Hot Springs Geology
The Thermal Springs are low temperature springs produced as a result of the fractured rock bed along the Hanmer Fault.
The thermal water originates from rainwater that fell 180 years ago, which seeped down through fractured rock in the Hanmer Mountains to a depth of about two kilometres below the Hanmer plains.
Heat radiating from the Earth’s core raises the temperature of the rainwater in the underground reservoir. This heated water then rises to the surface through a series of interconnecting fractures in the greywacke rock.
Where does the water come from?
When the springs were first discovered, the hot water rose to the surface under artesian pressure. Today, the natural upflow of heated water has diminished, and the water is pumped from a bore that was drilled into a fractured part of the rock in 1975 (original drilling was in 1911). The thermal water is drawn at a rate ranging from 4 litres per second to a maximum of 8 litres per second. It was originally drawn from a depth of 10 metres, but is currently drawn from approximately 28 metres.
What temperature is the water?
The water comes out of the bore at a temperature of 52°C, having lost some of its heat through convection on the way to the Earth’s surface. Heat is then extracted out of the water using a series of heat exchanges until it reaches bathing temperature of between 32-42°C.
Is the water treated?
Without some form of human intervention, water from the bore would be unpleasant to bathe in. So yes, the water is treated, but only very minimally. The water first passes through a series of gauze strainers which remove large pieces of debris such as leaves and twigs. It then passes through an exceptionally fine filter, to remove any smaller impurities.
Next, the water is tested for acidity and water purity, the former being controlled by the addition of carbon dioxide gas. Other measures to maintain high quality include the addition of chlorine at the ratio of 2.5 parts per million, and recirculating the water every 3-4 hours.
The three sulphur pools have no additional treatment. They remain completely natural - thermal water being drawn directly from the bore.
What is the chemical make-up of the water?
The geothermal water is described as a NaCI (sodium chloride) water. An average analysis shows about 380 mg/l Na, 480 mg/l Cl and 190 mg/l bicarbonate. Minor amounts of Boron, Calcium and Carbonate are also present, together with other elements.
What is the therapeutic value of the hot pools?
Communal bathing in hot water for relaxation and therapeutic reasons dates back to Roman times. Today, hot pools are becoming increasingly popular to relax and ‘unwind’ in and are used extensively to treat rheumatic conditions and paraplegia. They are also popular for treating orthopaedic conditions such as bone fractures and soft tissue injuries.
Who owns the pools?
Formerly part of the Queen Mary Hospital, the pools are on government reserve land, but are owned by the Hurunui District Council. The pools are controlled by an experienced management team, which reports to a committee. Profits not reinvested into the operation are returned to the Hurunui District Council and invested in the physical reserves of the wider Hurunui District.
Is there a health risk?
Maintaining the highest standards of water quality is our number one priority and is done by testing the water in our own laboratory three times daily. Testing is also carried out to determine the presence of the amoeba bug which can cause amoebic meningitis. This is despite the fact that the bug is unlikely to exist in the water given the way it is treated. An independent scientific laboratory in Christchurch also undertakes regular water checks. In their 125 year history, the hot pools have never had an incidence of the amoeba bug, but as an added precaution bathers are advised not to put their heads under the water.