Story: Geothermal energy
Page 5 – Effects on the environment
Depletion of resources
The process of extracting geothermal fluids (which include gases, steam and water) for power generation typically removes heat from natural reservoirs at over 10 times their rate of replenishment. This imbalance may be partially improved by injecting waste fluids back into the geothermal system.
Damage to natural geothermal features
Natural features such as hot springs, mud pools, sinter terraces, geysers, fumaroles (steam vents) and steaming ground can be easily, and irreparably, damaged by geothermal development. When the Wairākei geothermal field was tapped for power generation in 1958, the withdrawal of hot fluids from the underground reservoir began to cause long-term changes to the famous Geyser Valley, the nearby Waiora Valley, and the mighty Karapiti blowhole. The ground sagged 3 metres in some places, and hot springs and geysers began to decline and die as the supply of steaming water from below was depleted.
In Geyser Valley, one of the first features to vanish was the great Wairākei geyser, which used to play to a height of 42 metres. Subsequently, the famous Champagne Pool, a blue-tinted boiling spring, dwindled away to a faint wisp of steam. In 1965 the Tourist Hotel Corporation tried to restore it by pumping in some three million litres of water, but to no avail. Geyser Valley continued to deteriorate, and in 1973 it was shut down as a tourist spectacle. This story has been repeated many times where there has been geothermal development.
Extracting geothermal fluids can reduce the pressure in underground reservoirs and cause the land to sink. The largest subsidence on record is at Wairākei, where the centre of the subsidence bowl is sinking at a rate of almost half a metre every year. In 2005 the ground was 14 metres lower than it was before the power station was built. As the ground sinks it also moves sideways and tilts towards the centre. This puts a strain on bores and pipelines, may damage buildings and roads, and can alter surface drainage patterns.
Geothermal fluids contain elevated levels of arsenic, mercury, lithium and boron because of the underground contact between hot fluids and rocks. If waste is released into rivers or lakes instead of being injected into the geothermal field, these pollutants can damage aquatic life and make the water unsafe for drinking or irrigation.
A serious environmental effect of the geothermal industry is arsenic pollution. Levels of arsenic in the Waikato River almost always exceed the World Health Organisation standard for drinking water of 0.01 parts per million. Most of the arsenic comes from geothermal waste water discharged from the Wairākei power station. Natural features such as hot springs are also a source of arsenic, but it tends to be removed from the water as colourful mineral precipitates like bright red realgar and yellowy green orpiment.
Geothermal fluids contain dissolved gases which are released into the atmosphere. The main toxic gases are carbon dioxide (CO2) and hydrogen sulfide (H2S). Both are denser than air and can collect in pits, depressions or confined spaces. These gases are a recognised hazard for people working at geothermal stations or bore fields, and can also be a problem in urban areas. In Rotorua a number of deaths have been attributed to hydrogen sulfide poisoning, often in motel rooms or hot-pool enclosures. Carbon dioxide is also a greenhouse gas, contributing to potential climate change. However, geothermal extraction releases far fewer greenhouse gases per unit of electricity generated than burning fossil fuels such as coal or gas to produce electricity.