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Review

Environmental Implications of Shale Gas Hydraulic Fracturing: A Comprehensive Review on Water Contamination and Seismic Activity in the United States

by
Bohyun Hwang
1,
Joonghyeok Heo
2,*,
Chungwan Lim
3 and
Joonkyu Park
4
1
School of Earth Sciences, The Ohio State University, Columbus, OH 43210, USA
2
Department of Geosciences, University of Texas—Permian Basin, Odessa, TX 79762, USA
3
Department of Earth Science Education, Kongju National University, Kongju 32588, Republic of Korea
4
Department of Civil Engineering, Seoil University, Seoul 02192, Republic of Korea
*
Author to whom correspondence should be addressed.
Water 2023, 15(19), 3334; https://doi.org/10.3390/w15193334
Submission received: 28 July 2023 / Revised: 19 September 2023 / Accepted: 20 September 2023 / Published: 22 September 2023
(This article belongs to the Section Water-Energy Nexus)
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Abstract

:
Recent scholarship has highlighted the significant environmental impact of the rapidly expanding hydraulic fracturing industry, which is projected to grow from USD 15.31 billion in 2021 to USD 28.93 billion in 2028 at a Compound Annual Growth Rate (CAGR) of 9.5%. Recognizing the need for comprehensive, national-scale evaluations, this review of the literature investigates contamination and induced seismicity associated with shale gas hydraulic fracturing in the United States. Employing systematic reviews of the literature and federal reports up until July 2023, this study reveals multiple areas of concern, including water and soil contamination, seismic activity, and air pollution. A notable finding is the average use of 2.4 million gallons of water per well in hydraulic fracturing, of which only 15–35% is typically retrieved. However, ongoing studies are actively exploring remediation strategies, including advancements in monitoring and treatment technologies, as well as the potential of reusing wastewater for hydraulic fracturing, as exemplified by the Garfield County region in Colorado; they utilized 100% wastewater to mitigate the impact of contamination. These findings underscore the need for stringent regulations, sustained research, and effective management practices. This work emphasizes the importance of a collaborative approach that leverages field studies, experimental investigations, and computational advancements to ensure the responsible development of shale gas resources.

1. Introduction

1.1. Overview of Shale Gas and Hydraulic Fracturing

Shale gas, a type of unconventional natural gas, is trapped within formations composed of fine particles that are densely packed over time. It predominantly consists of methane and originates from sources such as heat-decomposed crude oil or the decomposition of organic matter by microorganisms. This organic carbon exists either as free gas interspersed among pores and fractures or adheres to the surface of organic matter. As a member of the unconventional gas category, which also includes coalbed methane (CBM), tight gas, and gas hydrates, shale gas presents unique extraction challenges due to its depth and wide dispersion within rock fractures. It is found in shale layers formed by fine-grained particles, which simultaneously act as a source rock, reservoir, and trap. Shale gas remains deep within shale formations, unlike natural gas, which migrates closer to the surface. This depth, along with the gas’s dispersion throughout rock fractures, poses a significant challenge to extraction. Despite these complexities, the composition of shale gas closely mirrors that of natural gas. Yet, for many years, shale gas was thought to not be productive since it is thinly dispersed in wide rock layers [1].
While shale gas has been known as far back as the 1800s, it has only become a viable energy source recently due to modern extraction techniques with significant reserves in the United States, Canada, and parts of Europe. Recent advancements, such as molecular simulations and X-ray tomography, have significantly improved our understanding of fluid behavior and rock formations in shale layers. These complementary approaches have directly addressed key challenges, like low permeability and gas dispersion, making shale gas a more viable energy resource [2]. There are an estimated 7257 trillion cubic feet (Tcf) of total world-proved reserves of natural gas, which is nearly on par with the 7299 trillion cubic feet of world shale gas resources [3,4]. These quantities could potentially cater to the world’s energy demands for over 60 years.
Until the 1990s, the lack of developed extraction techniques made shale gas production unfeasible. However, with the advent of improved techniques in the 2000s, mainly in the United States, production has surged. This surge enabled the U.S. to outstrip Russia, becoming the largest producer of natural gas post-2009. The increased attention to shale gas worldwide could be attributed to a few key factors: it is more cost-effective than other fossil fuels, produces only half the carbon dioxide of coal when burned, and has vast reserves that can address escalating energy demands. Efficient wastewater treatment and reuse in the extraction process could further enhance its cost-effectiveness [5]. While shale gas presents significant potential as a future energy source, its low cost and high availability could divert investments or focus away from renewable energy industries, such as solar power.
Unlike conventional gas sources such as petroleum and natural gas, which rely on their inherent high permeability for extraction via a single borehole, unconventional gases like shale gas necessitate unique techniques due to their extremely low permeability. Hydraulic fracturing, also known as fracking, was developed to overcome this challenge. The method involves injecting a fracturing fluid—a mixture of water, sand, and specific chemical additives—under high pressure to create fractures in the rock, enhancing its permeability. This approach has made it possible to tap into vast reserves of petroleum and natural gas that accumulate in shale formations. The widespread adoption of horizontal drilling and hydraulic fracturing technologies in the 2000s revolutionized shale gas extraction, starting in the United States. As a result, hydraulic fracturing emerged as a breakthrough technology that revolutionized the extraction process, enabling a rapid increase in petroleum and shale gas production. This rapid increase in energy production, attributable to hydraulic fracturing, has fortified the energy security of the United States, fueled economic growth, and mitigated consumer energy costs [6].
The workflow of shale gas reservoir development consists of distinct steps. Starting with exploration, experts utilize seismic surveys to identify potential reservoir locations [7,8]. After selecting a suitable site, drilling operations commence, first vertically and then transitioning to horizontal drilling to tap into the shale formation [9,10]. Upon successful drilling, hydraulic fracturing or ‘fracking’ is applied, a technique that involves injecting a fluid mixture under high pressure to create fractures in the rock, thereby enhancing its permeability. After fracturing, flowback fluids, which are a source of recycled fracturing comprising fluid and water, are managed, and may be reinjected as waste or treated for other uses. On-site wastewater treatment facilities have proven effective in enhancing the efficiency of managing these fluids, with studies indicating treatment success rates as high as 26% to 38% across varying methods like hydraulic fracturing and chemical-enhanced oil recovery [5]. As the gas is released from the fractured rock, it is collected and processed for use. After the productive life of the well, decommissioning procedures are carried out to safely close operations and mitigate the environmental impact [9].
However, this technique also poses environmental and health risks, such as potential water pollution from fracking chemicals, induced seismic activity, and impacts on water resources and wildlife [1,11]. In addition to these risks, elevated temperatures can significantly alter the chemical composition of surrounding water: a concern that may also be applicable to the high-pressure, fluid-intensive process of hydraulic fracturing [12]. These concerns necessitate a balanced evaluation of hydraulic fracturing’s environmental impact in the context of its economic benefits.

1.2. Shale Gas in the U.S.: Successes, Challenges, and Mitigations

The U.S. has experienced a surge in shale gas production, notably in regions like Barnett, Marcellus, and Eagle Ford, with an impressive potential of 26.2 trillion cubic feet [13]. Factors that determine the productivity of shale formation include organic matter content, maturity, and limestone barriers. Historically, the U.S. has drilled over 4 million oil and gas wells, and recently, hydraulic fracturing has been employed in over 95% of them to an estimated total of 2 million wells. This technique has bolstered economic growth, creating millions of jobs [14], and has also significantly contributed to petroleum and natural gas output.
A comparison of the contributions that different shales play in the overall shale gas production in the U.S. is depicted in Figure 1. This figure represents the average monthly gas production in billion cubic feet per day (bcf/d), which was obtained by averaging daily production values for each month. This highlights the overall increase in shale gas production over time in each play, underscoring the impact of hydraulic fracturing and horizontal drilling. The source for these data is the U.S. Energy Information Administration (EIA).
While the benefits of hydraulic fracturing are evident, it simultaneously presents notable environmental challenges throughout its lifecycle, particularly with the potential for water and soil contamination and induced seismicity. Starting with seismic surveys that can disturb local ecosystems, the subsequent deep drilling phase, extending more than a mile underground, is an intense operation accompanied by methane emissions—a potent greenhouse gas [15,16,17]. This fracturing process itself consumes millions of gallons of water per site and introduces chemicals deep underground [10,17,18,19,20]. At some locations, this presents considerable risk, with contaminants such as heavy metals and specific hydraulic fracturing chemicals like halides appearing in nearby groundwater and surface water [17,18,21,22,23]. Vengosh et al. (2014) highlighted the potential for shallow aquifer contamination and water depletion from these large-scale operations. Small but statistically significant regional correlations between groundwater chloride concentrations and the proximity and density of fracturing sites were reported [24].
An additional concern is the potential for induced seismicity, particularly during the phase where flowback water, a byproduct of hydraulic fracturing, is injected into specific disposal locations. The injection of these large volumes of fluid and waste increases pore pressure, which can lead to the movement of existing faults and trigger earthquakes. Studies have reported an increase in seismic activity near hydraulic fracturing sites, particularly in areas with pre-existing faults [25,26]. The need for consistent recognition of cases, proposed triggering mechanisms, geologically susceptible conditions, the identification of operational controls, effective mitigation efforts, and science-informed regulatory management are required to better understand and manage the earthquake rupture processes induced by hydraulic fracturing [27]. Concluding this process, the gas collection phase presents its own challenges, primarily through methane leaks, which bolster greenhouse gas concentrations. The journey wraps up with decommissioning: a phase that aims to restore the extraction site safely. Its magnitude varies depending on the operation’s scale, but the objective remains constant: achieving a pollutant-free restoration.
To mitigate these environmental concerns, a multi-faceted approach is essential. Stricter regulations concerning chemical usage and wastewater disposal are necessary. New materials like hydrogel additives that optimize production performance without using environmentally hazardous chemicals are being developed [28]. Ref. [29] also highlighted the beneficial applications of viscoelastic surfactants in wellbore hydraulic fracturing fluid, which are considered clean and do not leave deposits in reservoirs, making them suitable for use in tight reservoirs. In a similar vein, Sun et al. (2019) [20] focus on the characterization of hydraulic fracturing wastewater and establish strategies to mitigate environmental impacts. This paper identifies a number of potentially harmful organic compounds in hydraulic fracturing wastewater, predicts their associated toxicity to freshwater organisms, and suggests that partial treatment and reuse remain the preferred methods for managing wastewater from hydraulic fracturing where feasible. Additionally, emerging technologies like adsorption chillers offer the potential to utilize waste heat from the hydraulic fracturing process, producing both cool and desalinated water and showcasing another avenue for waste repurposing [30,31]. Ongoing research and monitoring of the environmental impact of hydraulic fracturing are also essential for ensuring the responsible development of shale gas resources.
This article integrates the latest data on shale gas production and recent high-magnitude earthquakes up to 2023, offering a comprehensive snapshot of the current landscape of shale gas development. By merging existing evidence with established approaches, we not only highlight previously overlooked connections but also encompass the newest research findings on environmental impacts and mitigation measures.

2. Environmental Problems Caused by Hydraulic Fracturing

Despite the abundant economic and resource benefits of shale gas, its exploitation has sparked heated discussions due to concerns about environmental pollution. In this review, we investigate the environmental impact of shale gas exploitation with a focus on the potential effects of hydraulic fracturing on groundwater quality, seismic activity, and soil health.

2.1. Surface and Groundwater Contamination

Hydraulic fracturing, a process used to extract natural gas and oil from shale rock formations, poses substantial threats to surface water and groundwater quality. As depicted in Figure 2, contamination can occur through various channels. These include well leakage, cracks in rocks, the spillage of flowback water, or faulty cementing work that is supposed to seal the space between the well casing and the surrounding rock to prevent leaks. Notably, numerous incidents of water contamination linked to poor well construction, leaks, and inadequate wastewater treatment and disposal have been reported across various regions in the United States [18].
While there has been limited direct evidence tying shallow potable aquifer contamination specifically to deep hydraulic fracturing [32], there are documented cases of contaminants, such as stray natural gases and drilling-related fluid spillages in close proximity to fracking sites [33,34,35]. Instances of contamination include the detection of various harmful compounds in the groundwater of the Pavillion gas field area in Wyoming [36]. Additionally, in areas overlying the Marcellus and Utica shale formations of northeastern Pennsylvania and upstate New York, studies have observed methane contamination in drinking water originating from bedrock aquifers rather than atmospheric sources [16]. Importantly, these methane concentrations in drinking water wells were found to increase with proximity to the nearest gas well. Building on this, numerical simulations have investigated the possible contamination pathways between tight-gas reservoirs and overlying freshwater aquifers. These simulations underscore the role of factors such as the permeability of the connecting pathway and the volume of this connecting feature when determining the risk of contamination. However, while such models provide insight, real-world evidence remains pivotal in confirming these risks [37]. For a detailed account of documented contamination incidents, refer to Table 1, which summarizes key studies on soil and water contamination related to hydraulic fracturing.
The depth of shale gas formations and aquifers is a critical factor in mitigating water pollution risks from gas exploitation. As shown in Figure 2, the depth of aquifers can vary greatly depending on their geological location; however, for the purpose of this illustration, an aquifer is depicted at around 1000 feet below the surface. By contrast, shale gas production zones are typically situated between 7000 and 10,000 feet underground. Table 2 provides further details on the average depth of the extraction well used for hydraulic fracturing in the United States, which is approximately 8300 feet. Regional variations exist; for example, in Texas, the average depth is approximately 8750 feet, 10,400 feet in North Dakota, and 7000 feet in Pennsylvania. These depth variations, elaborated upon in Table 2, are influenced by a range of factors, including the accessibility of shale gas, formation characteristics, and regional geology [44]. The proper consideration of these factors is essential for preventing water pollution during hydraulic fracturing operations.
Hydraulic fracturing for shale gas exploitation is a water-intensive process. On average, as of 2013, a well that is drilled for shale gas extraction used around 2.4 million gallons of water for horizontal drilling and hydraulic fracturing (Table 2). However, the actual amount of water used can vary significantly depending on a range of factors such as formation characteristics, regional environment, well depth, well spacing, the length of the well, and the number of fracturing stages. For example, around 0.7 million gallons of water are used in New Mexico, and approximately 2.5 million gallons are utilized in Texas. This large volume of water used in each well not only poses risks of becoming severely contaminated when retrieved but also impacts local water availability. Typically, only 15–35% of the water used for extraction is actually retrieved, increasing the risk of groundwater pollution and local water shortages. Recent studies have underscored the complexity of how hydraulic fracturing fluids interact with geologic formations, affecting both the chemistry of produced fluids and potential environmental impacts [45]. A comprehensive understanding of these chemical interactions is crucial to develop strategies that can mitigate the environmental risks associated with hydraulic fracturing.
Fracturing fluids require additional chemicals to facilitate the fracturing of underground formations [46]. These chemicals, as outlined in Table 3 and Figure 3, can pose a risk to groundwater quality. They can migrate into groundwater sources through cracks in the well casing or via surface spills. The flowback water from hydraulic fracturing can vary in its chemical composition, heavily influenced by the reservoir’s characteristics [1]. It can contain harmful substances, such as heavy metals, salts, and radioactive materials, posing a risk to nearby water sources if not properly managed. The composition of the input fluid for hydraulic fracturing significantly influences the chemistry of flowback water, which has considerable implications for managing produced water and its potential environmental impacts [47]. These findings emphasize the importance of the careful selection, monitoring, and management of input fluid composition.
Despite these risks, several mitigation measures can help protect groundwater quality. These include the use of high-intensity and multiple casings, cementing, and underground water monitoring. The public disclosure of the chemicals used in fracturing fluids facilitates better regulatory oversights by allowing agencies to monitor and evaluate the substances being used. Industrial activities are governed by federal and state laws, including the Safe Drinking Water Act, Clean Water Act, Clean Air Act, and National Environmental Policy Act [48]. Technological advancements could aid in water recycling and mitigate deep-well injection, thereby reducing the risks associated with water contamination from hydraulic fracturing operations [49]. Further insights into the transport of pollutants in subsurface water resources could significantly enhance the design of more effective remediation strategies [48,50]. These include innovative approaches to accurately estimate gas-in-place (GIP) in shale reservoirs, which could improve our understanding of potential gas losses through well leakage [51]. This knowledge could then inform the design of better well constructions and maintenance strategies, further minimizing the risk of groundwater contamination. These measures emphasize the potential to responsibly develop shale gas resources through ongoing research and advancements.

2.2. Soil and Environmental Contamination

Soil contamination is a significant concern associated with shale gas hydraulic fracturing operations. During the development and production of shale gas, various pollutants are generated, posing potential risks to soil quality [43,52]. The impact of shale gas exploitation on soil pollution (Table 1) is supported by evidence from the Barnett Shale in Texas. Fontenot et al. (2013) [40] conducted a study that provided explicit evidence of the relationship between shale gas exploitation and soil pollution. Their results align with those found in the Marcellus Shale region, where the contamination of groundwater via stray natural gas and the spillage of brine and other gas drilling-related fluids has been known to occur. For instance, a case study in Pennsylvania from Marcellus Shale reported an incident where natural gas wells caused the inundation of natural gas and foam in the initially potable groundwater used by several households [32]. This study revealed a clear correlation between the proximity to the hydraulic fracturing site and higher concentrations of total dissolved solids (TDS) in the soil, indicating increased contamination nearer to the well. The correlation between shale gas activities, surface water, and soil pollution is primarily attributed to the leakage of highly compressed fluids during the storage, transportation, and disposal processes associated with hydraulic fracturing. Notably, in 2009, a leak of the liquid gel used for hydraulic fracturing, along with its waste, resulted in the pollution of wetlands and rivers, leading to the deaths of fish in nearby rivers.
Soil pollution associated with hydraulic fracturing can result from the accumulation of various contaminants, including metals, salts, organic compounds, and naturally occurring radionuclides (NORM). The presence of NORM, such as radium isotopes, in wastewater fluids from hydraulic fracturing operations can pose significant risks to the environment and human health. Studies have emphasized the variability and complexity of these wastewaters. They contain a diverse mixture of fracturing fluid additives, geogenic inorganic and organic substances, as well as transformation products, underscoring the challenges of identifying and analyzing specific organic compounds [53]. Moreover, when these fluids are disposed of in freshwater streams or ponds, radium can adsorb onto sediments within disposal and spill sites. Studies have observed the accumulation of radium in stream sediments downstream of these sites, with levels sometimes exceeding regulatory limits [41]. Furthermore, elevated levels of NORM have been detected in soil near roads where conventional oil and gas brines are spread for deicing purposes, as well as in pond bottom sediments associated with hydraulic fracturing spills [42,54].
The accumulation of NORM-rich flowback and produced waters during hydraulic fracturing operations can lead to the buildup of radium and radiation, posing substantial risks to the environment and human health. Radium-bearing barite, a common component of scale and sludge deposits in oil and gas exploration, can contribute to elevated radium levels in the soil and pipe scale near production sites [55]. These reactive residuals in brine, including metals and radioactive elements, have the potential to accumulate in river and lake sediments, as well as in soil near shale gas drilling sites and brine treatment facilities. Over time, these accumulations can release toxic elements and radiation, posing long-term environmental and health effects in affected areas [18].
To mitigate the risk of soil pollution associated with hydraulic fracturing, it is crucial to implement effective measures. One approach is to ensure the secure storage of flowback water in heavy-duty storage tanks to prevent the leakage of fracturing fluid and pollutants into the soil. This method has been implemented for some horizontal wells, effectively preventing soil contamination. However, it is important to address potential sources of surface-level pollution during the storage, transportation, and disposal of flowback water. Measures such as using double-lined pits for storage and maximizing the reuse of flowback water can help minimize the risks of soil pollution. Additionally, strict leak prevention protocols should be followed at all stages, including storage tanks, transportation trucks, and pipelines, to ensure minimal environmental impact and protect soil quality. By adopting these measures, the risks of soil pollution associated with shale gas hydraulic fracturing operations can be reduced.
Beyond soil contamination risks, hydraulic fracturing activities also have broader ecological implications, particularly concerning biogeochemical changes and the impacts on the hydrologic system [56]. Hydraulic fracturing can alter the deep subsurface’s biogeochemical landscape, encouraging the growth of halotolerant microbial communities. This shift can potentially affect reservoir sustainability and infrastructure integrity [57]. Furthermore, the development of unconventional oil and gas infrastructure such as well pads, roads, and pipelines can lead to ‘catchment disturbance’, impacting the health of local ecosystems. For instance, a study conducted in the Upper Susquehanna River Basin in Pennsylvania found that only a small percentage of catchments in headwater streams exhibited medium to high levels of unconventional oil and gas disturbances [58]. This suggests that with responsible development and careful management, the environmental impact of hydraulic fracturing on catchments could be minimized. Moreover, studies have underscored the potential role of climate change in exacerbating water contamination issues, especially from nonpoint source (NPS) pollution, which can be influenced by activities such as hydraulic fracturing [59]. The implications of changing rainfall patterns on the dispersion and concentration of contaminants in catchment areas should be a factor in the design of sustainable management strategies for hydraulic fracturing operations.

2.3. Earthquakes

The process of hydraulic fracturing, while indispensable for enhanced oil and gas production, has been a topic of scientific and public debate due to its potential for inducing seismic activity. The injection of high-pressure fluids into shale formations, a crucial step in this method, creates cracks through which artificially generated seismic energy is released. This seismic energy is transmitted underground, similar to naturally occurring seismic waves. In some instances, these waves can cause ground shaking that is noticeable from the surface. Although the general consensus indicates a low risk of harm from such seismic events, this topic remains an active area of research and discussion [60].
Concerns have also arisen regarding potential earthquakes induced by the disposal of water after hydraulic fracturing. On average, approximately 10 barrels of brine water are formed per barrel of petroleum [61]. Brine water is generally processed through disposal wells, following the guidelines provided by the Underground Injection Control (UIC) Manual issued by the EPA (EPA Class III). In the United States, there are approximately 35,000 disposal wells in use to process the fluid generated from petroleum and natural gas production. The equipment involved in this process, which includes over 800,000 processing units throughout the United States, falls under the purview of the EPA, with enforcement typically carried out by state agencies. Despite the vast majority of these disposal wells not being associated with induced seismicity, a comprehensive record of anthropogenic activity leading to earthquakes, particularly in the context of extraction industries, underlines the importance of such considerations when analyzing the occurrence and severity of earthquakes [62].
While the hydraulic fracturing process itself does not typically pose a significant concern for induced seismicity, specific geological conditions combined with large-scale hydraulic fracturing and a concentrated number of injection wells could lead to earthquakes that are perceptible from the surface. This phenomenon, although rare, is evidenced in a select list of incidents provided in Table 4. The selection is based on incidents of induced earthquakes of relatively higher magnitudes associated with hydraulic fracturing and wastewater disposal wells. It aims to highlight notable occurrences, though it does not include all earthquakes associated with hydraulic fracturing. For example, the Barnett Shale region of Texas, which had experienced no seismic activities for over a century, saw several earthquakes during 2008 and 2009. This led residents to claim that these earthquakes were caused by the increase in hydraulic fracturing in the area [63]. Moreover, the increased detection of seismic activity in Oklahoma was also believed to be linked to the rise in shale gas exploitation [64,65]. While earlier research from the USGS (United States Geological Survey) suggested a lack of direct correlation between hydraulic fracturing and earthquakes [36,66], recent findings have associated seismic events in Oklahoma with wastewater injection practices [64,65]. Notably, changes in injection volumes and locations relative to known faults seem to have influenced these seismic patterns.
Though seismic waves are produced during hydraulic fracturing, the magnitude of such seismic activity is small in most cases, as measured using the Richter scale [69,70]. Although the occurrence of small-magnitude earthquakes during shale gas exploitation is rare, resulting in a very low risk, continuous monitoring is still necessary [69,70,71]. Induced earthquakes are extremely complicated to study, and the knowledge base relating to this phenomenon is quickly changing. Due to its complexity, an umbrella approach that could cover almost anywhere would not be appropriate as induced earthquakes vary greatly by population, building, infrastructure, and the region’s geological condition.
Understanding the underlying science is essential when evaluating the requirements for mitigating and managing potential earthquake risks. Historical records since 1920 indicate that induced seismicity primarily results from human activities and man-made structures, including large-scale water reservoirs behind dams, geothermal plant projects, mining, construction, and underground nuclear weapons testing [43,56,59]. Given this context, a scientific approach is imperative for discussing earthquake risk management and mitigation. Notably, seismic energy can be generated in areas with geological defects or stress. For example, increased pressure from fluid influx can expand a fault or even cause it to slip, leading to surface tremors.
To effectively manage and mitigate such potential risks from hydraulic fracturing-induced earthquakes, the accurate detection of high-risk earthquake locations is paramount. Comprehensive databases of human-induced earthquakes are vital in this endeavor [62]. The Traffic Light System, designed for cyclic injections, has been an innovative strategy for monitoring the seismic response to fluid injections: a concept that has been explored in various settings, including geothermal development [72,73,74]. Agencies such as the U.S. Geological Survey (USGS), the Environmental Protection Agency (EPA), and the Department of Energy (DOE) play crucial roles alongside state oil and gas regulators. Collaborative efforts between these entities have led to the formulation of protocols and guidelines to ensure safety while advancing technologies associated with induced seismicity. A holistic approach to safer energy production requires the development of eco-friendly disposal methods for water generated from hydraulic fracturing. Furthermore, a deeper understanding of induced seismicity and its intricate mechanism demands collaboration between industries, academia, and government.

2.4. Air Pollution

Air pollution is a significant environmental concern associated with shale gas hydraulic fracturing. Despite advancements in technology and emissions reduction within the petroleum and natural gas industry, the production of shale gas still contributes to air pollution, primarily through the release of methane gas. Methane can escape from various points in the production process, including wellheads, pipelines, and storage tanks. A summary of the key studies exploring various air pollutants associated with shale gas hydraulic fracturing is provided in Table 5.
Methane emissions from the global energy sector reached nearly 135 Mt in 2022, with the energy sector accounting for almost 40% of total methane emissions due to human activity, second only to agriculture. The IEA has indicated that significant opportunities exist to cut these emissions; about 70% of methane emissions from fossil fuel operations can be reduced using existing technology, and in the oil and gas sector, an over 75% reduction is feasible using deploying measures like leak detection and repair programs [81]. More than a 28% decrease in methane emissions has been observed from all fossil fuel systems since 1990, with petroleum production experiencing an 8% decrease by 2014. According to the U.S. Environmental Protection Agency, while there has been a significant increase in natural gas production, emissions from U.S. natural gas systems have shown a minor increase of 1.8% from 1990 to 2019 [55]. It is imperative to underscore that methane emissions from the petroleum and natural gas industry constitute just 4% of total U.S. greenhouse gas emissions. However, a study focused on the Boston urban region, spanning from 2012 to 2020, identified that methane emissions from natural gas distribution and end-use were approximately three times larger than previously calculated by usage-based inventories, with no observable change in emission rates over an 8-year period despite mitigation efforts [82]. This emphasizes that while significant strides have been made in certain areas, there are still gaps and challenges to address.
In addition to methane, other air pollutants are associated with shale gas extraction and processing. Volatile organic compounds (VOCs) are emitted, which contribute to air pollution and the formation of a ground-level ozone. Vinciguerra et al. (2015) [77] observed a significant increase in daytime ethane concentrations in the Baltimore, MD and Washington, DC areas since 2010 to 2015, correlating with the rapid expansion of natural gas production in neighboring states, particularly Pennsylvania and West Virginia.
Furthermore, the use of diesel-powered machinery and vehicles in drilling and hydraulic fracturing activities releases air pollutants, including nitrogen oxides (NOx) and particulate matter (PM). A 2023 study by Zhang et al. (2023) [80] found a causal surge in PM2.5 concentrations around over 20,000 wells in Pennsylvania, highlighting the significant environmental implications of shale gas extraction. Additionally, fracking operations also release toxic air pollutants, such as benzene, toluene, ethylbenzene, and xylene (BTEX) [83], which pose risks to respiratory and neurological health. The inhalation of silica, a crucial component of fracking sand, can cause lung-related diseases, particularly among workers exposed to silica dust and diesel fumes in the fracking industry [83].
Aware of these occupational threats, the National Institute for Occupational Safety and Health (NIOSH) has extensively researched and highlighted these risks. In response, the Occupational Safety and Health Administration (OSHA) implemented more stringent regulations on respirable crystalline silica exposure. Specifically, OSHA’s final rule, effective in 2016, established a new 8 h time-weighted average permissible exposure limit (PEL) of 50 micrograms per cubic meter (µg/m3) and an action level of 25 µg/m3 for all silica polymorphs. The commitment of this industry to adopting these regulations, combined with their proactive efforts to develop effective control measures, underscores a growing dedication to improving worker safety and health in the hydraulic fracturing domain [84].
To mitigate air pollution from shale gas hydraulic fracturing, strict regulations and best practices are crucial, involving the adoption of technologies that minimize methane leaks and improve equipment, detection systems, and vapor recovery. Additionally, transitioning from diesel-powered machinery to cleaner energy sources and implementing emission control technologies could help reduce the impacts of air pollution. Raheja et al. (2022) [79] and similar studies have demonstrated the significant air pollution effects of shale gas development, particularly in heavily affected regions like Belmont County, Ohio. This study revealed shortcomings in the current regulatory air pollution monitoring network, highlighting the importance of enhanced monitoring practices and collaboration among residents, including environmental advocates and regulatory agencies, to establish more effective air quality standards to protect public health.
Overall, air pollution is a significant environmental concern associated with shale gas hydraulic fracturing operations. It is crucial to implement comprehensive measures that minimize pollution to adopt cleaner technologies and practices that can mitigate these environmental impacts. In doing so, the sustainable development of shale gas resources can be ensured.

3. Conclusions

The United States is the leading producer of shale gas globally, with active exploitation in prominent regions such as Barnett, Eagle Ford, and Permian Basin of Texas, Marcellus in Pennsylvania, and Bakken shale formation in North Dakota. This leadership stems from a surge in shale gas production catalyzed by hydraulic fracturing, marking a modern renaissance in energy production.
However, this growth has not been without environmental implications. Groundwater contamination is a pressing concern, often attributed to fracturing fluid leakage—either from geological faults or insufficient cement and casing within drilling. With hydraulic fracturing averaging 8000 feet in depth and 2.4 million gallons of water per well, the potential environmental threats from defective storage tanks or pits resulting in water or soil contamination and inducing earthquakes due to wastewater injections persist. Further emphasizing these environmental challenges, several earthquakes registering up to M 5.8 were detected in Texas and Oklahoma. These seismic activities are attributed to the disposal of contaminated water through injection wells. Moreover, methane, a critical greenhouse gas, saw emissions in the energy sector peak at 135 Mt in 2022. And while there has been a commendable 28% reduction in fossil fuel methane emissions since 1990, studies, like the one conducted in Boston from 2012 to 2020, highlight that emissions from natural gas distribution total three times previous estimates, pointing to areas that still require focused mitigation efforts.
The environmental issues tied to shale gas exploitation, from groundwater and soil pollution to induced earthquakes and air pollution, are vast. These challenges have the potential to adversely affect local communities, ecosystems, and even result in loss of life. It is crucial that continuous environmental monitoring, proactive measures, and regulations be tailored toward local geological and regional specifics. This industry’s partnership with the government to lower shale gas pollution and protect local communities is indispensable. Our review also underlines the urgent need for research into the intersection of hydraulic fracturing, environmental degradation, and climate change. With consolidated efforts from all stakeholders, we can pave the way toward a more sustainable future in shale gas extraction.

Author Contributions

B.H. crafted the overall manuscript, calculated the surface and groundwater contamination, collected detailed information on the earthquakes and developed the arguments. J.H. designed the structure, developed the arguments, and contributed to the overall paper. J.P. calculated soil and environmental pollution and developed the arguments. C.L. analyzed air pollution and contributed to the overall paper. All authors have read and agreed to the published version of the manuscript.

Funding

This research received no external funding.

Data Availability Statement

The data presented in this study are derived from multiple publicly accessible repositories: For Figure 1 detailing shale gas production in the United States: Data are openly available from the U.S. Energy Information Administration (EIA) without a specific DOI. For Table 4 listing key earthquakes in relation to hydraulic fracturing and wastewater disposal wells: Datasets were sourced from the USGS Earthquake Catalog [67] and the Railroad Commission of Texas (RRC) [68]. The data can be accessed directly from the respective sites.

Conflicts of Interest

The authors declare no conflict of interest.

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Water 15 03334 g001
Figure 1. Shale gas production in the United States. Average monthly gas production in billion cubic feet (bcf) or approximately 2.83 × 107 cubic meters (m3) based on daily production data averaged for each month. Data source: U.S. Energy Information Administration (EIA).
Figure 1. Shale gas production in the United States. Average monthly gas production in billion cubic feet (bcf) or approximately 2.83 × 107 cubic meters (m3) based on daily production data averaged for each month. Data source: U.S. Energy Information Administration (EIA).
Water 15 03334 g001
Water 15 03334 g002
Figure 2. Hydraulic fracturing process and potential contamination paths. This diagram showcases pollution routes via flowback water, well leakage, and rock fissures, affecting groundwater, subsurface water, soil, and air quality. Note: the lengths of fractures represented are not to scale and are for illustrative purposes only. Natural faults could exist that might not conform to the represented lengths. A red circle along the vertical drilling path indicates the area where poor cementing processes in the well can lead to leaks and contamination. For reference, depths indicated in the figure, such as 1000 ft (~304.8 m), 2000 ft (~609.6 m), and 8000 ft (~2438.4 m), are not averaged numbers and could vary greatly depending on the geological characteristics of the drilling site.
Figure 2. Hydraulic fracturing process and potential contamination paths. This diagram showcases pollution routes via flowback water, well leakage, and rock fissures, affecting groundwater, subsurface water, soil, and air quality. Note: the lengths of fractures represented are not to scale and are for illustrative purposes only. Natural faults could exist that might not conform to the represented lengths. A red circle along the vertical drilling path indicates the area where poor cementing processes in the well can lead to leaks and contamination. For reference, depths indicated in the figure, such as 1000 ft (~304.8 m), 2000 ft (~609.6 m), and 8000 ft (~2438.4 m), are not averaged numbers and could vary greatly depending on the geological characteristics of the drilling site.
Water 15 03334 g002
Water 15 03334 g003
Figure 3. Average composition of materials injected for hydraulic fracturing in the Permian basin, Texas [48].
Figure 3. Average composition of materials injected for hydraulic fracturing in the Permian basin, Texas [48].
Water 15 03334 g003
Table 1. Representative studies on hydraulic fracturing-related water and soil contamination.
Table 1. Representative studies on hydraulic fracturing-related water and soil contamination.
Reference (Year)LocationContaminantNote
Osborn et al. (2011) [16]Northeastern Pennsylvania and Upstate New YorkMethaneGroundwater contamination in Marcellus and Utica formations
EPA (2011) [38]Wyoming, PavillionDiesel Range Organics (DRO), Gasoline Range Organics (GRO), and Total Purgeable Hydrocarbons (TPH)Soil and shallow groundwater contamination in Wind River formation
Vidic et al. (2013) [17]PennsylvaniaBarium, Strontium, and BromideSurface water contamination in Marcellus formation
Olmstead et al. (2013) [39]PennsylvaniaChloride (Cl−)Surface water contamination in Marcellus formation
Fontenot et al. (2013) [40]North TexasTotal Dissolved Solids (TDS)Soil contamination in Barnett formation
Warner et al. (2013) [41]Western PennsylvaniaChloride and BromideSurface Water contamination in Marcellus formation
Skalak et al. (2014) [42]Appalachian Basin, PennsylvaniaTotal Dissolved Solids (TDS), Total radium (specifically Ra-226), Extractable Ba, Ca, Na, SrNo significant contaminant increases from treatment facilities but the spread of road brine raised levels near roads.
EPA (2016) [36]Wyoming, PavillionBenzene, Toluene, Ethylbenzene, and Xylenes, as well as Methane and other HydrocarbonsGroundwater contamination in Wind River formation
Nelson and Heo (2020) [43]Permian Basin, Western TexasTotal Dissolved Solids (TDS), Chloride, Fluoride, Nitrate, and ArsenicGroundwater contamination in Wolfcamp formation
Table 2. Average depths (in feet|m) and water usage (in gallons|L) for hydraulic fracturing in the United States both overall and in individual states [44].
Table 2. Average depths (in feet|m) and water usage (in gallons|L) for hydraulic fracturing in the United States both overall and in individual states [44].
Hydraulic Fracturing Depth (ft|m)Water Volume Used (gal|L)
Alabama221067437,600142,331
Arkansas412012565,230,00019,797,694
California2960902158,000598,095
Colorado755023011,410,0005,337,428
Kansas491014971,230,0004,656,054
Louisiana11,95036425,140,00019,457,007
Montana953029051,650,0006,245,927
New Mexico68502088706,0002,672,499
North Dakota10,37031612,170,0008,214,340
Ohio781023804,310,00016,315,117
Oklahoma856026093,430,00012,983,956
Pennsylvania704021464,460,00016,882,929
Texas875026672,490,0009,425,671
Utah83602548382,0001,446,027
Virginia4720143942,100159,366
West Virginia687020945,040,00019,078,466
Wyoming93902862793,0003,001,830
United States
Mean829025272,430,0009,198,546
Min221067437,600142,331
Max11,95036425,230,00019,797,694
Standard deviation26578101,933,8777,320,518
Coefficient of variation0.320.320.800.80
Notes: The quantity of data points for each category are as follows: United States (42,388); Alabama (55); Arkansas (1473); California (918); Colorado (5261); Kansas (206); Louisiana (1111); Montana (268); New Mexico (1292); North Dakota (2748); Ohio (157); Oklahoma (2194); Pennsylvania (2794); Texas (20,267); Utah (1692); Virginia (91); West Virginia (278); Wyoming (1583). The table values are based on data from Jackson et al. (2015) [44].
Table 3. The purpose and general use of chemicals utilized in hydraulic fracturing (Source: FracFocus.org).
Table 3. The purpose and general use of chemicals utilized in hydraulic fracturing (Source: FracFocus.org).
ChemicalPurpose
AcidDissolving minerals and the initiation of rock fracturing (pre-fracturing)
Sodium ChlorideDelaying the decomposition of gel polymer chains
PolyacrylamideMinimizing the friction between the fluid and pipes
Ethylene glycolPreventing scale formation within the pipe
Boric acid saltMaintaining fluid viscosity during temperature increases
Sodium/Potassium carbonateMaintaining the effect of other ingredients as cross-linking agents
GlutaraldehydeElimination of bacteria in the water
Guar gumIncrease in water viscosity to keep the sand afloat
Citric acidPreventing the precipitation of metallic oxides
IsopropanolDecreasing the viscosity of fracturing fluid
Table 4. List of key earthquakes that have occurred in regions related to hydraulic fracturing and wastewater disposal wells.
Table 4. List of key earthquakes that have occurred in regions related to hydraulic fracturing and wastewater disposal wells.
DateLocationMagnitudeNotes
2008 and 2009Dallas-Fort Worth and Cleburne, Texas>M 3Increased the number of small-magnitude earthquakes observed
23 August 2011Trinidad, ColoradoM 5.3Largest earthquake linked to fracking in the state of Colorado
11 September 2011Snyder, TexasM 4.4Fracking-related earthquakes occurred over a period of two months
16 December 2011Youngstown, OhioM 4.0Led to the suspension of fracking activities in the area
17 May 2012East TexasM 4.8linked to wastewater injection.
Earthquake associated with fracking in the Haynesville Shale
2 November 2016Cushing, OklahomaM 5.0Largest earthquake ever recorded in Cushing: a major oil hub
7 November 2016Pawnee, OklahomaM 5.8The largest earthquake known to be induced by wastewater disposal and the strongest earthquake in Oklahoma history at the time, leading to the temporary shutdown of wells
7 April 2018Lucien, OklahomaM 4.6
9 April 2018Marshall, OklahomaM 4.6
31 May 2018Pecos, TexasM 4.5Part of a series of earthquakes linked to fracking activities in the Permian Basin
26 March 2020Mentone, TexasM 5.0Largest quake in two decades; linked to deep water injection seismicity
4 March 2021Weld County, ColoradoM 4.2Earthquake linked to fracking operations in the Denver–Julesburg Basin
28 December 2021Stanton, TexasM 4.6Shanton in the Permian Basin identified as Seismic Response Areas (SRAs); saltwater disposal injections contributed to the region’s seismic activity.
17 March 2021
25 March 2022
1 June 2022
21 July 2022
11 August 2022		Whites city, New MexicoM 4.5
M 4.6
M 4.6
M 4.9
M 4.5		In New Mexico, seismic events occurred up to magnitudes of 5.0, linked to the oil and gas industry’s wastewater injection wells
16 November 2022Mentone, Coalson Draw, TexasM5.3–5.4In a region known for oil and gas production
16 December 2022Range Hill, TexasM 5.2Over 120 significant earthquakes have occurred since 2018 in this area, predominantly due to human activities
9 March 2023Trinidad, ColoradoM 4.3Ten days later, a 3.8 magnitude earthquake struck, likely due to wastewater injections causing underground movement
Note: Data Source: USGS Earthquake Catalog [67], Railroad Commission of Texas (RRC) [68]; Accessed: 27 July 2023.
Table 5. Summary of key studies on air pollutants associated with shale gas hydraulic fracturing in this paper.
Table 5. Summary of key studies on air pollutants associated with shale gas hydraulic fracturing in this paper.
Reference (Year)LocationContaminantNotes
Colborn et al. (2014) [75]Western ColoradoNon-methane hydrocarbons (NMHCs) and Polycyclic aromatic hydrocarbons (PAHs)NMHC concentrations were highest during the initial drilling phase.
Field et al. (2014) [76]Several basins in the US where airborne assessments have been conducted *nitrogen oxides, particulate matter, volatile organic compounds, hazardous air pollutants (HAP), methaneSuggests that emissions of pollutants are possible close to well pads and are identified as pollutants of concern.
Vincigurra et al. (2015) [77]Baltimore, MD and Washington, DCEthane (a VOC)Significant increase in daytime ethane concentrations since 2010
Helmig (2020) [78]Denver–Julesburg Basin, Northern Colorado Front RangeNon-ethane VOCs, methane, and NOxBased on observations of VOC/methane ratios and methane flux estimates.
Raheja et al. (2022) [79]Belmont County, OhioParticulate matter (PM) and volatile organic compounds (VOC)Air pollution sensor network of 35 particulate matter and 25 volatile organic compound sensors.
Zhang et al. (2023) [80]PennsylvaniaParticulate matter (PM)Casual increase in PM2.5 concentration in the vicinity of over 20,000 wells.
Notes: * Includes the Haynesville Shale in eastern Texas and western Louisiana, the Fayetteville Shale in northern Arkansas, the Marcellus Shale in western Pennsylvania, the Denver–Julesburg Basin in Colorado, and the Barnett Shale in Texas.
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Hwang, B.; Heo, J.; Lim, C.; Park, J. Environmental Implications of Shale Gas Hydraulic Fracturing: A Comprehensive Review on Water Contamination and Seismic Activity in the United States. Water 2023, 15, 3334. https://doi.org/10.3390/w15193334

AMA Style

Hwang B, Heo J, Lim C, Park J. Environmental Implications of Shale Gas Hydraulic Fracturing: A Comprehensive Review on Water Contamination and Seismic Activity in the United States. Water. 2023; 15(19):3334. https://doi.org/10.3390/w15193334

Chicago/Turabian Style

Hwang, Bohyun, Joonghyeok Heo, Chungwan Lim, and Joonkyu Park. 2023. "Environmental Implications of Shale Gas Hydraulic Fracturing: A Comprehensive Review on Water Contamination and Seismic Activity in the United States" Water 15, no. 19: 3334. https://doi.org/10.3390/w15193334

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