Characterisation of intertidal springs in a faulted multi-aquifer setting

In intertidal zones, groundwater is often present as seepage that provides freshwater and nutrients to marine ecosystems. Point discharge or springs in intertidal zones have been observed in many locations, often in the form of sand boils. The spatial extent, temporal variability and source of intertidal springs are rarely documented and typically, not well understood. This study examined four intertidal groundwater springs at Sellicks Beach, South Australia, during May 2017, November 2019 and September 2020 using a combination of hydrogeophysical methods. A thermal infrared survey undertaken in 2017 showed springs as groupings of closely spaced sand boils that were warmer (15 °C) than the surrounding saturated beach sediments (7 °C). The four springs ranged in diameter from 0.20 to 0.45 m. Electromagnetic geophysical surveys identified a resistive anomaly (3.5 to 5.0 ohm.m), assumed to represent freshwater upwelling at the location of a spring, that extended 10 m horizontally and at least 6.7 m vertically. The average electrical conductivity of water discharging from the springs was 18.4 mS/cm, while seawater was 54.8 mS/cm. δ¹⁸O and δ²H data from the springs showed a variation between winter and spring, likely caused by variations in mixing ratios between seawater and groundwater. The springs are proximal to major regional fault systems that likely create preferential flow paths that control spring location and flow rates. The observations of spring characteristics highlight the critical role of seawater-groundwater mixing ratios, preferential flow paths and tidal variations in creating temporal variability in spring discharge and salinity.