Rowan T., 202x, Data & Code, Observations of tidal induced responses in Self-Potential data in a UK beach sand aquifer: Implications for monitoring seawater intrusion

<br><br>Affordable and effective early warning of saline intrusion into aquifers is a key challenge affecting coastal water supplies. This paper reports on the application of Self Potential (SP) monitoring close to a mobile saline-fresh water interface over a 3-month period. SP was monitored in 12 boreholes, distributed over a variety of depths and distances to the high-water mark (HWM). Logged borehole measurements of groundwater level, temperature and fluid electrical conductivity (FEC) were also collected. Power Spectral Density (PSD) analysis of SP data showed a clear semi-diurnal (principal-lunar, M₂) tidal component, with an amplitude of around 1 mV. The magnitude of the M₂ component linearly decreased with depth and distance from the shore. Whilst PSD analysis showed M₂ components in borehole water levels, none were detected in groundwater temperature and FEC. The results suggest that SP M₂ components were generated remote from the boreholes by the movement of a saline interface, most likely associated with the upper saline plume. Superimposed on these tidal signals were periodic rises and falls in mean SP of around 5 – 10 mV. These responses appear to be linked to storm surge events, during which seawater advanced up to and beyond the monitoring boreholes. Although detailed interpretation of the data is complex due to salinity changes at the site of the reference electrode, they provide further support for the use of SP as a precursor to saline intrusion.