Investigation of groundwater salinity using geophysical and geochemical approaches: heuningnes catchment coastal aquifer. Western Cape Province, South Africa

Abstract

Seawater intrusion is recognised as a major factor affecting the quality of groundwater in coastal aquifers around the world. To determine the occurrence and extent of saline groundwater intrusion prevailing in the Heuningnes Catchment, a 2D resistivity imaging survey was carried out in three sites within the catchment. Eleven resistivity profile lines were conducted using the multi-electrode ABEM SAS resistivity meter system, employing the Wenner Array to map the extent of saline groundwater. A 2D resistivity image was acquired for these resistivity profile lines and the data were processed using Res2DINV software to produce the 2D inverse resistivity models. The interpretation of the resistivity models revealed three resistivity zones. The first zone is characterised by low resistivity (1–3 Ω.m); this represents an aquifer filled with saline water. The second resistivity zone has values ranging from (3–30 Ω.m); this represents a brackish saturated formation. The third resistivity zone denotes a sandy layer/sandstone saturated with freshwater having high resistivity values above 30 Ω.m. Geochemical analysis results of water samples at 11 locations along the electrical resistivity profile lines also suggest high salinity, indicated by high levels of Cl−, Na+, Total Dissolved Solids, and Electrical Conductivity. It is concluded that some parts of the study area are impacted by seawater intrusion, based on the lower resistivity of < 3 Ω.m and ionic ratios which were close to the standard seawater ratios for Na+/Cl− and SO42−/Cl−. In addition, the high ionic ratios of Na+/Cl− and SO42−/Cl− suggest that salinity in the area is a result of dissolution of evaporates and dilution of marine clays rich in sodium. Therefore, salinization in the study area is the result of combined processes. Integration approach proved to be significant in mapping saline groundwater in coastal aquifers. Integrated assessment of resistivity data, ionic analysis and chemical interpretation indicate that the study area is a transition zone from freshwater to saltwater and only few locations are actually impacted by seawater intrusion. The overall groundwater quality is not apt for human ingestion.