Salinity in the Struisbaai aquifer

[摘要] English: The water-supply for Struisbaai has a historical reputation for its high salinity. Like most of the coastal holiday resortsalong the Cape South Coast there are a number of permanent residents. For Struisbaai these comprise owners ofsmall businesses servicing the holiday trade, fishermen and their families and retired folk. However, unlike othercoastal resort-towns, Struisbaai is notable for the lack of houses with attractive gardens. This is a direct reflectionof the relatively high salinity of the water-supply which in the past was not suitable for irrigation, having salinities ofover 100 mS/m and sometimes over 500 mS/m. For example a sample of town water-supply was collected in June1990 from a tap at Struisbaai Hotel and the EC was found to be 658 mS/m (4150 mg/L of dissolved salts). In 1990a replacement wellfield was developed which has an EC of about 90 mS/m. With this relatively low salinity attractivegardens may become a feature of the future for Struisbaai.Three previous groundwater investigations have taken place, and in all of these the high salinities have been ascribedto over-pumping with resulting seawater intrusion. However, none of these authors gave any conclusive evidencefor their theories. All of these reports gave field observations and then formulated theories for the origin of the salinitybut in none of these reports were the two linked by logical arguments. This thesis thus considered the variouspossible sources of salinity and made conclusions as to the most likely source.The three sources of salinity that were considered are:HH. Salinity derived from sea-spray causing high salinity recharge.H'. Geological factors which yield high salinity groundwater.H'. Hydrogeological factors which result in sea-water intrusion.Field work consisted of drilling of four new boreholes, geophysical down-hole logging of six boreholes, establishingrainwater sampling points, collecting of groundwater samples and processing water samples for isotopic analysis.The methodology used to analyse the data was to closely consider the water quality of rainwater and groundwatersamples as well as the isotopic composition of the groundwater. By considering ratios of the chemical composition,ratios of isotopic composition and using graphical plots the conclusion made was that the source of salinity is due tosea-spray which causes rain recharging to the aquifer to have a high salinity. This rain was measured to have asalinity of 10.5 and 12.5 mS/m respectively for the two rain collector stations. When a recharge to the aquifer of about10% is allowed the resultant salinity is similar to that measured for groundwater at that site.Using the same methodology it was concluded, except for two individual boreholes, that geological andhydrogeological factors are not the source of salinity. For these two individual boreholes it was shown that overpumpingand seawater intrusion was the probable cause of salinity.The decision to replace the old wellfield with the new wellfield has been a fortuitous decision. By doing so two positiveeffects have occurred. Firstly the wellfield is now in an area where the local recharge is less saline due to it beingfurther away from the sea and thus receiving less sea-spray. Secondly the possibility of over-pumping and inducingseawater intrusion is minimal.This thesis has contributed to South African hydrogeology in that as far as can be ascertained this is the firstdocumentation of the contribution that sea-spray makes towards causing salinity in South African coastal aquifers.

[发布机构] University of the Free State