Numerical modeling of tidal influence on density-dependent contaminant transport

Transport of contaminants in coastal aquifers subject to tidal fluctuation is an important topic in hydrogeology as a consequence of the significant development of human activities near the shoreline. Despite this, relatively little work has been done to investigate the joint effect of variable water density flow and tidal saltwater head fluctuation. In particular, numerical modeling results have rarely been validated with experimental data. The primary aim of this work was to develop and validate a modeling strategy to incorporate the effect of tides in a variable density simulation using a widely adopted simulation package. The numerical model reproduced data from a laboratory experiment designed to investigate the impact of tides on conservative contaminant transport in a coastal aquifer, where the polluted water is denser than the ambient groundwater. Both the plume shape and size were reproduced when tidal fluctuations were incorporated in the model. Model simulations were then used to examine in detail how the flow patterns are modified during one tidal cycle. According to the simulations, the contaminant was mainly discharged from the beach surface. The model was also used to investigate the contact between the contaminant plume and the saltwater intrusion in the freshwater aquifer. We observed that the two dense water phases never completely mix during the simulation. Instead, a slice was continuously removed from the front of the polluted plume because of the discharge of freshwater. The observed mechanism is cyclical with a frequency compatible with the tide.