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Environmental aspects of geothermal energy resources utilization

M. J. Heath

Bibliographic reference: HEATH, M. J., 2002. Environmental aspects of geothermal energy resources utilization. In: Chandrasekharam, D. and Bundschuh, J. (Eds). Geothermal Energy (Resources) for Developing Countries. A. A. Balkema, Rotterdam.

ABSTRACT: Geothermal energy is often advocated as one of a number of 'green', 'renewable' alternatives to fossil fuels for the supply of clean energy for the future. Geothermal energy is not without its environmental impacts, however, particularly on air, water, land use and the aesthetic qualities of the landscape in regions with geothermal energy potential. A range of socioeconomic impacts are also important. Geothermal energy is not always renewable, except over very long timescales, and geothermal energy exploitation often takes the form of heat mining with long term implications for site rehabilitation. These environmental factors can be addressed at different stages in the development of geothermal energy resources through environmental impact assessment (EIA) in advance of any development and through the implementation of an environmental management system (EMS) during the operation of a geothermal energy scheme. Approaches to optimizing environmental impacts are also considered.

1 INTRODUCTION

1.1 Geothermal energy - general features

Geothermal energy is energy derived from heat from the Earth's interior. This heat can be held in hot water or steam or in the rocks themselves and represents a potentially vast energy resource, estimated by Armstead & Tester (1987) to be more than 300 times the energy held in fossil fuels. Although sometimes included in lists of 'renewable' energies, most sources of geothermal energy are non-renewable, at least over human time scales, and the utilization of geothermal energy is sometimes referred to at 'heat mining', a useful term that serves as a reminder of the non-renewable nature of these resources. In view of the greatly reduced emissions of CO₂ that are associated with geothermal energy when compared with the use of fossil fuels, geothermal energy is often described as a 'clean energy' (Department of Energy, 1994) but there are still environmental impacts to consider in relation to its utilization. Geothermal energy associated with hot water or pressurized steam held in aquifers or with hot dry rocks requires different approaches to its exploitation.

1.2 Geothermal aquifer systems

In the case of geothermal aquifer systems, heat can be held as pressurized steam or in hot water in deep aquifers and can be exploited directly by drilling into the aquifer. In 'high enthalpy' geothermal systems, such as those of Iceland, New Zealand and California, pressurized steam or superheated water can be used to drive a turbine, either passing the steam directly into the turbine in 'direct cycle' systems, or using a heat exchanger in a 'binary cycle'. In 'flash cycle' systems, energy is extracted from hot water by passing it through a low pressure vessel to produce steam to drive the low pressure stages of a turbine. Waste steam from these systems can either be vented directly to the atmosphere (non-condensing systems) or can be condensed prior to reinjection into wells (condensing systems), both of which have potential environmental impacts.

In 'low enthalpy' geothermal systems, hot water aquifers at temperatures below boiling point can be used directly by pumping to district heating schemes, for horticultural and agricultural use (Rinehart, 1994) or for industrial purposes or to preheat water for electricity generation. Geothermal district heating schemes have been successfully developed in Southampton, UK (ETSU, 1994), for example, and at Melun, France (Armstead, 1978).

1.3 Hot dry rock systems

In hot dry rock (HDR) geothermal systems, heat is extracted from dry rocks rather than aquifers. In this case, water must be introduced into the dry system in order to transfer heat to the surface where it can be used to generate electricity or for space heating or other purposes. The extraction of heat from an HDR geothermal energy resource is essentially non-renewable as the rock is cooled in the process, but HDR resources are extremely large (Armstead & Tester, 1987). Research into HDR systems is currently taking place in Europe, the USA and Japan but none are as yet in production.

1.4 Geothermal energy in developing countries

Geothermal energy generally makes a small contribution to the energy needs of the major developed countries but makes a significant contribution in a number of developing countries, including the Philippines, Mexico, Indonesia, El Salvador and Kenya (Reed & Renner, 2002).

The geothermal electric power industry in the USA is the largest in the world, with an electrical generating capacity of over 2100 MW (megawatts of electricity), a little over half of which (1100 MW) is in The Geysers geothermal field in California. Generating capacity in the Philippines is currently around 890 MW and in Mexico is 700 MW; other developed countries with major geothermal capacity include Italy with 545 MW, and New Zealand with 460 MW (Reed & Renner, 2002). Interestingly, in Iceland, which has an overabundance of both hydroelectric and geothermal energy potential, most geothermal energy is used for space heating and hot water rather than electrical generation (Reed & Renner, 2002).  

2 ENVIRONMENTAL IMPACT ASSESSMENT FOR GEOTHERMAL ENERGY UTILIZATION

2.1 Scope of Environmental Impact Assessments

An Environmental Impact Assessment (EIA) is a formal procedure carried out as part of the planning process prior to any development. Its purpose is to assess the future impact of a proposed development on both the natural and human environments. For large projects with potentially significant environmental impacts, such as a major geothermal energy scheme, the EIA might be compulsory. For smaller projects, such a geothermal heat pump installations, the EIA might be optional but might be produced in support of a planning application to demonstrate that the environmental impacts have been considered.

The EIA will take into account any plans within the proposed development to address environmental impacts. Thus, the possible creation of water pollution problems, for example, might not be considered problematic if water treatment and monitoring procedures are also part of the plan. Similarly, plans for the restoration and aftercare of abandoned sites would be taken into account in the overall EIA.

The scope of an EIA is necessarily very broad and will vary from project to project. It will include impacts on both the natural environment (in terms of physical, chemical or biological impacts) and the human environment (in terms of economic development, employment, health implications, cultural aspects etc.).

An EIA will address the impacts of both the construction and operational phases of the proposed development. Indeed, the impacts associated with these different phases of the project might be significantly different. An EIA should include, short-, medium- and long-term impacts, local and global impacts, direct, indirect, secondary, cumulative, permanent and temporary, positive and negative effects of the project. Clearly, for a major geothermal energy development, the impacts are likely to be many, with local and global implications for the physical/chemical/biological and human environments.

2.2 Framework for Environmental Impact Assessment (EIA)

A framework for EIA is provided by the European Community's Council Directive on the assessment of the effects of certain public and private projects on the environment (85/337/EEC) which states that an EIA shall identify, describe and assess in an appropriate manner, in the light of each individual case ... the direct and indirect effects of a project on the following factors:

— human beings, fauna and flora;
— soil, water, air, climate and the landscape;
— material assets and the cultural heritage;
— the interaction between [these] factors (European Community, 1985).

Geothermal energy schemes are covered by Annex II of the Directive (proposed developments which may require EIA following a case-by-case examination or thresholds or criteria set by Member States). Here, there is specific reference to "Deep drillings, in particular: geothermal drilling", "Industrial installations for the production of electricity, steam and hot water" and "Industrial installations for carrying gas, steam and hot water; transmission of electrical energy by overhead cables". "Construction of overhead electrical powerlines with a voltage of 220 kV or more and a length of more than 5 km" is included in Annex I (where EIAs are always required). Annex IV of the Directive provides a detailed list of the information requirements of an EIA, which, for a geothermal energy development, should include the information outlined in Table 1.
 

3 ENVIRONMENTAL IMPACTS OF GEOTHERMAL ENERGY UTILIZATION

3.1 Global environmental factors

With increasing concern about climate change associated with increasing atmospheric CO₂ concentrations that are due, at least in part, to the burning of fossil fuels, geothermal energy, along with other 'renewable' energy resources, is considered to offer global benefits through the provision of clean energy with low associated CO₂ emissions (International Energy Agency, 1998).
 

These global benefits are shown in Table 2 and 3, in which life cycle emissions of carbon dioxide (CO₂), sulphur dioxide (SO₂) and nitrogen oxides (NO_X) associated with a range of renewable energies and with conventional fossil-fuelled electricity generation, are compared with emissions from geothermal electricity generation. These emissions values are based on life cycle analysis of the different energy sources and include emissions associated not only with energy generation itself but with the construction of plant and manufacture and transport of machinery and components, which can be greater for renewables than the energy expended in producing the plant and machinery for conventional electricity generation because renewables generally harness energy sources that are more 'dispersed' or 'dilute' (in contrast to the 'concentrated' energy represented by, say, coal or oil).
 

It is clear from these comparisons that geothermal energy offers significant reductions in emissions of CO₂, the main greenhouse gas, and of SO₂ and NO_X, both of which are toxic gases and major contributors to acid rain. These reductions represent the main argument offered in support of the further development of such energy sources. Locally, however, the balance of environmental costs and benefits is sometimes less clear.

3.2 Local environmental factors

Despite the emissions reductions that can be achieved by utilizing geothermal as opposed to conventional (fossil) energy sources, the local environmental impacts can sometimes be significant, especially with regard to air and water pollution, land use, and impacts on the aesthetic qualities of the landscape. The socioeconomic impacts on the local environment must also be considered.

Air pollution

Although emissions of CO₂, SO₂ and NO_X are lower for geothermal energy than for conventional fossil fuels, there are still important gaseous emissions associated with geothermal energy utilization. Of particular importance is the emission of hydrogen sulphide, which can represent an odour nuisance at low concentrations but which is toxic at concentrations above 0.001% v/v (Smith, 1991). There are, however, absorption and stripping techniques available for the removal of H₂S gas and there are no emissions at all if binary plant is used.

Other gaseous pollutants include traces of ammonia, hydrogen, nitrogen, methane, radon and the volatile species of boron, arsenic and mercury, though generally in very low concentrations (International Energy Agency, 1998). Silica is also sometimes problematic, as at Wairakei in New Zealand, where forest damage has been attributed to silica deposition (Armstead, 1978).

In addition to gaseous emissions, dust can be associated with the construction of the plant, with drilling and with the clearance of the land for site development. More visible are the plumes of steam which contribute to the overall visual impact of the site during its operation.

Water pollution

Both surface waters and groundwaters can be affected by geothermal energy schemes. The most important potential impacts on the water environment are associated with the management and disposal of wastewaters, notably geothermal brines, which are commonly disposed of by reinjection into wells (where they can contaminate groundwaters) or by storage in holding ponds (from which they can leak into surface waters).

Chloride brines of Na and Ca are particularly important as they can have very high concentrations of metals (Mattigod & Page, 1983). Of particular interest here are Fe, Mn, Pb, Zn, Ba and B. Other contaminants can include I, Sb, Li, H₂S, Hg, Rb, bicarbonate, fluoride, silicate and ammonia (Nicholson, 1992). Contamination of shallow groundwater reservoirs can also be caused by drilling fluids and as a result of well casing failure, which might also affect groundwater levels (Hunt & Brown, 1996).

The metals content of geothermal brines from Imperial Valley, California, are shown in Table 4 where they are compared for reference with maximum admissible concentrations recommended for drinking water under the European Community drinking water directive of 1980. The higher metal concentrations observed in the Imperial Valley brines clearly exceed maximum admissible concentrations for drinking water and, therefore, represent a potentially significant environmental hazard.

Geothermal brines can not only contaminate surface and groundwaters but can also affect soils, with implications for agriculture; phytotoxic boron is particularly important in this respect (Mattigod & Page, 1983).

The pollution impacts on the water environment can be mitigated through effluent treatment, the careful storage of waste water and its reinjection into deep (as opposed to shallow) wells and through careful monitoring of the condition of holding ponds and well casing (Hunt & Brown, 1986).
 

In addition to water quality impacts, the abstraction of geothermal waters can impact on groundwater levels. The most important consequence of this is ground instability and subsidence (considered below), but lowering the water table can also affect local water supplies (International Energy Agency, 1998).

Solid wastes

Apart from the general wastes associated with any commercial operation (office and workshop wastes, canteen wastes, general garbage etc.), there are few solid wastes produced during geothermal energy utilization. The main solid waste specific to geothermal energy results from the chemical deposition that takes place within the pipes and vessels of the plant, notably of calcite and silica (Armstead, 1978). These deposits also occur naturally in geothermal areas where they are deposited as travertine and siliceous sinter around hot springs and other geothermal manifestations. The problems caused by these deposits are usually operational as they can cause blocking of pipes and boreholes and can reduce the permeability of the aquifers being developed. Where they have to be removed in order to maintain the operation of plant (as at Wairakei where silica deposition occurs in open channels), the resulting waste is small in volume and is not considered to be a major environmental concern (Armstead & Tester, 1987).

Land use

As with any major energy development, a geothermal power plant will require land and that land will either already be in use for another purpose or might be valued as natural environment or might have other proposed uses. There is then an opportunity cost associated with the geothermal development (that is, the land cannot be used for any other purpose during the lifetime of the plant). Land take can be reduced by the use of directional drilling techniques, as advocated by the Sierra Club, a leading environmental organization in the USA, in its conservation policy on geothermal energy, as one of the measures for minimizing surface disturbance in resource production areas (Sierra Club, 1980).

Aesthetic impacts

Many geothermal energy resources are located in regions that are considered to be of great natural beauty (as at Rotorua, New Zealand, or the geysers of Iceland) or in National Parks (such as Yellowstone, USA) or in areas considered to be aesthetically valuable. Indeed, may geothermal phenomena (such at geysers and hot springs) are themselves valued as important environmental assets with both intrinsic value as natural phenomena and economic value for tourism. Among possible impacts of the exploitation of geothermal resources is a fall in reservoir pressure which can result in a reduction in the vigour of geysers and other geothermal phenomena and affect their economic value as tourist attractions. This potential loss of aesthetic value has been recognized by the Sierra Club, which noted the need to protect "hot springs, geysers, thermal pools, and other thermal features and their ecological, educational, aesthetic, and recreational values" (Sierra Club, 1980).

As well as the major impacts that a large geothermal station might have on the aesthetic quality of the landscape, local visual impacts from buildings, plant, pipework and plumes of steam might also be considered important, especially by local residents, but this can be reduced by careful screening of the site.

Other physical impacts

Among other physical impacts that might be encountered during the utilization of geothermal energy resources are noise, ground instability and heat pollution.

Noise can be associated with the exploration, construction and production phases and can be significant in terms of occupational exposure, requiring workers to be suitably protected. Noise as an environmental impact (along with many of the visual impacts) can be reduced by the screening of the site with earth bunds and/or trees (Hunt & Brown, 1996) and through the adoption of good working practices, such as restricting working hours.

Ground subsidence occurs when geothermal fluids a

re withdrawn from a reservoir at a rate greater than natural inflow back into the reservoir. This causes compaction of the rock formations at the site which, in turn, leads to surface subsidence, as observed at Wairakei in New Zealand. Ground subsidence can be reduced by reinjecting waste waters into wells to maintain well pressure, though associated risks of groundwater contamination need to be considered when designing the reinjection process. Subsidence might also result from thermal contraction of rocks associated with hot dry rock geothermal utilization (Taylor, 1983).

Because many geothermal resources are located in seismically active zones of the Earth's crust, there are sometimes problems of instability, both in the natural landscape and in association with geothermal energy utilization (DiPippo, 1991). The re-injection of fluids into the ground, for example, can enhance the seismic activity of the area affecting buildings and other structures and allowing seepage of fluids within the system, though this can be minimized by keeping reinjection pressures to a minimum. Landslide hazard might also be a feature of geothermal regions where steep slopes are susceptible to failure, perhaps leading to damage to well heads or pipes resulting in the release of steam and hot fluids. The likelihood of this can be minimized by stabilizing all slopes which may be prone to landslides (DiPippo, 1991).

Heat pollution of air and, particularly, water can represent a significant environmental impact as well as being energy inefficient. The discharge of hot water to rivers can damage aquatic wildlife (as in the Waikato River in Wairakei) and lead to unwanted growth of vegetation (Armstead, 1978). The effects of heat pollution are local, however, and the total amount of heat released in this way is negligible compared with solar radiation and does not in itself represent a significant global environmental problem (Armstead, 1978).

Rehabilitation of disused sites

Where geothermal energy resources become exhausted, the rehabilitation of sites becomes necessary and the same approach may be adopted to the management of disused geothermal sites as is employed in the assessment and remediation of land affected by other industrial activities. These aspects are covered by Cairney (1993), Pratt (1993) and others and are beyond the scope of this paper.

Socioeconomic impacts

In its overview of the socioeconomic impacts of geothermal energy, the International Energy Agency (1998), as an OECD organization, focused on the employment benefits to OECD members, noting that the investment of US$2.4 trillion (1 trillion = 10¹²) foreseen by the World Energy Council (1993) on renewable energy resource development as a whole represented a huge business opportunity with significant benefits in terms of employment. It was also noted that, as renewable energy developments are commonly in rural areas, these jobs are often made available in areas with few other employment opportunities (World Energy Council, 1998). This is a particularly important factor in parts of the world where agriculture is in decline (as in the UK, for example) and where alternative uses are being sought for agricultural land.

The tourist potential of geothermal regions and the need to protect geothermal phenomena like geysers has already been noted as a major economic consideration. At Svartsengi geothermal power station in Iceland, geothermal energy utilization has itself been used to the advantage of the tourist industry where geothermal waste waters have been used for the development of the Blue Lagoon, a spa facility developed adjacent to the plant itself (Blue lagoon Ltd., 2000).

The International Energy Agency (1998) also sees the restructuring of energy markets as an important economic impact of renewable energy sources which tend to take the form of many small, dispersed units rather than the large centralized power plants that have characterized the energy markets in recent decades. This is seen as contributing to increased competition and greater flexibility. The ability of local schemes to meet local demands (for heat, for example, in the case of geothermal energy) is also noted (International Energy Agency, 1998).
 

4 MANAGEMENT OF OPERATIONAL ENVIRONMENTAL IMPACTS

4.1 Environmental Management Systems

4.1.1 ISO 14001

During the lifetime of a geothermal energy plant, day to day environmental impacts can be managed through the implementation of an environmental management system (EMS). The international standard for environmental management systems is the International Standards Organization's ISO 14000 series, of which ISO 14001 is the specification with guidance for use (ISO, 1996). ISO 14001 provides a structure for carrying out the following key environmental management activities:

• establishment of an environmental policy;
• evaluation of the environmental effects of the business;
• identification of legislative and regulatory requirements;
• establishment of environmental objectives and targets;
• establishment and maintenance of an environmental management programme to ensure that the objectives and targets are achieved;
• employment of environmental management procedures and documentation;
• monitoring and measurement of operational environmental impacts in the field;
• implementation of operational control measures and preventive and corrective action;
• keeping of environmental management records;
• implementation of environmental management system audits; and
• arrangement of periodic environmental management reviews which provide the mechanism for modification of the EMS in response to changing conditions or the results of environmental audits.

In a formal EMS, control of the environmental impacts of any project is brought within the overall management structure of the organization. For this to be effective, appropriate management structures must be put in place. Equally important is an understanding of the environment itself and of the way the project impacts upon it. The ISO 14000 approach to EMS has been criticized (e.g. by Krut & Gleckman, 1998) as serving the interests of the company rather than those of the environment but, through the identification of (at least) the legal requirements for environmental protection, which allows the minimum environmental objectives and targets to be set, and regular monitoring of the environmental impacts in the field, an EMS ensures that the operational impacts are kept to within prescribed limits. In this way, an EMS under ISO 14000 will at least enable compliance with legislation.

It is of interest to note that ISO 14001, the standard for environmental management, is closely linked to the quality standard, ISO 9001 (ISO, 2000). If, therefore, an organization is already using the ISO 9000 structure (which is commonly the case in many countries), it is already well prepared to adopt an ISO 14000 environmental management system.

4.1.2 Eco-Management and Audit Scheme (EMAS)

An alternative to the ISO standard is the European Community's Eco-Management and Audit Scheme (EMAS) (European Community, 2001). The main provisions of EMAS are similar to those of ISO 14001 but there is greater emphasis on the publication of the results of environmental reviews and audits.

The purpose of EMAS, for which registration is obtained on a site-specific basis after independent (external) verification, is to help industrial (and local authority) sites to:

• minimize pollution, creating a cleaner and healthier environment;
• operate more efficiently, by minimizing energy and water usage, saving natural resources and reducing waste;
• minimize their production and processing costs, therefore improving profitability and enhancing competitiveness;
• openly report their environmental improvements in an environmental statement, ensuring that the general public are informed of their environmental achievements;
• develop new markets for their products and services from the competitive advantage that positive environmental management can achieve.

• formalizing an environmental policy;
• carrying out an environmental review;
• establishing an environmental programme to put the policy into practice;
• managing the programme, including organizing and documenting it, ensuring that responsible staff are trained, and integrating it into the company's existing management structure;
• audits of performance at regular intervals;
• issuing annual public environmental statements linked to the audit process.

There are clearly similarities between the EMAS and ISO 14001 approaches to environmental management and auditing, but they are considered to be different in several respects. EMAS was developed by governments acting through the European Community and thus represents many different interests, while the ISO is considered to be primarily an international industry organization. EMAS requires external verification while ISO 14001 has no external verification requirement and can be certified internally. EMAS requires disclosure of environmental information whereas ISO 14001 leaves this to the discretion of the organization itself. For these reasons, many (like Krut & Gleckman, 1998) favour EMAS as the more rigorous system.

4.2 Optimization of the environmental impacts of geothermal energy utilization

The environmental impacts of a geothermal energy scheme can be optimized through the adoption of environmental procedures at different stages in the development of a project.

Environmental questions are addressed in advance of the development of a geothermal scheme through the implementation of an environmental impact assessment (EIA), which addresses a range of questions relating to the impacts of a proposed project on the physical, chemical, biological and human environments. During the operational phase of a geothermal scheme, day to day environmental impacts can be managed through the implementation of an environmental management system (EMS).

The environmental impacts of a geothermal development can also be optimized by paying special attention to the following aspects (International Energy Agency, 1998):

• Site selection: care in choosing a site for a geothermal energy scheme is particularly important if adverse impacts on the natural and human environments are to be avoided. Avoidance, where permitted by geological considerations, of National Parks and areas of outstanding natural beauty, special scientific or archaeological interest or cultural value is particularly important in this respect. Choice of site and possible alternatives is a key consideration in an EIA.

• Environmental costs and benefits: geothermal energy, like some other alternative energy sources (such as wind power) is interesting in that it offers global benefits (through reduced use of fossil fuels) yet creates environmental impacts that cause concern at a local level. Information is, therefore, a key element of any debate about any proposed geothermal energy scheme, but it is still difficult to present the balance between the local impacts and the global benefits (as is the case for the further development of wind power schemes in the UK).

• Best technology and management practices: selection of technologies to minimize disruption to the natural and human environments is important along with the adoption of sound technical and management practices that are sensitive to environmental concerns and that minimize the adverse impacts of the scheme. A structure for good environmental management is provided by a formal environmental management system (EMS), such as those provided by ISO 14001 and EMAS.

• Public information and local involvement: with increasing environmental awareness among the general population, the involvement of the public through public consultation and the provision of information has become increasingly important in explaining the environmental aspects of any proposed geothermal energy scheme. Here, the importance of the socioeconomic impacts of any proposed project should not be underestimated and a clear understanding of local, regional and national policies relating to energy supply, conservation and planning are essential. The involvement of the local community in proposed schemes has also often helped to improve public acceptability. Other approaches suggested by the International Energy Agency (1998) include providing concrete benefits to the local population through, for example, the payment of rent for land occupied, inclusion of local people as owners of the scheme, offering lower electricity prices, providing local use of waste heat, and maximizing local employment opportunities.

The environmental impacts (physical/chemical/biological and socioeconomic) associated with the development of geothermal energy resources with vary from site to site according to the nature of the scheme itself and of the environment into which it is emplaced, and environmental impact assessments are needed for each proposed scheme in order to identify the specific environmental issues raised by the proposed development.
 

5 CONCLUSIONS

Geothermal energy offers considerable advantages over conventional fossil fuelled electricity generation through greatly reduced CO₂ emissions (with global implications with regard to climate change) along with lower SO₂ and NO_X emissions (with implications for local air quality and the generation of acid rain). Despite the global environmental benefits that can be claimed for geothermal energy, there may be important local impacts on the atmosphere, notably through the emission of H₂S and other minor gaseous pollutants, and on surface and groundwater, mainly through the disposal of contaminated waste water.

The impacts of geothermal energy utilization can be managed and minimized through their careful consideration as an environmental impact assessment (EIA) prior to site development and through the implementation of an environmental management system (EMS) during the operation of the scheme. Socioeconomic impacts are also important and can be optimized through the involvement of local communities in the development of geothermal resources.
 

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