Measuring and Monitoring Saltwater Intrusion in Shallow Unconfined Coastal Aquifers Using Direct Current Resistivity Traverses

Saltwater intrusion into coastal aquifers is potentially a major problem in New Zealand, as in other areas around the world. Saline intrusion is usually investigated using point samples, e.g., discrete borehole measurements, and their analysis involves considerable extrapolation. Resistivity traverses, however, produce a two- or three-dimensional image of the saline interface. The technique has considerable potential for monitoring the interface, thereby facilitating management of coastal aquifers. This study, using three examples from different field settings, shows that bulk electrical resistivity changes by two orders of magnitude across the fresh-saline boundary. The exact values depend on the interaction of the pore fluid with the aquifer media, i.e., the formation factor of the aquifer. At all sites a change in resistivity occurs at approximately the mean high tide mark, with significant mixing of fresh and saline water occurring inland of this position. There was no evidence of either the sharp boundary, or concave shape, to the interface predicted by simple models. Rather, the interface is a distributed zone through which saline and fresh waters mix. The shape of the interface depends on the thickness and stratigraphy of the aquifer, and its topographic profile and hydraulic gradient. Resistivity traverses from sites at Te Horo and Rarangi, which have significant groundwater abstraction, show that the mixing zone at the interface extends further inland, causing a significant reduction in resistivity. Monitoring changes in resistivity can therefore provide an early indication of saltwater intrusion. Resistivity traverses are relatively cheap and non-intrusive, and offer considerable potential for both locating the position of the saline interface and monitoring any saltwater intrusion.