title: Marine water from mid-Holocene sea level highstand trapped in a coastal aquifer: Evidence from groundwater isotopes, and environmental significance.
abstract: A multi-layered coastal aquifer in southeast Australia was assessed using environmental isotopes, to identify the origins of salinity and its links to palaeo-environmental setting. Spatial distribution of groundwater salinity (electrical conductivity values ranging from 0.395 to 56.1 mS/cm) was examined along the coastline along with geological, isotopic and chemical data. This allowed assessment of different salinity sources and emplacement mechanisms. Molar chloride/bromide ratios range from 619 to 1070 (621 to 705 in samples with EC >15 mS/cm), indicating salts are predominantly marine. Two distinct vertical salinity profiles were observed, one with increasing salinity with depth and another with saline shallow water overlying fresh groundwater. The saline shallow groundwater (EC=45.4 to 55.7 mS/cm) has somewhat marine-like stable isotope ratios (δ(18)O=-2.4 to -1.9 ‰) and radiocarbon activities indicative of middle Holocene emplacement (47.4 to 60.4pMC). This overlies fresher groundwater with late Pleistocene radiocarbon ages and meteoric stable isotopes (δ(18)O=-5.5 to -4.6‰). The configuration suggests surface inundation of the upper sediments by marine water during the mid-Holocene (c. 2-8 kyr BP), when sea level was 1-2m above today's level. Profiles of chloride, stable isotopes, and radiocarbon indicate mixing between this pre-modern marine water and fresh meteoric groundwater to varying degrees around the coastline. Mixing calculations using chloride and stable isotopes show that in addition to fresh-marine water mixing, some salinity is derived from transpiration by halophytic vegetation (e.g. mangroves). The δ(13)C ratios in saline water (-17.6 to -18.4‰) also have vegetation/organic matter signatures, consistent with emplacement by surface inundation and extensive interaction between vegetation and recharging groundwater. Saline shallow groundwater is preserved only in areas where low permeability sediments have slowed subsequent downwards propagation. The configuration is unlikely to be stable long-term due to fluid density; this may be exacerbated by pumping the underlying aquifer.
authors.affiliation: School of Civil, Environmental and Chemical Engineering, RMIT University, Melbourne, Australia.
authors.forename: Stephen
authors.initials: S
authors.lastname: Lee
authors.affiliation: School of Civil, Environmental and Chemical Engineering, RMIT University, Melbourne, Australia. Electronic address: Matthew.currell@rmit.edu.au.
authors.forename: Matthew
authors.initials: M
authors.lastname: Currell
authors.affiliation: Australian Nuclear Science and Technology Organisation, Kirrawee, Australia; Connected Water Initiative, School of Biological, Earth and Environmental Sciences, University of New South Wales (UNSW), Sydney, Australia.
authors.forename: Dioni I
authors.initials: DI
authors.lastname: Cendón
keywords: Coastal aquifer; Groundwater salinity; Holocene; Isotopes; Sea level; Trapped seawater