Présentations Colloques

Session 8.02: Groundwater Development and Protection in Coastal and Volcanic Environments with Complex Geological Structures
Graham (orateur)
Predicting Seawater Intrusion in Coastal Groundwater Boreholes Using Measurements and Modelling of Self-Potentials
Many coastal groundwater boreholes are under threat from seawater intrusion- this is exacerbated in summer by low water tables and increased abstraction. Existing hydrochemistry or geophysical techniques often fail to predict the timing of intrusion events. We investigate whether the proximity of seawater can influence self-potentials (SPs) measured within groundwater boreholes, with the aim of using SP monitoring to provide early warning of saline intrusion.**SP data collection- SP data were collected from a coastal groundwater borehole in the fractured Chalk of England. Downhole SP monitoring was conducted at a second site, located more than 60 km inland and also within the Chalk. Spectral analysis showed that semi-diurnal SP fluctuations were several orders of magnitude higher at the coast than inland, indicating a strong influence from oceanic tides. Seawater intrusion occurred in the coastal borehole on several occasions during the monitoring period. A characteristic increase in SP (c.100-300 microvolts) was observed within the borehole array, several days before saline breakthrough.**Modelling results- Hydrodynamic and geoelectric modelling suggest that observed pressure changes (associated with the streaming potential) are insufficient to explain the magnitude of semi-diurnal SP fluctuations, whilst modelling of the exclusion-diffusion potential closely matched the observed coastal data. Simulations of a homogenous aquifer consistently produced a negative precursor, although the inclusion of a fractured zone led to the development of strongly positive precursor signals, comparable with field observations. Sensitivity analysis suggests variations in the dispersion of the saline front, associated with aquifer heterogeneity, are an important control on the nature of SP precursors.**Conclusions- Our results show that combined SP monitoring and modelling holds considerable promise as an early warning device for seawater intrusion. We now aim to refine our understanding of the technique by applying it to a range of aquifer types. **
United Kingdom