together demonstrate the state of the art of the research in coastal hydrogeology. On the other hand, the aim is to present a review of the extent of saltwater-intrusion problems around the world. The study of groundwater flow in coastal areas can not be considered separately from the transport of dissolved salts. This is because spatiotemporal changes in salinity give rise to density differences that control flow patterns. Consequently, equations of flow and solute transport must be solved simultaneously. The development of models that simulate flow and solute transport under variable-density conditions has been an enormous achievement. Although, by now, many sophisticated codes have become generally available, research in this field is still ongoing, which is reflected by a number of contributions to this theme issue. Voss et al. present a methodology to benchmark threedimensional (3D) variable-density flow and transport simulators. A benchmark is a test to confirm that the simulation results are correct and that the governing equations of flow and transport are correctly solved. Previously, unambiguous 3D benchmarks for unstable systems (i.e., a system in which, in some part or everywhere, the density decreases with depth) were lacking. The benchmark is based on analytically derived stability criteria for steady-state convection in a 3D box in an unstable system. Different modes of convection develop depending on the inclination of the box and the Rayleigh number. By comparing the results of the variable-density code with the analytically derived stability criteria, it can be ascertained that the code correctly simulates the physics of the flow system. An application of a model of coupled density-dependent groundwater flow, and solute and heat transport of an unstable system is presented by Hughes et al. They present a first assessment of the aquifer salinization process occurring beneath an engineered, hypersaline cooling-canal system. Their simulations show the development of salt fingers which can reach the bottom of the aquifer in times that range between a few days and 5 years, depending on the hydraulic conductivity configuration. They conclude that the temperature gradient, in this case, has a mitigating effect on the rate of aquifer salinization. Downwards unstable salinization is also studied by Post and Simmons, who show the results of a combined laboratory tank experiment and numerical modeling study of free convective flow at the scale of individual lowpermeability structures. Their results provide insight into the flow patterns that evolve in both the aquifer material surrounding the lens and within the lens itself. In the system studied, upward vertical-flow components are Received: 9 November 2009 /Accepted: 17 November 2009 Published online: 1 December 2009