Salinity Distribution in Heterogeneous Coastal Aquifers Mapped by Airborne Electromagnetics

Understanding the hydrologic seƫ ng of any given area is a challenging task, especially considering that the amount of available informaƟ on is oŌ en very sparse relaƟ ve to the size of the area and the complexity of the problem. We used geophysical transient electromagneƟ c (TEM) data from an airborne SkyTEM instrument to invesƟ gate discharge of freshwater from a river catchment into a coastal lagoon area, in support of large-scale hydrologic mapping of the HOBE hydrology research center. ExisƟ ng hydrologic data indicate possible ouƞ low from the catchment underneath the survey area, a hypothesis that was indeed backed up by our geophysical results. The results revealed a straƟ fi ed geologic seƫ ng frequently incised by buried valleys beneath the lagoon. They also showed that the valleys are saturated with sea water, unlike the layers at the same depth that are probably saturated with brackish water. The salinity decreases with depth in large parts of the lagoon. The salinity distribuƟ on in the seƫ ng indicates the presence of intricate fl ow systems beneath the coastal area and the lagoon, and we propose that it is governed by the heterogeneity in the geologic seƫ ng. We also demonstrated how water depth as well as salinity can be accurately mapped using SkyTEM in combinaƟ on with a robust and fl exible inversion scheme.