Groote Schuur ground­wa­ter probe

Springs, bore­holes have enough good-qual­ity wa­ter to ir­ri­gate his­tor­i­cal es­tate, study shows

Weekend Argus (Saturday Edition) - - PROPERTY -

GROOTE Schuur Es­tate in Cape Town has been home to the VOC gra­nary, Ce­cil John Rhodes and South Africa’s pres­i­dents. The Groote Schuur Minute be­tween the ANC and FW de Klerk, then pres­i­dent of South Africa, was also signed here.

In a new devel­op­ment, to en­sure that these his­tor­i­cal grounds are well main­tained in an eco-friendly way, the es­tate has been the site of a study to de­ter­mine its ground­wa­ter po­ten­tial for ir­ri­ga­tion.

Ground­wa­ter is wa­ter found un­der­ground in por­ous sandy strata, and in the frac­tures and fis­sures of rock for­ma­tions. Where ground­wa­ter oc­curs in rea­son­able quan­ti­ties, it is re­ferred to as an aquifer.

This study is part of a larger ir­ri­ga­tion stor­age and sup­ply sys­tem

‘Two pos­si­ble bore­holes drilled in 1961 have been dis­cov­ered. These had been for­got­ten over the past decade due to work done’

up­grade to be un­der­taken by the Depart­ment of Pub­lic Works, the client. The lead con­sul­tant (civil en­gi­neer­ing) is Aure­con and the ground­wa­ter as­sess­ment and hy­dro­cen­sus have been un­der­taken by Muizen­berg-based wa­ter and earth s ci ences con­sul­tancy Umvoto Africa.

The hy­dro­cen­sus has in­volved in­ves­ti­gat­ing all cur­rent wa­ter points such as bore­holes and springs within and sur­round­ing the project area.

As much in­for­ma­tion as pos­si­ble has al­ready been gath­ered on fac­tors such as con­struc­tion of ex­ist­ing bore­holes – depth, di­am­e­ter, cas­ing – yield of the bore­hole, how much wa­ter is used, what it’s used for and sam­ples for hy­dro­chem­i­cal anal­y­sis. This gives a good idea of the aquifer char­ac­ter­is­tics in the vicin­ity.

This on-site re­search is be­ing sup­ple­mented with a desk­top-based ground­wa­ter as­sess­ment – us­ing data sources such as ge­o­log­i­cal and hy­dro­ge­o­log­i­cal maps, Google Earth map­ping, old re­ports and Depart­ment of Wa­ter Af­fairs databases.

Us­ing the Depart­ment of Wa­ter Af­fairs’ old Na­tional Ground­wa­ter Data­base, Umvoto has al­ready dis­cov­ered two pos­si­ble bore­holes drilled in 1961. These had been for­got­ten over the past decade due to the nu­mer­ous con­trac­tors that have been do­ing work on the site over the years, says Umvoto manag­ing direc­tor Rowena Hay.

“The project has been in­ter­est­ing be­cause of its his­tor­i­cal con­text, and also un­rav­el­ling the com­plex sur­face wa­ter-ground­wa­ter in­ter­ac­tions that oc­cur in the area. These in­ter­ac­tions have led to the devel­op­ment of nu­mer­ous springs, which have been used for many years, in the Wyn­berg-New­lands-Ron­de­bosch and city bowl ar­eas. Two springs – Nurs­ery Spring and Glenn Spring – are vis­i­ble in the es­tate.”

Springs oc­cur when ground­wa­ter emerges nat­u­rally on the earth’s sur­face. In this case, the springs are fed by ground­wa­ter leak­ing out of the Penin­sula For­ma­tion sand­stone that forms Ta­ble Moun­tain, which lies above the Cape Gran­ite Suite and Malmes­bury Group shale, which are rel­a­tively im­per­me­able.

Rain­wa­ter flows into open sur­face frac­tures and faults within the Penin­sula For­ma­tion aquifer, and then into larger frac­ture sys­tems that ex­tend from the Cape Penin­sula into the un­der­ly­ing Malmes­bury Group shale. Ground­wa­ter emerges at spe­cific points or dif­fuse ar­eas as springs or seeps, where these frac­ture and fault sys­tems have closed suf­fi­ciently in the Malmes­bury Group (ei­ther due to me­chan­i­cal weak­ness or weath­ered clay in­fill) to cause the pres­sure head within the wa­ter to force it to the sur­face.

The perennial, strong spring flow and high wa­ter qual­ity sug­gests the spring wa­ter is from the Penin­sula For­ma­tion aquifer, de­spite ex­it­ing from the Malmes­bury Group shales, which are gen­er­ally low-yield­ing with poor wa­ter qual­ity, says Hay.

Umvoto com­pleted the re­port last month. Early in­di­ca­tions are there is suf­fi­cient ground­wa­ter from both es­tate springs as well as cur­rent bore­holes to sup­ply a large por­tion, if not all, of the es­tate’s ir­ri­ga­tion re­quire­ments, keep­ing this his­tor­i­cal part of Cape Town in good con­di­tion for fu­ture gen­er­a­tions.

Fu­ture work will in­clude mea­sur­ing sea­sonal spring flow, as well as de­vel­op­ing in­fra­struc­ture to in­cor­po­rate the springs into the cur­rent ir­ri­ga­tion sys­tem. The sus­tain­able yields of the cur­rent bore­holes, as well as de­ter­min­ing whether new pump in­fra­struc­ture will be re­quired, will also be in­ves­ti­gated.

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FO­CUSED: Umvoto trainee hy­dro­ge­ol­o­gist Si­nawo Jack sam­ples wa­ter from the bore­hole next to Ge­naden­dal in the Groote Schuur Es­tate.

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