The use of specific-depth sampling technique to demonstrate groundwater quality variation different groundwater units of unconfined aquifers has not been widely published. To demonstrate the feasibility of such technique, the unconfined Cape Flats Aquifer (CFA) in Cape Town of South Africa was studied. The aquifer underlies an urbanised area which is vulnerable to contamination from industrial and agricultural activities, waste disposal sites, landfill sites, and formal and informal settlements. The study assessed Spatio-temporal and depth variation salinity levels in CFA using electrical conductivity (EC) as an indicator of salinity. Groundwater samples were collected using specific-depth sampling, and analysed using multi-parameters probes and standard laboratory methods for EC, temperature, pH and major ionic concentrations. Statistical analysis was used to compare mean concentrations of selected parameters to guidelines set by Department of Water and Sanitation and Food and Agricultural Organization to establish fitness for irrigation use.
The results showed high EC levels (212.26 mS/m) at shallow depths (9m) and low EC levels (78.53 mS/m) at greater depths (39m) proposing anthropogenic influence. Potassium, sodium, chloride, and the Sodium Adsorption Ratio (SAR) exceeded permissible ranges set for irrigation water suggesting that groundwater be used with caution. A conceptual diagram was developed to explain sources and processes contributing to groundwater salinization of the aquifer. The diagram illustrated that irrigation return flow, in residential and agricultural areas, contributed significantly to salinity levels. In conclusion, groundwater in the CFA is suitable for irrigation use but should be used with caution as shallow depths contain groundwater with elevated salinity levels. It is recommended that the specific-depth sampling technique be used to understand how the physical, chemical and microbiological constituents vary with depth in these groundwater units.