Electrical resistivity imaging for assessing groundwater resources in coastal aquifers

Introduction

Securing reliable water supplies—especially in areas where water is scarce—is key for socioeconomic development and food security. Irrigated agriculture from both surface and groundwater resources has been continuously expanding and today accounts for 20% (year 2010) of global cropland (FAOSTAT) and produces as much as 44% of total crop production (Alexandratos and Bruinsma, 2012). Groundwater extraction has facilitated significantly enhanced food security in many regions. However, groundwater development has also depressed water tables, degraded ecosystems, and led to the deterioration of groundwater quality, as well as to conflict among water users (Gleeson et. al, 2012).The management of groundwater in coastal areas across the globe is already in a critical state because urbanization and irrigated agriculture have claimed significant amounts of groundwater resources with associated higher risks of seawater contamination thus resulting in abandonment of wells (Lewis et al., 2003; Gaaloul et al., 2012). This study evaluates the benefit of using electrical resistivity method for water well development in coastal areas.

Methods

An ABEM SAS 1000 Terameter system and ES 10-64 switching unit was used for 2D Electrical Resistive Imagery (ERI) survey in the Heuningnes catchment. Four multicore cables and stainless steel pegs were used with the “roll-along” survey method. Eleven resistivity traverses were conducted using a Wenner array electrode spread of 240 m with 4 m electrode spacing which yielded a maximum investigation depth of 24 m. Finally, ERIs were created by inverting the measured resistivities using the software RES2DINV.

Results and conclusions

The ERI interpretation suggests a maximum of four sub-surface layers including a brackish water saturated layer (conductive blue layer), high resistive layers (red shale and sandstone), a moderate resistivity clay layer and a fresher water saturated layer. These layers coincide well with the geology of the area. The ERI method successfully located suitable areas for well development. The study results suggest that ERI can be used as a suitable support tool for planning boreholes/wells for groundwater supply development and for monitoring. ERI analysis also helps to prevent siting of borehole in a saline water saturated zone; thus minimising borehole abandonment in coastal areas. Results will be validated by borehole drilling.

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Last edited: 03 February 2016