Multi-tracer characterisation of saline groundwater bodies in coastal areas and implications for paleo-hydrology

Abstract:

In coastal aquifers a wedge of intruded seawater extends inland from the coastline and is separated from the freshwater part of the aquifer by a sloping transition zone. Few studies have provided a detailed characterisation of the chemical composition and the age of groundwater within the wedge. This paper presents the results of a field study from South Australia in which a series of multi-level observation wells were installed in a semi-confined aquifer along a transect that extends 1 km inland from the coast. An unexpected finding was that, apart from intruded seawater, a second saltwater type was present in the form of hypersaline groundwater residing in the bottom part of the aquifer. Using the conservative tracers chloride and the stable isotopes of water, a three-end member mixing model was developed. Age tracers, in particular ¹⁴C, revealed that the hypersaline end member is older than the intruded seawater by at least tens of thousands of years. It is postulated that the hypersaline water formed as a result of strong evaporation during a time with dryer climatic conditions than the present, and that the seawater intruded over the hypersaline body when sea level rose during the Holocene. The results of this study testify that the hydrological evolution of coastal areas often lead to much more complex salinity distributions than those based on the classical conception of a coastal aquifer in a steady equilibrium with the present sea level and coastline position.