Summary
Over the last thirty years, groundwater pumping to supply water to the community on Milingimbi Island has resulted in declining water levels and increasing groundwater salinity towards the end of the dry season. Recent studies have indicated that that the limited groundwater supply may not be capable of supplying future water needs of the community, which is planned to grow significantly in the next few decades. This study used geophysical and hydrogeological methods to characterise the hydrological system on the island, constrain recharge rates and evaluate the risk of saltwater intrusion. As part of this investigation local community members from the island were engaged in training and use of near-surface geophysical instruments, as well as to share their knowledge so that it could be integrated into the investigation.
Introduction
For the 1600 Indigenous residents who live in Milingimbi, a remote island off the coast of Arnhem Land in the Northern Territory, drinking water is a scarce and precious resource for which they rely solely on aquifers beneath the island. Studies have shown that, as the population grows (to a projected 2300 residents by 2030), the island’s limited fresh groundwater reserve – which is already vulnerable to saltwater intrusion from the surrounding Arafura Sea – will not be capable of supplying the future water needs of the community. Solutions such as seawater desalination are commonly unfeasible.
The annual rainfall is high (1090 mm) but there is uncertainty as to what proportion replenishes the freshwater lens and how much is lost to evapotranspiration and lateral flow to the coast. Groundwater recharge of the freshwater lens takes place during the wet season between November and April with very little rainfall occurring during the dry season. Groundwater monitoring has shown that there has been a notable increase in salinity. Recent drilling investigations identified that there is interconnection between the more saline aquifer that lies beneath the fresher shallower aquifer and that over pumping of the fresh aquifer could result in saltwater migration inland and/or contamination by the more saline aquifer below.
