Investigation of Potential Saltwater Intrusion Pathways in a Fractured Aquifer using an Integrated Geophysical, Geological and Geochemical Approach

Abstract

Borehole geophysics and horizontal loop electromagnetic profiling (Max-Min) were integrated with regional and site-scale geological and geochemical data to investigate the occurrence of, and possible pathways for, saltwater intrusion near fracture zones on a small island in British Columbia, Canada. An island-wide geochemical study identified a number of coastal wells that are contaminated by seawater; however, the occurrence of high salinity groundwater is spatially irregular due to variable fracturing of the bedrock. To investigate the influence of fracturing on the presence of high salinity groundwaters, geophysical investigations were undertaken at several sites. The nature of the bedrock permeability at these sites, with respect to lithology and fracture zone proximity, is described from geologic and hydrogeologic investigations and supported using surface EM profiling. Fractures and bedding contacts within boreholes, which were suspected to dominate bedrock permeability on the basis of outcrop studies, were identified using borehole video camera in conjunction with normal resistivity, spontaneous potential and natural gamma logs. Flow meter logs, acquired under a variety of aquifer stress conditions including static, tidal and pumping are used to identify potential water transmitting fractures and the locations of entry points for fresh and saline groundwater. The low flow rates measured under the various stress conditions confirm that groundwater flow is minimal and is restricted to mudstone units and single, generally sub-vertical fractures. The low natural groundwater discharge rates near the coast, even at close distances to fracture zones, may be key to the occurrence of saltwater intrusion on many parts of the island.