Modelling self-potentials as a predictor of seawater intrusion in coastal groundwater boreholes

Abstract

Seawater intrusion is pervasive on every inhabited continent, although existing monitoring techniques typically fail to predict the timing of its occurrence in groundwater boreholes. Recent monitoring of a coastal groundwater observation borehole suggests that a nearby saline front can affect self-potential (SP) signals and could be used to predict saline intrusion. The borehole displays a persistent SP gradient, which is absent in boreholes further inland. This gradient reduces several days before saline breakthrough, constituting a possible precursor to intrusion, and fluctuates at the same frequency as oceanic tides. The magnitude of the oscillations reduces prior to breakthrough. A numerical model of the surrounding Chalk aquifer explains the SP gradient and precursor for the first time. The gradient requires spatial variation in the exclusion-efficiency of the aquifer, a parameter that, in the context of seawater intrusion in the Chalk, describes the extent to which anions are excluded from the pore space and which has been linked previously to rocks with narrow pore throats. The modelled precursor is a response to seawater moving through fractures with a change in exclusion-efficiency across them. The fractures driving the precursor may either intersect the borehole or lie immediately beneath it. However, the observed reduction in SP oscillations can only be replicated in the latter scenario. The model relies on relatively high values of exclusion-efficiency in marl horizons, a parameter that had not been measured previously in this lithology. The results of marl testing carried out as part of this project closely agree with the simulated values and provide greater confidence in the model. These SP phenomena were observed in a single coastal groundwater observation borehole. To understand fully the applicability of SP monitoring and modelling for predicting seawater intrusion, it should now be applied to boreholes in different aquifer types, including operating abstraction boreholes.Open Acces