Shale development can introduce a broad range of local air quality concerns, some of which appear later in the development and production phases. Many of them begin with the drilling of exploratory wells and carry on through the later phases of development and production. The major sources of potential air quality impacts include venting and flaring of natural gas from wells and fugitive emissions from oil and natural gas processing equipment; diesel-powered trucks and machinery; road dust; evaporation from storage pits; and dust from silica sand (see Box 6 on silica dust). Depending on the people affected and the exposure levels and pathways, these emissions can potentially provoke a variety of health effects, ranging from a nuisance, to acute to chronic respiratory problems, to psychological stress caused by the perception of worsened air quality. For a summary of the potential health effects of air pollutants from shale development, see Table 3.
While there are few studies of air quality in the vicinity of shale development sites, there are numerous documented community complaints of odors and other symptoms consistent with exposure to contaminants from oil and gas operations, such as upper respiratory ailments and skin irritation. One Colorado study measured air samples near well pads during the well completion phase and found that volatile organic compounds (VOCs), an ozone precursor, were present more frequently and at higher concentrations than in regional ambient air samples. Residents nearest to the well pads were found to be at higher risk of acute and sub-chronic respiratory, neurological, and reproductive effects.
In another study in the Barnett Shale region of Texas, researchers established a regional air monitoring network to monitor for VOCs near Dallas-Fort Worth, an area of high-density shale development. They compared the monitoring data to a variety of regulatory health-based air comparison values (HBACVs) and found that none of the VOCs measured exceeded the HBACVs, concluding that the community was not being exposed to VOCs at a level that would cause a health concern. Given that this was a community-scale study, the authors noted that individual property owners could potentially be exposed at higher or lower levels than those measured.
In addition to monitoring location, the variability of air emissions at shale development sites (due to the intermittent use of equipment; the varying composition of shale formations and fracturing fluids; and the influence of weather patterns and terrain, among other factors) could be responsible for differing outcomes between the Texas and Colorado studies. Some researchers have concluded that further study – including community-based research – is needed in order to account for the potential cumulative impacts of the various sources of air pollution over time at shale development sites. ,
Venting and flaring
Prior to the installation of equipment for collecting natural gas at an oil or gas well site, operators historically vented or flared the natural gas produced by the exploratory well. Venting has the effect of releasing methane, the primary component of natural gas – along with VOCs like benzene, toluene, ethyl benzene, and xylene (the BTEX chemicals) – directly into the atmosphere. Methane itself is principally a safety hazard if it accumulates in closed spaces; it can cause asphyxiation or explosions at high concentrations. VOCs can cause health issues such as respiratory problems and eye and skin irritation and, under certain conditions, can combine with other hydrocarbons to produce ground-level ozone, which might cause lung damage at high exposure levels. Chronic and prolonged exposure to ozone can result in asthma, lung disease, and cardiovascular effects.
As an alternative, flaring can take place in a closed incinerator box or, more commonly, at the top of a tall flare stack. The operator may also flare the gas when testing well flow or in emergency situations to prevent explosions or fires. Flares have a destruction efficiency of at least 98%, thus significantly reducing methane and VOC emissions. Natural gas flaring principally forms carbon dioxide and water, but also results in some residual emissions of combustion byproducts, such as carbon monoxide and nitrogen oxides. Flaring typically lasts between three and ten days and can create loud noise and heat, often requiring companies to notify local communities and fire departments before the burn takes place.
To avoid the environmental and health issues associated with venting, incinerating, or flaring the gaseous materials during a well completion, many companies now capture the marketable gas in a process referred to as a green completion. Effective January 2015, new EPA regulations under the Clean Air Act (Amendment of New Source Performance Standards ) require 95% of VOCs from natural gas wells to be captured by green completions as the well is prepared for production. Under the EPA rules, venting, incinerating, or flaring may still occur under certain circumstances; for example, during periodic maintenance and emergencies.
In August 2015, the EPA issued additional proposed rules that apply green completion requirements to shale oil wells. The rules will apply only to sources newly constructed or modified after the date of proposed rule publication in the Federal Register (September 18, 2015). The agency intends to have the final rules in place in 2016. (For more information on laws and regulations, see Appendix C.)
Local air quality might not only be impacted through operational releases of gases, but also through fugitive emissions of methane and VOCs due to leakage at wellheads, pipelines, storage tanks, compressors, and other equipment. There is uncertainty about how much leakage occurs and studies have drawn varying conclusions, depending on the method used to calculate emissions. In light of the new EPA requirements for green completions and the reduction of fugitive emissions from equipment and infrastructure, fugitive emissions from shale development should be significantly reduced. , EPA’s August 2015 proposed rules require operators to locate and plug leaks from pneumatic pumps, pneumatic controllers, and compressor stations, among other sources.
Diesel-powered trucks and machinery
The estimated 1,148 one-way heavy truck trips during the early phase of well development can result in significant emissions from diesel fuel combustion. The preparation of drilling sites and construction of rigs and other industrial infrastructure require operation of heavy machinery, which is often diesel-powered. Once well drilling operations begin, diesel-powered generators usually power the drills and power the pumps and compressors that force hydraulic fracturing fluid down wells and funnel natural gas through pipelines.
Diesel fuel contains PM2.5, or very fine particulate matter, that can penetrate deep into the lungs if inhaled. Exposure to diesel fuel exhaust and its components (such as arsenic, benzene, formaldehyde, and nickel) can cause immediate health effects such as cough, headaches, lightheadedness, and irritation of the eyes, nose, and throat. It can exacerbate respiratory illnesses, and studies have indicated that long-term exposure can lead to the increased risk of lung cancer. For vulnerable populations, such as the elderly or those with respiratory conditions, exposure to high levels of fine-particle pollution is linked to increases in hospital admissions, emergency room visits, asthma attacks, and even premature deaths.
The many diesel-powered engines used in shale development also result in emissions of carbon monoxide (CO), nitrogen oxides (NOX), sulfur dioxide (SO2), and volatile organic compounds (VOCs). Under certain conditions, NOX and VOCs can combine to form ground-level ozone, which brings its own health concerns (see Table 3).
In 2007, EPA issued the “Highway Diesel Rule,” which set new emissions standards for heavy-duty vehicles. This new ruling is expected to reduce harmful emissions from diesel fuel by 90%. The NIEHS Working Group on Unconventional Natural Gas Drilling Operations indicated that the impact of this rule on diesel fuel emissions from shale development operations is unknown and an important subject for further study.
The construction and maintenance of oil and gas operations entails the transport of heavy equipment and truck traffic on local roads. New access roads may also be constructed to accommodate this traffic. The particulate matter (PM2.5 and PM10) generated can cause respiratory effects, particularly in vulnerable individuals. Dust can also worsen visibility conditions on roads, which can lead to traffic accidents.
Large surface pits that store produced water and other wastewater from the shale development process can be a source of emissions when VOCs and other hazardous air pollutants (HAPs) volatilize from the stored water. These pits were mostly used in Western states, and their use is declining as the industry transitions to the use of storage tanks for wastewater, either on the well pad or in a central location.