Detection of Tidal Signatures in Self-Potential Monitoring of UK Aquifers

Increased groundwater extraction, coupled with potential declines in recharge rates, is anticipated to negatively impact the sustainability of groundwater resources (Mehdizadeh, 2019). Coastal aquifers are particularly vulnerable, as these changes can result in a significant risk of saltwater intrusion (SI). While the fundamental mechanisms of SI are well-understood, tracking the encroachment of saline water into coastal aquifers and its risk to water extraction sources remains a complicated and expensive task. Studies have shown that self-potential (SP) could serve as an effective tool for remotely monitoring the movement of saline-freshwater interfaces due to SI (Graham, 2018).

Self-potential voltages, which originate from subsurface pressure and concentration gradients, occur when these gradients lead to ion separation. This separation results in an electrical potential and a subsequent electron flow to preserve electrical neutrality. These potentials, usually in the millivolt range, can be observed and recorded in the field using electrodes. SP primarily consists of two types: electro-kinetic potentials (VEK), arising from differing flow velocities, and exclusion-diffusion potentials (VED), stemming from ion concentration gradients with varying mobilities. A previous study recorded tidal signatures in SP within a Chalk borehole located less than 2 kilometres from the English Channel (MacAllister et al., 2016). More recent research has detected similar signatures in a sand aquifer on the north coast of Northern Ireland and in a gravel aquifer on the south coast of England.

In each instance, the tidal signature most prominently reflected the M2 (Principal lunar-semidiurnal) component, although in some cases other, less prominent, elements were also identified. Analysis of water level, electrical conductivity and temperature data suggests that these signatures were not due to electrokinetic potentials from tidally induced flows in and around the borehole. Rather, a more likely explanation is that nearby saline-freshwater interfaces, inducing exclusion-diffusion potentials, are responsible. Whilst further investigation is necessary to quantify, model, and fully comprehend these signals, the detection of tidal signatures in an increasing and diverse number of aquifers suggests that self-potential might be a viable technique for monitoring and providing an early warning of saline intrusion.

Bibliography
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