Remote Detection of Saline Intrusion in a Coastal Aquifer Using Borehole Measurements of Self‐Potential

Two years of self‐potential (SP) measurements were made in a monitoring borehole in the coastal UK Chalk aquifer. The borehole SP data showed a persistent gradient with depth, and temporal variations with a tidal power spectrum consistent with ocean tides. No gradient with depth was observed at a second coastal monitoring borehole ca. 1 km further inland, and no gradient or tidal power spectrum were observed at an inland site ca. 80 km from the coast. Numerical modeling suggests that the SP gradient recorded in the coastal monitoring borehole is dominated by the exclusion‐diffusion potential, which arises from the concentration gradient across a saline front in close proximity to, but not intersecting, the base of the borehole. No such saline front is present at the two other monitoring sites. Modeling further suggests that the ocean tidal SP response in the borehole, measured prior to breakthrough of saline water, is dominated by the exclusion‐diffusion potential across the saline front, and that the SP fluctuations are due to the tidal movement of the remote front. The electrokinetic potential, caused by changes in hydraulic head across the tide, is one order of magnitude too small to explain the observed SP data. The results suggest that in coastal aquifers, the exclusion‐diffusion potential plays a dominant role in borehole SP when a saline front is nearby. The SP gradient with depth indicates the close proximity of the saline front to the borehole and changes in SP at the borehole reflect changes in the location of the saline front. Thus, SP monitoring can be used to facilitate more proactive management of ion and saline intrusion in coastal aquifers. Plain Language Summary Self‐potential (SP) measurements in coastal boreholes respond to the movement of seawater, which occurs remotely from the borehole. Thus, monitoring changes in SP in coastal boreholes could assist the management of ion of fresh water from coastal aquifers. Key Points SP data obtained from a monitoring borehole in the coastal UK Chalk aquifer display a persistent gradient with depth, and temporal variations with an ocean tidal power spectrum, caused by the movement of a saline front in close proximity to, but not intersecting, the base of the borehole The spatial and temporal variations in borehole SP are dominated by the exclusion‐diffusion potential arising from concentration gradients across the remote saline front Borehole SP monitoring can be used to monitor remotely the moveme