Monitoring saltwater influx into freshwater aquifers using measurements of spontaneous potential

Coastal aquifers are valuable resources of fresh water for domestic and industrial use. However, over-abstraction leaves them at risk of contamination by saltwater, which migrates into the aquifer in response to abstraction. Detecting and monitoring the movement of saltwater is difficult, as the methods currently available rely on data acquired at production or monitoring wells. Consequently, it is typically of low spatial resolution. Moreover, once saltwater reaches the production well(s), it is too late to take action; that region of the aquifer has already been contaminated. This problem is exacerbated in chalk aquifers owing to their dual porosity behaviour and high transmissivity. The aim of this project is to develop new technology, based on measurements of electrical potential (the so-called spontaneous- or self-potential), to detect and monitor the movement of saltwater during freshwater abstraction. The electrical potential is measured using electrodes installed at the earth surface and/or in production or monitoring wells. The advantage of the technique is that saline fronts may be monitored while they are several tens to hundreds of metres away from the monitoring location. Consequently, saline water moving into an aquifer may be detected before it reaches the abstraction well(s), allowing abstraction to be managed proactively so as to avoid widespread contamination of the aquifer. The innovative, multidisciplinary project builds on existing work undertaken at Imperial College, in which measurements of the spontaneous potential, acquired from electrodes permanently installed downhole, are used to monitor water movement in oil or gas reservoirs during production. The underlying rationale in hydrocarbon applications is similar, in that waterfronts may be monitored while they are several tens to hundreds of metres away from the production well, allowing production to be managed proactively so as to avoid excessive unwanted water. This is contaminated and so expensive to treat and dispose of. The project will provide new experimental data, which is required to model and interpret measurements of spontaneous potential in coastal chalk aquifers. The project will also use numerical modeling to determine whether the saline front can be detected and monitored, and over what distances and at what spatial and temporal resolution. Finally, the utility of the method will be demonstrated at a well-characterized chalk aquifer test site, via field experiments in which small volumes of saline water are injected and the resulting electrical signals are measured. This is an essential step to establish credibility prior to deployment in a commercial abstraction project. The results will be of direct benefit to UK plc, because they contribute to the sustainable management of water resources, helping to preserve and maintain these as demand continues to increase. See the attached document 'Case for Support' for further details of the scientific case, aims and objectives, and scope of work.