Monitoring experimentally induced Saline Intrusion through vertical Self Potential profiling at costal aquifer in Northern Ireland

Due to greater ground-water abstraction, rising demand for water, possible reductions in recharge rates and rising sea levels, costal aquifers are under ever increasing threat of Saline Intrusion (SI) (Mehdizadeh, 2019). Though the mechanismins of SI have long been understood, the ability to monitor and warn in advance of the ingress of saline water into costal aquifers has remained costly and complex (Graham, 2018). The work reported here describes initial efforts to develop and results from, a vertically profiling Self Potential (SP) device. The device was used to monitoring the position of a well parametrized saline front in a costal aquifer, located on Benone Strand, Co. Derry,  on the northern tip of Northern Ireland, UK, as part of the SALine INtrusion in coastal Aquifers project.

Naturally arising voltages, Self Potential (SP), are formed when pressure and concentration gradients move though the subsurface. The gradients cause ion separations, which create electrical potentials and a flow of electrons in order to maintain electrical neutrality. The SP signals (usually in the millivolt range) can be detected, relatively inexpensively (in comparison to resistivity imagining) with reference electrodes and a high impedance voltage logger. The positioning of the electrodes is key as it has only been possible, until now, to measure the voltage between two points. There are two key types of SP, in hydrology, electro-kinetic potentials (V_EK), due to differential flow velocities, and exclusion-diffusion potentials (V_ED), due to ion concentration gradients with different mobilities. Understanding the source mechanims in these voltages is complex, but evolving. Previous work has shown that self-potential rises before a saline breakthrough into a borehole (Graham, 2018).

A novel vertically travelling (or trolling) SP electrode was repeatedly used in a number of satellite boreholes during a pumping test; in order to look at the changes in the vertical gradient of SP. The pumping test took place over three days, during which initially fresh water was abstracted from the main pumping well. Resistivity imagine was used as a benchmark. It was shown that the vertical SP profile changed as the salt content of the pumped water increased (i.e. the saline front moved inland). This change in SP could not be explained by pressure changes – gradients of 50mV inside a single borehole were observed. The data showed SP profiles that varied widely before, during and after the pumping test, as saline water is drawn progressively towards the pumping well, offering far more data than a single stationary electrode. Demonstrating that these signals change in advance of the saltwater arriving at the pumping well, but also that this method could be used as an inexpensive way to safeguard costal aquifers in the future.

References

Graham, M. T. (2018). Self-Potential as a Predictor of Seawater Intrusion in Coastal. Water Resources Research.

MacAllister, D. a. (2016). Tidal influence on self-potential measurements. Journal of Geophysical Research: Solid Earth.

Mehdizadeh, S. a. (2019). Abstraction, desalination and recharge method to control seawater intrusion into unconfined coastal aquifers. Global Journal of Environmental Science and Management, 5, 107-118.