Investigating Coastal Groundwater Discharge from the Willunga Basin, using hydrogeophysical techniques

Author: Marianna Ramirez Lagunas

Ramirez Lagunas, Marianna, 2017 Investigating Coastal Groundwater Discharge from the Willunga Basin, using hydrogeophysical techniques, Flinders University, School of the Environment

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Coastal groundwater discharge (CGD) is a noteworthy source of freshwater and nutrients to oceans. It is acknowledged as an important component in groundwater budget calculations and in freshwater-seawater interaction studies. Nevertheless, information regarding CGD is still limited due to the difficulty in identifying discharge zones and the complex processes that control this phenomenon.

This study aims to identify CGD 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. Limited hydrogeological information available from the coastal area of this Basin has resulted in the misinterpretation of the discharge to the sea component of the water balance of the Willunga Basin. To assess the CGD at Sellicks Beach a number of methods have been used in this study. A discharging site (seep) was located and analysed by piezometers at, and in proximity, to the site. A range of different investigative techniques provided considerable insight into the subsurface processes. These techniques included stable environmental isotopes to investigate the origin of the discharging water. In order to examine the composition of the water, the water level fluctuation and water temperature, hydrochemistry analyses were conducted at Sellicks Beach. A near surface geophysical survey to obtain the spatial distribution of the ground conductivity and thermal imagery to locate the freshwater seeps and their alleged preferential flowpaths.

The piezometers measurements indicated that the average temperature of the water at a discharging site ( ̴ 18.5 °C) was higher than that of the sites where no discharge was visible ( ̴ 16.4 °C). Additionally, the water level measurements showed the highest upward gradient at the discharging site. The hydrochemistry assessment indicates that the water at the visible discharging site is composed of freshwater and that during high tide periods is influenced by seawater. These results suggested that the water at the discharging site can be associated to freshwater and also correlates to the tidal cycle variations.

The near surface geophysical survey provided a 3D resistivity model that along with the previous methods, showed the groundwater flowing towards the sea. The thermal imagery was able to identify three additional discharging sites in the surrounding area. The temperature registered with the thermal images indicated that the water flows at ̴ 18-19 °C, these values are supported by the temperate results obtained with the piezometers and indicate the presence of groundwater. This method was also able to identify a preferential flowpath towards the sea.

Even though the techniques applied in this study are reasonably different, the results are consistent and revealed the presence of CGD at Sellicks Beach, as it was confirmed that the use of multiple techniques provides a good way to understand near surface water flow in coastal environments.

Keywords: groundwater, geophysics, hydrogeology, Willunga

Subject: Environmental Science thesis

Thesis type: Masters
Completed: 2017
School: School of the Environment
Supervisor: Ilka Wallis