Abstract
Population growth and changing climate continue to impact on the availability of natural resources. Urbanization of vulnerable coastal margins can place serious demands on shallow groundwater. Here, groundwater management requires definition of coastal hydrogeology, particularly the seawater interface. Electrical resistivity imaging (ERI) appears to be ideally suited for this purpose. We investigate challenges and drivers for successful electrical resistivity imaging with field and synthetic experiments. Two decades of seawater intrusion monitoring provide a basis for creating a geo-electrical model suitable for demonstrating the significance of acquisition and inversion parameters on resistivity imaging outcomes. A key observation is that resistivity imaging with combinations of electrode arrays that include dipole–dipole quadrupoles can be configured to illuminate consequential elements of coastal hydrogeology. We extend our analysis of ERI to include a diverse set of hydrogeological settings along more than 100 km of the coastal margin passing the city of Perth, Western Australia. Of particular importance are settings with: (1) a classic seawater wedge in an unconfined aquifer, (2) a shallow unconfined aquifer over an impermeable substrate, and (3) a shallow multi-tiered aquifer system over a conductive impermeable substrate. We also demonstrate a systematic increase in the landward extent of the seawater wedge at sites located progressively closer to the highly urbanized center of Perth. Based on field and synthetic ERI experiments from a broad range of hydrogeological settings, we tabulate current challenges and future directions for this technology. Our research contributes to resolving the globally significant challenge of managing seawater intrusion at vulnerable coastal margins.
Highlights
Of the world population in 2003, an estimated 1.2 billion people lived within 100 km of the coast (Small and Nicholls 2003)
We found no systematic comparisons of numerical modeling and field Electrical resistivity imaging (ERI) experiments for a diverse set of aquifer systems, which include detailed topography and accurate models of salinity distribution based on seawater intrusion monitoring (SIM) wells
Of particular interest in our study is the scenario where a high-quality, high-permeability coastal aquifer overlies a shallow clayey substrate. This is common along coastal margins, and we will show that this presents particular challenges for the application of electrical resistivity imaging
Summary
Of the world population in 2003, an estimated 1.2 billion people lived within 100 km of the coast (Small and Nicholls 2003). The demand on accessible fresh groundwater from shallow coastal aquifers will increase, potentially resulting in the loss of potable water to seawater intrusion. This will have significant human impact (Vorosmarty et al 2000). Of particular interest in our study is the scenario where a high-quality, high-permeability coastal aquifer overlies a shallow clayey substrate This is common along coastal margins, and we will show that this presents particular challenges for the application of electrical resistivity imaging. The second is the transition from fresh potable groundwater to the impermeable substrate on the landward side of the seawater interface Superimposed on these four high-contrast geo-electrical boundaries are contrasts linked to variation in lithology. Resistivity imaging and any near-surface hydrogeophysical method should assist predictive transport modeling processes and contributed to decision making for long-term management of high-quality coastal groundwater resources
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