2nd WARFSA/WaterNet Symposium: Integrated Water Resources Management: Theory, Practice, Cases; Cape Town, 30-31 October 2001
The use of garden boreholes in Cape Town, South Africa: Lessons learnt from Perth, Western Australia 334
Perth is a growing city that experiences a continued increase in water demand. Presently the city
relies on surface reservoirs and groundwater for domestic supply. Annual domestic consumption is
about 250 million m3. Of this 50 % is derived from groundwater. However, it is estimated that an
additional 184 million m3 is abstracted from 150 000 private boreholes for garden irrigation (Water
and Rivers Commission, pers. comm., 2001, Appleyard, et al., 1999). It is estimated that 1 house
in every 4 has a private borehole that is used for garden irrigation (Appleyard, et al., 1999). Many
of these boreholes were drilled during the water restrictions of the late 1970s, and reflect the value
that is attached to private gardens. Through the use of private boreholes a great weight has been
taken off the scheme supply. In this way the city saves in the amount of treated supply needed,
thus extending the life of water supply schemes.
The use of private boreholes has not been without problem. Of particular concern is the
environmental impact of over-abstraction. One reason for this is that individuals with private
boreholes tend to use more water than those using mains water for gardens. On an average 1000
square meter house block water use for garden irrigation from mains supply ranges between 300-
600 kL/year, compared to about 1000 kl/year average borehole abstraction Water and Rivers
Commission, pers. comm., 2001).
The private boreholes are primarily used for garden irrigation and abstract water mainly from the
shallow unconfined aquifer. The City’s groundwater supply is abstracted from both the unconfined
aquifer and from deeper semi-confined aquifers. The boreholes drilled into the deep semi-confined
aquifers often exceed depths of 800m. The quality of water in both the unconfined and the deeper
aquifers is generally good, with total dissolved solids content below 250 mg/L (Appleyard, et al.,
Garden boreholes play an important role in the city’s water supply infrastructure. Through the use
of private boreholes the city’s water supply burden is lightened, saving other water resources and
money. Despite the free access that private citizens have to the Perth coastal plain aquifers,
concerns over environmental impacts and a period of exceptionally dry summers has resulted in
Perth water authorities imposing restrictions on water use, irrespective of its source. Such
restrictions include appropriate garden watering times, with a ban on daytime sprinkler use.
GARDEN BOREHOLES AS AN ALTERNATIVE
The experience of Perth in the use of private boreholes for garden irrigation has been a good one.
With so many similarities between Perth and Cape Town it begs the question on whether a similar
initiative in Cape Town would be of benefit to the city.
The use of private boreholes for garden irrigation has in Perth resulted in a reduction in the amount
treated domestic water used. The increased use of private garden boreholes in Cape Town would
similarly result in less treated domestic water being used in garden irrigation. It is estimated that
about 30% of the treated domestic supply to Cape Town water users is applied to garden irrigation
(Parsons, 2000). With annual water consumption of about 300 million m3, the potential water
saving that could be achieve should only groundwater from private wells be applied to gardens
would thus amount to about 90 million m3. The potential monetary saving to the city that this
represents is vast. Such saving result from the smaller amounts of water that will be pumped and
treated, as well as in reducing the need to invest in new bulk water supply schemes.
The fact that Cape Town receives winter rain makes it an ideal candidate for groundwater
abstraction during the dry summer months. This would relieve stress on other water sources during
the dry part of the year. With winter rains the levels in the aquifer should recover, if the aquifer is
not over exploited. By lowering water levels in summer storage in the aquifer is increased, which
reduces the risks of flooding in some of the low-lying parts of Cape Town.