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.

Review: Saltwater intrusion in fractured crystalline bedrock

“…hydrodynamic dispersion and molecular diffusion (e.g. Cooper 1959;Allen et al 2002;Werner et al 2013). Along the gradient where concentration and fluid density varies between the two end-members (Lu and Luo 2010), groundwater flow is not static but flows in a vertical convection cell (density driven) transporting deep saltwater into the mixing zone (Cooper 1959).…”

“…Particularly heterogeneous aquifers and hence fractured aquifers show a high variation in the location and spatial extension of the mixing zone (Allen et al 2002). Especially on a local scale, system heterogeneity can have a strong effect along preferential flow paths (Werner et al 2013), which needs to be assessed on a regional scale, e.g.…”

“…industrial effluents, road de-icing salts and land use activities like irrigation) and natural dissolution of rock salt (e.g. Allen et al 2002;Allen 2004;Barlow 2003;Barlow and Reichard 2010).…”

Evaluating the Application of Electrical Resistivity Tomography for Investigating Seawater Intrusion

“…Lipfert et al [2] successfully used long-term resistivity to monitor seawater intrusion. Allen et al [3] used geophysics, geology and geochemistry to investigate seawater intrusion potential. Hodlur [4] used the ERT and monitoring well data to successfully divide the aquifer partition between seawater and freshwater.…”

Groundwater Aquifer Characterization Using Geophysical Methods

“…Geophysical methods have been used for groundwater management for a long time. Such methods are very efficiend in solving groundwater-related problems such as; find groundwater aquifers (Inverarity et al, 2011), seawater intrusions (Allen, et al, 2002), managing groundwater aquifers (Skokan and Munoz, 2010;Abraham et al, 2010), monitoring aquifers (Eigenberg and Woodbury, 2012), track flow paths (Kofoed et al, 2011), etc. Kirsch (2009 presents a good review of different geophysical methods and how we can use it in groundwater applications.…”