If you were in Melbourne on Thursday 11 March this year, you were probably oblivious to a natural earthquake rumbling away 5 kilometres beneath your feet. The tremor came and went beneath the densely populated southeast suburbs unfelt by all, but recorded at magnitude 2.3 by seismometers. A subsequent tremor of the same magnitude, two weeks later, was reportedly felt by a few residents near Frankston, described by one resident as “like a truck driving past on the road”.

This highlights a common phenomenon – earthquakes are a part of our daily life in Australia, but we either do not feel them or their effect is indistinguishable from everyday transient events, such as passing traffic. However, induced seismicity is commonly one of the first environmental concerns raised by communities as they engage with companies that hope to tap into the local potential of geothermal energy, or ‘hot rocks’.

This article addresses this and other concerns about environmental risks and hazards associated with geothermal projects. As well as induced seismicity, other environmental concerns often voiced include radioactive gases, groundwater pollution and cooling of the earth. In all but a few cases, initial concerns far overestimate the true risk, and can be allayed with clear and comprehensive information.

Exploration companies in Australia are focusing on two main ways of developing unconventional geothermal energy resources – Engineered Geothermal Systems, (EGS), commonly referred to as ‘Hot Rocks’, where companies artificially engineer a reservoir within hard, high-temperature rocks buried at depths of between 3 and 5 kilometers; and Hot Sedimentary Aquifers, or HSA, where companies target natural permeable reservoirs of hot water deep within sedimentary basins.

Each has its own individual risks and rewards, and both will be required if geothermal energy is to make a significant contribution to Australia’s future power generation. The environmental concerns covered below are common to the two types of development (EGS and HSA), although the relative weight of concern may vary from project to project.

Induced seismicity

The risk of induced seismicity, or causing earthquakes, is one of the most widely voiced concerns regarding geothermal energy.

All geothermal operations involve changes in fluid pressure at depth in the earth. Engineers use high-pressure water to artificially increase permeability of the rocks in EGS projects, while HSA fluid production involves extracting and reinjecting large volumes of water through rocks. Any such changes in fluid pressure can potentially alter the natural equilibrium of stresses within the uppermost crust to the point where small movements might be triggered along existing fractures. Exactly the same phenomenon is observed worldwide in other extractive industries, such as oil and gas production, and engineering operations that significantly alter the mass distribution in the upper crust, such as large dams and mining operations.

In almost all geothermal operations, the volume of rock affected is tiny compared to that required to trigger an ‘earthquake’, as we commonly think of them.

Consider this – a magnitude-5 earthquake, the minimum magnitude typically required to cause damage to buildings, requires about one square kilometre (1,000,000 square meters) of fault surface to slip by about 1 metre. The overwhelming majority of seismic events triggered by geothermal projects involve slippage of just a centimetre or less over a surface area of perhaps a few hundred square metres, thousands of metres underground. These events, less than magnitude-2, are described as ‘micro-seismicity’. Any event less than about magnitude-3 is rarely felt at surface except by the most sensitive of instruments.

The risk of significant seismic events – that is, events felt by humans at the earth’s surface – resulting from geothermal operations, although small, can be reduced even further by careful management and monitoring, especially of the rates at which fluid is pumped into and out of the rocks.

It is for this reason that the Australian geothermal community (government, academia and industry) has been actively engaged with other geothermal authorities around the world to produce a set of protocols for minimising seismic risk. The protocols outline best practice for seismic risk mitigation, and recommend such things as studies to determine the location and orientation of faults prior to any development activities, controlling the water injection rates, and aiming to produce smaller fracture lengths and thus smaller microseismic events. If followed, these protocols will ensure the induced seismicity risk is kept to a minimum.

Ultimately, though, the influence of geothermal projects on seismicity must be viewed in the context of Australia’s natural seismicity. Australia may be located some distance from active tectonic plate boundaries, but earthquakes are common and occur sporadically over most of the continent. Two natural magnitude-4 earthquakes were felt over large swathes of Victoria, including Melbourne, within weeks of each other in March last year. Both of these events had epicentres close to the town of Korumburra in Gippsland. One can only imagine the media headlines if a geothermal company had been undertaking activities in Gippsland at that time.

Radon emissions

High rock temperatures within non-volcanic portions of the Earth’s crust are mostly associated with the breakdown of naturally occurring radioactive elements within the Earth’s crust; specifically isotopes of potassium, uranium and thorium.

Radon gas is a product of the normal radioactive decay of uranium and some recent media reports have reflected community concern over uncontrolled radon gas emissions from geothermal fluids brought to the earth’s surface.

To date, all planned EGS and HSA geothermal projects within Australia involve closed loop systems. This means that water used to bring heat to surface is reinjected back into the reservoir once the heat has been extracted. It is highly unlikely that any gases dissolved in the fluid will be released to the atmosphere. Even under a worst-case scenario, if an accident did result in the release of geothermal fluid into the atmosphere, a study jointly conducted by the University of Adelaide, Geoscience Australia and the South Australian Department of Primary Industries and Resources found that radon levels are likely to remain lower than acceptable workplace levels.

By their very nature, HSA projects are unlikely to have radon gas associated with their operations since they do not target rocks with elevated radioactive element concentrations.

Water issues

Australia is recognised as the world’s driest continent. Given that many geothermal companies are exploring in arid to semi-arid localities, they are fully mindful of water requirement issues.

The Australian Federal Government, together with the various state authorities, has in recent years recognised water as a valuable commodity with increasing competition for access to the resource from competing interests. The response has been an implementation of both state and federal legislative and regulatory requirements to ensure water resources are used and protected from pollution in an ecologically sustainable manner. No geothermal project will be allowed to proceed unless it implements the most stringent of protection measures for groundwater aquifers currently relied upon for agricultural or other purposes.

Cooling of the Earth

Finally, concern about geothermal activities leading to a cooling of the Earth has sometimes been voiced.

In 2008, the total world primary energy usage – all energy used for electricity generation, heating, transport – amounted to about 474 x 1018 joules. The thermal energy contained in the earth amounts to about 8 x 1030 joules. That means that there is currently enough heat in the planet to run the whole of human society for about 10 billion years. And that does not take into account the ongoing generation of new heat in the earth by natural decay of radioactive isotopes.

It is clear then that tapping geothermal resources for energy production will not interfere with the usual planetary geological processes.

Demystify the unknown to unlock a huge resource

In conclusion, the best way to allay fears of geothermal energy is for geothermal companies, regulators and researchers to actively engage with communities and fully explain the processes that will occur during the exploration, development and production stages of geothermal power generation.

Geoscience Australia has estimated that just 1 per cent of the heat stored within the uppermost 5 km of the Earth’s crust in Australia can generate approximately 26,000 years of Australia’s energy supplies, based on 2005 levels of demand.

This important resource should be developed and managed responsibly so as to reduce Australia’s greenhouse gas emissions and enhance our energy security while minimising our impact on the environment.