Evidence for the Application of Self-Potential (SP) Monitoring to the Management of Abstraction in Coastal Aquifers.

Abstract:

The aim of this work is to investigate whether measurements of self-potential (SP) can be used to monitor seawater intrusion into coastal aquifers with application to the fractured UK chalk aquifer. The SP arises to maintain electrical neutrality when a separation of charge occurs due to gradients in pressure (electrokinetic (EK) potential) and concentration (exclusion-diffusion (ED) potential).

We have installed a borehole monitoring array near Brighton on the UK south coast comprising 14 electrodes and 3 FEC/T/P (conductivity/temperature/pressure) probes. Variations in head suggest that short-term changes in the borehole are controlled by semi-diurnal and fortnightly tidal cycles. FEC suggests that salinity is governed by the inland freshwater head. SP monitoring reveals semi-diurnal and diurnal fluctuations which are observed without any saline water entering the borehole.

Laboratory measurements to investigate the contribution of the EK and ED potentials to the overall SP indicate that tidally-driven changes in head at the borehole generate an EK potential fluctuation of c.260μV. This is not sufficient to explain the c.2mV semi-diurnal SP signal observed. Therefore, the ED potential across the saline front is suggested to be the main mechanism generating the tidal SP response at the borehole. In this model, which is currently being tested numerically, the SP response is caused by the tidally-driven lateral movement of the saline front within the aquifer.

Saline water was observed to enter the borehole in August 2013. Prior to this a systematic increase in the SP of c.300μV was observed, starting c.5 days before saline water breakthrough. In this case saline water entered the well owing to a seasonal decrease in inland head; however, the results suggest that SP could also provide early warning of saline water breakthrough in abstraction wells, allowing for improved management of abstraction rates from coastal aquifers.