Laboratory and field measurements of the self-potential (SP) in chalk, with application to monitoring of saline intrusion

Saline intrusion is a global phenomenon, affecting the availability of freshwater in coastal aquifers. The aim of this work is to investigate whether measurements of self-potential (SP) can be used to monitor the intrusion of seawater into coastal aquifers, with specific application to the UK chalk aquifer in the vicinity of Brighton on the south coast of the UK. The SP arises to maintain electrical neutrality when a separation of charge occurs due to gradients in pressure (electrokinetic (EK) or streaming potential) and concentration (electrochemical (EC) potential). Concentration gradients are a characteristic feature of saline intrusion and may give rise to a measureable EC potential. In addition, an EK potential will arise during abstraction. Laboratory and field SP measurements are used to investigate the magnitude of the EK and EC potentials in the UK chalk aquifer during saline intrusion. Laboratory measurements yield an EK coupling coefficient, relating the gradient in voltage to the gradient in pressure when the total current is zero, of -60 mV/MPa in samples saturated with groundwater, and -1 mV/MPa in samples saturated with seawater. This result agrees with earlier work suggesting the EK potential is suppressed at high salinity due to a compressed electrical double layer. The EC coupling is dominated by diffusion potentials arising from the concentration gradient across the saline front. Field experiments suggest that the EK component of the SP in the chalk is very small under ambient conditions, even in freshwater zones, because gradients in hydraulic head are small owing to the high conductivity of the pervasive fracture networks. However, a pumping test conducted in the chalk aquifer at a field site in Berkshire induced a measureable EK response with a coupling coefficient of magnitude consistent with the freshwater EK coupling coefficient obtained in the laboratory. SP monitoring at an observation borehole near Brighton reveals semi-diurnal SP fluctuations with an amplitude of c. 2mV over a tidal cycle. Long term conductivity monitoring of this borehole suggests that short-term salinity variations are controlled by the semi-diurnal tidal cycle and fortnightly spring and neap tides, while long-term control appears to be governed by the inland freshwater head. We hypothesise that tidal fluctuations control the short-term lateral movement of the saline-freshwater interface within the aquifer. This gives rise to the SP observed at the monitoring well some distance up-gradient of the interface. On-going monitoring will help test this hypothesis, as a decline in freshwater head over the summer should cause the mean position of the saline-freshwater interface to move inland. Consequently, we expect to observe an increased SP signal as the oscillating front approaches the monitoring borehole. Our results suggest that EC potentials across a saline front in the UK chalk aquifer are expected to dominate the measured SP at ambient conditions, but EK potentials are also likely to contribute to the SP measured during groundwater abstraction in coastal aquifers where saline intrusion is occurring.