Assessing the impact of saline intrusion with density dependent flow modelling for the fractured Peninsula Aquifer in Hermanus, South Africa

In the study area of Hermanus, South Africa, the Gateway Wellfield is used to augment municipal water supply. Due to the coastal location, the impact of saline intrusion needs to be considered. The confined Peninsula Aquifer comprises a complex fault system in a fractured rock environment. Analytical equations do not indicate any negative impact of saline intrusion. The impact of different parameters on the interaction between fresh and sea water was tested in a 2D sensitivity analysis. The hydraulic gradient and dispersivity were identified as crucial parameters. In addition, the impact of discrete fractures within a porous medium was tested. Fracture apertures in the likely range of the case study (b < 1 mm) showed a negligible effect. The geology of the Peninsula Aquifer was modelled as an Equivalent Porous Medium (EPM) whereby highly fractured zones around the faults were assigned high hydraulic conductivity. A maximum increase in salinity of 30 mg/l was predicted for the first 20 years of groundwater abstraction. An impact of vertical fractures with b > 1 mm was detected that is hardly predictable. In order to prove the gained conclusions and completely eliminate a harmful impact, further investigations are recommended.