Effects of a low-permeability layer on unstable flow pattern and land-sourced solute transport in coastal aquifers

Abstract

The tide-induced upper saline plume (USP) in coastal aquifers plays a critical role in affecting the fate of land-sourced solutes. Early studies focused primarily on a quasi-steady USP, namely the size and salinity distributions of the plume remain unchanged over consecutive tidal cycles. In certain situations, the USP may become unstable. However, the effects of a low-permeability layer (LPL) on the pattern of fingering flow and land-sourced solute transport in coastal aquifers have not been examined. This problem was addressed numerically based on a 2-D cross-section of a tide-influenced layered coastal aquifer. The results reveal that the LPL can significantly reduce the size and accelerate the formation of salt fingers associated with the unstable USP. Compared with the homogeneous case, the maximum solute discharge rate and the width of discharge zone are decreased by the presence of the LPL, and the solute plume is stretched horizontally and squeezed vertically. Salt fingers may force part of the solute plume into the LPL, leading to an extended solute discharge duration. A sensitivity analysis shows that variations of the hydraulic conductivity, thickness and elevation of the LPL would greatly alter land-sourced solute transport. The trapped solute mass in the LPL, the maximum discharge rate and the residence time of the solute plume are more sensitive to the hydraulic conductivity and elevation than to the thickness of the LPL. These results have important implications for improving our understanding of flow and solute transport in layered coastal aquifers with an unstable USP.