Abstract
Abstract. Surface electrical resistivity tomography (ERT) is a widely used tool to study seawater intrusion (SWI). It is noninvasive and offers a high spatial coverage at a low cost, but its imaging capabilities are strongly affected by decreasing resolution with depth. We conjecture that the use of CHERT (cross-hole ERT) can partly overcome these resolution limitations since the electrodes are placed at depth, which implies that the model resolution does not decrease at the depths of interest. The objective of this study is to test the CHERT for imaging the SWI and monitoring its dynamics at the Argentona site, a well-instrumented field site of a coastal alluvial aquifer located 40 km NE of Barcelona. To do so, we installed permanent electrodes around boreholes attached to the PVC pipes to perform time-lapse monitoring of the SWI on a transect perpendicular to the coastline. After 2 years of monitoring, we observe variability of SWI at different timescales: (1) natural seasonal variations and aquifer salinization that we attribute to long-term drought and (2) short-term fluctuations due to sea storms or flooding in the nearby stream during heavy rain events. The spatial imaging of bulk electrical conductivity allows us to explain non-monotonic salinity profiles in open boreholes (step-wise profiles really reflect the presence of freshwater at depth). By comparing CHERT results with traditional in situ measurements such as electrical conductivity of water samples and bulk electrical conductivity from induction logs, we conclude that CHERT is a reliable and cost-effective imaging tool for monitoring SWI dynamics.
Highlights
Seawater intrusion (SWI) increasingly affects the evergrowing populations near coastlines
We display the bulk electrical conductivity (EC) model obtained by the inversion of the cross-hole ERT (CHERT) and surface-based electrical resistivity tomography (ERT) data (Fig. 3a), the result obtained when only considering the complete CHERT (Fig. 3b) and only the surface ERT (Fig. 3c) next to the calculated coverages for each model (Fig. 3d–f)
The bulk EC model obtained from the surface ERT campaign shows resistive layers in the first 5 to 10 m below the land surface, while the model obtained from the complete CHERT data alone is unable to resolve them
Summary
Seawater intrusion (SWI) increasingly affects the evergrowing populations near coastlines. SWI has been studied for many years but, even today, remains an open research topic because of the complex physical, chemical, mechanical and geological processes involved. The equations that govern interactions between fresh- and seawater are well established, and models. A. Palacios et al.: Time-lapse cross-hole electrical resistivity tomography of simplified generic scenarios are commonly used to predict and assess the risks linked to SWI and to define appropriate management strategies (Abarca et al, 2007; Henry, 1964). Real field conditions are much more complex, and detailed case studies are less common in the SWI literature
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