Salinity in the Struisbaai aquifer - Weaver, John M. C. (University of the Free State, 1998)
English: The water-supply for Struisbaai has a historical reputation for its high salinity. Like most of the coastal holiday resorts along the Cape South Coast there are a number of permanent residents. For Struisbaai these comprise owners of small businesses servicing the holiday trade, fishermen and their families and retired folk. However, unlike other coastal resort-towns, Struisbaai is notable for the lack of houses with attractive gardens. This is a direct reflection of the relatively high salinity of the water-supply which in the past was not suitable for irrigation, having salinities of over 100 mS/m and sometimes over 500 mS/m. For example a sample of town water-supply was collected in June 1990 from a tap at Struisbaai Hotel and the EC was found to be 658 mS/m (4150 mg/L of dissolved salts). In 1990 a replacement wellfield was developed which has an EC of about 90 mS/m. With this relatively low salinity attractive gardens 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 ascribed to over-pumping with resulting seawater intrusion. However, none of these authors gave any conclusive evidence for their theories. All of these reports gave field observations and then formulated theories for the origin of the salinity but in none of these reports were the two linked by logical arguments. This thesis thus considered the various possible 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, establishing rainwater 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 groundwater samples 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 to sea-spray which causes rain recharging to the aquifer to have a high salinity. This rain was measured to have a salinity of 10.5 and 12.5 mS/m respectively for the two rain collector stations. When a recharge to the aquifer of about 10% 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 and hydrogeological factors are not the source of salinity. For these two individual boreholes it was shown that overpumping and 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 positive effects have occurred. Firstly the wellfield is now in an area where the local recharge is less saline due to it being further away from the sea and thus receiving less sea-spray. Secondly the possibility of over-pumping and inducing seawater intrusion is minimal. This thesis has contributed to South African hydrogeology in that as far as can be ascertained this is the first documentation of the contribution that sea-spray makes towards causing salinity in South African coastal aquifers.

Afrikaans: Struisbaai se watervoorsiening is histories van n hoë soutgehalte. Soos die meeste vakansie oorde langs die Kaapse Suid Kus, is daar 'n aantal permanente inwoners. Struisbaai se permanente inwoners is eienaars van klein besighede, vissemanne en afgetredenes. In teenstelling met die ander vakansie dorpies langs die Suid Kus is daar nie erwe met mooi tuine nie. Die rede hiervoor is die relatiewe hoë soutgehalte van die dorpswater. In die verlede was die water se soutgehalte meer as 100 mS/m en in sommige gevalle was die soutgehalte selfs hoër as 500 mS/m. 'n Monster wat in Junie 1990 in die Struisbaai Hotel geneem is, het 'n elektriese geleiding van meer as 658 mS/m gehad (dit is gelykstaande aan 4150 mg/L se opgeloste soute). In 1990 is 'n nuwe grondwaterbron ontwikkel. Die water van hierdie bron het 'n elektriese geleiding van omgeveer 90 mS/m. Hierdie verbetering in die soutgehalte van die watervoorsiening mag dalk daartoe lei dat mooi tuine in Struisbaai nog 'n algemene verskynsel sal word. In die verlede is drie onafhanklike geohidrologiese ondersoeke op Struisbaai uitgevoer. In al drie die ondersoeke is die hoë soutgehalte van die grondwater toegeskryf aan seewater indringing as gevolg van te hoë pomptempos. Nie een van hierdie skrywers het oortuigende bewyse gelewer van hierdie teorie nie. AI drie verslae het veldwaarnemings voorgelê, waarvandaan gevolgtrekkings gemaak is. Geeneen kon die twee logies verbind nie. Hierdie tesis kyk na al die moontlike bronne van versouting en verwys daarvolgens na die mees waarskynlikste bron. Die drie moontlike oorsake van versouting wat in ag geneem is: Versouting as gevolg van seesproei wat daartoe lei dat afloopwater wat die grondwaterbron aanvul 'n hoë soutgehalte het. Geologiese faktore wat grondwater van 'n hoë soutgehalte lewer. Hidrogeologiese faktore wat gelei het tot seewater indringing. Vir die ondersoek is vier addisionele boorgate geboor, geofisise waarnemings is uitgevoer in ses bóorgate, reënwater monsterneming punte is opgestel en grondwater monsters is ingesamel vir chemiese en radio-isotoop analiese. Die metodiek vir hierdie ondersoek was om die watergehalte van die reën met die van die grondwater te vergelyk en te kyk na die isotoop samestelling van die grondwater. Deur die verhoudings van die chemiese samestelling asook die verhoudings van die isotoop samestellings grafies te vergelyk kon die gevolgtrekking gemaak word dat die seesproei die bron van versouting is deurdat die reênval afloopwater aanvullingswater met n hoë soutgehalte lewer. Die chemiese analise van die reënwatermonsters het bewys dat die soutgehalte van die reën by die monsterpunte wissel van 10.5 mS/m tot 12.5 mS/m. As'n 10% aanvullingsyfer in ag geneem word is die soutgehalte van die grondwater dus gelykstaande aan die gekonsentreerde soutgehalte van die reênwater. Deur van dieselfde metodiek gebruik te maak, kon die gevolgtrekking gemaak word dat behalwe vir twee van die boorgate, die geologiese en geohidrologiese faktore nie die bron van versouting is nie. In die geval van die twee boorgate is seewater indringing as gevolg van 'n te hoë pompontrekkings tempo waarskynlik die bron van versouting. Die besluit om die produksieveld met 'n nuwe een te vervang het gely tot twee goeie resultate. Eerstens is die watervoorsieningsbron verder van die see geleë in 'n area waar die soutgehalte van die afloopwater wat as aanvullingswater dien baie laer is. Tweedens is die waarskynlikheid van seewater indringing as gevolg van verhoogde pomptempo minimaal. Hierdie tesis het'n bydrae gelewer tot die Suid Afrikaanse hidrologie in die sin dat sover moontlik vasgestel kon word is dit die eerste skrywe wat die rol van seesproei verbind met 'n verhoogde soutgehalte in Suid Afrikaanse kusakwifere.
Keywords: Aquifers -- South Africa -- Struisbaai; Salinization -- Environmental aspects; Groudwater -- Sodium content -- South Africa -- Struisbaai; Dissertation (M.Sc. (Geohydrology))--University of the Free State, 1998