Geothermal studies probing the depths at Hanmer Springs

Press, 24 February 1987, Page 21

Geothermal studies probing the depths at Hanmer Springs

Story by

STAN DARLING

Pictures by

JULIEANNE MYERS

Simon Leech crouches on rocks beside a waterfall spouting from a streambank below the grounds of Queen Mary Hospital at Hanmer Springs.

He sticks a hand into the water, which is still warm after travelling through a pipeline from thermal pools on the far side of the grounds. The geothermal water, which may have come from a heat source as deep as 2km under Hanmer Springs, falls just a short distance before it joins Hospital Creek. Instead of just flowing away, could the water be used again in some way? That is one of the things Simon Leech, a University of Canterbury engineering geology post-graduate student, is looking at, along with the North Canterbury Catchment Board and the Amuri County Council, which owns the popular thermal pools. The geothermal production bore’s water level is 10 to 15 metres lower now than it was when the well was first drilled in 1975.

John Talbot, the Catchment Board’s groundwater group leader, says he became interested in the geothermal resource and its future management when he heard that the pool owners were interested in expanding someday. The pool manager had asked in the early 1980 s what could be done about the level. He suggested that measurements of the hot water should be taken for a year or two.

Mr Talbot says the lower water level may be due to an overuse of the well’s resources by the pools; or it may be the lack of a screen at the mouth of the bore, allowing siltation to become a problem. Several studies have been made in the past, but little data has been collected. In his work toward a master’s degree, Simon Leech will help to fill the information gap by monitoring measuring equipment on geothermal and freshwater wells. He has already produced a geomorphology map of the Hanmer Basin, working from ground observations and a study of aerial photographs, which will help researchers understand the basin’s evolution and potential hazards, such as earthquakes, flooding, and slope stability. He will be looking partly at the Hanmer Fault, which runs generally south of the township — partly through housing — and has a branch through the hospital grounds. The geothermal water source could be associated with a crush zone on either side

of that fault. Coming into Hanmer Springs from the south, the faultline can be seen to the left where there is a gentle, overgrown drop-off to swampy ground. Further west, it can be seen running below the golf course. Mr Leech says it has become eroded over the years, but its escarpment is still quite high, about five metres in some places. The Hanmer Fault is associated directly with movement along the Hope Fault, which comes down the Waiau River south of Hanmer.

That fault is probably the most active in New Zealand, he adds, and it is possible that future earth movements could disrupt the geothermal water source. “But in the 1888 earthquake, at Glen Wye, the resource was initially increased at Hanmer Springs,” he says. After the Waiau earthquake in 1948, the flow from the production bore which had deepened in 1936 nearly stopped, but it recovered over the following few weeks.

Although Mr Leech says that

no major movements have been recorded along the Hanmer Fault, future movement can be expected because the area is so active. To determine movement recurrence intervals along the fault, deep trenching and geophysical surveys might be needed, as well as some radiocarbon dating. In a 1973 report on water resources of the Hanmer Basin, Len Brown said that an alarming feature of the hot springs was "the lack of systematic collection of flow, temperature, and chemical data over the 100 years the springs have been known and used. Mr Brown is a hydrogeologist with the New Zealand Geological Survey. Work being done and planned now could meet that criticism. “If they are looking at expanding the pools, the mere fact that there were four or five wells drilled which haven’t found very much hot water tells me we’re dealing with quite a small resource,” says Mr Talbot. On the other hand, it is always possible that future work by the Geophysics Division of the

D.S.I.R. could help pinpoint other potential geothermal resources — ones that have not resulted in springs — deep under the Hanmer area. Seismic reflection and refraction measurements may be done soon. A tool known as a whacker pounds on the earth’s surface and measures offsets where geothermal water sources might exist. .

“Because of the geological nature of the area, you might have to be dead on the spot to put down a successful well,” adds Mr Talbot. The Catchment Board will place an on-line flow meter on the thermal pools production bore to record the amount of water being used. Pumping tests will also be done on wells, to give researchers an idea of the quantities of water available.

The area’s “basement” could be anything up to 2km deep, with a heat source for the water that is associated with a fault. The heat is not volcanic in origin, but is created by the natural geothermal gradient. Down-hole logging could also be done in the production bore, using a probe to determine what type of material is outside the bore casing. Such a probe is lowered slowly down the well, measuring temperature and radioactive decay of the ground material, such as clays and PFAVWa plrp A bore drilled in 1936, at the same time the former 1911 production bore was deepened because the flow of hot water had declined to about a quarter of its original yield, is unused and capped even though it had hit hot water at 76 metres (the present production bore is also at 76 metres). Mr Talbot says the Catchment Board plans to open and clean that bore, using it as an observation bore fitted with a temperature and level recorder. It may have just silted up, with the silt becoming fairly hard. Some water samples have already been taken from the production well and the boardedover original bathhouse springs nearby. They are being analysed chemically, and for isotopes. Samples have been sent to the Nuclear Institute in Wellington to be analysed for tritium, an isotope of water, and Oxygen 18. Oxygen 18 results could point to the origins of water that drops low enough to be heated. Tritium testing can provide clues about the water’s age. Nuclear atmospheric tests in the 1950 s and 1960 s produced a lot of tritium,

which has been an excellent groundwater tracer, says Mr Talbot.

So far, no studies have been done to see what effect the geothermal water, at its outfall, has on the water of Hospital Creek. “Of course, we don’t know what conditions were like before the discharge was started,” says Mr Talbot.

It is known that the discharged water has a high boron content, and one or two other substances that might be harmful to life in a freshwater stream. Catchment Board water quality people might do that part of the study. Simon Leech says it seems there should be some way to use the still-warm water again. John Talbot agrees. “One way to help the system could be to put that water back down into a recharge well,” he says, “while it is still at 30deg to 40degC. It would probably come in contact with the heat source again.” It might be possible, for instance, to use the now-unused 1936 bore as a recharge well. In his 1973 report, Len Brown also mentioned that the quantity of water available to the heat source might be increased by injecting cold groundwater into the plains alluvium near the hot springs.

He said that method could not be recommended unless there was a dramatic and sudden decline in the yield and temperature of the present geothermal supply, and then only if a complete shutdown of the thermal bores failed to produce a recovery in the hot springs and bores. He said the method “can only be regarded as a desperate move to rectify overwithdrawal problems.”

Mr Talbot says that Amuri County might also look at ways of reusing the outfall water once it has been through the pools, “but they would probably have to look at a quite expensive filtering system” to treat the water

before it was used again. But it could be looked at if there is not enough geothermal water there to carry on the way they have been using it. Simon Leech says it is a shame that methane from the geothermal water is not being used these days. It is being released into the atmosphere.

One of the early controversies at Hanmer Springs centres on the use of natural gas from the water. A gasometer was built on the grounds in 1898 to replace two tanks which had stored gas collected from the springs for lighting. The gas was then reticulated to the tea kiosk and sanitarium, and used in gas rings and stoves.

Some critics said that springs water could have lost some of its healing powers because gas was being captured. It was doing harm in the public mind, said

one letter writer, even if no actual harm was being done. Mr Talbot knows of nq measurement of how much methane is being lost to the atmosphere now, or whether there could be a potential user for it. Mr Leech says the county is interested in finding potential groundwater sources in the Hanmer area to bolster the present water supply from the Rogerson River, via an old 6km pipeline. “That system for the town’s water supply has been used since the early 19205,” he says. “That is why there are not many wells in the area, because the river has supplied the water. But they run short during the summer because the Rogerson runs low.” The last groundwater investigations in the area were in 1953. Mr Leech will be monitoring wells in the Chatterton River floodplain, along with other sites. Mr Talbot says that piezometers, small pipes piercing the ground to a water-bearing layer, could also be monitored. In ef-

feet, they create small wells that allow researchers to build up a contour map of the water table. In time, you can obtain enough information to tell the groundwater’s direction — where if comes from and where it is going.

The 1953 groundwater report said that shallow test bores, less than 5m deep, had been sunk near Hanmer on the Chatterton flood plain. They encountered fairly permeable material that yielded good quantities of groundwater, but two deeper bores had been unsuccessful.

It has been suggested, by some geophysical information, that the groundwater basement may be at least 100 m deep. Another aspect of Mr Leech’s thesis work will be tests of foundation materials in yet-un-developed subdivision land on the lower slopes of Conical Hill. That geotechnical investigation could provide a guide for the suitability of such materials for building. The work will include trenching and mapping.