The Impact of Layered Heterogeneity on the Ability of Subsurface Dams to Protect Groundwater Pumping in Coastal Aquifers

This study examined the ability of subsurface dams to protect freshwater abstraction against seawater intrusion in both homogeneous and layered heterogeneous aquifers. Laboratory experiments were conducted in a synthetic aquifer where a subsurface dam was simulated in a homogeneous scenario (case H), and in another scenario where a top low permeability layer was placed in the upper part of the aquifer (case LH). We then conducted numerical simulations using the SEAWAT model to validate the experimental results and examine other numerical cases where a low K layer existed at the middle (case HLH) and the bottom of the aquifer (case HL). Case LH needed 52% more pumping than case H for the wedge to spill over the dam into the landside. The existence of a low permeability layer has generally delayed the upconing, and it took longer for the SWI to contaminate the abstraction well. The clean-up time varied substantially from one case to another, with the case HL taking longer than the other cases for SWI removal.  The cleanup time was reduced by 23% in the presence of a top low-K layer compared to the homogeneous aquifer. The study demonstrates that the presence of the low-K layer on the top of the aquifer contributed positively to improving the ability of the subsurface dams to obstruct SWI, limit saltwater upconing and, therefore, allow more optimal freshwater abstraction.  A feature of this study was it examined the ability of dams to prevent seawater intrusion in the existence of freshwater pumping, which has not been discussed in previous studies, at least in laboratory experiments.