Numerical analysis of density-driven fluctuation in groundwater caused by saltwater intrusion

Abstract

Density-driven flow in groundwater environment is concerned with various fields such as saltwater intrusion in coastal areas, groundwater contamination due to non-aqueous phase liquid from garbage disposal plants, and designing of geothermal utilization systems. It is important to appropriately consider density-driven flow when dealing with these objectives, because density-driven flow changes solute and heat distribution as well as groundwater flow. In recent years, salt damage to farmlands caused by storm surge or tsunami is a serious problem. In order to prevent salt damage to crops, salt is often leached intentionally by freshwater. However, the effect of leached saltwater on groundwater environment is not considered adequately. It is known that flow instability is caused by unignorable density difference between intruding saltwater and groundwater. This instability changes groundwater flow and salinity distribution, and sometimes produces fluctuant flow that keeps oscillating. Therefore, it is important to comprehend these phenomena for management of groundwater resources. The purpose of this study is to ascertain the condition in which flow instability occurs when saltwater intrudes into groundwater. Systematic numerical experiments were conducted to specify a subdomain where flow keep oscillating in a parameters space. The equations governing groundwater flow and solute transport were discretized spatially by the Galerkin finite element method and temporally by the Crank-Nicolson method. The obtained results showed that the oscillatory flow depends not only on Rayleigh number concerning density difference and permeability, but also on both of basal groundwater flow and saltwater intrusion rates.