Public Health Concerns of Shale Gas Development
Natural gas has recently come to the forefront of the world's energy discussion. Hydraulic fracturing, used in combination with other recent key technologies, has enabled gas extraction from geologic formations of low permeability (e.g. shale) that were previously too expensive to exploit. Due largely to its abundance, shale gas has been viewed both as a transition fuel and as a way for the United States to become independent from foreign oil.
Drilling for shale gas entails injecting fluid underground to open pre-existing fractures in the shale formation to coax out the natural gas. In order to optimize access to thin shale formations, this type of gas extraction uses directional or horizontal drilling with laterals up to two miles in length, unlike conventional, vertically drilled gas wells. And unlike traditional forms of hydraulic fracturing, the unconventional methods used for shale formations require large volumes of fluid to extract gas from many existing fractures. These fluids are laced with chemicals used as friction reducers, biocides, corrosion inhibitors, etc., which allow for the use of the “slickwater” drilling technology that enables large volumes of fluids to be pumped at high pressures quickly and over long distances through a small-diameter casing. Also, instead of a single well pad at each site, shale gas extraction uses clustered, multi-well pads to further extend the area of extraction, thereby accessing as much of the resource as possible under leasing and capital constraints.
This modern form of hydraulic fracturing (hereafter referred to as HVHF high-volume, horizontal fracturing) differs from previous forms in both technique and magnitude, carrying with it a host of new public health and environmental concerns. Public health concerns about HVHF focus primarily on pollutant exposure from air emissions and water contamination; however, the entire life-cycle of unconventional shale gas extraction is potentially polluting. This includes everything from clearing the land for the gas well pad, to initial hydraulic fracturing, subsequent recompletions, and the final capping of the well years or decades later after it is no longer productive. The hydraulic fracturing of shale occurs deep underground and although there are some legitimate concerns that this may cause direct problems through the propagation of cracks, events closer to the surface provide clearly demonstrated pathways of exposure. Water contamination can occur in a variety of circumstances, such as transportation spills of fluids and produced waters; well casing leaks; leaks through fractured rock; drilling site discharge; and wastewater disposal. Drilling chemicals and fracturing fluids used in various steps of the drilling process can be highly toxic and include known carcinogens (e.g. benzene, butoxyethanol, boric acid, methanol). Endocrine-disrupting chemicals are also present in significant amounts; the EPA defines these as “exogenous agent(s) that interferes with the synthesis, secretion, transport, binding, action, or elimination of natural hormones in the body which are responsible for the maintenance or homeostasis, reproduction, development and/or behavior.”
“Flowback water” withdrawn from the well after the fracturing process, and “produced water” returned to the surface with the natural gas, introduce other toxic substances. In addition to the toxins put into the ground, these returned waters contain heavy metals (e.g. lead, arsenic), naturally occurring radioactive materials (e.g. radon, uranium, chromium), bromide, and chloride (brine). Containment of these returned waters remains a problem and recycling (i.e. reuse for other drilling operations) accounts for only a portion of these toxic fluids. Flowback and produced waters are often put in evaporation ponds, which have been known to leak, contaminating water and soil and leading to documented instances of fish and livestock deaths. In some parts of the country (e.g. Texas), wastes are injected into old, abandoned conventional gas wells or other underground storage facilities for disposal. However, suitable disposal wells are rare in the northeast and much waste has been brought to municipal sewage treatment plants, which are not designed to handle these types of toxins. No suitable or concrete plans have been made for the treatment and storage of wastewater from shale gas extraction in the Marcellus Shale region.
Water contamination due to natural gas operations associated with hydraulic fracturing has been documented. At the request of residents in Pavillion, Wyoming, the United States Environmental Protection Agency (EPA) conducted a three-year water quality investigation of private drinking water wells. The EPA released a draft analysis of its data in December 2011 which indicated ground water contamination by compounds likely associated with natural gas production activities. In addition to chemical contamination, methane, the principle constituent of natural gas, has been found in aquifers at concentrations well above hazardous levels. A Duke University study in Pennsylvania that tested sixty-eight water wells found that groundwater near drilling areas contained methane concentrations seventeen times higher than wells where drilling was not taking place. Methane is also emitted into the atmosphere by leaking and venting throughout various stages of natural gas production, processing, transmission, and distribution. Methane is a potent greenhouse gas, and while it doesn't stay in the atmosphere as long as carbon dioxide, it is 105X and 33X as strong as CO2 over 20-year and 100-year timeframes, respectively. A field study air sampling in the Denver-Julesburg Basin of Colorado suggest natural-gas producers are losing about 4% of their gas to the atmosphere. In the atmosphere, methane contributes to global climate change, which in turn affects human health in a number of ways, including heat waves, extreme weather events, flooding, water contamination, sea level rise, expansion of insect ranges and populations, worsening air quality, crop damage, and social instability and conflict.
The fugitive emissions of natural gas production matter significantly in the context of climate change and also have other implications for public health. Methane atmospherically transforms to toxic ground-level ozone when it interacts with sunlight. Methane and other ozone precursor emissions such as nitrogen oxides, and volatile organic compounds (VOCs) can lead to increased respiratory morbidity and mortality. Natural gas operations also produce other conventional air pollutants such as benzene, xylene (BTEX), formaldehyde, toluene, hydrogen sulfide, and ethylbenzene that pose both carcinogenic and non-carcinogenic health risks. A recent health risk assessment of air emissions showed that residents living a half-mile or less from gas wells are at a greater risk for health effects than those living more than a half-mile away. The study found subchronic exposures to air pollutants during well completions to pose the greatest risk, with benzene being the major contributor to cumulative cancer risks.
Significant air pollution emissions come not only from natural gas drilling and processing operations, but from transportation as well. Each hydraulic fracturing event requires on average an estimated 5 million gallons of water in addition to large quantities of proppants (silica, quartz sand) and chemicals. Based on fluid transportation and other well pad activities (e.g. construction, equipment), a total of 3,950 heavy and light-duty truck trips are required for each horizontal well, given that each well can be hydraulically fracked multiple times during its productive life. Fine diesel particulate matter as well as nitrogen oxides and VOCs are emitted into the atmosphere during transportation, posing additional concerns for public health.
The recent boom in shale gas production has left the public health community scrambling to catch up. There is a current paucity of scientific information on the health effects of HVHF and many barriers stand in the way of creating solid epidemiologic studies. For one, data collection and analysis have been hampered by regulatory exemptions at the federal level. The 2005 Energy Policy Act exempted the natural gas industry from seven major federal laws designed to protect public health, including the Safe Drinking Water Act and the Clean Air Act. The United States Environmental Protection Agency (EPA) still does not regulate the injection of fracturing fluids and the industry is not required to disclose chemicals it considers to be proprietary. At the local level, the signing of non-disclosure agreements by landowners when drilling companies settle and pay for illnesses suffered and contamination also obstructs the availability of necessary data. In addition, questions regarding the level, length, and source of exposure are difficult to answer in the context of natural gas operations. This makes it hard to link contamination incidents to drilling activities, creating a significant hurdle for scientific research. Furthermore, the toxicological complexity of the chemicals used in natural gas operations creates problems for causal inference.
Public health studies have been further hindered by other factors as well. Epidemiologic studies often require significant time and resources (e.g. prospective cohort studies), which have been far outpaced by the rate at which shale gas operations have developed. Environmental and health-related governmental agencies have lacked the capacity to adequately investigate public health considerations of shale gas extraction. There are political interferences as well, and currently there are no members on state and national advisory committees with recognizable public health expertise.
Given that there exists only a handful of peer-reviewed scientific publications on the public health dimensions of HVHF, more research is needed for the public health risks of shale gas development to be adequately assessed. Consequently, states that permit HVHF may be said to engage in an on-going experiment to determine the effect of a nascent drilling technology on human health. The risks associated with shale gas extraction are substantial and the level of magnitude at which it is carried out is unprecedented. To provide context, in Pennsylvania alone 5,364 wells have been drilled since 2007, a number expected to rise to over 100,000 within the next few decades. Regardless of the strength of regulation and safe practice, accidents will occur and water and air will become contaminated.
While no energy production method is completely benign, the large-scale development of shale gas resources and their potential impacts on human health and world climate call for the utmost precaution. Thus, the continuation of current moratoria and bans is prudent until the appropriate science documents HVHF's effects on public health. Potential exposure pathways must be further investigated, and epidemiologic research is needed to quantify short-term and long-term risks to human populations. Only then will policy makers have the necessary information to conduct an appropriate cost-benefit analysis or Health Impact Assessment. And only then will our nation be able to design a policy for shale gas extraction that adequately balances energy development with protection of human health.
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 HIAs are conducted in land use decisions to determine how human health will be impacted before a project or policy is enacted.
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