Managed aquifer recharge (MAR) is recognized as a potentially effective method for controlling seawater intrusion in coastal aquifers. However, the effect of layered aquifer heterogeneity associated with geological layering on the performance of MAR remains poorly understood, particularly under tidal forcing. Layering is typical of sedimentary and volcanic coastal aquifer systems, whose groundwater resources are crucial to populations in many coastal areas. Lab-scale sandbox experiments were employed as a benchmark for application of numerical simulations considering different scenarios of layered heterogeneity in tidal unconfined coastal aquifers. Results showed that the presence of a low permeability (K) interlayer increased the effectiveness of well injection in controlling seawater intrusion, whereas a high permeability interlayer decreased it, as tracked by the position of the seawater wedge toe which was used as an indicator. Both low and high K interlayer scenarios decrease the effectiveness of well injection in retreating the upper saline plume (USP). In scenarios without well injections, both low and high K interlayer lead to thinner mixing zones between seawater and freshwater than in homogeneous conditions. Injections in aquifers with high K interlayer result in a much thicker mixing zone compared to aquifers with low K interlayer. The presence of a low/high K interlayer may also alter the sensitivity of the retreat process of seawater intrusion, induced by well injection, to tidal conditions and injection rates. This study provides insights that have implications for the design of effective MAR systems using freshwater injection wells in layered coastal aquifers.
 

Managed aquifer recharge; Tidal coastal aquifer; Layered heterogeneity; seawater intrusion; Physical experiments and numerical simulations