Self Potential Monitoring of Saline Intrusion Dynamics in a Coastal Sand Aquifer

Greater groundwater abstraction combined with possible reductions in recharge rates are likely to be detrimental to the long-term viability of groundwater resources (Mehdizadeh, 2019). An additional issue specifically affecting coastal aquifers is saltwater intrusion (SI). The key processes governing SI have been long understood but monitoring the ingress of saline water into coastal aquifers and especially its risk to abstraction sources is still a complex and costly exercise (Graham, 2018). Here we build on evidence that self potential (SP) could be a useful tool for remotely tracking the movement of saline-freshwater interfaces associated with SI.  The work reported describes SP response, along with water level, temperature and electrical conductivity measurements from an array of piezometers under ambient and pumped conditions on a beach aquifer located on Benone Strand, on the northern tip of Northern Ireland, UK. These data are supplemented by time-dependent electrical resistance tomography (ERT) obtained from the BGS PRIME system.

Self potential voltages arise from subsurface pressure and concentration gradients (Jackson et al., 2012). These gradients can cause ion separation, which gives rise to an electrical potential and a flow of electrons in order to maintain electrical neutrality. The potentials (typically in the millivolt range) can be detected and logged in the field using installed electrodes. There are two main types of SP; electro-kinetic potentials (V_EK), due to differential flow velocities, and exclusion-diffusion potentials (V_ED), due to ion concentration gradients with different mobilities. SP has been shown to have a response to pumping tests in (Jackson et al., 2012), though this was limited in scope. In a longer-term study, tidal signatures in SP were recorded in a Chalk borehole less than 2 km inland from the English Channel (MacAllister, 2016). Separating out these two sources of SP can be challenging.

Comparing SP and ERT responses coupled with groundwater level changes show tidal responses with are related to depth below surface and distance from the sea. In addition, results pumped well water levels appears to indicate that the drop in SP is not correlated with the expanding cone of depression from pumping, as the high pressure gradients that occur at the start of pumping has not induced an electrokinetic response. This is in contrast with the results obtained from (Jackson et al., 2012) at an inland site on the Cretaceous Chalk. This, therefore, points to the change in SP being induced by local movements of the saline-freshwater interface in the vicinity of the pumping wells, where a more progressive response is induced by changes in groundwater flow.