Estimating freshwater lens volume in highly permeable aquifers

Abstract

A unique freshwater lens shape observed in Tarama Island, Japan, in which hydraulic conductivity is on the order of 10⁻² m s⁻¹, has posed a question as to how well we can estimate the fresh groundwater volumes in extremely permeable aquifers. We applied both an analytical model and numerical simulations with various hydraulic conductivities, including extremely permeable conditions, and compared their results. The simulation showed that, when the hydraulic conductivity was extremely high, saline groundwater existed near the coast. The analytical model overestimated the freshwater volume compared with those estimated from the numerical simulations, and the discrepancy became more significant with increasing hydraulic conductivity. These findings imply that, when hydraulic conductivity is extremely high, numerical simulations considering density-dependent flow and dispersive mass transport processes should be applied to better assess the shapes and volumes of freshwater lenses.