The natural gas industry is a boon to the economy of the United States and will continue to expand in the following decades. Hydraulic fracturing (fracking), however, produces much waste and it must be determined how to dispose of unwanted byproducts of natural gas drilling, such as produced wastewater, solid scale, and oil. Radionuclides such as uranium were deposited in the Marcellus Shale millions of years ago and are now being returned to the surface in produced water from fracking. The presence of radionuclides creates a policy conflict between laws that protect public health and the economics of disposing of produced water. This case study will help readers understand how geologic history, hydrology, and present policy are intricately related in Pennsylvania. It will address possible methods for handling wastewater—storage, reuse, treatment, injection wells, and transport—and the degree to which state and federal policies protect drinking water from produced water. In addition, the Radium Girls factory case from California helps readers consider how the mode of exposure matters for the effects of human contact with radionuclides. Students of environmental policy will be better able to understand the linkages between policy and the physical sciences.

INTRODUCTION

Advanced techniques in hydraulic fracturing (fracking) have brought rapid growth to the natural gas industry. The production of large deposits of natural gas offers economic advantages for the United States. However, the growth of fracking raises the concern of industrial wastewater management. Untreated produced water contains hundreds of chemicals that can negatively impact humans and the environment [1]. Produced water is a combination of resident brine and fracking fluid that flows back to the surface after being used to break up a rock formation. The aquatic chemistry of the produced water is dependent both on the known chemicals in fracking fluid and the geology of the fractured rock formation. Thus, a proper understanding of shale geology is necessary for developing effective water treatment policies.

Radionuclides, whose atoms have an unstable nucleus, are one group of chemicals found in produced water [2]. Radionuclides like radium-226 and polonium-210 bioaccumulate in fish and then cause a cancer risk to people who eat fish [3]. Humans can also be directly exposed through drinking water. We use the case of fracking in the Marcellus Shale region of Pennsylvania to discuss the linkages between the geology of shale formations and policies related to fracking wastewater. Additionally, we use the example of the Radium Girls to illustrate the long-term risk of exposure to radium. After reading and discussing this case, readers should be able to make connections between hydrogeology, geologic history, and current policy debates.

CASE EXAMINATION—FRACKING IN PENNSYLVANIA

According to the Energy Information Administration, “Pennsylvania is the fastest-growing hydraulic fracturing state in the U.S.” [4]. Over 8,000 unconventional wells were drilled in Pennsylvania between 2008 and 2014 [5]. Natural gas waste falls mostly outside federal regulations of radioactive materials, leaving this responsibility to state and local governments. Thus, the Pennsylvania Department of Environmental Protection (DEP) issues permits and enforces regulations for conventional (drilled vertically) and unconventional (drilled vertically, then horizontally underground) well drilling. Pennsylvania, for example, has setback rules, which require that well bores must be at least 300 feet away from any stream (58 Pa. Cons. Stat. §3215(b)). Pennsylvania is not uniformly affected by shale drilling (Figure 1).

FIGURE 1.

Map of the Marcellus Shale, John G. Van Hoesen (CC-CC0), https://commons.wikimedia.org/wiki/File:Marcellus_Shale.png

FIGURE 1.

Map of the Marcellus Shale, John G. Van Hoesen (CC-CC0), https://commons.wikimedia.org/wiki/File:Marcellus_Shale.png

Geological History of the Marcellus Shale

The deposition of mineral resources, such as natural gas, is due to mountain forming events (i.e., orogenies) that took place between 1 billion and 300 million years ago. The Marcellus Shale formed from the shallow sea floor of the Appalachian Basin (Figure 2) roughly 420 to 360 million years ago. The basin likely had an aerobic (oxygenated) surface, overlaying an anaerobic (oxygen-less) bottom. When plants and animals died, they would sink to the bottom of the basin and mix with sediment. High pressure transformed the deposited carbon to coal and natural gas during the Alleghany orogeny [6].

FIGURE 2.

In the high oxidative environment of the Cambrian eon, uranium sediments exposed by the mountain forming events were dissolved and deposited in the Appalachian basin along with dead plant and animal matter. This latter formed the deposits of coal and natural gas. Adapted from Dr. Ron Blakey, CC BY-SA 3.0.

FIGURE 2.

In the high oxidative environment of the Cambrian eon, uranium sediments exposed by the mountain forming events were dissolved and deposited in the Appalachian basin along with dead plant and animal matter. This latter formed the deposits of coal and natural gas. Adapted from Dr. Ron Blakey, CC BY-SA 3.0.

Uranium was also found in Appalachian Basin deposits [7]. Uranium mineral was eroded by the weather and transported to the Basin by the northern river in Figure 2. The reason for this deposition is that uranium is soluble in the oxidative conditions of shallow water, but forms solid particles in anaerobic conditions, such as a sea floor [2]. The resulting geological formation was a mix of fossil fuel, shale, salts, and uranium. In fact, uranium is present in such concentrations that there was once a uranium mine at Mt. Pisgah, Pennsylvania [8]. Since the uranium in shale has been decaying for hundreds of millions of years, much of it has transformed into other elements, including radium [2]. Radium is important because it is water soluble, like other alkali/alkaline salts (such as NaCl, table salt); a factor in its presence in produced water.

Fracking and Produced Water

After drilling a well and establishing a wellhead, small explosive charges then fracture pathways perpendicular to the fracking pipe and high-pressure fracking fluid allows gas to return to the surface (see Figure 3). The explosive charges are a mixture adjusted for detonation at a specific ignition temperature. Fracking fluid may contain sand, solvents, guar, water, oils, gels, acids, biocides, and alcohols. Between 60% and 90% of the fracking fluid remains underground. The flowback water is the fracking fluid that returns to the surface before the well starts to produce natural gas. Produced water is the solution that then returns along with natural gas. Fracking a single well requires millions of gallons of water, an amount that increased 770% from 2011 to 2016 for U.S. wells [9]. Flowback during the first year of well use increased 1,440% in the same time period [9]. Thus, wastewater treatment and disposal are growing concerns.

FIGURE 3.

Process of hydraulic fracturing, U.S. Environmental Protection Agency (CC-CC0), https://commons.wikimedia.org/wiki/File:Hydraulic_Fracturing-Related_Activities.jpg.

FIGURE 3.

Process of hydraulic fracturing, U.S. Environmental Protection Agency (CC-CC0), https://commons.wikimedia.org/wiki/File:Hydraulic_Fracturing-Related_Activities.jpg.

Hydrology of Pennsylvania

A key challenge of regulating fracking wastewater is the fact that water can physically transport pollutants across long distances. Transport of water occurs primarily in surface water and groundwater. In surface water, weathering, rain, and wind erode solids to create a mixture between colloidal and suspended particles in fluid. Transport of solids is dependent on both the properties of the particles and the properties of the fluid. Properties of the particle include size, shape, dimension, and density. For instance, round particles tend to settle faster [10]. Influential fluid characteristics include turbulence, flow, and density. Turbulence is a complex phenomenon that exhibits chaotic mixing and motion. When a chemical mixes with water it can dissolve if the chemical is soluble. Soluble compounds usually stay within the water mixture longer than suspended particles or colloids. For instance, rain erodes shale, limestone, and sandstone and transports the ions used to construct those minerals into our drinking water supply. Calcium, magnesium, sodium, and potassium then harden after the water evaporates, leaving a white residue on bathtubs and sinks [11]. The solubility radium is dependent on the water’s pH. Radium is found in higher concentrations in low pH water [12], like the acidic water in mining areas.

Groundwater is an important resource in Pennsylvania, reaching 30% of potable water after 1984 [7]. This is why evidence of fracking waste in groundwater is such a concern in Pennsylvania [13]. Seasonally, the greatest draw from wells is in the winter and spring, reaching groundwater flows of up to 3,000 to 8,000 gallons per minute [7]. Groundwater flows in the direction of least energy and is dependent on permeability, such as differences in the size of openings between rocks, rock formation, pressure difference, and gravity. It is actually possible for underground water to flow uphill when the pressure is lower in that direction.

The fact that water can move waste chemicals from fracking wells and treatment discharge pipes to other parts of the state makes analysis of its effects, and consequent policy choices regarding disposal, dependent on several factors that vary greatly, such as weather, pH, distance, and topography [14]. Isolation of waste and careful protocols regarding waste disposal are thus a necessary liability of fracking. When discharged directly into the environment, compounds in fracking wastewater prove difficult to track. This creates a diffuse pollution, policy, and, ultimately, political problem that is challenging to solve with effective policy.

RADIOACTIVE PRODUCED WATER

The quality of fracking produced water depends on the original fracking fluid and dissolved minerals from the shale rock formation. Water dissolves salt because the polarity of water pulls apart the differently charged salt ions. As shown in Table 1, most of the salt found in produced water is calcium, sodium, magnesium, strontium, and barium. The U.S. Environmental Protection Agency regulates all point source discharge to waterways under the Clean Water Act. PA DEP issues permits for discharge from municipal sewer treatment systems. Table 1 displays the concentration of salt in produced water compared to amounts allowed in these regulations, which is hundreds, and sometimes thousands, of times greater. When the concentration is too salty, the dissolved salt will try to find locations, such as dirt particles, for forming solids. Because radium is a larger atom than other salts, it is blocked from forming solids on dirt particles in high salinity produced water. Therefore, radium stays in the produced water without binding to anything [15].

TABLE 1.

Hydraulic Fracturing and Salt Related Regulations based on DEP Form 26R (Resources for the Future, 2013)

ParametersType of RegulationRegulation (mg/L)Observed* (mg/L)
Total dissolved solids PA wastewater effluent 500 30,000–80,000 
Chloride PA wastewater effluent 250 10,000–60,000 
Strontium EPA finished municipal 100–1,000 
Barium PA wastewater effluent 10 20–2,000 
ParametersType of RegulationRegulation (mg/L)Observed* (mg/L)
Total dissolved solids PA wastewater effluent 500 30,000–80,000 
Chloride PA wastewater effluent 250 10,000–60,000 
Strontium EPA finished municipal 100–1,000 
Barium PA wastewater effluent 10 20–2,000 

*Values observed in produced water are represented by the midspread, i.e., the middle 50% of values.

The radium, alpha radiation, and beta radiation levels for produced water from wells in Pennsylvania are shown in Table 2. It includes a comparison between the maximum contaminant level set for drinking water and the median and maximum values found in produced water. It is evident in Table 2 that the median values for each substance found in produced water are up to 1,000 times that of the EPA’s maximum contaminant limit for drinking water. Tables 1 and 2 illustrate the toxicity of produced water and the consequent need for proper treatment before water is returned to the environment.

TABLE 2.

Hydraulic Fracturing Wastewater Radionuclides (DEP, 2016)

SubstanceType of RegulationMCL1 RegulationMedian (pCi/L)Max (pCi/L)
Radium-226 EPA MCL 5 pCi2/L 4,550 25,500 
Radium-228 EPA MCL 30 ug/L 643 1,710 
Gross Beta EPA MCL 4 mrems3/yr 2,440 21,300 
Gross Alpha EPA MCL 15 pCi/L 10,700 71,000 
SubstanceType of RegulationMCL1 RegulationMedian (pCi/L)Max (pCi/L)
Radium-226 EPA MCL 5 pCi2/L 4,550 25,500 
Radium-228 EPA MCL 30 ug/L 643 1,710 
Gross Beta EPA MCL 4 mrems3/yr 2,440 21,300 
Gross Alpha EPA MCL 15 pCi/L 10,700 71,000 

1MCL is Max contaminant level for EPA drinking water regulations.

2Curie (Ci) is a measure of radioactive decay.

3Rems is a measure of radioactive exposure, 1 rem represents about a 0.055% chance increase of cancer due to radiation using the linear threshold model.

Health Risk and Exposure: The Radium Girls

The Radium Girls study illustrates how exposure to even low levels of radioactive material can harm humans [16, 17]. At the beginning of the 20th century, clock dials were painted with radium to make the dial luminescent in the dark. Factory workers (i.e., the Radium Girls) sharpened the tip of the brush used to paint the dial with their lips. Many of the factory workers contracted head and bone cancers. It is logical that radium would accumulate in bones, because it is in the same chemical group as the bone nutrient calcium. Thus, ingestion or inhalation of radium can cause lymphoma, bone cancer, and leukemias [18].

Dr. Harrison Martland was one of the first to study the Radium Girls and observed that their cases fell into two general groups: acute and chronic. Martland measured the body burden, or the part of radium that remains in the body. The acute group had a radium body burden of 10 to 100 μg over about a 1-year period and died within 1 to 7 years of ingesting the paint due to bone cancer or anemia. The chronic group of workers were exposed to lower doses over a longer time period and had a body burden of 0.7 to 23 μg of radium. They suffered bone irregularities and often fatal cancer 12 to 23 years later [19]. In 1941 Dr. Robely Evans of MIT conducted breathalyzer tests on the Girls for radon, a daughter product of radium. Based on observed symptoms, he deduced that the standard of acceptable radium exposure is below 0.5 μCi. Including a safety factor, 0.1 μCi was the first radium standard used by the National Bureau of Standards. In 1981, Evans reported that he noticed no significant ill symptoms below the standard of 0.1 μCi of over 2,000 radium patients [19]. The California Department of Environmental Protection also performed a review of the Radium Girls. It concluded that the lowest dose to cause an increase of health risk due to radium-226 is 200 pCi/L of water [18]. The EPA’s drinking water regulation for the level of radium is therefore set intentionally low at 5 pCi/L. The medical effects of low doses of radiation are still debated, as displayed by the different curves in Figure 4. Overall the linear model (a) is accepted as the most common way of analyzing radiation versus cancer, though some individuals are more (b,e) or less (c,d) sensitive to exposure. In summary, radium in produced water entering drinking water systems is a concern. Both acute exposure to high levels of radium and chronic exposure to low levels increase the risk of cancers.

FIGURE 4.

Proceedings of the National Academy of Sciences suggest that overall the linear model (line a) for modeling dose of radiation versus cancer risk is the most reliable across a large population [20]. Other models exist (b,c,d,e), some more sensitive (b, e), some less sensitive (c,d), depending on radiation types, exposure, and biological resilience, Dudley T. Goodhead (CC-CC0).

FIGURE 4.

Proceedings of the National Academy of Sciences suggest that overall the linear model (line a) for modeling dose of radiation versus cancer risk is the most reliable across a large population [20]. Other models exist (b,c,d,e), some more sensitive (b, e), some less sensitive (c,d), depending on radiation types, exposure, and biological resilience, Dudley T. Goodhead (CC-CC0).

THE CHALLENGE: DISPOSING OF PRODUCED WATER

From 2006 to 2012, the Pennsylvania oil and gas industry increased disposal of waste from 270 million gallons of produced water to about 1 billion gallons [21]. In the early days of the fracking boom, much of the industrial wastewater was sent to municipal treatment facilities that were not engineered for radioactive fracking water. In 2011, Pennsylvania banned fracking operations from sending their wastewater to municipal sewer treatment plants. Afterward, industrial treatment was reengineered specifically to treat wastewater for reuse. On site storage and deep well injection have also become popular as alternatives to treatment. Though, the geology of Pennsylvania is not suited for deep well injection, so much of the fluid is trucked to Ohio. We will discuss each of these potential methods of disposal in turn.

Centralized Waste Treatment

Initially, fracking wastewater was taken to publicly owned treatment works (POTWs) for disposal. The Clean Water Act regulates normal sewage treatment POTWs that discharge to U.S. waters, but the treatment plants are not designed to handle radionuclides and high total dissolved solids. One study found that effluent in Western Pennsylvania contained barium and radium that was about 200 times higher than the background concentration upstream and above the nuclear waste disposal threshold [22]. Elevated levels of radium remain for years after treatment activity is stopped [23]. Produced water from fracking is not currently regulated as hazardous waste by the Resource Conservation and Recovery Act (RCRA). The RCRA regulates the disposal of nonhazardous waste, such as trash, or hazardous waste, such as wastewater treatment sludge [24]. The oil and gas industry is currently exempt from these regulations.

Industrial pretreatment is a similar method to POTWs, but is engineered specifically to treat produced water with high total dissolved solids. Industrial pretreatment discharge flows into a municipal treatment work and is regulated by the local municipality (40 CFR §435). In 2016, EPA ruled to disallow discharge of produced water, scaling, or sludge from industrial pretreatment facilities into local bodies of water. In Pennsylvania, facilities must establish action plans for radioactivity monitoring (25 Pa. Code §95).

The Josephine facility operated by Fluid Recovery Services (Creekside, PA) illustrates problems with oversight of industrial pretreatment. Table 3 shows the average monthly, daily maximum, and instant maximum allowable levels of gross alpha and radium from the facility’s 2015 permit. Note that no maximum amount is officially regulated (hence the XXXs), though the facility does have to report maximum daily concentrations to DEP. While one study found that the facility reduced radioactivity in fracking effluent by 90%, the constituent material still accumulates in high levels as sludge that goes to a landfill. Additionally, stream sediments at the discharge site still exhibit higher levels of radioactivity [25]. These sites voluntarily stopped receiving fracking produced water in 2011, but continue to receive conventional oil wastewater, which also leads to a higher level of radium in stream sediments [23].

TABLE 3.

Permit for the FRS Josephine Facility

Concentration (mg/L)

ParameterAverage monthlyDaily maximumInstant maximum
Gross Alpha (pCi/L)    
Interim Report Report XXX 
Final XXX Report Max XXX 
Radium 226/228, Total (pCi/L)    
Interim Report Report XXX 
Final XXX Report Max XXX 
Concentration (mg/L)

ParameterAverage monthlyDaily maximumInstant maximum
Gross Alpha (pCi/L)    
Interim Report Report XXX 
Final XXX Report Max XXX 
Radium 226/228, Total (pCi/L)    
Interim Report Report XXX 
Final XXX Report Max XXX 

Open Pit Storage

After disposal through wastewater treatment plants was banned in 2011, some companies turned to wastewater impoundments—otherwise known as open pits—for storing produced water. In this case, wastewater lays on the surface and looks like a lake or a pond. The pits are lined to prevent leakage. There are presently at least 25 such impoundments in Pennsylvania [25, 26]. Over time, water can evaporate from these pools, leaving behind only chemicals and sand. This means that the chemicals, including radioactive isotopes, increase in concentration as they reside in the pits. For example, beta radiation in impoundments is more than eight times the regulatory limit [25]. Leakage to groundwater may also occur in cases where the ground cover is not well maintained or not properly installed. Thus, while providing storage, impoundments do not actually address the presences of radioactive materials in fracking waste.

A number of toxic organic chemicals are also found in produced water such as xylene, toluene, ethylbenzene, and styrene [27]. These chemicals are known carcinogens and are volatile, meaning that they also evaporate. Oil, salt, solvents, and surfactants from oil and natural gas pits can result in bird mortality, particularly in older poorly managed facilities [28]. Oil pits do not have to be covered or monitored due to the exemption from hazardous waste classification in the RCRA. Most problems with impoundments are due to construction and maintenance [27]. Several possible tactics to raise safety include testing impoundment slope stability, geomembrane liners, and developing emergency response to leakages.

Reuse

Pennsylvania fracking operations reuse the majority of produced water. This reuse requires treatment, such as that performed by Eureka Inc. (Williamsport, PA). Eureka’s industrial treatment plants take in produced water and either treat it for discharge to a POTW or treat it for reuse in the fracking industry. The treatment technology includes bag filtration, membrane biological reactor, reverse osmosis, and mechanical vapor recompression crystallizer. Eureka’s two crystallizer treatment plants claim to have a capacity of 0.42 million gallons per day [29].

There are political difficulties involved in the permitting process for reuse treatment facilities. Not all members of a community or local officials want treatment facilities in their back yards. For instance, in southwestern Pennsylvania, an attempt to build a wastewater treatment plant is underway by JKLM Energy, LLC (Franklin Park, PA). The brine treatment plant was initially denied due to lack of public notice and conflict with the local government. Thus, it is important to recognize that effectively permitting a wastewater treatment plant requires community awareness and cooperation with local officials [30].

Deep Well Injection

Deep well injection is a viable alternative, but expensive, with a capital costs of $100,000 for each new well opened, plus an additional $20,000 spent per day per well to transport produced water [31]. Injection wells operate by a similar concept as unconventional drilling. In contrast to unconventional drilling, the main purpose of injection wells is to dispose of the produced water underground, instead of removing gas from the ground. Figure 5 shows a diagram of an injection well, where the fluid is trapped below a solid layer of rock, theoretically unable to reach groundwater. The proper capping of an injection well is necessary so that when the well has reached capacity, the fluids do not travel vertically. Pennsylvania only has 8 injection wells, whereas Texas has over 34,000 active injection and disposal wells [32].

FIGURE 5.

Disposal wells inject fracturing fluid into the rock layer far below the water table, U.S. Government Accountability Office (CC-CC0), https://www.flickr.com/photos/usgao/14791477504.

FIGURE 5.

Disposal wells inject fracturing fluid into the rock layer far below the water table, U.S. Government Accountability Office (CC-CC0), https://www.flickr.com/photos/usgao/14791477504.

The main concern with using many injection wells is the apparent association between seismic activities (i.e., earthquakes) and injection well sites. The extreme pressure of injection can take nearly a year to dissipate [25]. Also, the viscous produced water slips and faults, analogous to oil lowering the inertia between car parts. One study examined this phenomenon by tracing, over a 2-year period in Texas, 24 earthquakes back to epicenters that were all within 3.2 km of an injection well [33]. This activity was traced to only 9 of the 27 total injection wells in Johnson County (where the study took place). The other 18 injection wells did not have earthquakes around them, suggesting that earthquakes due to injection wells are related to the specific tectonics, faults, and other geological phenomena underground. More research is needed to determine the exact causes and relationship between injection wells and earthquakes.

CONCLUSION

This case study demonstrates how the geologic formation of the Marcellus Shale included not only carbon-based lifeforms, but other materials like uranium. While heat and pressure created shale rock containing natural gas, the decay of uranium produced an array of radioactive materials that are still present in that rock. Given the high salinity of fracking water, produced water contains high levels of radium, which is dangerous to human health, if ingested. Thus, policies are necessary to prevent produced water from entering freshwater sources. Geology and hydrology constrain policy choices for dealing with problems like fracking. Overlooking both can lead to accumulation of radionuclides and hazardous organics, as well as misinformation on the fate and transport of produced water. Thus, policymakers in shale states must consider the trade-offs of different methods for addressing hazardous bi-products of the industry.

It is also important for students to consider the political and economic context of shale gas extraction in the United States. Multiple states are aggressively pursuing the development of shale gas resources due to their great economic potential. From a national perspective, there is also the push to reduce dependence on foreign sources of energy and, in fact, the United States has become a net exporter of natural gas [34]. Each of the above options for disposing of wastewater have their own costs, ranging from the low cost of direct discharge to much higher costs for deep well injection or proper treatment. Combined with the diffuse nature of this problem—thousands of wells geographically dispersed—effective policy is challenging to develop.

CASE STUDY QUESTIONS

  1. Discuss how radionuclides can impact long-term health.

  2. For this question, consider the different effects of chronic and acute exposure to radium. Go to: https://radiationcalculators.cancer.gov/radrat/model/inputs/. Enter your gender and year of birth at the top. Under “Exposure Information” enter the following: Year: ‘2018’, Organ: ‘Oral Cavity and Pharynx’, Exposure: ‘Chronic’, Distribution Type: ‘Normal.’ Change the parameter to rad and try the following pairs of mean (parameter 1), standard distribution (parameter 2): (0,0), (1, 0.2), (10, 2), (100, 20), (1000, 200). Consider that the exposure (0,0) is your negative control and (1000, 200) is your positive control. How does the dose of radiation for the three test pairs compare and contrast to the positive and negative controls? Find the mean, upper, and lower percent lifetime increase in cancer for each radiation dose. Repeat the process, except change exposure to ‘acute.’ How does this effect the mean, upper and lower intervals? One way to compare the results would be to plot them in Excel.

  3. Do you believe Dr. Evans’s study is definitive in setting the level for radium regulation at 0.1 μCi? Research other ill effects of radiation such as brain development in children. How might this effect regulations?

  4. What is the acceptable contaminant level for radium? What level of risk should be assumed: 1 cancer case per 100, 1 per 1,000, 1 per 10,000, 1 per 100,000, etc.? How should we decide what level of risk is acceptable?

  5. How should geologic and hydrologic history shape current policy debates in hydraulic fracturing? What are the potential dangers in not considering the geology of a state when creating statewide or local regulations of natural gas drilling?

  6. How do politics, economics, and the diffuse nature of the problem of fracking wastewater also shape policy debates over wastewater disposal policy?

  7. What type of treatment should be used in the long term: injection wells, storage, centralized treatment, and/or reuse? Compare and contrast the benefits and drawbacks of each.

  8. What are the federal, state, and local government roles in produced waste water treatment and fracking? Where do they overlap? Where might there be gaps?

  9. Should the limit for radionuclide concentration be set at the state, federal, or local level? What are the trade-offs of setting limits at each level?

  10. What are the implications of the findings of this case study for other locations across the United States where there are shale deposits?

AUTHOR CONTRIBUTIONS

Conceptualization – ANB, MAH, DJM, KK

Investigation – ANB, MAH, KK

Project Administration – ANB, MAH

Supervision - DJM

Visualization – ANB

Writing – original draft – ANB, MAH, KK

Writing – review & editing – ANB, MAH, DJM

COMPETING INTERESTS

No competing interests to report.

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