Origin and geochemical evolution of saline groundwater in the Brisbane coastal plain, Australia

Abstract Fresh, brackish and saline groundwaters of variable acidity and redox potentials occur in a coastal floodplain area, north of the Brisbane River in Southeast Queensland, Australia. The complicated spatial and vertical distribution of these groundwaters, and their tendency towards a seawater chemistry, reflect that the local groundwater system has undergone repeated episodes of hydrological adjustments in response to Quaternary sea-level fluctuations and coastal landscape modifications. Since the last major Holocene transgression, topographic changes have permitted repetition of evaporative concentration, aeration and acidification processes in near-surface groundwaters. The surface and subsurface occurrences of gypsum, pyrite and jarosite minerals, in the Brisbane coastal plain, testify to the intrinsic relationship between sea-level and topographic variations, and hydrogeochemistry of the local groundwater system.