The Protective Effect of Mixed Physical Barrier in Heterogeneous Aquifers: Experimental and Numerical Study

The ability of mixed physical barrier (MPB) as a seawater intrusion countermeasure was explored in heterogeneous coastal aquifer settings. The performance of MPB was examined in a synthetic aquifer containing a low permeability interlayer sandwiched between two layers of high permeability (case HLH). The performance of the MPB was compared to that of a single cut-off wall for various hydraulic gradients in a laboratory setting, whereby performance was measured in terms of measuring the percentage of reduction of the intrusion length. Also, numerical simulations were conducted using SEAWAT to validate and further examine the effects of various layering patterns on MPB performance. In total, five additional heterogeneous scenarios were simulated, including a scenario where a low permeability layer was set at the top of the aquifer (case LH), at the lower part of the aquifer (case HL), at the top and bottom part of the aquifer (case LHL), and two cases with monotonically increasing/ decreasing permeability from top to bottom. The sensitivity of the percentage reduction to the MPB design and hydrogeological parameters was examined thereafter. Experimental results demonstrate that the MPB could perform better than the single cutoff wall, with up to 55 % more reduction of the intrusion length. Also, the numerical results showed that the MPB remained effective regardless of the stratification patterns adopted, whereby it achieved at least around 70% SWI length reduction. The results also showed that the effectiveness of MPB was very sensitive to the thickness of the middle layer as well as the permeability ratio. While increasing the thickness of the interlayer induced a negative impact on the MPB performance, a reduction in the permeability of the interlayer induced better reduction. The findings of this study provide insight into the main parameters affecting the performance of the MPB system in realistic layered heterogeneous coastal aquifer scenarios and further evidence of its reliability as a practical countermeasure for SWI.