While implementing emergency water-supply schemes for the drought-impacted southern Cape coastal region of South Africa, boreholes were drilled directly adjacent to the Knysna Estuary to provide saline feed water to the desalination plant. The siting of the boreholes was based on a conceptual understanding of the position of the saline water – freshwater interface provided by the Ghyben-Herzberg relationship. The water yielded by the boreholes during testing and production was of lower salinity than expected. It was found that low-salinity groundwater initially provided about half the water to the boreholes, gradually decreasing to 28% over a period of 7 months of continuous pumping. The salinity gradually increased from 2 600 mS/m to 4 000 mS/m as the relative contribution of seawater from the estuary increased. These observations highlighted the fact that the Ghyben-Herzberg relationship is based only on the density of two fluids, and it fails to recognise the hydrodynamics of the subsurface environment, particularly during periods of groundwater abstraction.