Submarine groundwater discharge (SGD) is a noteworthy source of freshwater and nutrients to oceans. It is acknowledged as an important component in groundwater budget calculations, in freshwater-seawater interaction studies, and in the evaluation of nutrients loads to marine systems.
Objectives
This project aims to identify SGD zones occurring as freshwater seeps, in Sellicks Beach, South Australia, located in the south-western part of the Willunga Basin. The basin comprises a multi-aquifer system that provides freshwater to the McLaren Vale region.
Design and methodology
The study characterised a groundwater discharge site, in the form of a bubbling seep, at Sellicks Beach. Techniques included the installation of piezometers to monitor water level and temperature fluctuations, in addition to hydrochemical and stable isotope analyses to investigate the origin of the discharging water, and a near-surface geophysical survey to obtain the spatial distribution of the ground conductivity. Thermal imagery was also used to locate the distribution of groundwater expressions within the intertidal zone.
Original data and results
Salinity measurements showed that discharging groundwater is approximately 10,000 mg/L during low tide but increases to 28,000 mg/L during high tide periods due to the influence of seawater. The near-surface geophysical survey mapped the subsurface distribution of freshwater, showing an upwelling freshwater plume significantly wider than the width of surface seepage zones. Thermal imagery was able to identify three additional discharge sites in the surrounding area.
Conclusion
The use of multiple techniques to characterise these intertidal springs provided a robust characterisation of the near-surface conditions and demonstrated complementary aspects of each method in identifying freshwater-seawater interactions in a dynamic setting.