Naturally-Occurring Radioactive Materials (NORM) - ACS Publications

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Chapter 4

Naturally-Occurring Radioactive Materials (NORM) Associated with Unconventional Drilling for Shale Gas Andrew W. Nelson,1 Andrew W. Knight,2 Dustin May,3 Eric S. Eitrheim,2 and Michael K. Schultz1,4,* 1Interdisciplinary

Human Toxicology Program, University of Iowa, Iowa City, Iowa 52242, United States 2Department of Chemistry, University of Iowa, Iowa City, Iowa 52242, United States 3State Hygienic Laboratory, University of Iowa, Coralville, Iowa, 52241, United States 4Department of Radiology, Department of Radiation Oncology, Free Radical and Radiation Biology Program, Medical Scientist Training Program, Biosciences Program, University of Iowa, 500 Newton Road, ML B180 FRRB, Iowa City, Iowa 52242, United States *E-mail: [email protected]

As unconventional drilling has emerged as a major industry in the U.S. and around the world, many environmental health and pollution risks have surfaced. One emerging concern is the risk of environmental contamination arising from unconventional wastes that are enriched in naturally-occurring radioactive materials (NORM). Although NORM has been a well-documented contaminant of oil and gas wastes for decades, there are new challenges associated with unconventional drilling. This chapter will present the origin of NORM in black shale formations. In addition, we present the fundamentals of radioactive decay and ingrowth, so as to provide a foundation for a discussion of the potential for environmental impact of NORM. Finally, within this context, we highlight key-relatively-unexplored aspects of unconventional drilling that point to the need for further environmental radiochemistry research.

© 2015 American Chemical Society In Hydraulic Fracturing: Environmental Issues; Drogos, Donna L.; ACS Symposium Series; American Chemical Society: Washington, DC, 2015.


Introduction Hydraulic fracturing and horizontal drilling for shale gas has emerged as an important technology for supplying energy to the United States and the rest of the world (1–3). Amongst its benefits (for example, reduced land impact compared to conventional natural gas (4), lower direct CO2 emissions than coal (5), etc.), unconventional drilling has many unknown and uncharacterized potential environmental pollution risks (6–14). Of the least characterized environmental pollutants generated by new natural gas extraction techniques are naturally-occurring radioactive materials (NORM) (15). At depth, gas-bearing formations such as the Marcellus Shale (northeastern United States), often contain elevated levels of NORM, relative to the terrestrial environment at the surface (16); Thus, the identification of elevated levels of NORM is frequently used as an environmental marker of petroleum rich reserves and used to guide drilling operations (17). However, as mining operations proceed to the extraction stage of well development, the relative enrichment in NORM in these materials has the potential to result in co-extraction of these NORM (often referred to as “technologically-enhanced naturally occurring radioactive materials” (TENORM) (18). Co-extraction has the potential to lead to environmental contamination, bioaccumulation of radionuclides and radiation exposure to humans that would have otherwise been confined at depth. Thus, the proliferation of new horizontal drilling and hydraulic fracturing practices and technologies is leading to challenges associated with management of NORM. Some of these challenges, such as management of produced fluids, disposal of solid waste, and bioaccumulation of contaminants from oil and gas operations are well known from off-shore drilling operations (19, 20). Other challenges are new and uncharacterized as drilling technologies originally developed for offshore operations are brought into the interior of the country (1). Developing understanding of and management strategies for NORM in these uncharacterized wastes will require input from interdisciplinary teams comprising specialists in industrial hygiene, toxicology, health physics, epidemiology, geology, petrology, atmospheric chemistry, environmental and civil engineering, policy, sociology, and radiochemistry. As radiochemists, our goal is to provide accurate and precise models of past, present, and future levels of radioactivity concentrations to inform appropriate waste management decision-making. Three related concepts are critical to understanding radioactivity concentrations in relation to NORM liberated by unconventional oil and gas mining: (1) geochemical partitioning of radionuclides in the natural decay series; (2) the resulting potential for disruption of natural steady-state radioactive decay relationships in the decay series (referred to as disequilibrium); and (3) the subsequent radioactive ingrowth that occurs after a disequilibrium event. To explain the concept of partitioning, we introduce the physicochemical properties of NORM and how these properties can result in geochemical partitioning (disequilibrium) of specific NORM in solid, liquid, and gas phases. Next, we describe radioactive ingrowth to explain how the radioactivity concentration of radioactive wastes have the potential to increase over time after a disequilibrium event. We conclude with a discussion on the major phases of drilling as they 90 In Hydraulic Fracturing: Environmental Issues; Drogos, Donna L.; ACS Symposium Series; American Chemical Society: Washington, DC, 2015.

relate to radioactivity and provide three research areas that are relevant to environmental public health. Thus, the goal of this perspective is to highlight some of the known challenges and to guide future investigations in environmental radioactivity related to unconventional drilling, with the goal of informing appropriate decision-making for waste management.


NORM The observation that certain elements are naturally-radioactive dates back to 1896, when Henri Becquerel discovered that uranyl-sulphate crystals spontaneously emitted radioactivity similar to X-rays (21). Madame Marie Curie later observed that all uranium and thorium isotopes were radioactive regardless of their chemical composition (22, 23). She also observed that uranium-containing mineral pitchblende possessed a higher level of radioactivity than compounds prepared from recently purified uranium. This observation prompted Marie Curie and her husband (Pierre Curie) to purify pitchblende and to characterize many of the radionuclides that are now collectively referred to as NORM. Many of the chemical properties that allowed the Curies to separate the various elements are important factors in environmental partitioning. We have provided a brief description of the physicochemical properties of the elements that comprise NORM so as to provide a basis for the subsequent discussion on environmental partitioning (see Addendum: Basic Properties of NORM). We restrict our discussion to elements and isotopes from the three primordial decay series: (1) the actinium (235U) series, (2) the thorium series (232Th series), and (3), the uranium series (238U series, sometimes referred to as the radium series). Note, that another important natural isotope 40K (t1/2 = 1.248 x 109 years) can be considered TENORM (24). 40K will be present in all natural sources of K with a natural abundance of 0.0117% (25). Each of the primordial decay series is supported by an isotope with an extremely long half-life (235U, 7.04 x 108 y, 232Th 1.4 x 1010 y, 238U 4.47 x 109 y) (25). Each of the three primordial NORM decay series noted above comprises a radiogenic ‘progenitor’, which supports a series of ‘progeny’ radionuclides with varying half-lives (μs to millions of years) and different physicochemical properties (gas, particle reactive, redox sensitive, etc.). In total amongst the three decay series there are 41 radionuclides from 12 different elements, where each radionuclide has unique decay modes and half-life, and each element has unique chemistry (26). Note, due to these physicochemical and radiochemical properties, each isotope has a different detection strategy (Table 1). All three of these decay chains will ultimately decay to one of three stable lead isotopes (208Pb, 207Pb, and 206Pb) (Figure 1) (21). In general, the specific activity (radioactivity per gram of natural material; e.g., soil, sediment) of radionuclides in the actinium series is substantially lower than the specific activity of 232Th and 238U series radionuclides – owing to the low natural abundance of 235U (0.7% the mass of natU) (25). Thus, we focus on NORM in the 238U series (14 radionuclides, 8 elements) and 232Th series (11 radionuclides, 8 elements) (26).

91 In Hydraulic Fracturing: Environmental Issues; Drogos, Donna L.; ACS Symposium Series; American Chemical Society: Washington, DC, 2015.


Figure 1. The primordial decay series of interest in shale formations: actinium (235U) series (left), uranium (238U) series (middle), and thorium (232Th) series (right). All half-lives are from the National Nuclear Data Center (NuDat).


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Table 1. Naturally Occurring Radioactive Materials of Interest in Unconventional Drilling Wastes Element Element Uranium (U)

Protactinum (Pa)

Thorium (Th)

Isotopes

Expected forms U4+ reduced, particle reactive, expected in shale formation; U6+ oxidized, mobile, expected to form under some conditions at the surface of the earth Pa5+ particle reactive

Th4+ particle reactive

α/β/γ

a

Half-life

S/L/G

b

Isotopic Detectionc

238d

α

4.468x109 y

S

MS, AS, HPGe (234Th, 234mPa)

235

α, γ

7.04x108 y

S

MS, AS, HPGe

234d

α

2.455x105 y

S

MS, AS

234m

γ

1.159 m

S

HPGe

234

β

6.7 h

S

HPGe (234mPa)

234

β, γ

27.1 d

S

MS, HPGe

232

α

1.4x1010 y

S

MS, AS

230

α

75,400 y

S

MS, AS

228d

α

1.9116 y

S

AS

Actinium (Ac)

Ac3+ particle reactive

228

β, γ

6.15 h

S/L

HPGe

Radium (Ra)

Ra2+ soluble at depth and surface, solubility dependent on salinity and chemical matrix

228d

β

5.75 d

S/L

HPGe (228Ac)

226d

α, γ

1600 y

S/L

HPGe (direct or 214Bi, 214Pb)

224d

α, γ

3.6319 d

S/L

HPGe (direct or 212Bi, 212Pb) Continued on next page.

In Hydraulic Fracturing: Environmental Issues; Drogos, Donna L.; ACS Symposium Series; American Chemical Society: Washington, DC, 2015.

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Table 1. (Continued). Naturally Occurring Radioactive Materials of Interest in Unconventional Drilling Wastes Element Element Radon (Rn)

Polonium (Po)

Bismuth (Bi)

Lead (Pb)

Isotopes

Expected forms Rn, nobel gas

Po2+, Po4+ unclear which form, particle reactive

Bi3+

Pb2+

particle reactive

particle reactive

a

Half-life

S/L/G

b

Isotopic Detectionc

222d

α

3.8235 d

S/L/G

Emanation

220

α

55.6 s

S/L

Emanation

218

α

3.098 m

S/L/G

N/A

216

α

0.145 s

S/L

N/A

214

α

164.3x10-6 s

S/L/G

N/A

212

α

0.299 x

10-6

S/L

N/A

210d

α

138.376 d

S/L/G

AS

214

β, γ

19.9 m

S/L/G

HPGe

212

β, γ

60.55 m

S/L

HPGe, AS

210

β

5.012 d

S/L/G

N/A

214

β, γ

26.8 m

S/L/G

HPGe

212

β, γ

10.64 h

S/L

HPGe

β, γ

22.2 y

S/L/G

HPGe

210d a (α) alpha-emitter;

α/β/γ

s

b (S) expected to be present and/or generated in solid wastes (bit cuttings);

(β) beta-emitter; (γ) gamma-emitter. (L) expected to be present and/or generated in liquid wastes (flowback/produced fluids); (G) expected to be present and/or generated gas streams (flared-waste gases and natural gas). c (MS) mass-spectrometry; (AS)alpha-spectrometry; (HPGe) high purity germanium gamma-spectrometry Emanation—radon emanation. d key isotopes of interest in environmental fate and transport.

In Hydraulic Fracturing: Environmental Issues; Drogos, Donna L.; ACS Symposium Series; American Chemical Society: Washington, DC, 2015.


NORM in Marine Black Shales The term “shale” generally refers to fine-grained-sedimentary clays and rocks that have widely differing biogeochemical morphologies and geologic ages (27). The types of shale of most interest to the petroleum and natural gas industries are the so-called, “black shales,” which are sedimentary rock deposits that became enriched in organic materials and metals originating from marine or brackish water (28). Black shales are characteristically dark in color owing to enrichment of organic content (which in some cases may exceed 10%) via multiple biogeochemical processes (Figure 2) (29, 30). Over millennia, different biogeochemical processes (thermogenic and microbial), resulted in the reduction of the various organic materials into methane (natural) gas (31).

Figure 2. Uranium (U) trapped in marine organisms and sediments as an explanation for the increased levels of U and U decay products in marine black shale formations. The relatively high level of organic content in black shales is important not only for the production of natural gas, but also has implications for enrichment of NORM. The marine environment from which black shales typically arise are enriched in natural U isotopes (natU: 238U, 235U, 234U), with an average of 3.3 μg natU/L depending on ocean salinity in which the sediment was formed (32). While natU behaves conservatively in oxic marine environments resulting in a conservative natU and salinity correlations, this dissolved natU is 95 In Hydraulic Fracturing: Environmental Issues; Drogos, Donna L.; ACS Symposium Series; American Chemical Society: Washington, DC, 2015.


concentrated in organic-rich ocean sediments (for example, black shales) by the following mechanisms: (1) inorganic precipitation as U-rich marine waters pass across suboxic, organic-rich sediments (33), (2) microbiological reduction and immobilization of natU (34), and (3) sedimentation of marine organism detritus (35). As hexavalent uranyl (UO22+) is reduced to tetravalent (U4+) at the anoxic/oxic boundary, insoluble complexes/minerals are formed that lead to enrichment of U in black shales (35). Thus, over millions of years, black shales are enriched in U and radioactive decay products in the U-series and are likely to be found in steady-state radioactive equilibrium with long-lived progenitors (26, 36). Thus, at depth in the formation, one can expect that the decay rates of primordial progenitors and radioactive progeny are essentially the same (in secular equilibrium). On the other hand, anthropogenic activities, such as natural gas and petroleum mining in these formations have the potential to disrupt these steady state conditions by co-extraction of specific radionuclides in the primordial series – i.e., partitioning of the specific progeny into the extracted liquid and gas phases in the mining process. Although shale formations vary in mineral composition and age, one can describe four overarching biogeochemical properties that can be used to predict the likely behavior (partitioning) of radionuclides from formations at depth: (1) Formations are generally ancient (fossil, > 100 million years old, particularly in the case of the Marcellus Shale), suggesting that radioactive decay products are present and in equilibrium with the supporting progenitor (37); (2) Formations are likely anoxic, reducing environments at depth (38). This has direct implications for the fate and transport of select redox-sensitive radionuclides (e.g., U, Po) and indirect implications by altering the key adsorptive surfaces (for instance, Mn and Fe minerals) (39–41); (3) At depth, microbial reduction of sulphate (SO42-) to form sulfides (H2S, S2-) enhances the solubility of heavier alkaline earth metals (Sr, Ba, Ra). The implication is that in environments low in sulphates, there is a higher probability that Ra will be soluble (RaSO4 Ksp = 4.25 x 10-11) (42) as evidenced by low SO42- brines from the Marcellus Shale region having high levels of Ra isotopes (43–46); and (4) Interstitial fluids in some black shales (Marcellus Shale in particular) are likely high in salinity, which creates conditions that enhances the solubility of Ra (47, 48).

Partitioning in the Subsurface Based on our investigations of NORM in Marcellus Shale produced fluids we developed a general model to predict partitioning of U-series and Th-series progenitors and radioactive progeny between solid-phase materials (bit cuttings; solid waste generated by drilling) and interstitial brine (flowback/produced fluids) extracted through the unconventional drilling/hydraulic fracturing process. To illustrate the principles of our generalized model, we describe the partitioning of the radionuclides in these series at depth by examining the physico-chemical events that govern the biogeochemical behavior of the radionuclides in these series. We begin with a single atom of apical progenitors 238U and 232Th, and 96 In Hydraulic Fracturing: Environmental Issues; Drogos, Donna L.; ACS Symposium Series; American Chemical Society: Washington, DC, 2015.


examine differences in geochemistry and decay modes that govern the separation of progenitors and radioactive progeny (Figure 3).

Figure 3. Theoretical partitioning model of U and U decay products at depth in shale formations. (Adapted with permission from reference (63)). Uranium Series Partitioning at Depth Owing to the geologic history and chemically-reducing conditions of the Marcellus Shale, one can expect 238U (t1/2 = 4.5 x 109 years) to be contained in the crystal lattice of minerals in a reduced-immobile (U+4) oxidation state (35). As 238U decays by alpha-particle emission to 234Th, it imparts a large amount of energy (often referred to as alpha recoil energy) to the Th nucleus. Alpha recoil energy is sufficient to break chemical bonds in the crystal structure of the rock (49), potentially leading to enrichment of the 234Th atoms located at the solid-phase interstitial aqueous-brine interface (26, 50). Although the geochemistry of Th is not susceptible to changes in oxygen concentration that might arise under environmental conditions, Th is known to be highly particle reactive. These properties predict Th to be relatively immobile via adsorption to mineral surfaces at depth in shale formations (51). Thus, the reducing environment of the Marcellus Shale and particle reactivity of Th predict that U and Th radionulclides are likely to be relatively immobile and unlikely to be extracted into aqueous-phase hydraulic fracturing fluids used for natural gas mining (34, 52). Conversely, radioactive decay of solid-phase bound 230Th results in 226Ra species that are much more soluble in the interstitial brine due to low sulfate concentrations and high salinity of Marcellus Shale formation water. The decay of 226Ra leads to the formation of the radioactive noble gas 222Rn, which adds complexity to predictions of the fate of successive decay products. Because 222Rn is an inert gas with a half-life of nearly four days, gaseous diffusion and partitioning of 222Rn (and its subsequent decay progeny) from 226Ra 97 In Hydraulic Fracturing: Environmental Issues; Drogos, Donna L.; ACS Symposium Series; American Chemical Society: Washington, DC, 2015.


via diffusion through fissures created by hydraulic fracturing events is possible (53). In general, 222Rn only travels 0.1 m in wet soils by diffusion alone, but long-range transport is possible when coupled with a carrier fluid (54). Given that 222Rn has increased solubility under high pressure at depth, it is likely that 222Rn will be contained in the interstitial fluids of the formation only to decay through a series of short-lived isotopes to insoluble radioactive decay product 210Pb (t1/2 = 22.2 years) (54). More research is needed to determine the effects of the divalent-rich saline environment on the solubility of 222Rn (55). Careful sample preparation and measurements of flowback water and produced fluids from hydraulic fracturing at the time of extraction have the potential to elucidate more precisely the radioactive steady-state/non-steady-state relationship of 226Ra and progeny 222Rn in liquid waste products of unconventional drilling. Differences in the geochemistry of 210Pb and 226Ra have provided an opportunity to develop a temporal understanding of geochemical phenomena for decades (56). However, as 210Pb decays to the redox active element, 210Po, the fate of 226Ra progeny becomes less clear. Studies of anoxic marine environments suggest that 210Po can partition from 210Pb and concentrate in organic rich particulate phases (57). Other studies show that under low-oxygen conditions, 210Po is particle reactive and its transport is associated with the migration of Fe or Mn minerals (40, 57–59). Although it is currently unknown whether reagents used in hydraulic fracturing fluids will mobilize 210Po, its transport in the subsurface will likely be coupled with changes in redox conditions, pH, and bulk movement of Fe- and Mn-containing particulates. Given that 210Po and 210Pb can separate from one another under certain environmental conditions (60), researchers should measure 210Pb and 210Po independently. Use of 210Po levels as a proxy for 210Pb radioactivity concentrations may not be appropriate. The potentially high levels of 210Po in unconventional drilling wastes present a unique opportunity to study fundamental 210Po processes at depth and in the terrestrial environment. For example, future studies could focus on the possible relationships between microorgamisms, sulfur-cycling, and polonium partitioning (61). Thorium Series Partitioning at Depth The fate and transport of the 232Th series is similar to the previous discussion of the 238U series. The progenitor radionuclide, 232Th (t1/2 = 1.4 x 1010 years), is insoluble in environmental waters and brines. As 232Th decays by alpha emission to 228Ra (t1/2= 5.75 years), the resulting 228Ra progeny is soluble in the sulphatedeficient, divalent-rich brine. As 228Ra decays to 228Ac (t1/2 = 6.15 hours) by beta emission, the fate is uncertain. Generally, Ac forms insoluble complexes and quickly adsorbs to mineral surfaces, but given its short half-life it can be difficult to discern the exact mechanism (62). Then 228Ac decays by beta emission to the insoluble 228Th (t1/2 = 1.91 years). Note, the large difference in solubility for 228Ra and 228Th gives rise to a chronometer (transient equilibrium model) that has potential to assist in determining the time when samples were removed from the Marcellus Shale (63). The nucleus then undergoes decay to form 224Ra (t1/2 = 3.63 days), which again solubilizes into the brine. 220Radon (t1/2 = 55.6 s) then decays by alpha emission to form 216Po (t1/2 = 0.145 s), which rapidly emits another alpha 98 In Hydraulic Fracturing: Environmental Issues; Drogos, Donna L.; ACS Symposium Series; American Chemical Society: Washington, DC, 2015.


particle to form 212Pb (t1/2 = 10.64 hours). Similar to 210Pb in the 238U decay series, 212Pb is expected to be insoluble and particle reactive. Next, 212Pb decays by beta emission to 212Bi (t1/2 = 60.55 min), which is also likely insoluble (64). Then 212Bi branches to two different decay products; 64% of decays are by beta emission to form the very short-lived 212Po (t1/2 = 0.299 μs) while the other 36% of decays form 208Tl (t1/2 = 3.053 min) (25). Both 212Po and 208Tl decay, by alpha and beta particle emissions respectively, to 208Pb (stable).

Radioactive Decay, “Equilibrium”, and Ingrowth This section describes the fundamental properties (decay, equilibrium, ingrowth) of radioactive materials and how these properties relate to unconventional drilling wastes in general. Radioactive Decay Radioactivity (A) is a measurement of the number of decaying atoms in a sample in a given period of time. Radioactive decay of any given atom is a spontaneous event; however, mathematical models can predict the decay rate of large groups of atoms (N) of the same isotope. It is observed that isotopes decay by first order kinetics at a rate related to their half-life (t1/2) according the following equations (eq.: 1-3):

Within the context of NORM, progenitor atoms decay by alpha particle (charged helium nucleus, 4He2+, α) or beta particle emissions (electron, β-) to form progeny atoms. After one half-life, exactly 50% of the progenitor of radioactive atoms will exist. After two half-lives, exactly 25% of the progenitor atoms exist. After three half-lives, exactly 12.5% of the progenitor atoms remain. And the pattern continues, such that after every half-life, the continual radioactive decay results in exactly 50% of the remaining progenitor atoms having decayed by the time the next half-life begins. This concept is the foundation for describing the decay of a single radioactive element. However, when considering a radioactive decay series, the radioactive-progenitor atom decays to a radioactive-progeny atom, which in turn, decays to a radioactive-second-progeny atom. This pattern 99 In Hydraulic Fracturing: Environmental Issues; Drogos, Donna L.; ACS Symposium Series; American Chemical Society: Washington, DC, 2015.

continues with progenitors providing the input of atoms to progeny and the progeny providing the input of atoms for successive progeny, until reaching a stable decay product. This relationship can be quantified by a differential equation described by Bateman in 1910 (65).


Radioactive Equilibrium For the purposes environmental monitoring, the concentration of progeny, second progeny, or atom further along in the decay series may be of more interest than the progenitor. For instance, the concentrations of 226Rn (t1/2 = 1600 y), 222Rn (t1/2 = 3.824 d), 210Pb (t1/2 = 22.2 y), 210Pb (t1/2 = 138.4 d) may be of greater interest in the natural gas industry than the progenitor, 238U (t1/2 = 4.5 x 109 y). In a closed system that has not been disturbed for millions of years (such as in an ancient, fossil shale formation at depth) the radioactivity concentrations of 226Ra and 222Rn are in radioactive “equilibrium” (steady state) with the progenitor, 238U, where the radioactivity associated with 226Ra, 222Rn and 238U are equal. However, during an event where this closed system is disturbed (such as unconventional drilling to extract natural gas) the physicochemical differences of certain NORM can result in a radiochemical “disequilibrium” (non-steady state), where the subsequent progeny radioactivity concentrations are not equal to the progenitor. During a disequilibrium event, progeny may partition from progenitor atoms, as well as other progeny atoms further in the decay chain. For instance, 226Ra experiences elevated solubility in shale formations in hydraulic fracturing flowback water, while its progeny (222Rn, 210Pb, 210Po) and supporting atoms higher in the decay chain (238U, 234Th, 234Pa, 234U, 230Th) remain insoluble, resulting in partitioning of 226Ra from its progenitors and progeny (63). Given that the progenitors have been removed, the radioactivity of 226Ra will decrease at the rate of its half-life (1600 y). Because 226Ra is unsupported by its progenitor radionuclides, its decay can be modeled using the basic radioactive decay equation described above (eq. 2). However, the activities of the progeny (including 222Rn, 210Pb, 210Po) will increase through a process known as radioactive ingrowth. Radioactive Ingrowth Radioactive ingrowth is important to consider when estimating the long-term risks associated with radioactivity liberated by unconventional drilling (63). The most precise way to describe radioactive ingrowth is through derivations of the Bateman equation (eq. 6 & 7), yet in practice, there are two scenarios where simplification of the Bateman equation may prove useful: secular equilibrium and transient equilibrium. Secular equilibrium refers to a closed-system scenario in which the half-life of a supporting isotope is much longer than the decay product (e.g., 226Ra: t1/2 = 1600 y; and 222Rn: t1/2 = 3.824 d). Immediately after a partitioning event that results in disequilibrium, time (t0), the progeny (222Rn) will begin to grow in at a rate determined by its own half-life until it equals the activity of the supporting atom (226Ra). Note, the progeny will grow into equilibrium with the progenitor with an activity ratio of 1:1, but the atomic ratio will not be 1:1 as this ratio related to half-lives (eq. 4): 100 In Hydraulic Fracturing: Environmental Issues; Drogos, Donna L.; ACS Symposium Series; American Chemical Society: Washington, DC, 2015.


When the activities of the two nuclides become equal, which occurs after approximately 6-10 half-lives of the progeny (e.g., ~30 days for 222Rn and ~150 years for 210Pb, Figure 4A and 4B), the progeny is said to be in secular equilibrium with the progenitor (e.g., 226Ra). This relationship can be modeled using a simplified secular equilibrium equation (eq. 5):

In contrast, transient equilibrium refers to a scenario in which the half-life of the progenitor is only slightly greater than that of the decay product (e.g., 228Ra, t1/2 = 5.75 y; and 228Th, t1/2 = 1.91 y). In this case, as with secular equilibrium, immediately after a partitioning event that results in disequilibrium, time (t0), the progeny (228Th) will begin to grow in at a rate related to its own half-life (21). However, the progenitor (228Ra) in this case is decaying on a time scale of similar magnitude as the progeny, and although the activity of the progenitor is initially larger than the progeny (228Th), over time the activity of the progeny will exceed the activity of the progenitor (Figure 4C). The significance of this phenomenon is that several years after disequilibrium occurs, the activity of 228Ra will be less than the activity of its decay products (228Th, 224Ra, 212Pb, 212Bi in particular). Note, 228Th (228Th t1/2 ≫ 224Ra t1/2) will then support subsequent progeny in the decay series (Figure 4D). This relationship between 228Ra and 228Th can be modeled using a simplified transient equilibrium equation (eq. 6):

Although these simplifications may be useful to explore the relationship between any two isotopes in a series, they are limited when modeling the complete decay series is desired. For instance, if concentrations of 210Pb and 210Po (decay products of 238U) or 228Th and 224Ra (decay products of 232Th) are needed, then the Bateman equation must be used. The following equations (eq. 7 & 8) can provide the radioactivity concentrations of any decay product in any decay series using standard spreadsheet software: 101 In Hydraulic Fracturing: Environmental Issues; Drogos, Donna L.; ACS Symposium Series; American Chemical Society: Washington, DC, 2015.


By assuming that the activities/atoms of decay products are zero at some starting time, (t0), one can easily model how long it will take each decay product to reach “equilibrium” with a progenitor. For example, when hydraulic fracturing flowback fluids are initially captured at the surface of the earth they are enriched in 226Ra. Investigations from our laboratory indicate that decay products may be absent from the flowback; however, given the nature of radioactive materials the decay products will ingrow (63). By modeling 226Ra-decay product ingrowth using the Bateman equation, we can show that 222Rn activities steadily ingrow, approaching equilibrium in ~30 days (in a closed-system where 222Rn cannot escape). The decay products of 222Rn with short half-lives (218Po, 214Pb, 214Bi, 214Po) quickly follow the ingrowth of 222Rn. The result is that the total radioactivity due to 226Ra and its decay products will increase by a factor of approximately six in 30 days as the short-lived progeny approach secular equilibrium with 226Ra (Figure 4E). Further, the total radioactivity will continue to increase for nearly 100 years as the long-lived 210Pb grows into the sample (Figure 4F). The 210Bi will relatively quickly establish equilibrium with 210Pb since it has a relatively short half-life. The final radioactive decay product in the series— 210Po — with a half-life of 138.4 days takes approximately 3 years to reach equilibrium with 210Pb and will then will continue to increase until establishing secular equilibrium with 226Ra. Although this theoretical discussion of a closed-system is useful for illustrating how radioactivity behaves, closed-system models may not be suitable for environmental systems. Shale formations at depth likely behave as a closed-system, until disturbed by unconventional drilling. The drilling process is quite extensive and comprises multiple stages that generate, reuse, or dispose of large volumes of solid or liquid materials that open the system to different environmental conditions.



Figure 4. Equilibrium characteristics of selected NORM after a disequilibrium event as modeled by the Bateman equation. (A) 222Rn grows into secular equilibrium with 226Ra in less than 30 days. (B) 210Pb grows into secular equilibrium with 226Ra after 100 years. (C) 228Th establishes transient equilibrium with 228Ra. (D) 224Ra approaches secular equilibrium with 228Th. (E) 226Ra decay products increase total radioactivity approximately 6-fold within 30 days. (F) Activity of 226Ra decay products increase for more than 100 years formations. (Adapted with permission from reference (63)).


Major Stages of Unconventional Drilling


Unconventional drilling can be broken down into six major stages, including: (1) water acquisition, (2) chemical mixing, (3) drilling, (4) injection of hydraulic fracturing fluids, (5) releasing well pressure (flowback/produced fluids and gas flaring), and (6) treatment or storage of liquid and solid wastes (Figure 5) (66). Each of these stages has distinct characteristics and environmental considerations that help determine which radionuclides to expect throughout the drilling process.

Figure 5. Major stages of unconventional drilling as they relate to environmental radiochemistry. Water Acquisition One hallmark of unconventional drilling operations is the tremendous volume of water required (67, 68). Some operations in the Marcellus Shale region (Eastern US) have been documented to use up to 40,000 m3 of water for a single fracture (16). Unsurprisingly, the potential for straining fresh water resources is a concern, particularly at the local scale and in the arid Western US (69). In addition to these concerns, the cost of acquiring water has led many operators to pursue new wastewater purification technologies that allow for the reuse or recycling of flowback/produced fluids (70). One drawback with reusing such fluids is that flowback and produced fluids may be enriched in Ra isotopes (43, 44). Although some of the treatment technologies may remove Ra isotopes, they may not simultaneously remove other NORM (71). For recycling technologies to effectively remove radioactivity from produced fluids, they must consider the nature of radioactive ingrowth. Undertreated/untreated recycled fluids may contain 228Ra-decay products such as 228Ac, 228Th, 220Rn, 212Pb, 212Bi, 208Tl and 226Ra-decay products such as 222Rn, 214Pb, 214Bi, 210Pb, and 210Po. 104 In Hydraulic Fracturing: Environmental Issues; Drogos, Donna L.; ACS Symposium Series; American Chemical Society: Washington, DC, 2015.


Chemical Mixing A major source of controversy surrounding unconventional drilling in the United States is the large volume of unspecified chemicals used in hydraulic fracturing fluids (72). At the federal level in the United States, these chemicals and chemical blends have largely been exempted from disclosure due to trade secret protection (73). In recent years many companies have identified the majority of the chemicals, which are listed well-by-well in publicly accessible databases (for example, FracFocus.org) (74). Some of the disclosed constituents of fracturing fluids (including acids, reducing agents, organics, chelators) are known to interact with NORM (72). For example, uranium mobility is enhanced when complexed with citrates—a known constituent of hydraulic fracturing fluids (75). Additionally, hydraulic fracturing fluids are acidified with hydrochloric acid (11), a reagent that is commonly used in laboratories to solubilize NORM. Without detailed information on the chemicals introduced to the formation, it is difficult to predict how NORM will interact when it comes in contact with hydraulic fracturing fluids. Further, the complexity, quantity, and diversity of chemical blends used as hydraulic fracturing fluids suggest a case-by-case analysis of each well is necessary. Drilling Drilling operators have used gamma-ray-log-detectors for decades to find target formations due to the well known correlation between gas productivity and radioactivity (76). Historically, natural gas wells were vertical (conventional well), and thus only a relatively small portion of the well was in the target formation of higher radioactivity. Advances in horizontal drilling now allow operators to drill down and laterally through the formation for thousands of meters (1). The result is significantly larger surface area of the unconventional well in the formation in comparison to conventional, vertical wells. In order to make space for the well, material must be removed from the depth. The material that is removed is referred to simply as “cuttings (77),” or commonly as bit cuttings or drill cuttings. Although values vary from well-to-well, one report indicated a single horizontal well may produce 250,000 kg of bit cuttings (77). Since a large portion of these bit cuttings comes from the higher radioactivity formation, the bit cuttings can be expected to be enriched in radioactivity. A recent report indicates that radioactivity concentrations of 238U in vertical cuttings were between 40 and 70 Bq/kg, whereas concentrations in horizontal cuttings exceeded 300 Bq/kg (78). Horizontal bit cuttings can similarly be expected to be enriched in insoluble U-series decay products, such as Pa, Th, Po, and Pb isotopes. Injection Once the well has been drilled and the casing has been installed, hydraulic fracturing fluids are pumped into the well at tremendous pressures (up to 800 kPa) (11). In some cases industry will inject radioactive tracers into the well to check for inter-well connectivity or to measure flow rates (79). Most of the radioactive 105 In Hydraulic Fracturing: Environmental Issues; Drogos, Donna L.; ACS Symposium Series; American Chemical Society: Washington, DC, 2015.


materials used at this stage are gamma-emitting radionuclides that decay to a stable product. Thus, the phenomenon of radioactive ingrowth that is observed for the natural decay chains is not observed; radioactivity associated with these tracers will decrease over time. Additionally, many of the tracers have relatively short half-lives (ex: 131I t1/2= 8.025 days) and will consequently decay to stable decay products in a short period of time, unlike the natural decay chains which will produce radioactive decay products for millennia. The greatest potential for lasting radioactive contamination during the injection stage is in the event the high pressure causes the well casing to fail. The frequency of casing failure in unconventional gas wells, albeit debatable, is more likely than in conventional wells (80). Estimates of casement failure rates are quite variable with rates suggested from 1-2% to as high as 6.3% (11, 81). If a casing failure were to occur, there is the potential for fluids from the well, containing NORM (from recycled fluids and interstitial fluids) or radioactive tracers, to leach into aquifers (82). Although this scenario is unlikely, there have been reports of signatures unique to the Marcellus Shale formation appearing in shallow domestic wells near hydraulic fracturing operations (83, 84). To our knowledge, no comprehensive studies have been performed to identify levels of NORM present in groundwater around drilling operations. Due to the natural occurrence of NORM in most groundwater the impacts of drilling operations will be difficult to assess (85). Without a well-controlled, longitudinal study that has pre- and post- drilling data, conclusions may be prone to confirmation bias. Flowback and Flaring After the well has been fractured, the pressure at the wellhead is lowered to allow gases and fluids to return to the surface. Initially, returned fluids, termed flowback, consist largely of the hydraulic fracturing fluids that were injected (11). Over time, the well will continue to release fluids, termed produced fluids. These fluids are typically much higher in total dissolved solids (TDS), salinity, and NORM (44, 46). Over time, a well releases increasingly complex fluids that may be enriched in Ra isotopes naturally present in the fractured formation. Although Ra isotopes may be selectively solubilized in flowback/produced fluids, over time, 228Ra-decay products such as 228Ac, 228Th, 220Rn, 212Pb, 212Bi, 208Tl and 226Ra-decay products such as 222Rn, 214Pb, 214Bi, 210Pb, and 210Po will ingrow (63). In addition to liquid wastes, natural gas wells produce large volumes of gaseous waste. This gaseous waste includes hydrogen sulfide, volatile organic compounds (VOCs), natural gas, and radioactive Rn gas (86). Flaring and/or venting are common and necessary practices at natural gas extraction sites, for safety, environmental, and economic reasons (87). Although the extent to which flares reduce the environmental impact of produced gases is debatable (88), it will have no effect on the radioactivity of Rn gas. Surprisingly little attention has been paid to the extent/impacts of Rn gas during the flaring stage, even though Rn is a well-known contaminant of natural gas streams (89–91). One article has suggested that increased 222Rn levels in natural gas extracted from shale will increase radioactivity concentrations in homes (from use of stoves 106 In Hydraulic Fracturing: Environmental Issues; Drogos, Donna L.; ACS Symposium Series; American Chemical Society: Washington, DC, 2015.


and space heaters) in New York State, resulting in an additional 1,182-30,448 lung cancers (92). More data on the levels of 222Rn in natural gas streams at the source and end-point are needed to validate this assessment. Very little is known about the levels Rn on drilling sites, though a recent report indicates that levels of 222Rn on Marcellus Shale drilling sites are in the range of ambient background 222Rn concentrations in the US (7-26 Bq/m3) (78). As studies are designed to address radiological exposures associated with 222Rn, both on drilling sites and downstream, careful consideration of 222Rn decay products is needed. The long-lived progeny of 222Rn, 210Pb and 210Po, can adsorb to dusts and accumulate into higher organisms (93–96).

Treatment The final stage of unconventional drilling is treatment of solid and liquid wastes. Little peer-reviewed information is available about the composition of NORM in wastes from the Marcellus Shale region and their potential to migrate into the environment (15). To date, reports on NORM associated with unconventional drilling have largely focused on three components of the radioactivity: gross alpha/beta levels (97), analysis of Ra levels (43, 44, 98–100), and gamma spectrometry (101). Gross alpha/beta levels are a simple screening technique for radioactivity in environmental samples (102, 103), but they do not indicate which radionuclides are present in the waste. Thus, it is not feasible to determine whether the level of radioactivity will increase, persist, or decrease over time without subsequent analyses. Analysis of Ra levels in flowback and produced fluids liquid waste from Marcellus Shale unconventional drilling has proven challenging, and can be underestimated (in some cases > 100 fold) due to matrix interferences (43). Traditional drinking water methods and other wet chemistry methods for Ra isotopes do not work on the complex brines from the oil and gas fields (43). Methods such as gamma spectroscopy or radon emanation are superior for these samples as they are less affected by matrix composition (43). However, analysis of Ra isotopes alone does not provide information on the total radioactivity, which can increase substantially for over 100 years resulting from the ingrowth of the radiogenic progeny (63). Gamma spectroscopy is used to measure select gamma-emitting radionuclides in the natural decay series (63, 101). In some cases, measurements of gamma-emitters can be used to infer radioactivity concentrations of radionuclides that are not gamma-emitters (example, gamma emissions from 228Ac can be used to infer 228Ra levels). However, due to partitioning events in the subsurface, analysts cannot assume all radionuclides will be in equilibrium (example, 228Ac levels cannot be used to infer levels of its decay product 228Th) (63, 85). Gamma spectrometry alone cannot fully characterize the levels of NORM present, particularly with respect to Ra decay products (228Th, 222Rn, 210Pb and 210Po). Without comprehensive analyses of NORM in these wastes (i.e. gamma spectrometry and alpha spectrometry), the levels of exposure will remain relatively unknown. Most environmental monitoring reports of Marcellus Shale waste have focused on 226Ra associated with liquid waste (i.e. flowback and produced fluids) 107 In Hydraulic Fracturing: Environmental Issues; Drogos, Donna L.; ACS Symposium Series; American Chemical Society: Washington, DC, 2015.

and dose rates from solid waste (i.e. bit cuttings and drill cuttings) (43, 44, 46, 63, 98, 104).


Liquid Waste In the US, oil and gas operations are exempt from many federal environmental protection regulations, such as the Safe Drinking Water Act (the so-called “Halliburton Loophole”) (105). Further, the levels of NORM in oil and gas wastes are not regulated at the federal level, but rather at the state level (106). Thus, treatment options for liquid wastes differ from state-to-state due to a combination of regulations, economics, and local geology (107). The major treatment options in the US for liquid wastes from unconventional drilling are: (1) direct discharge (i.e. spraying on roads) (2) chemical treatment at wastewater treatment plants, (3) deep surface injection, and (4) recycling (77). The Marcellus Shale region provides an interesting case study on how state regulations can affect the handling of liquid wastes. In Pennsylvania in particular, radioactivity in liquid wastes has proven to be a controversial issue (15). Since unconventional gas exploration and production in the Marcellus Shale region began in 2003 (76), there was a rapid surge in drilling and waste generation across the region (107). In 2014, for example, over 8000 active wells generated an estimated 5 billion liters of flowback and produced fluids (107). Liquid wastes in Pennsylvania are largely disposed of at wastewater treatment plants as there are no suitable deep-well injection sites (107). This practice has resulted in cases of contaminated sediments downstream from these facilities (78, 98). Despite high profile publications on Ra contamination from untreated or undertreated flowback and produced fluids, a recent report from the State of Pennsylvania indicates that waste treatment facilities are still poorly equipped to remove Ra from unconventional drilling wastes (78). However, it is important to note that many drilling operators in the Marcellus Shale region are moving towards flowback recycling practices (70, 108). As liquid waste management continues to shift towards recycling, the volume of produced liquid waste containing Ra to be treated and disposed at the surface will decrease. Radium isotopes appear to be liberated from the Marcellus Shale and soluble in the liquid waste (44, 46, 63), which is consistent with historical observations of disequilibrium in oil and natural gas brines from conventional operations (109). Two major complications with handling these wastes are (1) the large volume of brines produced (>5 billion liters in 2014 in PA alone) (107), and (2) the high levels of dissolved solids and divalent cations (Sr, Ca, Ba) present in the liquids, which can interfere with treatment processes aimed at removing Ra (67). When wastewater treatment plants are not equipped to handle these high levels of divalent cations, flowback and produced fluids wastes may flow through the wastewater treatment plant untreated or undertreated (98, 110–112). Discharges of undertreated waste may result in the accumulation of these divalent cations in sediments of riparian environments (98). This was recently evidenced by a report, in which the investigators documented that levels of 226Ra and 228Ra in sediments immediately downstream of the Josephine Wastewater Treatment 108 In Hydraulic Fracturing: Environmental Issues; Drogos, Donna L.; ACS Symposium Series; American Chemical Society: Washington, DC, 2015.


Facility in Pennsylvania, USA, were several orders of magnitude higher than background levels upstream of the facility (98). Due to the high levels of Ra isotopes, the plant operator now plans to dredge the contaminated sediments (113). Other waste operators in Pennsylvania have experienced similar challenges in removing Ra from complex hydraulic fracturing wastes (78, 114). In response to these challenges, several groups have suggested mixing in high-sulphate coal mine drainage as an approach to precipitate Ba and Ra isotopes as insoluble sulphate complexes from the liquid waste of the Marcellus Shale (115, 116).

Solid Waste There are numerous methods available for the disposal and treatment of solid wastes generated in the drilling process, including biological and non-biological treatments (117). In some states, solid waste is disposed of in landfills, though in other states, like Oklahoma, bit cuttings and drilling muds are tilled into agricultural soils (77, 118). Given the mass of solid waste generated by unconventional drilling (up to 250,000 kg/well) (77), there is surprisingly little information available on their radioactivity content. Most readily available information available arises from newspaper reports of trucks turned away from landfills after tripping radiation alarms. For example, one truck carrying Marcellus Shale bit cuttings was turned away from a Pennsylvania landfill, because the radioactive emissions from the load exceeded the allowable radiation threshold. The dose rate from these bit cuttings was measured at 0.96 μS/hr, which exceeded the Pennsylvania threshold of 0.5 μS/hr (104, 119). In another case at the Meadowfill Landfill in West Virginia, a truck was turned away when bit cuttings measured 2.12 μS/hr, exceeding the allowable limit of 1.5 μS/hr (120). While these events of solid waste exceeding the allowable dose thresholds invariably raise criticism and concern from citizens, the risk of radiation exposure (including Rn) to the general public is likely minimal (78, 121). Although assessments of radioactivity dose rates are useful from a health physicist’s perspective, dose rates provide little information about the elemental and isotopic composition of these materials. Only recently has limited information about the composition (natU, 232Th, 228Ra, and 226Ra levels) of the bit cuttings become available from a report by the State of Pennsylvania (78). More detailed radiochemical assessments of the elemental and isotopic composition are critical to determine the equilibrium status, ingrowth potential, and likelihood for NORM to migrate into the surrounding environment.

Research Needs There are many important research questions concerning NORM that have recently surfaced as unconventional drilling expands around the world. Three unanswered questions in the context of environmental radiochemistry are: (1) the fate and transport of 226Ra decay products in freshwater environments, (2) the behavior and composition of NORM in solid waste from NORM-enriched shale 109 In Hydraulic Fracturing: Environmental Issues; Drogos, Donna L.; ACS Symposium Series; American Chemical Society: Washington, DC, 2015.

formations, and (3) regional impacts of 222Rn gas released from 226Ra containing wastes, flares, and natural gas streams.


Fate and Transport of 226Ra-Decay Products in Aqueous Environments As mentioned earlier, Ra from unconventional drilling waste can enter riparian environments (98). This observation has raised numerous questions about the fate and transport of Ra and methods to remove it from complex liquid wastes produced by unconventional drilling (115, 116, 122); however little research has been performed on Ra decay products. Due to radioactive ingrowth processes, wastes that contain natural Ra isotopes (228Ra, 226Ra, 224Ra, and 223Ra) will generate Ra decay products. Importantly, because Ra decay products have different physicochemical properties than Ra, methods that are designed to remove Ra from liquid wastes will not necessarily remove Ra decay products. The three Ra decay products of greatest interest are: 222Rn, 210Pb, and 210Po, because these isotopes possess radiochemical (sufficiently long half-lives) and physicochemical (unique chemistry from the supporting 226Ra) properties that allow for partitioning in the environment and possible bioaccumulation in higher organisms (94, 95, 123). On-going studies in fresh waters in West Virginia by our laboratory indicate that Ra decay products have accumulated in sediments to a level nearly five times that of 226Ra (Figure 6) (124). The mechanism of 210Pb and 210Po enrichment in sediments is still under investigation. One possibility is that 226Ra is more soluble in this environmental system and is constantly removed from the system, but steady inputs of 210Pb and 210Po into the lake readily accumulate onto mineral surfaces in lake sediments. Currently, we are investigating the role of seasonal fluctuations in the observed disequilibrium. Alternatively, dissolved 222Rn may be transported in effluent discharge pipes as the result of 226Ra scale formation (125). A steady stream of 222Rn could result in the enrichment of 222Rn-decay products, including 210Pb and 210Po, in water columns and sediments of seasonally anoxic lakes (126, 127). Previous research indicates that excess 210Po (disequilibrium with 210Pb) is likely to occur in the summer in anoxic lake bottoms as Fe and Mn minerals are reduced to soluble phases (126). Further studies on 210Pb and 210Po levels are needed to elucidate their speciation, potential for migration, and exposure risks.



Figure 6. Anticipated fates for 226Ra and 226Ra decay products in fresh water environments. Behavior and Composition of NORM in Solid Waste To date, there have been no peer-reviewed scientific investigations on the radiochemistry of unconventional drilling solid waste. Numerous questions beckon, such as how redox sensitive elements (U, Fe, Mn, etc.) will behave when removed from depth, and how alteration of redox sensitive elements will affect the mobility of NORM. For example, as described earlier, U was trapped in shale formations in its reduced, immobile, +4 oxidation state. When brought to the surface, bit cuttings will be exposed to an oxidizing environment, which will likely result in the oxidization of U4+ to hexavalent UO22+ (Figure 7) (128). Once in the 6+ oxidation state, U may more readily leach off of bit cuttings into the surrounding environment (129). It is difficult to assess the extent and rate of U oxidation as well as the potential for U6+ to leach from bit cuttings without more data. Though, some lessons may be gleaned from the American West, where U mine tailings were stored along the banks of the Colorado River (130). Oxidized U6+ from these tailings traveled into ground water, prompting years of research and attempts to reduce the mobile U to the immobile, U4+ with bioremediation (131). In areas, such as Oklahoma, where bit cuttings and drilling muds are directly applied to fields, we expect a similar fate of U as described in mine tailings from CO. In other regions where bit cuttings are stored in landfills, such as the Marcellus Shale, we suspect that leachates will contain measurable quantities of U though the risk to the public is likely minimal (78). Non-radioactive, redox sensitive elements are also important to consider in assessing the fate and 111 In Hydraulic Fracturing: Environmental Issues; Drogos, Donna L.; ACS Symposium Series; American Chemical Society: Washington, DC, 2015.

transport of NORM. Further research is needed to investigate the potential for and 210Po to migrate through the environment. As an aside, measurements of 238U are sometimes performed by mass spectrometry. Yet, given the alpha recoil enrichment processes that we described above, 234U will likely not be in secular equilibrium with 238U. As a result, determining 238U activity by its mass and applying the assumption that 234U activity is equivalent could underestimate the true radioactivity attributed to U isotopes (85). Furthermore, activity ratios of 234U/238U may provide valuable information for predicting NORM migration at contaminated sites (132). Thus, we recommend that when possible, isotopic natU levels be measured by alpha spectrometry or another suitable method (63).


222Rn, 210Pb,

Figure 7. Oxidation of bit cuttings and other solid wastes may mobilize U and other NORM into ground water and other water resources. (Adapted with permission from reference (63)). Potential Regional Impacts of 222Rn 226Ra

sources constantly produce 222Rn gas. Considering that liquid waste from the natural gas industry is known to contain enriched levels of 226Ra, very little attention has been given to 222Rn. Researchers have known that 222Rn is present in natural gas, though it is currently unclear what levels are present in shale gas and whether these levels pose a hazard (92, 133, 134). Although a recent report from the State of Pennsylvania indicates levels of 222Rn are low in commercial gas, the levels of 222Rn released during fugitive gas emissions and flaring of unwanted gases were not investigated (78). Since 222Rn is not combustible, flaring will not remove its radiologic hazards. Once delivered to the atmosphere, 222Rn will form decay products, which are known to fallout in particulates and in precipitation (Figure 8) (135, 136). The progeny of 222Rn, such as 210Pb and 210Po, could then 112 In Hydraulic Fracturing: Environmental Issues; Drogos, Donna L.; ACS Symposium Series; American Chemical Society: Washington, DC, 2015.


be taken up by plants (such as tobacco), accumulate in lake bottom sediments, and ultimately organisms in the region (126, 137). We suspect any increases in 222Rn levels and 222Rn products (210Pb and 210Po) will require well-controlled longitudinal studies, as many drilling operations occur in areas with relatively high levels of background 222Rn (138).

Figure 8. Flaring of unwanted gases may result in regional increases of product fallout. (Adapted with permission from reference (63)).

222Rn-decay

Conclusion NORM is a well-documented contaminant of conventional oil and natural gas equipment and wastes. Many of the challenges associated with NORM management in conventional wastes apply to the management of unconventional drilling wastes. The complexity and scale of wastes produced by unconventional drilling have proven difficult to handle in even the most developed of nations with decades of experience in natural gas production. When waste management protocols are inadequate, enriched levels of NORM from unconventional drilling 113 In Hydraulic Fracturing: Environmental Issues; Drogos, Donna L.; ACS Symposium Series; American Chemical Society: Washington, DC, 2015.


activities can enter the environment. Most attention to date with respect to NORM in unconventional drilling waste has focused on the risks of Ra isotopes entering aqueous environments. Given the natural ingrowth processes of radioactive decay products, wastes that are enriched in Ra isotopes will either contain Ra decay products or produce Ra decay products. Thus, studies on the fate and transport of NORM liberated by unconventional drilling operations should include Ra decay products. Similarly, studies of solid waste should include progenitors of Ra isotopes. NORM is comprised of multiple elements and isotopes of different physicochemical and radiochemical properties that play a role in environmental partitioning and any potential human exposure. Further studies are needed to characterize NORM solid, liquid, and gaseous wastes generated by unconventional drilling operations.

Addendum: Basic Properties of NORM Uranium Although uses of uranium (U, [Rn]5f36d17s2) date back to 79 CE, when the Romans used U to add yellow to ceramic glazes, the discovery is credited to a German chemist, Martin Heinrich Klaproth, in 1789 (62, 139). Klaproth dissolved pitchblende in nitric acid then neutralized the solution with NaOH and precipitated yellow sodium diuranate and concluded this was a new element. He later named the new element after Uranus, the primordial Greek god of sky, because of its yellow colour and the recent discovery of the planet Uranus eight years earlier (139). There are three naturally occurring isotopes of U, 238U (t1/2= 4.468x109 years, α), 235U (t1/2= 7.04x108 years, α), and 234U (t1/2= 2.455x105 years, α) (25, 140). The behavior of U in the environment is greatly dependent on the redox conditions. While in U6+ (as UO22+) is the most stable cation in oxidizing conditions, it is readily reduced to U4+ in anoxic conditions. In general, U forms strong inorganic and organic complexes, but the strength of the species depends on the oxidation state (26).

In the case of Marcellus Shale, the formation is rich in U due to the accumulating U6+-carbonate (UO2CO3) species from the ancient ocean (35, 141). Now, the conditions are very reducing, immobilizing U and reducing it to U4+, therefore, U will be recovered with the bit cuttings. Once U is exposed to ambient conditions, it will be oxidized to U6+ and will be soluble. Protactinium Protactinium (Pa, [Rn]5f26d17s2) was first isolated by William Crooks in 1900 when he dissolved uranyl nitrate in ether but he was unable to characterize it as a new element, so he named it uranium-X (142). In 1913, Fajans and Göhring fully characterized uranium-X as new element and named it brevium because of the short half-life of 234mPa (142). Finally in 1917, a German group (Hahn and Meitner) and 114 In Hydraulic Fracturing: Environmental Issues; Drogos, Donna L.; ACS Symposium Series; American Chemical Society: Washington, DC, 2015.


a British group (Soddy and Crankston) independently discovered another isotope of Pa with a longer half-life and named it proto-actinium (later changed by IUPAC to protactinium), because it was the progenitor of actinium (4). There are two naturally occurring radionuclides of Pa, 234Pa (t1/2= 6.7 hours, β-) and 231Pa (t1/2= 32,760 years, α) (26, 143). In general, Pa exists as a pentavalent cation and forms strong affinity for inorganic complexing ligands:

In the environment, Pa is known to readily hydrolyze to form insoluble colloidal species, but in the presence of a high concentration of strong inorganic ligands, Pa may remain soluble (26, 142). In the conditions such as the Marcellus Shale, it is likely that Pa will be largely insoluble due lack of strong complexing ligands (i.e. F- and SO42-), and would be potentially recoverable in the bit cuttings. Thorium Thorium (Th, [Rn] 6d27s2) was first documented in 1823 in Norway. Morten Thrane Esmark found a black mineral on the island of Løvøya and presented it to his father, mineralogist Jens Esmark, and he could not identify the sample (144). Jens Esmark sent the sample to a Swedish chemist, Jöns Jakob Berzelius, who in 1828, concluded it was a new element and named it after Thor, the Norse god of thunder (144). There are six naturally occurring isotopes of Th, 234Th (t1/2= 27.1 days, β-) 232Th (t1/2= 1.4x1010 years, α), 231Th (t1/2= 25.52 hours, β-), 230Th (t1/2= 75,400 years, α), 228Th (t1/2= 1.9116 years, α), and 227Th (t1/2= 18.68 days, α) (26, 145). Generally, Th exists as a tetravalent actinide and is redox inactive in the environment. Because Th4+ is the dominant species, Th remains largely insoluble and but its mobility is greatly controlled by the ability to form complexes with organic and inorganic ligands:

While Th remains insoluble, it can coordinate strongly with particles in the environment and be mobilized by their transportation (26). In the Marcellus shale, Th will remain insoluble and be recoverable with the bit cuttings. However, 228Th, originating from 228Ra, will grow into secular equilibrium in the recover fluids. Actinium Actinium (Ac, [Rn] 6d17s2) was discovered in 1899 by a French chemist, named André-Louis Debierne, when he isolated it from pitchblende residues of Marie and Pierre Curie radium extraction. The name originates from the Greek word aktis, meaning beam or light, because of the eerie blue glow of Cerenkov radiation emitted from actinium (146). There are two naturally occurring isotopes of Ac, 228Ac (t1/2= 6.15 h, β-) and 227Ac (t1/2= 21.1772 years, β-) (26). Chemically, Ac behaves as a trivalent cation similar to the lanthanide elements, remaining mostly insoluble and pH inactive (146). Within the conditions in the Marcellus Shale, Ac will remain largely insoluble and is expected to be associated with 115 In Hydraulic Fracturing: Environmental Issues; Drogos, Donna L.; ACS Symposium Series; American Chemical Society: Washington, DC, 2015.

solid/particulate phases. However, 228Ac, originating from secular equilibrium in the recovered fluids.

228Ra,

will grow into


Radium Radium (Ra, [Rn]7s2) was first discovered by Marie and Pierre Currie in 1898 in the form of radium chloride by extraction from pitchblende (21). The name radium originates from the Latin word radius, meaning ray, referring to radium’s intense production of energy rays (147). There are three common naturally occurring isotopes. 224Ra (t1/2 = 3.66 d) and 228Ra (t1/2 = 5.76 y) are found in the 232Th decay series; 226Ra (t1/2 = 1599 y) is found in the 238U decay series. Ra is not redox sensitive and is found in the +2 oxidation state in nature (42). Owing to its highly basic behavior, Ra is not easily complexed. Ra does however form simple ionic salts. Radium sulphate and Ra carbonate are very insoluble, while Ra hydroxide, chloride, bromide, and nitrate are all soluble (42). Radium tends to precipitate (and coprecipitate) with all barium, most strontium, and most lead compounds as ionic salts. Due to the low sulphate concentrations in the Marcellus Shale, Ra isotopes are expected to remain soluble in interstitial fluids, flowback, and produced fluids. Radon Radon (Rn, [Xe]4f145d106s26p6) was discovered in 1900 by Dorn, who called it radium emanation (139). Yet, since 1923 the element has been known as radon. Each of the primordial decay series includes an isotope of Rn: 219Rn (t1/2 = 3.96 s, sometimes referred to as actinon) belongs to the actinium series, 220Rn (t1/2= 55.6 s, sometimes referred to as thoron) belongs to the thorium series, and 222Rn (t1/2= 3.8235 days, commonly referred to as simply as radon). Radon is the heaviest noble gas, and is thus relatively chemical inert. Rn is relatively soluble in water, though the effects of high saline environments may significantly alter its partitioning between various phases in the subsurface (55). In the subsurface Rn is expected in brines (supported and unsupported) (148), organic layers (149), and in natural gas streams (89). Although Rn is relatively soluble in aqueous solutions, when liquids containing Rn are exposed to the atmosphere (open system) Rn gas will partition into the air as predicted by Henry’s Law. Furthermore, solids containing 226Ra will produce 222Rn (150). Polonium Polonium (Po, [Xe]4f145d106s26p4) historically referred to as Radium F, was the first element discovered by Madame Marie Curie during her investigations of pitchblende (139). Polonium was named after Poland, the home country of Marie Curie. It is one of the rarest elements, with natural abundances of only 100 μg of 210Po per ton of uranium ores (151). Investigations of the speciation and chemistry of Po is difficult as all known isotopes and isomers are radioactive. Furthermore, analysis of its chemistry is complicated by its volatility—at temperatures over 100°C, Po is volatized, thus preventing the use of high temperature environmental 116 In Hydraulic Fracturing: Environmental Issues; Drogos, Donna L.; ACS Symposium Series; American Chemical Society: Washington, DC, 2015.


sample preparations (152). There are several naturally-occurring Po isotopes, most of which have extremely short half-lives: 218Po (t1/2 = 3.098 min), 216Po (t1/2 = 0.145 sec), 215Po (t1/2 = 1.781 x 10-3 sec), 214Po (t1/2 = 164.3 x 10-6 sec), 212Po (t1/2 = 0.299 x 10-6 sec), and 211Po (t1/2 = 0.516 sec). The longest-lived naturallyoccurring Po isotope is 210Po (t1/2 = 138.376 days). Po is theorized to form the -2, +2, +4, and +6 valence states (151), though the environmentally relevant valence states are likely +2 and +4 (153). Po is readily dissolved in dilute acids, but can be easily concentrated on manganese oxides or iron hydroxide surfaces even in complex samples. Interestingly, some investigators have discovered that only a portion of total Po is extracted by iron hydroxides, suggesting that multiple valence states and species of Po may coexist under certain conditions (61). Po is an elusive element that often behaves in unexpected ways; however, under the strong reducing conditions of shale formations it is expected that Po will be particle reactive and mostly associated with bit cuttings/drill cuttings. Importantly, Po isotopes will ingrow into phases that contain either supporting Ra or Rn isotopes. We believe the environmental transport mechanism and ultimate fate of 210Po (and its progenitor, 210Pb) liberated by unconventional drilling will be one of the most interesting and challenging research questions in the coming years. Bismuth Bismuth (Bi, [Xe] 4f14 5d10 6s2 6p3) was first discovered in the 15th century and identified as a distinct element by Potts and Bergmann in 1739 (154). The name bismuth originates from the German words, weisse masse, meaning white mass. For centuries, Bi was confused with Pb (139). There are several naturally-occurring radioactive isotopes including: 215Bi (t1/2 = 7.6 min), 214Bi (t1/2 = 19.9 min), 212Bi, (t1/2 = 60.55 min), 211Bi (t1/2 = 2.14 min), and 210Bi (t1/2 = 5.102 day). Until recently, 209Bi was thought to be the heaviest stable isotope; however, new evidence suggests that this isotope emits low-energy α-particles with an extremely long half-live (t1/2 = 1.9 x 1019 year) (155). Bi is most commonly found in the +3 and +5 oxidation states and tends to form insoluble complexes (64, 139). Given this tendency, Bi is expected to adsorb to particulate and mineral phases. In practice 214Bi and 212Bi are important gamma emitting isotopes for determining levels of supporting Ra isotopes (156). Lead Lead (Pb, [Xe] 4f14 5d10 6s2 6p2) has been in common use for thousands of years and is renowned for its toxicity (139). The word ‘lead’ has Anglo-Saxon roots, yet the abbreviation ‘Pb’ is derived from the Latin word plumbum. Radioactive Pb has numerous applications in radiochemistry, geology, and medicine. naturally-occurring radioisotopes of Pb include: 214Pb (t1/2 = 26.8 min), 212Pb (t1/2 = 10.64 hour), 211Pb (t1/2 = 36.1 min), and 210Pb (t1/2 = 22.2 year). The 238U, 235U, and 232Th decay series all decay to a stable Pb isotope (206Pb, 207Pb, and 208Pb, respectively). Pb exhibits two oxidation states in solutions, the +4 oxidation state, or more commonly the +2 state (157). Pb is insoluble when complexed with halides, sulphates, carbonates, phosphates, and sulphides, 117 In Hydraulic Fracturing: Environmental Issues; Drogos, Donna L.; ACS Symposium Series; American Chemical Society: Washington, DC, 2015.


but soluble when complexed with nitrates, nitrites, citrates or acetates (157). In the environment, Pb tends to be relatively immobile, thus making it a useful pollution indicator (158). 214Pb and 212Pb are important isotopes for determining gamma emissions of supporting Ra isotopes (156). Due to (1) the difficulty in measuring 210Pb in comparison to other gamma-emitting NORM, and (2) the natural ingrowth of 210Pb from 226Ra source, we believe the fate and transport of 210Pb associated with unconventional drilling wastes is one of the most interesting and challenging areas of environmental radiochemistry research.

Thallium Thallium (Tl, [Xe] 4f14 5d10 6s2 6p1) was discovered independently by Lamy and Crookes in 1861 (159). The name thallium is derived from the Greek word thallos, meaning ‘green shoot’ in reference to the green light it emits in spectrometers (139). There are two relevant radioactive isotopes of Tl: 208Tl (t1/2 = 3.053 min) of the 232Th series and 207Tl (t1/2 = 4.77 min) of the 235U series. Thallium has two observationally stable isotopes; 203Tl and 205Tl. Tl is most commonly found in a +1 or +3 oxidation state as ionic salts. In the +1 state Tl behaves similarly to potassium (K), which in part explains its chemical toxicity. In reducing environments, Tl is expected in the +3 state, where it behaves similarly to aluminium (III, Al) (159). Tl is commonly soluble, even in the carbonate form, but can be precipitated as a +1 ion with hydrogen sulfide, potassium chromate, potassium iodide or thionalide. Co-precipitation of thallium (III) in small amounts is possible with iron (III) hydroxide. Given the low atomic abundances and short half-lives of 207Tl and 208Tl, their potential to partition likely plays a minimal role in the gross transport of NORM from unconventional drilling wastes through the environment. In practice, the reliable gamma emissions from 208Tl are important for monitoring 224Ra and associated progeny.

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