Characterising the hydrodynamic interactions due to natural and pumping-induced saltwater intrusion in a coastal aquifer

This has impacts for coastal water supply design, potentially rendering traditional shallow-wells inappropriate. The processes by while the IRC forms have been reported in other work, and include beach slope, tidal amplitude and aquifer recharge. No field studies to date have however attempted to characterise the effect of an aquifer with an IRC in terms of pumping.

This thesis deals with the installation of a world-class test site to investigate the effect of pumping in a homogenous, pristine coastal aquifer with an IRC. A unique range of characterisation and monitoring techniques were deployed to establish the hydrodynamic interactions present under natural conditions. This represented the first field-scale study to do this.

Furthermore, the high homogeneity present in the coastal aquifer permitted a unique opportunity to compare characterisation methods. A bespoke four-dimensional electrical resistance tomography (ERT) system was installed at the site and used to compare the hydrodynamic interactions due to pumping with the natural state of the aquifer. Investigations form part of a wider multidisciplinary study in which geophysical data collection has been complimented by hydraulic measurements and water quality sampling/monitoring to permit the source of saltwater entering pumping wells to be identified.

In bringing together two important spheres of hydrogeological research – aquifer responses to pumping, and tidal impacts on coastal aquifers – this study represents the first time that ERT has been applied to monitoring the response of a pumping test in the intertidal zone of a coastal aquifer.

Background

ERT monitoring identified that over the course of one lunar cycle, the IRC waxes and wanes predominantly around the high water mark, but overall expresses little change. Pumping test instrumentation enabled the identification of aquifer dewatering due to pumping, while well head and major ion chemistry indicated a shift toward pumped water which is more saline and from a shallower source in the latter part of the test.

Time-lapse ERT over the course of pumping enabled the source of saline water to be tracked and linked to the IRC. Consequently, test findings suggest that future water resource development at the site should focus on pumping deeper parts of the aquifer. The wider implications of this study lie in the supply of water to vulnerable coastal communities, as water resources become increasingly stressed.

Studies of this nature are at the nexus of anthropogenic requirements of coastal aquifers and the need for a deeper understanding of tidal processes within coastal aquifers, which have heretofore been studied almost exclusively through modelling efforts.<br><br><i>Thesis is embargoed until 31 December 2028</i>.<br><br><br>