Abstract

Coastal groundwater resources face mounting pressure under sea-level rise and population growth. Under active seawater intrusion (SWI), where the coastal hydraulic gradient slopes landward, onshore groundwater resources will eventually be salinized. This study aims to expand the limited understanding of the influence of alluvial aquifer heterogeneity on the rate and characteristics of active SWI. Using the Central Canterbury Plains aquifer system in New Zealand as an example, we simulated active SWI conditions caused by a decrease in onshore groundwater head. We considered 25 realizations of a 3D heterogeneous confined coastal aquifer that extends offshore. Salinization patterns in the heterogeneous aquifers are influenced by preferential flow through high hydraulic conductivity gravel deposits. Our results indicate that movement of the salt/fresh water interface was up to ten times faster than expected from an analytical model that uses the effective hydraulic conductivity of the aquifer, or in an equivalent homogeneous model that included the effects of dispersion. The rate of interface movement also varied spatially along the width of the aquifer. While similar findings have been observed in models of coastal volcanic and fluvio-deltaic aquifers, the hydraulic conductivity contrasts in the alluvial aquifer models studied here are much lower. Our results indicate that aquifer heterogeneity should be carefully considered when studying SWI in alluvial settings, even when the salt/fresh water interface is located offshore. If the heterogeneity of a coastal alluvial aquifer is overlooked, salinization of onshore groundwater under seawater intrusion may occur considerably earlier than expected.

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