Self-potential monitoring of seawater intrusion in a fractured coastal aquifer.

Abstract:

We investigate whether borehole measurements of self-potential (SP) can be used to monitor seawater intrusion into the UK chalk aquifer. The SP, a naturally occurring voltage, arises in water saturated porous and fractured media due to gradients in pressure (streaming potential) and concentration (exclusion-diffusion potential), both of which occur during seawater intrusion. We use an electrode array installed in an observation well c. 1.7km from the coast and 1.3km from an active abstraction well. Head fluctuations in the observation well are primarily controlled by tidal processes and seasonal changes in groundwater recharge.

SP monitoring over a period of 1.5 years has revealed semi-diurnal fluctuations in voltage with larger amplitude (c.200μV) than those observed at a comparable inland site in the same chalk aquifer (c.0.2μV from spectral analysis). Numerical simulation of the coupled hydrodynamic and electrical processes in the coastal aquifer suggests that the streaming potential generated by tidal processes is one order of magnitude too small to be responsible for the semi-diurnal fluctuations in voltage. Instead, the signal is caused by the exclusion-diffusion potential across the saline front, the location of which moves in response to the tidal cycle. Thus the SP signal recorded at the well arises from the saline front which is some distance away.

In August 2013 and 2014, tidal processes coupled with a decline in inland head caused saline water to enter the observation well. Electrical conductivity logging over a tidal cycle showed that entry was via a fracture near the base of the well. Prior to each occurrence of saline breakthrough, an increase in the SP of c.300μV was observed, commencing c.5 days before saline water was detected in the well. Numerical simulation suggests that the SP precursor is dominated by the exclusion-diffusion potential across the saline front as it advances through the fracture.

Although we focus here on an observation well, SP monitoring arrays could be installed in abstraction wells and our results suggest that the data may be used to provide early warning of saline water breakthrough, allowing for improved management of groundwater resources in coastal aquifers.