Logging of self-potential gradients to track saline intrusion in chalk, gravel and sand aquifers around the United Kingdom

Coastal aquifers, a vital drinking water (and agricultural) resource for over a billion people, are facing increasing risks of seawater contamination due to the dual challenges of population growth and climate-induced sea-level rise. With water stress expected to intensify and water use consumption growing annually, there is an urgent need for innovative methods to track saline intrusion in these aquifers.  Current monitoring techniques like observational boreholes are limited in their warning capabilities, while resistivity imaging, despite its effectiveness, is prohibitively expensive and logistically challenging, (MacAllister et al. 2016). Self-Potential (SP), naturally occurring voltages arising from ion separation in the subsurface, is a promising geophysical technique to identify and manage saline intrusion, provided its source mechanisms are well understood. There are two key sources of SP in hydrology. Electro-kinetic potentials (VEK), due to fluid flow induced by pressure gradients, and exclusion-diffusion potentials (VED), due to ion concentration gradients in the subsurface. The balance of these effects depends on a variety of variables including the physical and chemical properties of geological material in which the saline-fresh water interface is located. Spatial and temporal changes in these potentials provide  insight into the location and behaviour of the saline-fresh interface. This study introduces a novel SP profiling approach that employs both fixed and movable electrodes within a borehole, greatly enhancing the data acquired from SP measurements. Observations across various UK locations have revealed SP profiles exceeding 50mV. Coastal aquifers including those of Chalk, Gravel and Sand have been investigated. This talk presents not only gathered results but also details insights in the practical assessment of these gradients. Notably, these SP gradients are dynamic, with changes seemingly connected to the movements and proximity of the saline interface. The findings are corroborated by laboratory experiments and numerical models, showing that the dynamics of SP gradients can serve as an early indicator of saline intrusion in coastal aquifers.   Bibliography MacAllister, DJ., Jackson, M. D., Butler, A. P., and Vinogradov, J. (2016), Tidal influence on self‐potential measurements, J. Geophys. Res. Solid Earth, 121, 8432– 8452