As the international community comes to terms with the realities of anthropogenic climate change, the need to reduce carbon dioxide emission levels is driving demand for clean, efficient, and safe energy (Finkel & Hays). Due to this rising demand for alternatives to oil and coal, some deposits of natural gas that were previously deemed either insufficiently accessible and/or economically inefficient due to the complexity of production are now being considered as viable options to meet the world’s energy needs (Carey, Redmond, & Haswell, 2014). Unconventional gas development is an example of demand driven expansion in the mining and energy sector to find alternatives and/or supplementary sources of energy, made possible by advances in extraction technology (Rafferty & Limonik, 2013).
Unconventional gas exploration and extraction is described by its proponents as an efficient method to access hitherto untapped pockets of natural gas, helping to address the problems of energy supply (Mackie, Johnman, & Sim, 2013). However as the practice has expanded, the published literature shows an increasing level of alarm over the potential risks to both the natural environment and human health (Rafferty & Limonik, 2013). This paper attempts to provide a summary of the evidence on the health and environmental impacts of unconventional gas extraction, limitations of current research, and possible future directions from contemporary published literature.
2. What is unconventional gas and how is it extracted?
To fully explore the health and environmental issues related to unconventional gas extraction, it is useful to both define some of the terms used to describe the process and also understand the extraction methodology. Conventional natural gas is readily accessible for extraction, resting in subsurface natural reservoirs (Geoscience Australia, 2014). Conversely unconventional gas is an umbrella term for gas resources that are stored in more complex systems that require capital, energy and technology intensive extraction methods (Rafferty & Limonik, 2013). Unconventional gases include coal seam gas, shale gas, gas hydrates and tight gas, all of which are only able to be exploited with significantly greater effort, time and technology than has traditionally been the case (CSIRO, 2013).
Shale and coal seam gasses are contained within the source rock, rather than having shifted to a reservoir deposit as is the case with conventional gas. Tight gas refers to gas that is contained in reservoir, but one with low permeability such as sandstone and limestone which hampers access (CSIRO, 2013). Extraction of these resources is made possible via hydraulic fracturing, a process sometimes known as fracking (Chen, Al-Wadei, Kennedy, & Terry, 2014).
The purpose of hydraulic fracturing is to create fissures and fractures in the source rock so the gas therein can be released. This is achieved by drilling a series of deep boreholes and then pumping vast volumes of fluid at high pressure into the bores. Subsequently the gas is carried back to the surface via a well bore where is it captured and processed (A Kibble, 2014). The fluid that is pumped into the bores is called fracturing fluid, and is comprised of a combination of water, sand and various chemicals (A Kibble, 2014; Finkel & Hays).
3. Environmental and Health Impacts
There are hundreds of studies in the literature that examine various aspects of unconventional gas development. Upon examination, the majority of research in this area coalesces around three main aspects of the extraction process, namely the potential for water contamination, air pollution and issues associated with waste management (A Kibble, 2014; Chen et al., 2014; Gordalla, Ewers, & Frimmel, 2013; Mackie et al., 2013; McKenzie, Witter, Newman, & Adgate, 2012; Swarthout, 2014). Each of these aspects are considered below. Impacts on both human health and on the natural environment are discussed contemporaneously as environmental hazards are frequently linked with both direct and indirect effects on public health (Mackie et al., 2013).
Water management is one of the key issues that dominate debate around unconventional gas development, both in terms of health and environment (Vidic, Brantley, Vandenbossche, Yoxtheimer, & Abad, 2013). There are a number of issues related to water that are raised in the literature – namely the impact of withdrawal of local water required to support the mining operation, potential for contamination of drinking water via the fracturing process and the complications associated with wastewater treatment and disposal after the extraction has been completed (Chen et al., 2014; Gordalla et al., 2013; Vidic et al., 2013).
In current practice, large volumes of water are required to undertake hydraulic fracturing, so access to local water reserves as part of the production process is pivotal (Chen et al., 2014). Finkel and Hays in their 2014 paper posit that the potential for aquifer depletion is a risk of fracking - stating that droughts and diminished water levels may represent a serious unintended consequence, both in terms of maintaining the ecology of the natural environment and ensuring access to adequate water for human consumption (Finkel & Hays). Similar concerns are raised by Chen et al (2014) who outline that because water is usually extracted from single location proximate to the hydraulic fracturing operation, there is the possibility of a real and significant impact on the amount of water available for crops, livestock and human consumption, particularly in locations with distinct wet and dry seasons (Chen et al., 2014). Further to this, the potential for water withdrawals to affect water quality is examined by Cooley and Donnelly (2012), who suggest that groundwater quality may be detrimentally impacted by the migration of naturally occurring substances into the water supply, and also by the promotion of bacterial growth, when aquifers are depleted. In turn, the authors advance, this not only affects the quality of the remaining water but also changes the hydrological profile of the source water (Cooley & Donnelly, 2012)
Various studies demonstrate that both the fracturing fluid used to stimulate gas extraction and the waste water produced subsequently – ‘flowback water’ - contain material that can have deleterious effects at high concentrations (Chen et al., 2014; Gordalla et al., 2013; Rafferty & Limonik, 2013; Vidic et al., 2013). Gordalla et al (2013) advance that the components found in flowback water are the most problematic in terms of human health, because of the fluids’ exposure to organic contaminants via the fossil deposit during the fracking process, and the presence of heavy metals. Flowback may include a number of known or suspected carcinogens and the chemicals benzene, toluene, ethylene and xylene (‘BTEX’), all of which are considered hazardous at high concentrations or with chronic exposure in drinking water (Chen et al., 2014; Rafferty & Limonik, 2013). Whilst unconventional gas deposits are located well below sources of ground drinking water, access to the gas is gained only by drilling through drinking water sources (Cooley & Donnelly, 2012). This being the case, despite safeguards such as steel pipe that is inserted into a recently drilled section of a borehole to stabilize the hole, prevent contamination of groundwater, and isolate different subsurface zones - there remains the possibility of the very large volumes of toxic material contaminating both ground and surface water (Finkel & Hays; Vidic et al., 2013).
Studies undertaken in the US and Europe suggest that unconventional gas extraction produces air pollutants at a number of points during production, including both direct emissions (venting, gas capture) and fugitive emissions (unplanned leaks from valves and pumps) (Kibble, 2014). Pollutants include volatile organic compounds, methane, nitrogen dioxide, carbon monoxide and particulate matter (Kibble, 2014). Volatile organic compounds in particular are problematic not only because of the detrimental effects on human health but also their impact on the climate via ground-level ozone or secondary organic aerosols (Swarthout, 2014).
The current evidence suggests that taken individually, the level of pollutants emanating from unconventional gas wells are intermittent and relatively low, however when there are a number of wells located within reasonable distance of each other, cumulative exposure increases risk significantly (Kibble, 2014). In terms of human health, there is evidence that proximity to unconventional gas production sites is a significant risk factor. McKenzie et al. (2012) advance that the greatest potential for health effects is the subchronic exposure that can occur during the well completion phase, and further, that data collected during their study indicates that residents that live less than half a mile from wells are at greater risk than those living at a greater distance. More compelling is their analysis that estimates that less than half mile proximity to an unconventional gas well increases the lifetime risk of cancer from 6/1 million to 10/1 million (McKenzie et al., 2012) Studies referenced by Finkel et al demonstrate that up to 7.9% of the methane generated in unconventional gas production - a gas that is considered a serious contributor to climate change – escapes via venting and leaking. Jointly considering the above studies, not only are there conditions for medium term health impacts such as cancer, endocrine disruption and respiratory illness, but contributions by unconventional gas development to climate change will impact human health in a broader and more global sense – water scarcity, population displacement, and food security (Finkel & Hays; Kibble, 2014; McKenzie et al., 2012).
Both Vidic et al. (2013) and Cooley & Donnelly (2012) provide an overview of the risks associated with the storage of waste water after the gas extraction is complete. Appropriate wastewater management is required to prevent heavy metals leaching into soil – if this does occur it can have long term health (cancer, neurological, endocrinogical) and environmental (soil degredation, food contamination) impacts. In the United States, current practice is to first store the drilling muds and waste fluids, and then treat them for recycling and reuse (Kibble, 2014). Usually, storage is via underground injection into pits, but sometimes in open pits or ponds on site (Cooley & Donnelly, 2012; Schmidt, 2013). Harmful substances from the stored waste water have the potential to contaminate surface water, and can contribute to air pollution when volatile organic compounds evaporate (Finkel & Hays; Kibble, 2014).
4. Limitations of current research
Action to further regulate unconventional gas development to protect the health of the environment and the community is hampered by a lack of large epidemiological studies that explore the association between extraction activities and adverse impacts (Finkel & Hays). Whilst anecdotal evidence is myriad, as extensively outlined in Rafferty and Limonik (2013) the rapid growth of unconventional gas development has meant that the evidence base is lagging and it is evident that further work is required (Rafferty & Limonik, 2013). In particular, as highlighted in Chen et al. (2014) the full disclosure by unconventional gas operators about the chemicals that they are using as part of the extraction is pivotal – to date the composition of fracturing fluid has been confidential for competitive and commercial reasons. Long term monitoring and data dissemination is identified by many studies as important in managing risks both to environment and health (Chen et al., 2014; Finkel & Hays; Gordalla et al., 2013; Rafferty & Limonik, 2013). Some of the activities that are argued in the public health literature as causative are disputed elsewhere, for example (Warner et al., 2012) present evidence that pathways between shale formations and the aquifers above occur naturally in some locations, unrelated to drilling activity. Interestingly, Mackie (2013) provides a counterintuitive but nonetheless thought provoking question, that is, what is the interaction between the likely public and environmental health consequences of unconventional gas development and other known health impacts associated with the absence of affordable fuel? This broader approach is worthy of consideration when moving forward with the next phase of research.
Concern over unsustainable, high carbon output energy sources has driven a rapid increase in demand for unconventional natural gas (Coram, Moss, & Blashki, 2014). There is a strong argument that this has occurred without a full exploration of the potentially serious public health and environmental issues (Finkel & Hays). Current evidence suggests unconventional gas development activities have the potential to contaminate groundwater, release air pollutants and generate toxic waste all of which represents a hazard both to the environment and to human health, and a challenge for regulators and mine operators (Ladd, 2013). Both the immediate, long term and broader cumulative effects need to be examined and further data collected, however as argued by Finkel & Hays (2013) a precautionary approach should adopted, as ‘it should not be concluded that an absence of data implies that no harm is being done’
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