Seasonal saline intrusion monitoring of a shallow coastal aquifer using time-lapse DC resistivity traversing

Long-term time-lapse DC resistivity measurements along six 145- to 275-m-long transects have been carried out to image seasonal changes in the resistivity distribution within a shallow coastal aquifer on the Kapiti Coast, New Zealand. Two-monthly repetition of the measurements shows a continuous decrease in the bulk resistivity at depth over an extended horizontal portion of the profile lines when coming into the drier summer season. This is most distinct in more rural locations and less pronounced along urban profiles, and is interpreted to be the result of a decrease in the fluid resistivity resulting from lower freshwater pressure that allows saltwater to intrude. Coming into the wetter winter season, the bulk resistivity at depth increases again as freshwater pressure in the aquifer increases. A 24-hour tidal cycle monitoring along one of the northern profiles, observations of seawater level stages, and tidal ranges show that the observed resistivity changes are seasonally driven rather than tidal overprints. Time-lapse monitoring using electrical resistivity tomography has thus proven itself as a suitable tool for imaging both the saline interface shape and extent and the changes in the subsurface resistivity distribution in an unconfined coastal shallow sand aquifer over a seasonal time frame. This significantly adds to the knowledge about the dynamic behaviour near the saline interface in coastal aquifers and can be used for groundwater assessment in this and other similar geological and hydrological environments around the world.