Shale's Impact: Groundwater Pollution Risk?

Shale oil and gas development has been linked to groundwater contamination, particularly in regions with a history of intensive energy extraction. This contamination can occur through various mechanisms, such as erosion, spills, leaks, and the underground migration of gases and chemicals. For example, studies have found elevated levels of chloride in groundwater near unconventional wells in southwestern Pennsylvania, likely due to leaks or spills. The decline of freshwater tables in drilling regions also increases the risk of groundwater contamination. In addition, shale development poses risks to air quality and land resources, and it consumes significant amounts of water. While the environmental impact of shale oil and gas development is a growing concern, the understanding of its effects on groundwater remains limited, and further studies are needed to fully assess the potential risks.

Groundwater contamination from shale development

Shale oil and gas development can contaminate groundwater through various mechanisms. One significant concern is the injection of fluids, including freshwater, chemical additives, and proppants, during the fracking process. This high-pressure injection can lead to underground migration of gases and chemicals, potentially reaching groundwater sources. Leaks and spills from unconventional drilling can result in the release of toxic substances, such as thallium, which may exceed safety standards and pose risks to human health.

In southwestern Pennsylvania, studies have found a link between elevated levels of chloride in groundwater and regions with a history of intensive energy extraction, such as coal mining and conventional oil and gas drilling. This contamination is attributed to leaks or spills from unconventional wells, with localized increases in chloride, barium, and strontium concentrations found near these wells. The Marcellus Shale region, spanning Pennsylvania and New York, has experienced groundwater contamination incidents attributed to shale gas development, with stray natural gas and brine spillage impacting water sources.

The Permian Basin in the US desert southwest has witnessed rapid growth in shale oil and gas development, contributing to groundwater contamination concerns. The decline of the freshwater table in this region increases the risk of contamination from disturbed formation water associated with shale development. While most studies focus on shallow freshwater aquifers, there remains a lack of understanding regarding the impact on other groundwater resources, such as deep carbonate aquifers.

To address these issues, improved monitoring, data dissemination, and field-based hydrogeological research are crucial. Additionally, the implementation of practical measures to reduce environmental contamination risks in shale energy production is essential for mitigating the impact of shale development on groundwater quality.

The impact of shale production on shallow freshwater aquifers

Shale gas is natural gas trapped in shales and clays, which are fine-grained, sedimentary rocks predominantly comprised of consolidated clay-sized particles that were originally deposited as muds. The environmental implications of shale gas development are a rapidly developing area of research.

Groundwater is part of the water cycle and consists of water that has filtered into the ground from rainfall, snowmelt, and rivers. It forms the largest available store of freshwater in the UK, far in excess of river or lake waters. Over lowland England, about half of all freshwater supplies come from groundwater, and in parts of the southeast, this rises to more than 70%. About three-quarters of all groundwater taken is used for public water supply, with the remainder being used for agriculture, industry, and other purposes. One reason for this is that, compared with surface waters, groundwater is of relatively high quality and requires less treatment prior to use.

The scarcity of surface water resources in arid and semi-arid regions has led to a reliance on groundwater to sustain anthropogenic activities. In some of these regions, economic development has become conflicted with water conservation goals, leading to groundwater contamination and over-extraction. The Permian Basin in the US desert southwest is one example. The region has experienced rapid growth in shale oil and gas development since the mid-2000s following recent innovations in drilling and fracking technologies. Shale fracking injects freshwater mixed with chemical additives and proppants (ceramic or sand) under pressure. During the sequential oil or gas recovery process, produced water (PW) emerges as a byproduct containing mainly formation water (FW) and a small portion of the fracking fluids.

Most existing studies concerning the impact of shale production on groundwater focus on shallow freshwater aquifers. There is little understanding of shale development's impact on other groundwater resources (e.g., deep carbonate aquifers and deep basin meteoric aquifers). The possible natural hydraulic connections between shallow aquifers and formation water suggest that the impact of shale production on shallow aquifers can be consequential.

Studies have shown a strong positive statistical correlation between shale production and the levels of TDS (total dissolved solids), chloride, sodium, and calcium in PW, even after controlling for well age, well depth, geological features, and temporal effects. The implications of their positive correlations with shale production can be significant because of the existing natural hydraulic connections between shallow aquifers and deep formations. These are common constituents that occur naturally in groundwater.

Other contaminants linked to shale gas include metals, naturally occurring radioactive materials (NORM), and organic compounds. Contaminants enter surface water primarily through spills or leaks and infiltrate downward into shallow aquifers. There is no evidence supporting aquifer contamination by the upwelling of fluids from production zones. Recent investigations have contributed to a growing consensus that stray gas in aquifers results primarily from casing failures in older production wells, rather than migration from zones where hydraulic fracturing was conducted in horizontal wells.

One environmental concern is the potential risk of groundwater contamination from the fluids used in the fracking process. A particular question is whether fracking fluids injected into the subsurface at depths of several kilometers can migrate via natural geological pathways to shallow aquifers at depths of tens to hundreds of meters. In the Marcellus shale play, USA, the deepest reported drinking-water level is ∼600 m below the surface. Similarly, groundwater abstractions in England typically do not descend more than 200 m below ground level, and it has been suggested that a reasonable maximum depth of ∼400 m may be considered for conventional freshwater aquifers. English regulations ensure that fracking only occurs 1,000 m below the surface and 1,200 m below the surface in specified groundwater areas, National Parks, Areas of Outstanding National Beauty, and World Heritage Sites.

The environmental risks of shale gas extraction

Shale gas extraction has been a growing industry in the United States, with estimates of shale gas resources increasing over the last few years. However, there are several environmental risks associated with the process of extraction that have been documented.

Firstly, shale gas extraction poses risks to water quality. The process of hydraulic fracturing involves injecting water, sand, and chemicals at high pressure to create fractures in underground formations and release trapped gas. The wastewater generated by this process contains high levels of pollutants, including deep brine salts such as chloride, barium, strontium, and trace elements like thallium and arsenic. Improper treatment of this wastewater can lead to serious groundwater contamination, as evidenced by studies in southwestern Pennsylvania, where elevated levels of chloride were found in groundwater near unconventional wells.

Secondly, the extraction process can impact surface water bodies and groundwater resources. In regions with limited surface water, such as the Permian Basin, there is a conflict between economic development and water conservation, leading to over-extraction and contamination of groundwater. The decline of the freshwater table increases the risk of contamination from disturbed formation water. Additionally, spills and leaks of chemicals and other fluids during shale gas development can further contaminate surface water and groundwater.

Thirdly, shale gas extraction poses risks to air quality. Engine exhaust from increased truck traffic, emissions from diesel-powered pumps, flaring or venting of gas, and unintentional releases of pollutants from faulty equipment contribute to air pollution. The release of methane, carbon dioxide, volatile organic compounds (VOCs), and particulate matter during the extraction process also poses risks to air quality and human health.

Furthermore, shale gas development impacts land resources and wildlife habitats. The construction and maintenance of infrastructure, such as well pads, impoundments, and compressor stations, result in extensive surface disturbances, impacting agriculture, tourism, outdoor activities, and ecosystems. The use of toxic chemicals and the injection of fluids underground can also contaminate soil and affect wildlife.

Lastly, there are concerns about the unknown extent and severity of environmental risks across different shale basins due to varying location-specific factors, geological characteristics, climatic conditions, and regulatory practices. While some studies suggest that hydraulic fracturing is unlikely to directly cause water contamination, the complex composition of flow-back fluids and the potential for well integrity issues highlight the need for continuous monitoring and assessment of pollutants.

The water-rock interaction in oil shale mining

Oil shale mining and processing have raised several environmental concerns, including land use, waste disposal, water use, waste-water management, and air pollution. Oil shale is an organic-rich sedimentary rock that lacks a definite geological definition or a specific chemical formula. Its seams do not always have discrete boundaries, and its mineral content, chemical composition, and depositional history vary significantly.

In-situ oil shale mining technologies, such as Shell ICP and ExxonMobil's Electrofrac, involve fracturing the oil shale using explosive or hydrostatic pressure and then heating it to release shale oil and gas. This process may have detrimental effects on groundwater due to the water-rock interaction under high temperature and pressure.

The impact of shale production on groundwater resources has become a growing concern, particularly in regions with rapid shale oil and gas development, such as the Permian Basin in the US. Studies have found increased levels of TDS (total dissolved solids), chloride, calcium, and sodium in groundwater after the transition from conventional to unconventional wells.

Overall, the water-rock interaction in oil shale mining can lead to the release of organic pollutants, including phenol, BTEX, TOC, and TPH, into groundwater. The high temperatures and pressures associated with in-situ mining technologies may exacerbate these effects, potentially impacting the quality and safety of groundwater resources.

The role of organisations in protecting groundwater from pollution

Shale oil and gas development has been linked to groundwater contamination. This contamination can be caused by spills and leaks, the underground migration of gases and chemicals, and the injection of fluids and toxic chemicals underground. The development of shale oil and gas can also impact the quality of surface water.

To combat this, organisations like the UK's Environment Agency and California's State Water Resources Control Board play a crucial role in protecting groundwater from pollution. These organisations implement regulations and standards to prevent and mitigate groundwater pollution.

The Environment Agency in the UK requires any activity that could affect groundwater quality or quantity to obtain an environmental permit or exemption. They assess permit applications by considering the geological characteristics of the location. The agency also applies the precautionary principle in cases where an activity may cause serious or irreversible damage to groundwater.

Similarly, the State Water Resources Control Board in California adopts water quality objectives (WQOs) to protect the beneficial uses of groundwater. They set statewide policies, coordinate with Regional Water Boards, and review petitions contesting Board actions. The Board also offers financial assistance programs to support projects that prevent or clean up groundwater pollution.

In addition to regulatory measures, organisations like the Groundwater Ambient Monitoring and Assessment (GAMA) program in California focus on monitoring and assessment. GAMA collects samples from community and domestic water supply wells to evaluate regional groundwater quality. This data helps prioritise cleanup work and permitting decisions to protect high-quality groundwater resources.

The role of these organisations is crucial in safeguarding groundwater resources from the potential impacts of shale oil and gas development, ensuring that any pollution is prevented, mitigated, or addressed through appropriate measures.

Frequently asked questions