Time-lapse cross-hole electrical resistivity tomography (CHERT) for monitoring seawater intrusion dynamics in a Mediterranean aquifer

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.

The Issue of Groundwater Salinization in Coastal Areas of the Mediterranean Region: A Review

“…Many studies have used indirect techniques to monitor the spatial extent of saline groundwater in coastal aquifers, such as geophysical methods. For example, the electrical resistivity tomography (ERT) and geological data have been largely used [33][34][35][36] to map zones characterized by high salinities, while ERT coupled with hydrochemical data was less commonly employed [37][38][39][40][41][42][43], although it provides more robust information in comparison to former. Moreover, the time domain electromagnetic method (TDEM) has been widely employed as a characterization method to delineate the salinization of coastal aquifers with good results at large-scale areas [44][45][46][47][48], while only an example of airborne electromagnetics methods (AEM) has been present [49] due to the high cost of instrumentation and survey.…”

Imaging the Structure and the Saltwater Intrusion Extent of the Luy River Coastal Aquifer (Binh Thuan, Vietnam) Using Electrical Resistivity Tomography

“…Second, the difficulty is further enhanced by the regularization used for the ERT inversion, often leading to over smoothing [6] and the loss of resolution of surface ERT at depth [14,15,21], which affects the petrophysical relationship [22,23]. It often leads to discrepancies when comparing inverted results with borehole data [14,24,25]. The salinity is often overestimated in the low-salinity zone and underestimated in the high-salinity zone [6].…”

“…The salinity is often overestimated in the low-salinity zone and underestimated in the high-salinity zone [6]. This effect can be reduced but not eliminated by integrating crossborehole [25] or surface-to-hole measurements [14] or by choosing more appropriate regularization [21]. Third, the heterogeneity of the deposits affects the ability to derive accurate salinity estimation.…”

Combining resistivity and frequency domain electromagnetic methods to investigate submarine groundwater discharge in the littoral zone

“…Lapointe et al, 2005), and it can result in a significant loss of freshwater. RSGD can also have a significant impact on the dissolved nutrient input when it interacts with the sediment (Rodellas et al, 2018). On the other hand, SI threatens potable water in coastal zones.…”

“…The technique has been previously used to characterize pore water salinity in a coastal wetland (Greenwood et al, 2006) and to map saltwater intrusion on Long Reef Beach (Davies et al, 2015). It has also been combined with (continuous) vertical electrical and TDEM soundings to investigate the distribution of saltwater and freshwater on a sandy beach between Egmond aan Zee and Castricum aan Zee (Pauw, 2009), combined with 2D resistivity imaging to detect plumes of freshwater on a beach in northern Wales (Obikoya and Bennell, 2012), and combined with TDEM and onshore electrical resistivity tomography (ERT) to discriminate freshwater and saltwater in the Albufeira-Ribeira de Quarteira coastal aquifer system (Francés et al, 2015). However, when applying FDEM in saline environments, it is important to know that ground conductivity instruments designed to operate under low-induction-number (LIN) conditions (e.g.…”