By Mike Wills
They gamble on everything in Las Vegas except the water supply. To ensure that the booming city never runs dry, water engineers in Nevada have established one of the world’s biggest and most successful artificial recharge schemes, pumping treated water underground into natural aquifers and then extracting 0, 59-million cubic metres a day when needed.
Artificial recharge (known as AR) is an important weapon in effective supply management in water-poor regions around the world. It is widely used in the US, Australia and Europe, has attracted R33-billion in investment from the Indian government and has been enthusiastically pursued by the Namibians, but it has been underused in South Africa. The Department of Water Affairs & Forestry (Dwaf) is now trying to change that. It has developed a strategy and a legislative framework to enable local water authorities to make wider use of the technique.
AR takes many forms but in its most basic applications involves transferring surface water underground into suitable natural aquifers, either through borehole injection or sand infiltration ponds, and then extracting it on demand. In some instances, groundwater storage can be used instead of expensive dams or lengthy pipelines; in others, it can be used as an adjunct to surface storage and become a ‘water bank’ to meet seasonal demand or in response to the strains of a drought.
AR can work on almost any scale. In Southern Africa, two recent successful applications have been in the city of Windhoek and the tiny, isolated Namaqualand community of Karkams. Dr Peter Dillon, of Australia’s widely respected Commonweath Scientific and Industrial Research Organisation, says “the success of both those sites cannot be overstated – potential for reversing declines in groundwater storages, expansion in water supplies, and in increasing security of supply are clearly evident, and the technology has been decisively proven effective”.
The Windhoek case study is especially impressive. Needing to raise supply capacity from 21-million cubic metres a year to more than 30-million cubic metres a year by 2019 and to improve water supply security against drought, local authorities were faced with the prospect of spending R1,7-billion on a pipeline from the Kavango river to supplement their three existing dams. Instead, they opted to expand their existing groundwater extraction scheme and make better use of the aquifer in the surrounding mountains to store water that would otherwise be lost to evaporation from their dams or be wasted as overflow during high rainfall years. The aim is to increase groundwater yield potential from 1,7-million cubic metres a year to an estimated 19-million cubic metres a year.
Windhoek’s quartzite aquifer is highly fractured and complex but extensive pilot testing established the viability of rapid replenishment and large-scale abstraction.
The capital cost of the artificial recharge scheme was R242-million, one-seventh of the pipeline, and it could be spread across four phases, compared with the massive upfront payment needed for the pipeline. AR also scored considerably better on any measure of environmental impact. Further benefits were the ability to rectify a worrying drop in groundwater levels in the old wellfield areas and engineers being able to operate the existing dams at higher risk levels.
Ten new deep boreholes have been drilled and initial injection and abstraction results far exceed expectations.
Not all geological formations are suitable for groundwater storage owing, primarily, to poor hydraulic conductivity and the quality and compatibility of the source water and the existing groundwater, but a survey of aquifer storage potential in South Africa, conducted for Dwaf, shows many untapped areas, both in primary aquifers suitable for infiltration, and fractured and weathered aquifers that would involve borehole injection. The Limpopo, Crocodile West Marico, Lower Vaal and Lower Orange Water Management Areas show the highest potential AR storage volumes but small-scale opportunities exist all over the country.
Even if the hydrogeological conditions are suitable, careful consideration needs to be given to a number of other factors before proceeding with AR, including the quality of the water mix, the potential for clogging and the environmental impact of artificially raised or lowered groundwater levels. These pitfalls can all be avoided with proper research and scheme management
No one is suggesting that AR can be a complete solution for water management in every region, but engineers definitely need to fully understand its potential in the constant battle for conservation and sustainable supply.
* Wills is a Cape Town radio talk show host and journalist who is working with Dwaf to ensure widespread awareness of issues around water conservation and management – email@example.com