Articles

Tapping the hidden reserves of the land for our thirsty city.


January 18, 2005 Edition 1
John Yeld

Encouraging householders to install wellpoints or boreholes so that they can tap Cape Town's rich groundwater sources for their gardens is a sensible policy for the city.
But the city must also appreciate that even groundwater well below the surface has an important ecological function, and it should introduce proper monitoring to ensure that this precious water source is not over-exploited or polluted.
This is the opinion of Hout Bay environmental hydro-geologist Ritchie Morris, who was asked to comment after a flurry of contradictory arguments in the media recently about the impact of hundreds of new boreholes and wellpoints since the introduction of water restrictions.
Residents using groundwater drawn from their wellpoints or boreholes are exempt from the water restrictions, although the city has moved to make users subject to the ban on watering between 10am and 6pm.
But many of their neighbours are unhappy at what they see as often wasteful watering habits of those lucky enough to have their own groundwater.
There is also concern that the rush to install, and use, devices to tap groundwater could be having a negative impact on the overall underground resource which Cape Town is likely to rely on more heavily in the future.
Morris dispelled one claim, to the effect that household wellpoints or boreholes are illegal. "Since the new Water Act came into effect in 1998, all water is actually owned by the state," he confirmed.
"The act also has a section which stipulates there is no requirement for small-scale users to register boreholes." (But the council requires you to put up a sign indicating your water source.)
small-scale use is defined as household use, garden irrigation and even some small-scale stock farming.
So groundwater is effectively available free for everyone in Cape Town who lives in an area where it occurs, and some of the biggest aquifers (underground water sources) are under the poorest areas .
A good example is the Cape Flats aquifer, which stores water in unconsolidated, or loose, sands.
The area with the highest yield of this aquifer is around Swartklip, just south of the old Swartklip landfill site, to the east of Mitchell's Plain.
The Cape Flats aquifer could deliver as much as 10 million cubic metres of water a year - enough for 55 litres a day per person for the 500 000-odd people living in informal settlements in this area, and well above the nine litres a day which these residents now use on average.
There was a serious fear that the Cape Flats Aquifer could have been irreparably polluted by contamination from the Swartklip landfill site, but Morris said that according to information to date this had not happened, although further long-term monitoring was needed.
Another major aquifer is the Newlands aquifer, which is already tapped to the tune of about 10 million litres a day. It could probably safely supply about 30 to 50 million litres a day if properly developed and managed. This is equivalent to the daily consumption of a medium-sized town such as Paarl or Worcester.
This aquifer is a transitional zone that covers the Newlands-Rondebosch-Kenilworth areas, grading
towards Wynberg and Plumstead, Morris explained.
"What happens there is that, historically, rock has fallen from the slopes of Table Mountain.
"This build-up of rock becomes the talus, or rock scree, and that gets covered by sand. Below that you've got underlying weathered granite or Malmesbury Group rocks - mudstones and sandstones.
"These contact zones between sand, rock and granite or baked Malmesbury are quite complex.
What happens is that water comes off Table Mountain, down the slopes, and into these weathered contact zones.
"Essentially you've got a sponge, or storage, of water above impermeable rock - that's why you've got all those springs popping out in the Newlands and Rondebosch areas. And there is a massive annual recharge potential during winter."
Morris believes there is no harm in the city promoting the use of ground
water.
"It's a very good policy to have, but you must understand the implications, and to do this one needs scientific information."
For example, a householder in a suburb like Plumstead with an erf of 600m2, who installed a wellpoint at a cost of about R3 000, would offset his or her costs in probably as little as two or three years. And, contrary to popular belief, that householder might not be using substantial volumes of water.
"A typical wellpoint will produce anything between 1 000 litres to perhaps as much as 5 000 litres an hour - it obviously depends on the size of the pipe and of the pump, and the depth of the water," said Morris.
Before the water restrictions came into effect, he and his wife, environmental consultant "MJ" Morris, had done a water-use test in their garden using a tap connected to the municipal water system.
"In the front we have a fat, 30mm diameter hosepipe which produced 1 400 litres an hour. In the backyard we have a thin (20mm) hosepipe, which gave us about 680 litres an hour," he explained.
"Then we put normal sprinklers on them. The thin hosepipe with the sprinkler didn't give any change in water use, while water use with the thick hosepipe and the sprinkler actually went down by about one third - it was throttling it back - although there were yield variations due to pressure fluctuations, depending on the time."
That experiment is now academic, as sprinklers are banned - the motive being because people put them on and simply leave them running for hours, and much of the water is lost to evaporation because a sprinkler atomises the water.
But the Morrises' findings help prove the point that householders with a wellpoint in the garden could be using less water than their neighbours with their hoses attached to taps.
"And wellpoint water is not expensive - treated municipal water is," said Morris.
But he also said that while an individual with a wellpoint was not using a lot of water, there could be problems with the cumulative effects - "for example, if everyone in a suburban block was using them".
In such a scenario, the water table would drop during any extended drought.
"This is why the city should actually have a certain number of monitoring wells in groundwater-use areas."
This should be a joint effort with the Department of Water Affairs and Forestry, Morris suggested.
"I know there is a certain amount of monitoring done, but I don't think it's pro-active enough, as there is little knowledge of use patterns."
The ecological function of groundwater must be taken into consideration, Morris stressed.
"Although groundwater is underground, it does have 'boundaries', commonly called 'flow basins', and it flows into rivers and vleis and other water bodies, especially down-catchment."
He pointed to the "very interesting" work of fellow hydro-geologist Roger Parsons at Zeekoevlei, who proved that there was a reverse seasonal flow of groundwater into the vlei and then from the vlei back into groundwater.
Many vleis rely on groundwater for their functioning.
"So groundwater works both ways - it flows from the ground into rivers during their low-flow periods, and then from rivers back into the groundwater during higher-flow periods."
While promoting the use of groundwater was good policy, the city had also to manage this process adequately, Morris said.
This is legally possible because the Department of Water Affairs can declare "water-control areas".
Once a water-control area has been declared, several requirements apply in terms of the use of that water, and water users must register that use.
But there wasn't enough information about Cape Town's groundwater resources to take a decision on whether the city needed water-control areas, Morris said.
He noted that the city had been warned as long ago as 1992 that a water crisis was looming - as shown in newspaper articles at the time.
"Now we're 12 years down the line, and nothing has been done really."
Cape Town should take a lesson from Perth, which was a "brilliant example" of a city using its groundwater resources effectively and protecting them.
The Australian city pumps about 65% of its water supply from the ground, and suburban blocks include a vacant erf into which all rainwater run-off gets channelled so that the aquifers can be recharged, he said.
"Admittedly there is a slightly different geology there, but the Cape Flats is very typical of a place where you get a lot of water in sands."
After presenting a paper in Australia six years ago, Morris brought Perth's water-management document and gave it to a city official with the idea that Cape Town should perhaps initiate a similar study to understand its groundwater resources, and how they need to be preserved and managed for long-term sustainable yields.
"For example, the city needs to understand the nature and extent of available groundwater resources and the potential pollution inputs to it, whether from leaking sewers, pipes or service stations or whatever, which must be stopped.
"Because in five years this water resource is going to be so valuable that we could be developing wellfields in public open spaces and plugging the water straight into the reticulation supply."
So Cape Town needs to be more proactive by initiating the necessary research and monitoring programmes to ensure the long-term sustainability of its groundwater which could be a vital part of its lifeblood, Morris suggests.
And it also needs to build greater awareness of the need for holistic water use and management.
"I fear that should good rains fall in early winter, all the lessons of this summer will be forgotten. Some form of water restrictions should be here to stay," he said.