Fluids used in hydraulic fracturing are a mixture of water, proppant, and chemical additives. Additives typically include gels to carry the proppant into the fractures, surfactants to reduce friction, hydrochloric acid to help dissolve minerals and initiate cracks, inhibitors against pipe corrosion and scale development, and biocides to limit bacterial growth (23). The exact mix of additives depends on the formation to be fractured. Chemical additives typically make up about 0.5% by volume of well fracturing fluids, but may be up to 2% [17, 23]. Some potential additives are harmful to human health, even at very low concentrations (24). Unless diesel is used, the fracturing fluids are not regulated by the Safe Drinking Water Act (SDWA). Underground disposal of oil and gas wastes, however, is regulated by the SDWA(25).
Water cycle in hydraulic fracturing; from U.S. EPA's Study of the Potential Impacts of Hydraulic Fracturing on Drinking Water Resources, Progress Report, 2012.
Diagram of possible fluid migration pathways and other environmental concerns with hydraulic fracturing. Source: Mike Norton, Wikimedia Commons.
Potential pathways for the fracturing fluids to contaminate water include surface spills prior to injection, fluid migration once injected, and surface spills of flowback and produced water (fig. 9). Because the fracturing fluids are injected into the subsurface under high pressure, and because some of the fluids remain underground, there is concern that this mixture could move through the well bore or fractures created in the reservoir rock by hydraulic pressure, and ultimately migrate up and enter shallow formations that are sources of freshwater (aquifers). There is also concern that geologic faults, previously existing fractures, and poorly plugged, abandoned wells could provide conduits for fluids to migrate into aquifers .
The potential to contaminate groundwater due to hydraulic fracturing is an environmental risk being studied (figure 10)[26, 28, 29]. At present, there have been possibly two confirmed cases of groundwater contamination caused directly by the hydraulic fracturing process; in one location the fractured rock is within 420 feet of the aquifer[6, 20, 30]. One challenge is to distinguish natural contaminants that seep into groundwater unrelated to oil and gas development, from contamination due to oil and gas development. There often are no water quality samples prior to hydraulic fracturing to provide a baseline comparison [6, 32, 33].
Horizontal Well Construction; from U.S. EPA Study Progress Report, December 2012, modified by Kansas Geological Survey .
For example, methane has been detected in some water wells in areas with oil and gas development [33, 34]. Some researchers suggested hydraulic fracturing may be responsible for methane in water wells in northeastern Pennsylvania and upstate New York, although leaky well casings is a more likely possibility [29, 32]. In some geologic settings, methane can naturally originate from gas-producing rock layers below and close to the aquifer and be unrelated to the deeper fractured zone [20, 31]. Analysis of the gas can be used to identify the origin of gas occurring in groundwater [31, 35]. In one study of drinking water wells near shale gas well sites in Pennsylvania and Texas, wells were sampled for hydrocarbon gas to determine if contamination had occurred. . The researchers concluded that contamination has locally occurred, and, for those wells with elevated gas levels, the fugitive gas appeared to have migrated from shallower rocks through cracks in the cement around the well (annulus), leaks in the well casing, or from other well failures , rather than from the artificial hydraulic fractures in the reservoir rock. An analysis of a large database on dissolved methane in domestic wells and proximity to pre-existing oil and gas wells in Pennsylvania indicated no statistically significant relationship , although the study had criticism for its industry support for the study .
Figure 12. Groundwater Water Quality Sampling from a small diameter, temporary borehole.
Kansas Department of Health and Environment, 2012.
Figure 13: Measuring groundwater depth before sampling, from a non-pumping well installed to
monitor water quality conditions. Kansas Department of Health and Environment, 2012.
There have been confirmed cases of groundwater contamination from improperly constructed, oil and gas wells. To protect groundwater, proper well design, construction, and monitoring are essential. During well construction, multiple layers of telescoping pipe (or casing) are installed and cemented in place, with the intent to create impermeable barriers between the inside of the well and the surrounding rock . It is also common practice to pressure test the cement seal between the casing and rock or otherwise examine the integrity of wells. Wells that extend through a rock formation that contains high-pressure gas require special care in stabilizing the well bore and stabilizing the cement or its integrity can be damaged . As with any mechanical device or barrier, failures can occur. There is significant variability in the estimated failure rates of the integrity of oil and gas wells [60, 61]. Local regulations, the technology, the geologic setting and the prevailing operational culture influence the well completion, abandonment and monitoring [60, 61], and these evolve over time. Differences in the type and sizes of well integrity datasets adds to the challenge of generalizing well integrity failure rates [60, 62, 63].
The physical separation between the relatively shallow freshwater aquifer and the typically much deeper oil- and gas-producing rock layer provides protection to shallow aquifers. Typically there are thousands of feet of mostly low- to very low-permeability rock layers between an aquifer and oil or gas reservoir rocks that prevent fracturing fluids and naturally migrated hydrocarbons from reaching the aquifer. In areas where there is concern about faults, fractures, or plugged wells, various geophysical methods can be used to locate and avoid faults, although such surveys are time consuming and expensive. There is also renewed interest in the need to locate and plug abandoned or “orphaned” oil and gas wells, and unused water wells, as a further measure to protect near-surface aquifers. It will also be prudent to develop technologies to monitor deep groundwater. In some regions, identifying and properly plugging all the abandoned wells is a significant undertaking .
Proper storage and disposal of fracturing fluids and produced water is important to ensure that both surface water and groundwater are protected. Most fracturing fluids and produced water are re-injected into Class II wells  drilled specifically for deep disposal, treated in wastewater treatment facilities, or recycled . Wastewater treatment facilities, designed primarily for municipal waste, can be overwhelmed with the volume and treatment of fracturing fluids and produced water; a number will not accept such waste [39, 40]. Disposal wells inject waste water deep into formations that originally produced the oil and gas, or into different formations that generally contain highly saline and otherwise unusable water. Water is generally co-produced in equal or larger volumes than petroleum throughout the life of a well. Fluid handling and disposal are important issues for all oil and gas activity. Appropriate management practices and regulatory oversight help assure that accidental leaks and spills are minimized.
Baseline water-quality testing, carried out prior to oil and gas drilling, helps to document the quality of local natural groundwater and may identify natural or pre-existing contamination, or lack thereof, before oil and gas activity begins [37, 41, 42]. Without such baseline testing, it is difficult to know if contamination existed before drilling, occurred naturally, or was the result of oil and gas activity. Many natural constituents, including methane, elevated chlorides, and trace elements occur naturally in shallow groundwater in oil- and gas-producing areas and are unrelated to drilling activities . The quality of water in private wells is not regulated at the state or federal level, and many owners do not have their well water tested for contaminants. States handle contamination issues differently. For instance, Colorado and Ohio require baseline sampling of wells in oil- and gas-producing regions as part of its regulatory process [23, 41, 43]. Pennsylvania places the presumptive burden of proof on oil and gas companies if groundwater contamination of drinking-water sources is found . In most states, however, such baseline sampling is not required .
Although there is little evidence of groundwater contamination due to hydraulic fracturing itself, there are still many questions about the risks to aquifers with the rapidly expanding industry developing tight oil and gas reservoirs using modern hydraulic fracturing techniques [6, 20, 26, 27, 28, 30]. There are few long term, peer-reviewed scientific studies. The U.S. Environmental Protection Agency’s (EPA) Scientific Advisory Board study Potential Impacts of Hydraulic Fracturing on Drinking Water Resources (projected to be finalized in 2016) will be an important contribution. Local baseline testing of groundwater quality prior to hydraulic fracturing operations can provide valuable data for later assessing claims of contamination.
Contamination risks to surface water during development of tight oil and gas plays has led to increased regulations in some U.S. states. Potential pathways for contamination include surface spills, waste disposal, and surface spreading of well cuttings. A study of the gas shale development in Pennsylvania documented increased chlorides downstream of the waste treatment plant and elevated total suspended solids downstream of shale gas wells . The elevated suspended solids appear related to the land clearing for the well pad, roads, and related infrastructure.
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