Geological Control upon Groundwater Flow and Major Ion Chemistry with Influence on Basin Management in a Coastal Aquifer, South Australia

Abstract

Estimation of natural recharge and potential for seawater intrusion are critical considerations for management of coastal freshwater aquifers. We show hydrochemical signatures of groundwater to identify the influence of geological control on chemical processes in a coastal groundwater system. We used dominant hydrochemical facies, salinity and magnesium ions to determine two main groundwater flow paths with different origins and ages. Mixing of groundwater with different origins and ages results in unreliable recharge estimates using chlorofluorocarbon (CFC) and chloride mass balance (CMB) methods, thus limiting available methods for recharge assessment. Interpretation of hydrochemical data suggests that calcium carbonate dissolution, ion exchange processes and mixing with sea aerosol in coastal zones are the main influencing factors on groundwater chemistry. Restricted groundwater flows due to occurrence of a basement high at the southern side of the basin boundary influences the distance to the toe of the saline wedge. Thus, knowledge of geological control over groundwater systems forms an important part of characterising basins and contributes toward effective management of groundwater resources.