Effect of Tidal Fluctuations on Contaminant Transport in Coastal Unconfined Aquifers

Abstract

The environment of the coastal aquifers becomes worse and worse, due to the rapid development of economy, especially for the unconfined aquifers which direct connects with the contaminates from the land surface. In this study, a 2D finite-difference model describing variably saturated flow and solute transport (VS2DT) was modified to simulate solute transport in unconfined coastal aquifers with periodic tidal boundary conditions. In order to improve the understanding of tidal effects on contaminant transport, simulations with a simplified seaward boundary condition at a fixed position were compared to simulations with periodic moving boundary condition. Tidal fluctuations were modeled respectively as diurnal tide, semi-diurnal tide, and multi-periods semi-diurnal tide. In comparison with a fixed seaward boundary condition, a periodic moving boundary condition induced a larger water table over height and causes a lower transport velocity of the contaminant plume. Therefore, neglecting the horizontal movements of the seaward boundary underestimated the migration rate of the contaminant plume towards the sea and incorrectly describes the temporal and spatial distribution of contaminant plumes in costal aquifers. The results found with three different kinds of tidal fluctuations revealed that the multi-periods of tide also affected the movement of contaminant plume. There were lower migration rates during neap tides and faster transportation rates during spring tides. Our results indicated that it was necessary to simulate both the vertical tidal fluctuation and the horizontal movement of the seaward boundary condition to predict the movements of contaminant plumes through unconfined coastal aquifers.