Feature

Groundwater Hydro Resources

Groundwater trapped under the Southern Alps may provide a solution for low lake levels.

By Earl Bardsley

Low water levels in South Island lakes have emphasised the importance of water storage for New Zealand's hydroelectric system. Unfortunately, there appears to be little hope of any short-term solution to the immediate need for more lake water.

Most of this winter's precipitation in the lake catchments will, as always, fall as snow, unavailable until the spring thaw. Cloud seeding can at best generate more snow in the upland areas along the main divide, and seeding will almost certainly fail at locations further east, due to insufficient frequency of the required cloud types.

An alternative, rather desperate, measure might be to blacken large areas of snow and ice, in the hope of inducing a brief winter thaw. Quite apart from environmental considerations, this approach has the drawback of yielding meltwater only until the darkened layer is covered by the next snowfall.

Short of unexpectedly liquid precipitation falling into the catchments, it would seem then that for this winter we are stuck with whatever electrical consequences the low lake levels imply. The lakes will refill sooner or later, but it would be dangerous to dismiss this winter as just the bad luck of a 1-in-100 event manifesting itself.

The lesson of 1992 should be that there is a serious need to develop an ability to increase winter inflows to the hydro lakes. Any new water source must have a known water volume available as a certainty to increase otherwise low lake inflows for a few months.

There is a good chance that mountain groundwater could provide an emergency supply. Not a lot is known about groundwater in the Southern Alps. However, it is a reasonable guess that the required water volumes exist in the thick sequences of intensely-jointed greywackes and schists, where groundwater would have accumulated over many years from stream seepage, summer rains and spring thaws.

Verifying the existence of the groundwater volume is only part of the requirement. The immediate question is whether part of this could be discharged into mountain rivers and streams at a sufficient rate to offset declining lake levels, without incurring high costs.

The ranges' dissected nature is of some help, as local areas of high elevation could generate high water pressures in the lower portions of an uplifted block, and even beneath adjacent valley floors. This would apply particularly to intensely-shattered zones of rock with hydraulic connections to higher-altitude streams or subglacial meltwater.

Production Bores

Given the right locations, there should be a good chance of setting up major production bores, with water yields maintained by elevation-induced high water pressure. Two kinds of bore are possible: the traditional vertical bore which take artesian water at the mountain bases, and high-yielding horizontal bores which might extend hundreds of metres into fractured rock. Both types would extract water by simple gravity flow.

Considerable set-up costs would be involved, particularly for horizontal bores, but gravity-flow bores, once established, require little maintenance as the problems of mechanised pumping are avoided. The establishment costs would also be in part offset by the fact that groundwater is highly reliable in its flow characteristics.

A deep bore giving a high flow over a period of, say, two months could be relied on to produce the same discharge again when required, after a period of recovery. Such discharges will not be at risk of zero yield in winter due to freezing, unlike the natural exits of mountain groundwater at seeps and springs.

Further advantages of groundwater storage include no evaporation loss and minimal environmental impact. Nor does groundwater storage require the presence of a controlled storage lake to function -- supply bores could be established, for example, in the upper portions of the catchments of lakes Wanaka and Wakatipu.

Whatever the establishment costs, they must be seen in the larger context of the cost of loss of electricity supply during an extended drought, and the associated need to use or build more expensive thermal power stations.

Some common sense would have to be applied to the selection of bore locations. There would be no point in setting up a high-yielding shallow bore at the expense of drying up a stream of the same discharge. It has to be remembered, too, that no extra water is being supplied by the groundwater. The water is, in effect, being borrowed against future groundwater replenishment in the years following the end of the period of water shortage.

To put the engineering requirements into perspective, some 430 bores, each yielding a gravity flow of 10,000 m³ of water per day, would be sufficient to offset a 5cm/day fall in Lake Tekapo. A full hydrogeological investigation with exploratory bores would be required to verify the feasibility of providing such groundwater discharges to the rivers draining into Lake Tekapo and the other hydro lakes.

A variation on the theme would be to install a much larger number of lower-yielding shallow bores -- perhaps tapping into the groundwater of the old fluvioglacial deposits which mantle much of the lower slopes in the western Mackenzie basin.

Dr Earl Bardsley is senior lecturer in hydrology at Waikato University.