Keywords

1 Introduction

Naturally occurring radioactive material (often abbreviated as NORM) is abundant in the terrestrial environment and presents a potential risk to human health. Most NORM of addressable public health concern exists as part of decay series (illustrated in Figs. 13.1 and 13.2) headed by extremely long-lived isotopes of uranium (U) and thorium (Th), specifically 238U and 230Th. It is thought that U and Th radionuclides were created during the supernovae or the collision of neutron stars creating these heavy elements via rapid, successive neutron capture events (Burbidge et al. 1957; Sneden et al. 2008). Due to the extremely long half-lives of these radionuclides, on the order of billions of years, much of this material is still present in the long after its initial creation. NORM from other sources (such as potassium-40 [40K] and cosmogenic radionuclides tritium [3H], Beryllium-7 [7Be], and Carbon-14 [14C]) is present in the environment but is not considered an addressable public health concern either due to its ubiquity in the environment or the very low risk posed by these radionuclides (Boxes 13.1 and 13.2).

Fig. 13.1
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Thorium decay series (Data retrieved from NNDC (National Nuclear Data Center) 2014)

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Fig. 13.2
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Uranium decay series (Data retrieved from NNDC (National Nuclear Data Center) 2014)

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Box 13.1 Definitions: Units

Becquerel (Bq): Number of radioactive disintegrations per second (s−1), SI unit for radioactivity.

Curie (Ci): 3.7 × 1010 radioactive disintegrations per second, approximately the activity of 1 g of 226Ra.

Picocurie (pCi): 1 × 10−12 Ci is the common U.S. regulatory unit for environmental radioactivity.

Gray (Gy): Amount of radioactive energy transferred to matter, J kg−1, SI unit for absorbed dose.

Rad (R): Absorbed dose, 100 erg g−1, 0.01 Gy, common U.S. unit for absorbed dose.

Box 13.2 Definitions: Units

Sievert (Sv): Absorbed dose multiplied by a weighting factor dependent on the type radiation, SI unit for equivalent dose.

Roentgen Equivalent Man (REM): Equivalent dose, 0.01 Sv, common U.S. unit for equivalent dose.

Working Level (WL): Concentration of potential alpha particle energy emitted by radon and its progeny equivalent to 1.3 × 108 MeV m−3, or 1.602 × 10−13 J m−3 (ICRP 1993).

Working Level Month (WLM): Exposure equivalent to 1 month, or 170 h, of exposure at the WL, or 3.54 × 10−3 mJ η m−3 (ICRP 1993).

2 Decay Series and Basic Radioactivity Overview

Radioactive decay can be defined as the spontaneous and random disintegration of an unstable atom. The rate of radioactive decay is inversely related to the radionuclide’s half-life (the amount of time it takes for half of the atoms of the radionuclide in question to decay) and is proportional to the number of atoms present such that,

$$ A=\lambda N, $$
$$ \lambda =\frac{\ln 2}{t_{1/2}}, $$

where A is the disintegration per unit time, λ is the decay constant, N is the number of atoms, and t1/2 is the half-life.

With the decay of radionuclides, we must also consider the relationship between progeny and progenitors (i.e., the conversion of a radionuclide via radioactive decay into another radionuclide or stable isotope). As defined by the Bateman equations derived in 1910, the relationships between radionuclides in decay chains in a closed system vary based on the ratio of the half-lives of the progeny to the progenitor (Bateman 1910). If the half-life of the progeny is much shorter than that of its progenitor (roughly less than one-tenth), the two radionuclides will reach an equilibrium and decay with the same activity, and at the same rate, as the progenitor (referred to as “secular equilibrium”). If the half-life of the progeny is shorter than that of its progenitor (between one and one-tenth), the progeny will reach a maximum activity greater than its progenitor and will decay at a rate related to both its half-life and that of its progenitor (“transient equilibrium”). In the case where the progeny’s half-life is longer than that of its progenitor, no equilibrium will be reached between the two radionuclides, but the decay rate of the progeny is altered from its normal half-life. These relationships are especially important when dealing with NORM, as some very hazardous radionuclides biologically (see Polonium-210 [210Po] and Lead-210 [210Pb]) have relatively short half-lives when separated but can exist in a supported relationship in the environment, leading to higher activities than might be expected based on half-lives alone.

Radiation can be defined as the energy emitted or transmitted in the form of waves or particles and can be divided into ionizing (such as gamma rays or X-rays) and nonionizing forms (such as microwaves or visible light). Ionizing radiation can create ions in matter, either directly via charged-particle interactions with atoms (such as with alpha and beta particles) or via indirect interactions (such as with neutrons and photons). With NORM, alpha, beta, and photon radiation (gamma/X-rays) are of concern. Alpha particles are radioactive emissions composed of two neutrons, two protons, and have positive charges. Beta particles are electrons or positrons ejected from a nucleus at high speed; these particles can be either negatively or positively charged, but nearly all are negatively charged with natural radionuclides (the lone exception is a small probability, 0.01%, of a positron emission from 40K) (NNDC (National Nuclear Data Center) 2014). Positrons are rapidly converted to photon radiation in matter via particle–antiparticle annihilation with an electron, generating two gamma rays. Photons are electromagnetic radiation emitted during radioactive decay via a number of processes. These emissions can be in the gamma ray or X-ray region of the electromagnetic spectrum and are often emitted together with beta and, to a less extent, alpha emissions (Box 13.3).

Box 13.3 Not All Fission Products Are Anthropogenic

One interesting example of the variability of NORM deposits is the Oklo uranium deposit in Gabon. Within this deposit, two regions of high-grade (20–60% UO2) U ore were found depleted in 235U. Further investigation elucidated the presence of 235U fission products within these regions, indicating the occurrence of natural fission events approximately 2.1 billion years ago. These natural fission reactors functioned sporadically for hundreds of thousands of years, with infiltrated water acting as a neutron moderator. Fission events lead to the concentration of U ore in these regions via depletion of silicates from the region around the fission events due to the intense heat generated during the periods of fission (Gauthier-Lafaye et al. 1989; Gauthier-Lafaye et al. 2006).

2.1 Thorium, Uranium, and Actinium Series

The two primary sources of terrestrial radiation are the Th and U series, illustrated in Figs. 13.1 and 13.2, respectively. A third series, the actinium series, is also present in the environment but is not of major concern to health without anthropogenic enrichment, as it is of very low abundance in the environment; the progenitor of this series, 235U, makes up on 0.72% of all U by mass and, as the head of this decay series, is unsupported in its decay (de Laeter et al. 2000). These groups of radionuclides decay from one to another until they reach a stable lead isotope end product (208Pb for the Th series, 207Pb for the U series, and 206Pb for the Actinium series). The half-lives of the radionuclides in these series vary from billions of years to sub-microseconds leading to complex relationships in the environment. The duration of half-lives can have a number of effects, potentially impacting mobility in the environment and health impact of radioactive emissions.

3 Radionuclides of Interest

3.1 Radium

Radium (Ra) possesses no stable isotopes and four natural radioactive isotopes , 223Ra (α), 224Ra (α), 226Ra (α), and 228Ra (β), with half-lives varying from days to thousands of years. As an alkaline earth metal, Ra possesses similar chemical characteristics to calcium, magnesium, and barium, and one, 2+, oxidation state. Its chemical similarities to these other elements and its high solubility, especially in chloride and nitrate forms, lead to substantial dissolution and mobilization in the environment (Nelson et al. 2014, 2015a). These properties lead to Ra being a major concern in drinking water. With internal exposure, 228Ra and its progeny (including 224Ra) are considered to be more hazardous per decay than 226Ra; this is due to the short half-life of 228Ra progeny and their likelihood to decay inside the body following ingestion.

3.2 Radon and Thoron

Radon (Rn) possesses three alpha-emitting radioactive isotopes, 219Rn, 220Rn (often referred to as Thoron), and 222Rn (often referred to as radon). Radon is the heaviest noble gas and is relatively inert, but adsorbs readily to certain filter materials such as activated charcoal. Due to these physical properties, Rn can migrate from its progenitor and thus exist unsupported in the environment. Radon can also exist dissolved in water, especially when confined with its progenitor (i.e., “closed system”). Exposure to radon is thought to be the greatest contributor to NORM dose for the general public (NCRP 2006).

3.3 Thorium

Thorium possesses four naturally occurring radioactive isotopes, 228Th (β), 230Th (α), 232Th (α), and 234Th (β). Nearly all Th by mass is 232Th, the head of the Th series, and is two to three times more abundant in the terrestrial environment than U (Harmsen and De Haan 1980). Due to Th insolubility under many environmental conditions, it is found in less abundance in aqueous environments, mostly concentrating in sand and sediment. Thorium is generally found in a (IV) oxidation state in the environment, often existing as ThO2 in the environment, but has also been found in mineral deposits forming Th3(PO4)4 and Th(SO4)2 compounds.

3.4 Uranium

Uranium possesses three natural alpha-emitting isotopes, 234U, 235U, and 238U, with nearly all U being 238U by mass. Due to ingrowth relationships, U found in geologic deposits is typically found at equal activities of 234U and 238U, though interestingly, these radionuclides often exhibit disequilibrium in aqueous environments due to alpha recoil ejection of 238U progeny in addition to the normal dissolution processes. This ejection occurs when 238U decays and the momentum transfer from alpha particle emission forces the 234Th decay product out of the formation into the surrounding water where 234Th then decays to 234U (through 234mPa) enriching the groundwater in 234U versus 238U. Uranium is found in a number of oxidation states, but most commonly as (IV) and (VI) with the (VI) state existing as uranyl ion (UO22+). The U(VI) state tends to predominant under oxic conditions, with the U(IV) state predominating under anoxic, reducing conditions leading to the formation of insoluble U(IV) complexes. Uranium deposits exist in a number of chemical forms, with the primary ore form being UO2 (often existing in an oxidized U3O8 form commonly called pitchblende), but can also be found co-located in sedimentary rock formations (Rich et al. 1977).

3.5 Potassium-40

Potassium-40 is the most abundant radionuclide, making up 0.0117% of all potassium found on earth and decays with a half-life of 1.26 billion years (de Laeter et al. 2000). This radionuclide decays by both beta emission (89.27% of all 40K decay events) and electron capture (10.72% of all 40K decay events) and emits a 1461 keV gamma ray during electron capture decay (NNDC (National Nuclear Data Center) 2014). Potassium-40 is also the only natural positron emitter, decaying to 40Ar (0.0010% of all 40K decay events) (Engelkemeir et al. 1962). Potassium-40 is found in all materials containing K, including human beings, food, soil, and water. With its ubiquitous presence in the environment and K’s vital nature to biological systems, 40K provides a considerable exposure that cannot be controlled or decreased.

3.6 Overall Exposure Estimates

The average person in the U.S. receives 6.2 mSv year−1 in combined radiation dose, whereas the average person worldwide receives 3.0 mSv year−1 (NCRP 2006; The United Nations Scientific Committee on the Effects of Atomic Radiation 2008). The difference in these doses is primarily due to greater exposure to radon/thoron gas and a greater use of radiation in medicine in the U.S. Of this exposure, in the U.S. and worldwide, 3.1 and 2.4 mSv are absorbed per year from natural sources, respectively. The remainder of annual absorbed dose is due to manmade radiation, with medical use making up the majority of this dose (3.0 and 0.6 mSv, year−1, respectively). Of all sources of radiation exposure to the public, exposure to radon and thoron makes up the largest dose source. Exposure to these radionuclides in air and water makes up 36% and 42% of total annual absorbed dose in the U.S. and worldwide, respectively.

Once in the body, radionuclides are distributed differentially based on their chemical properties, forms, and routes of exposure. This leads to variable accumulation and excretion that must be accounted for when evaluating the potential effects of exposure. It is important to note that NORM, on average, is only responsible for approximately 50% of radiation exposure to the general public (Fig. 13.3), with the other half coming from man-made sources, primarily due to medical procedures (NCRP 2006). This distribution varies greatly across the population due to the variability in medical procedures, with some individuals receiving most of their radiation exposure from these procedures and other receiving none.

Fig. 13.3
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Average public exposure data in the U.S., natural exposure in green, medical exposure in purple. This exposure is highly variable across the population

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3.7 Uptake

The route providing the greatest dose from NORM is inhalation of radon and thoron into the lungs. Once inhaled, the decay of these radionuclides leads to direct exposure of the mouth, airway, and lung tissue to emitted alpha particles. In addition to the initial decay of radon and thoron in the lungs, the decay products of these radionuclides can also expose the lung tissue to additional radiation due to their extremely short half-lives (microseconds to minutes) and can even be dissolved into the airway surface fluid and absorbed into the blood stream through the lung (Marsh and Bailey 2013). Longer-lived progeny of radon and thoron can also be transported via mucociliary transport into the digestive system.

Ingestion of radionuclides mostly occurs via the consumption of contaminated drinking water and foods containing radionuclides. Once in the gastrointestinal tract, radionuclides can be absorbed via passive or facilitated diffusion, as well as active transport. Some radionuclides exploit processes used to transport chemically similar elements; examples being the active transport of Ra by calcium transport mechanisms and uranyl ion by iron transport processes (Taylor et al. 1962; Perewusnyk et al. 1989). The absorption of 40K is mostly constant for individuals, as K absorption of K is tightly controlled and excreted 40K is continually replaced.

3.8 Biokinetics

The biokinetics of NORM is complex and varied owing to the wide variety of radionuclides present in the environment and the variability of individual physiology. Most NORM is quickly (in hours to days) excreted via urine or not absorbed in the GI tract, passing through feces. Of the absorbed radionuclides, many accumulate in bone tissue, with large quantities of U, Th, and Pb deposited in bone (20%, 70%, and 15%, respectively) (Perewusnyk et al. 1989; Agency for Toxic Substances and Disease Registry 2007). Radium is also thought to accumulate in bone due its chemical similarity to calcium, but exact distribution to bony tissues is unknown. Accumulation in bone leads to very long biological half-live (approximately 30 years in adults), creating a continuous internal dose source and generating other radioactive progeny.

Radionuclides not only accumulate in bone but also in soft tissues. Uranium concentrates in the kidneys and Th as accumulates in the liver (Perewusnyk et al. 1989). Polonium-210, an alpha-emitting radionuclide with a moderately long half-life of 130 days, has been shown to concentrate in the spleen, liver, and kidneys of mice with highly variable biological half-lives depending on the tissue type and absorption rates up to 50% (Stannard 1964; Thomas et al. 2001). Generally, radionuclide half-lives in soft tissues are much shorter than those in bone, with most material being excreted in urine and feces in a relatively short time span (in hours to days) (Stannard 1964; Agency for Toxic Substances and Disease Registry 1990, 2007).

Clearance of radionuclides, such as U and Po, from the body is often accomplished via thiol-rich, metal-chelating proteins such as metallothionein (Hao et al. 2016; Aposhian and Bruce 1991). With this clearance mechanism and extremely long-lived radionuclides such as U, chemical toxicity can be an issue as well radiological toxicity due to the high mass to activity ratio associated with long-lived radionuclides. Uranium is known to be nephrotoxic via chemical interference with cellular metabolism, independent of its radioactive properties (Vicente-Vicente et al. 2010). Kidney dose is often the limiting factor when dealing with absorbed radionuclides and can be used to set dose limits for the public (Box 13.4).

Box 13.4 Definitions: Cancer Biology

Apoptosis : A type of cell death in which a series of molecular steps in a cell lead to its death. This is one of the methods the body uses to get rid of unnecessary or abnormal cells. The process of apoptosis may be blocked in cancer cells. This is also called programmed cell death (National Cancer Institute n.d.).

Oncogene : A gene that is a mutated (changed) form of a gene involved in normal cell growth. Oncogenes may cause the growth of cancer cells. Mutations in genes that become oncogenes can be inherited or caused by being exposed to substances in the environment that causes cancer (National Cancer Institute n.d.).

3.9 Cancer

Exposure to radiation has long been known to act as a initiator for carcinogenesis (Little 2000). Cellular exposure to ionizing radiation can cause a number of genetic modifications, including cell cycle checkpoint, tumor suppressor gene, and oncogene mutations. These alterations lead to genetic instability in both the cells that are directly impacted by the radiation and cells that are not directly impacted, leading to the development of cancer (Zhou et al. 2000). Evidence also exists that ionizing radiation can act as a tumor promoter via the production of free radicals, an atom or compound with an unpaired electron, within cells (National Research Council 1990). Additionally, substantial evidence exists that early exposure to radiation increases the potential risk of carcinogenesis (Little 2000). Data from the victims of the nuclear weapons detonations at Hiroshima and Nagasaki have demonstrated a strong dose–response relationship (i.e., increased dose correlates with an increased risk of cancer) between radiation exposure and carcinogenesis (Land 1995) (Box 13.5).

Box 13.5 Definitions: Cancer Biology

Tumor Suppressor Gene : A type of gene that makes a protein called a tumor suppressor protein that helps control cell growth. Mutations (changes in DNA) in tumor suppressor genes may lead to cancer. This is also called antioncogene (National Cancer Institute n.d.).

Cell Cycle Checkpoints : Points during the cellular growth and division where division of the cell may be stopped by complex series of regulatory genes and proteins. Mutations to genes governing these checkpoints may allow the growth of cancer cells (National Cancer Institute n.d.).

Genetic damage from radiation can be divided into two categories, namely, direct and indirect effects. Direct effects are the result of direct DNA damage by radioactive particles via kinetic or charged particle interaction. Indirect effects are the result of the generation of a free radical that damages DNA. Most commonly this is via the ionization of water, generating a hydroxyl radical. Damage from either effect can initiate cancerous mutations in DNA if the damage is not repaired correctly.

A prime example of radiation-induced carcinogenesis may involve the tumor suppressor gene, p53, the most common gene mutation involved in lung cancer and the target of a study involving radon exposure in U miners and lung cancer (Taylor et al. 1994). This gene is involved in the production of p53, a signaling molecule that is involved in apoptosis and cell-cycle checkpoint activation (cell cycle arrest) (Muller and Vousden 2013). In this particular study, the same AGG to ATG mutation to codon 249 was observed in 31% of the U miner patients with large-cell and squamous-cell lung cancer in the study versus 0.4% occurrence in other studies looking at this particular gene mutation.

When evaluating exposure to NORM, we are mostly considering low-level exposure leading to rare, random developments of cancer in a population, often referred to as stochastic effects. This should be differentiated from deterministic effects, often seen in high-level exposures, where exposures lead to predictable outcomes, with a threshold for effects, on the individual level. When evaluating low-level radiation exposure, a linear, no-threshold model is often employed to relate exposure to effect (illustrated by the solid line in Fig. 13.4). With this model, observations at higher levels of exposure are extrapolated through the origin, assuming that there is no level at which radiation will not have a negative effect. This is the most common model used to establish protective limits on exposure to workers and the public from potentially hazardous levels of radiation.

Fig. 13.4
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Example exposure models

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Along with this model, another potential relationship in which low-level radiation is actually beneficial, called hormesis, has been proposed (illustrated by the dash-dotted line in Fig. 13.4). This theory proposes that exposure to low doses of radiation stimulates protective biological responses that can increase cell and organism survival. Although there is some evidence in support of this theory, mostly involving cellular and epidemiological studies , the results have been mixed and the effects of extremely low-level radiation exposure have been difficult to discern (Mossman 2001). With this uncertainty, hormesis has failed to gain widespread acceptance in the radiation protection community, whereas the more protective linear no-threshold model is still considered the standard.

3.10 Radon Health Impacts

The single largest health outcome from NORM exposure is initiation and promotion of cancer. There is strong evidence that environmental exposure to NORM can cause cancer, with the best evidence coming from 220Rn and 222Rn exposure. Extensive epidemiological studies performed both on U miners and the general public show a likely causal link between 220Rn/222Rn exposure and lung cancer (Lubin et al. 1995; Field et al. 2000). Radon and thoron exposure is thought to be the second largest cause of lung cancer, estimated by the BEIR VI report to be 15,400–21,800 cases each year in the U.S. attributable to this exposure (National Research Council 1999). These estimates are based on models of radon and thoron exposure and the mortality risk in U miners. Negative outcomes from exposure to other radionuclides have been much more difficult to discern at very low levels of exposure, with risk estimated by evaluating higher level radiation exposure.

In one retrospective cohort study (which was evaluated by the BEIR VI report), a population of 3238 white male Colorado Plateau U miners was evaluated for mortality from a variety of causes and was compared to similar populations of white males from New Mexico, Arizona, Utah, and Colorado (Roscoe 1997). Overall, 11% of the study population died from lung cancer, compared to an expected mortality rate of 2% from the control population. A significant dose–response trend with radon exposure was also observed in this study, with a standardized mortality ratio (SMR, actual cases divided by expected cases based on control population) of 2.5 observed for the lowest exposure quartile and an SMR of 11.7 observed for the highest quartile. It is important to note that the cases of lung cancer attributable to radon exposure in this study are likely less than the total observed cases due to co-exposure to mining dust (with crystalline silica a known Class I carcinogen) (IARC 2012).

In another important study, residential exposure to radon and its relationship to lung cancer was evaluated in the state of Iowa in the U.S. In this case–control study of Iowa women, 413 lung cancer cases were paired with 614 controls (Field et al. 2000). Radon exposure was reconstructed for the 5–19 years prior to diagnosis using a combination of field measurements, survey data, and statistical models to create an estimate of exposure. The study showed a statistically significant positive trend between lung cancer and cumulative radon exposure across the quintiles of exposure. Additionally, a statistically significant excess risk of 0.50 for lung cancer in cases exposed to greater than 11 Working Level Months (WLM5–19) (i.e., 1.5 times more likely than the control group to develop lung cancer), was observed. This exposure level is roughly equivalent to residential exposure at 148 Bq m−3 (4 pCi L−1, the USEPA action level for indoor radon) for 15 years. Overall, the median exposure in this study’s case group was 7.9 WLM over the evaluation period, compared to the lowest and highest quartiles of exposure in the previously mentioned Colorado miners study of <120 WLM and <1000 WLM. Where the BEIR VI models extrapolate high-level miner exposure to a low-level public exposure, this study provides direct evidence of the positive correlation between radon exposure and lung cancer incidence at much lower exposure levels. As the leading cause of lung cancer, the relationship between smoking and radon has also been studied extensively. In both the BEIR VI report and many other subsequent studies, a synergistic effect between smoking and radon exposure has been observed, meaning the combined effect of co-exposure to these hazards is greater than a simple sum of their effects (Moolgavkar et al. 1993; National Research Council 1999; Barros-Dios et al. 2002). This effect has mostly been found to be supra-additive, but sub-multiplicative. Unfortunately, due to the uncertainty involved in estimating radon exposure, often-incomplete documentation of smoking status, and the much higher risk of developing cancer associated with smoking versus radon exposure, there is a high degree of uncertainty in the magnitude of this effect.

4 Sources of NORM

The root source of all terrestrial NORM that reaches the earth’s surface is igneous rock that is formed from the mantle as the main component of earth’s crust. This NORM is then mobilized from these rock formations via weathering and erosion processes; these processes include chemical, radiochemical, and physical interactions that both solubilize minerals and break the rock into fine particles that can be incorporated into the greater environment. These weathered minerals go on to form sedimentary rock and become components of granular materials such as sand, clay, silt, and soil.

4.1 Air

Air is an important carrier of NORM, as radioactive 220Rn and 222Rn gases enter air from geologic sources and are important components of radiation exposure. Additionally, dust carried by wind can also contain NORM, which can then be inhaled. Dust generated by activities such as coal, uranium, and phosphate mining, as well as particulates generated by the burning of coal can contain substantial quantities of radioactive materials.

Exposures to radioactive materials in air from industrial process are primarily a risk to workers, rather than the public. Although the amount of radioactive materials contained in coal fly ash is much greater than ambient background concentrations, only a small percentage of the generated ash is released into the air (~1–3%) with a massive dilution effect and decrease in concentration with distance from the source (Papastefanou 2010). In two independent studies of radon exposure discussed above, a median radon exposure level of 430 WLM with a median monthly exposure of 10 WLM was observed in a cohort of uranium miners versus 8.6 WLM over 7 years to the general public in an area of high radon exposure in the U.S. (Field et al. 2000; Roscoe 1997). Radon is also the primary concern for exposure in air with phosphogypsum piles (waste products from phosphate ore processing), with one study in Cyprus showing radon emanation rates ranging from 0.35 to 1.1 Bq h−1, as much as two orders of magnitude higher than that of the surrounding environment (Lysandrou et al. 2007; Tayibi et al. 2009).

4.2 Soil

Uranium and thorium series radionuclides are ubiquitous in soil , with U concentrations averaging 3 μg/g in U.S. soils and Th concentrations averaging 6 μg/g (NCRP 1984; Harmsen and De Haan 1980). The concentrations of NORM in soil are highly variable, and the liberation of these radionuclides is dependent on soil composition, with some types of soil much more likely to release nuclides such as radon than others. Additionally, fallout of radon decay products can lead to enrichment of these radionuclides (particularly with 210Po and 210Pb, see Fig. 13.2) on soil surfaces. A special case exists in some areas that may lead to enrichment of radon progeny, 210Pb and 210Pb, in lake sediments due to atmospheric fallout of these radionuclides (Nelson et al. 2017). These radionuclides are particle-reactive, meaning they tend to adhere to soil and may wash into bodies of water and collect in sediments. In lakes both in Iowa and West Virginia, 210Po was observed in lake sediments at 2.5 times the concentration 226Ra in the same sediment (Nelson et al. 2017).

Deposits of Th and U also exist throughout the world and are actively mined for these radionuclides. These elements exist in a wide variety of mineral types, with the largest deposits of recoverable U being concentrated in Australia (29% of the world’s currently identified U resources) (NEA 2016). As of 2014, the largest active mining operations for U are located in Kazakhstan, with Kazakhstan, Canada, Australia, Niger, and Namibia accounting for 79% of global U production (NEA 2016). Brazil, Turkey, and India are home to the world’s largest reserves of recoverable Th as of 2005, with reasonably assured reserves estimated to be 0.60, 0.38, and 0.32 million tons, respectively (IAEA 2005).

4.3 Water

Due to contact with water and the chemical properties of NORM, some radionuclides may be readily mobilized from soil and various porous rock formations into groundwater, either in dissolved or particulate phases. Generally, groundwater aquifers are divided into confined and unconfined systems. Unconfined aquifers are under the influence of surface water and compose the so-called “water table,” consisting of alluvium, sand, gravel, and buried river valleys. Confined aquifers are separated from the surface by a confining layer or aquitard, often composed of clay and shale. These confined aquifers are mostly composed of sedimentary rock, such as sandstone and carbonate rocks, and fractured igneous rock (USGS n.d.). Water derived from confined aquifers typically contain higher concentrations of NORM than unconfined aquifers, due to the increased contact time between the water and formation materials and the higher concentration of NORM within this constituent material versus unconfined aquifer substrates.

Surface waters may also contain measureable quantities of NORM, with accumulation occurring from: the dissolution of minerals from sediment, sand, and rock; the emanation and fallout of various radionuclides, such as Rn, Rn progeny, and cosmogenic radionuclides (3H, 7Be, and 14C), and pollution from human activities, such as mining and material processing (Technologically Enhanced Naturally Occurring Radioactive Material [TENORM]).

The chemical properties and variable solubility of naturally occurring radionuclides greatly affect their distribution and transport in water. Radium nuclides tend to be extremely soluble under most conditions, whereas the solubility of other radionuclides, such as U and Th, is greatly influenced by environmental conditions such as pH and oxygenation (Registry 1990; Nelson et al. 2015a). The solubility of U and Po is greatly affected by aqueous conditions and oxidation state (while Th exists in a stable 4+ oxidation state).

5 Exposure Vectors

A number of potential routes of exposure to NORM exist. These exposure routes impact both the potential locality and severity of the effect to persons exposed. The vectors of exposure can generally be divided into two categories, namely, external exposure and internal exposure. External exposure is mostly limited to the photon and beta emissions from NORM. Internal exposure contains contributions from alpha, beta, and photon emissions, but alpha and beta exposures tend to have the most impact due the short distance in which these radioactive particles dissipate their energy in matter.

5.1 External

External exposure to radiation comes primarily from two sources, namely, cosmic radiation and terrestrial photons. Cosmic radiation is a complex mix of neutrons, photons, and charged particles, with the primary dose contribution coming from muon exposure (Sato 2016). This dose increases with altitude, such that at high altitudes, as during commercial air travel, the exposure rate to cosmic radiation can be as much as 100 times that at the sea level, but averages 0.33 mSv year−1, or 5%, of annual U.S. public exposure (NCRP 2006; The United Nations Scientific Committee on the Effects of Atomic Radiation 2008).

Terrestrial photon radiation is primarily the result of 238U and 232Th progeny and 40K. The main contributions outside of 40K are from the gamma rays associated with the Th series radionuclides (Fig. 13.1): 228Ac, 212Pb, and 208Tl; and U series radionuclides (Fig. 13.2): 214Pb and 214Bi. The proportion of dose related to these contributors will vary highly depending on the soil and building composition in a given area, with Th series radionuclides emitting much more gamma radiation per gram of 232Th present versus 238U, primarily due to the high gamma emission probability associated with the decay of 228Ac and 208Tl. U.S. public exposure from natural external sources averages 0.21 mSv year−1, 3% of the annual total (NCRP 2006).

5.2 Internal

Exposure from air is the most important route of exposure to NORM. The primary contributors to this exposure route are 220Rn and 222Rn (NCRP 2006; National Research Council 1999). Radon concentrations are extremely variable depending on region (Fig. 13.5) with concentration in air dependent on a number of factors, including concentrations of U, Ra, and Th in the surrounding environment (soil, bedrock, and water), as well as the moisture content, density, and porosity of the emanating and overlaying material (Porstendörfer 1994). Complicating this exposure pathway, the inconsistent construction of dwellings and other buildings affects the concentration of Rn in indoor air. Radon gas can enter structures from outdoor air through gaps in buildings, from water utilized inside buildings, or from the primary source of indoor radon, the soil adjacent to the structure’s foundation via cracks and gaps (Mäkeläinen et al. 2001). These factors can lead to Rn concentrations much higher in indoor air versus outdoor air due to the capture and circulation of radon gas inside buildings via flow from the higher-pressure outdoor air/soil to the lower-pressure indoor air. Exposure from 220Rn and 222Rn averages 2.29 mSv year−1, 36% of U.S. exposure to all ionizing radiation and 73% of all exposure to NORM (NCRP 2006).

Fig. 13.5
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Map of EPA radon zones (U.S. EPA 1993b)

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Another important, controllable internal exposure route to NORM is via drinking water. Groundwater resources are of particular concern due to colocation of radionuclides with other salts in groundwater aquifers. The concentration of radionuclides in drinking water is often directly correlated with the increased dissolved solids in drinking water; this is especially true with Ra nuclides where iron and other competing divalent cations are also elevated (Szabo et al. 2012). Variable water chemistry, such as oxic versus anoxic, acidic versus alkaline conditions, and variability in coordinating anion composition (i.e., NO2, NO3, Cl, S2, SO3, and SO4), can affect the solubilization of NORM. This variability in water chemistry can lead to differential solubilization of radionuclides in different regions, as evidenced by enhanced concentrations of polonium-210 found in anoxic reducing groundwater in both Nevada and Florida (Seiler et al. 2011; Harada et al. 1989). Based on the available data, a thermochemical model of Po behavior may indicate that this is due to the predomination of the soluble HPo species under reducing, aqueous conditions (Ram et al. 2019).

In one well-known example, the Cambrian-Ordovician aquifer, a majority carbonate rock-based system in the upper Midwest of the U.S., has long been known to contain high concentrations of Ra. In one study performed by the U.S. Geological Survey (USGS) on this aquifer, 64% of the samples taken exceeded the 185 Bq L−1 (5 pCi L−1) U.S. limit for 226Ra and 228Ra in drinking water. This study also determined a correlation with locally iron-reducing and anoxic conditions within the formation; this is likely due to a reduction in adsorption to the formation solids (due to dissolution of iron and manganese hydroxides), as well as competition for adsorption sites between Ra and other alkaline earth metals (Stackelberg et al. 2018).

Water treatments that remove alkaline earth metals, such as calcium, magnesium, and iron, also remove radionuclides due the similarities in their aqueous chemistry. This similarity in aqueous chemistry is often exploited to concentrate radionuclides such as Ra and U from bulk solutions for analysis. Unfortunately, this treatment is expensive, leading water treatment systems to perform minimal treatment with radionuclides often the limiting factor. Evaluation of violations of the EPA Primary Drinking Water standards in 2008 showed that radionuclides (including combined Ra) were the third most common violation of these standards for ground water supplied by public water systems (Job 2009). In addition, the unregulated nature of private drinking water wells creates further concerns for the safety of and potential exposure to NORM.

Food products comprise another vector of NORM exposure. As most food products contain potassium, measurable amounts of 40K are found in most foods. Some foods show enrichment in NORM, including, but not limited to, Ra in brazil nuts (Penna-Franca et al. 1968); 210Po and 210Pb in various leafy vegetables, marine and terrestrial animals (Thomas et al. 2001; Heyraud and Cherry 1979), and various radionuclides in mushrooms (Guillén and Baeza 2014). It is estimated that adults receive an average dose of 0.12 mSv year−1 from consumption of NORM , mostly due to 210Po. The received dose can vary greatly and is estimated to be higher in populations consuming large amounts of seafood, with one Portuguese study estimating a body burdens of 210Po and 210Pb in Portuguese adult males at 3.5 times (70 Bq vs. 21 Bq) that of males consuming a reference diet (Carvalho 1995).

6 Methods of Concentration and Liberation

6.1 Drinking Water Withdrawals

Drinking water is a major exposure pathway of NORM in the general public. Nearly all water, both surface and ground waters, contains low levels of NORM. Although the concentrations of radioactive materials in drinking water are usually low, the large amount of water required to sustain life, often estimated to be two liters per day for adults, creates a measurable cancer risk to the general public. Guidelines and/or regulations for radionuclides in drinking water have been established by a number of organizations, including the U.S. EPA, WHO, and IAEA (WHO 2017; IAEA 2016; EPA 2001).

The dissolution of NORM into water is complex, mostly concentrating in water not only via simple weathering of minerals and sediment by ground and surface waters but also via alpha recoil ejection, among other processes (Porcelli and Swarzenski 2003). Generally, longer contact time and, as a result, higher levels of dissolved solids correlate well with concentrations of NORM in ground water. Other water chemistry parameters, such as oxygenation, pH, temperature, and anion availability/concentration, also affect the radionuclide solubility. Overall, the most important concerns are Ra and U due to their observed solubility and prevalence in subsurface minerals.

Although Ra and U are of primary concern in drinking water, other radionuclides, such as 210Po, appear to be of an intermittent concern. In certain areas of Finland, Nevada, and Florida, high concentrations of this radionuclide (in the context of chronic exposures) have been observed in wells, with maximum concentrations of 13, 17, and 23 Bq L−1, respectively (Harada et al. 1989; Seiler 2011; Lehto et al. 1999). For reference, a regulatory limit in the U.S. for 210Po (based on the same methodology as other limits for other radionuclides , described the section on NORM Regulations), though not currently regulated, would be set at 41 mBq L−1, or nearly 500 times lower than the concentrations observed in these studies.

6.2 Mining

Mining for valuable minerals is often a mechanism for the liberation and technological concentration of NORM (TENORM). Besides the obvious mining operations specifically for U ore, other operations, such as phosphate mining for the production of fertilizer, mining for coal, and rare earth elements, can liberate measurable amounts of NORM. Although the wastes and dust generated by the extraction of these minerals have relatively local impacts through plume transport, their processing and downstream products can further release NORM into the environment. For example, coal fly ash has been shown to contain concentrations of radioactivity with public health concern (with a 1000-Mw plant liberating 1–37 GBq of radiation from 226Ra and 228Ra each year) and both the produced fertilizers and wastes from the processing of phosphate ore become enriched in Ra and Ra progeny (Eisenbud and Petrow 1964; Rutherford et al. 1994). The release of this coal ash can have serious environmental consequences, with one spill in Tennessee in the U.S. releasing 4.1 million cubic meters of ash into a waterway in 2008. According to the Tennessee Department of Environment and Conservation, this ash contained 160 Bq/kg of 226Ra and 110 Bq/kg of 228Ra. This compares to local environmental levels of 41 Bq/kg and 52 Bq/kg for 226Ra and 228Ra, respectively (Ruhl et al. 2009).

Extraction of U ore for the production of nuclear fuels is also of great concern, but its scale is somewhat limited. Interestingly, U extraction can take a number of forms, including direct mining of ore and in situ and heap leaching, each with their own concerns. The ore itself is not extremely radioactive, but the processing of the ore liberates and concentrates U, whereas the leftover materials, known as mill tailings, still contain NORM that must be disposed of and, due to their high sulfide concentrations (from sulfuric acid leaching to extract U), can create acidic conditions that promote leaching of U, Th, and Ra into the environment (Abdelouas 2006). Uranium mill tailings have been involved in hundreds of releases to the environment worldwide, mostly due to improper disposal of these materials (Strachan 2002).

6.3 Oil and Gas Drilling

It has long been known that high concentrations of NORM can be co-located with oil and gas deposits. The formation of these deposits from the organic matter accumulated in the sediments of ancient seas leads to the concentration of Th, especially U via sedimentation and biological reduction, leading to the formation of insoluble U (IV) (Nelson et al. 2015b). With wastes, especially aqueous wastes, associated with oil and natural gas extraction, 226Ra and 228Ra are the major concerns due to their high solubility in brines and interstitial fluids, and the high concentration of other alkaline earth metals leads to easy mobilization in solution. For example, analysis of Marcellus Shale flowback wastes yielded very high concentrations of 226Ra, 59 Bq L−1, with comparatively low concentrations of U, Th, and other radionuclides (Nelson et al. 2014). This is likely due to the extremely high concentrations of alkaline earth metals present (gram per liter concentrations have been observed in Marcellus shale flowback fluid) and the associated competition for adsorption sites within the formation between Ba and Ra (Nelson et al. 2015a).

Very large volumes of wastewater, as well as drill cuttings, are generated through these operations, with complex partitioning of NORM that must be considered. Drill cutting wastes may be partially depleted in Ra and Ra progeny, whereas aqueous wastes may be enriched in Ra (Eitrheim et al. 2016). With the extreme partitioning of Ra into aqueous wastes, the ingrowth of their radioactive progeny must be considered during disposal and storage, as the progeny of 226Ra can result in total activity levels, under confined conditions, quadrupling in a relatively short time span (Nelson et al. 2014). The majority of the aqueous wastes, 71.5%, from drilling in the U.S. are recycled or reused, whereas drill cuttings are mostly disposed of in landfills (Maloney and Yoxtheimer 2012). The disposal of solid materials in landfills may pose a concern, with measured concentrations of U series radionuclides in these formation materials exceeding 200 Bq/kg and 3–5% of U shown to be leachable under landfill conditions (as determined in TCLP leaching studies) (Eitrheim et al. 2016).

6.4 Natural Emanation

Under many conditions, 220Rn and 222Rn gases can migrate out of minerals, soil, and water, moving toward the surface either via diffusion or the flow of radon-containing air due to pressure differentials in the soil (Tanner 1968). The compositions of the soil, deposits, and confining layers play a major role in the migration of radon to the surface; the upper Midwest of the U.S., for example, possesses gas-porous soil, carbonate-rich mineral deposits, and glacial till leading to higher levels of radon reaching the surface (illustrated in Fig. 13.5). Hot springs are another important source of natural emanation, as this water can carry a large quantity of Rn to the surface from deep geologic deposits. Once Rn reaches the surface it decays, liberating relatively long-lived progeny (210Pb and 210Po) that can migrate through the environment, including wash out and deposition from the air onto outside surfaces, often concentrating in sediment and plants (Nelson et al. 2017; Zagà et al. 2011).

Of major concern with relation to Rn is the concentration of 220Rn and 222Rn in buildings via flow from the adjoining soil through cracks and gaps in the structure foundation. This is the result of the negative pressure gradient between the building and the surrounding soil (U.S. EPA 1993a). This flow of Rn results in accumulations at much higher concentrations inside structures than outside, with the accumulated radon then being circulated throughout the structure via forced air HVAC systems.

7 NORM Regulations

Environmental regulations vary greatly across the world, but all environmental regulations with regard to NORM seek to minimize exposure to workers and the public. These regulations can be divided into two categories, limitations on exposure and limitations on discharges to the environment. Exposure limits differ for the public and radiation workers, with radiation workers allowed more exposure due to the projected benefits of the work done while receiving this exposure. The ICRP and NCRP have recommended limits of 1 mSv year−1 for public exposure, whereas they differ with regard to worker exposure, setting limits of 20 and 50 mSv year−1, respectively (NCRP 1993; ICRP 2007).

7.1 Drinking Water

Due to the high potential of exposure over a lifetime, drinking water is often heavily regulated with regard to NORM. In the U.S., drinking water is regulated for a number of naturally occurring radioactive components, including gross alpha particles excluding U at 0.56 Bq L−1 (15 pCi L−1), gross beta and photon emitters at 0.05 mSv year−1 (50 mrem year−1), U at 30 μg L−1, and combined Ra-226/228 at 0.2 Bq L−1 (5 pCi L−1). These limits are based on a 10−4 public risk of developing cancer if water at the limit is consumed over a lifetime (1 in 10,000 persons developing cancer). Internationally, limits vary, but the WHO has also created guidelines for radioactivity in drinking water, where the combined dose from drinking water should not exceed 0.1 mSv year−1, corresponding to an lifetime cancer risk of 5.5 × 10−6 (1 in 180,000 persons developing cancer) (WHO 2017).

7.2 NORM and TENORM

Regulation of wastes associated with NORM is highly variable, inconsistent, and often inadequate both internationally and in the U.S. Wastes associated with the mining of U are generally more controlled in the U.S. due to the presence of many U mining sites on U.S. federal lands and specific regulations being applied to U processing wastes (40 CFR Part 440). Wastes from oil and gas extraction are often either unregulated or the radioactive component (TENORM) is not considered in the regulation. With the absence of federal regulations in the U.S., states have begun to regulate TENORM wastes, but these regulations vary greatly, leading wastes to be moved from heavily regulated states to be disposed of in states with weaker regulatory oversight. This includes disposal of wastes from West Virginia and Ohio in Kentucky, and Marcellus Shale wastes from Pennsylvania being disposed of in New York, both states with comparatively weaker disposal limitations for TENORM than the source states in question (Ann Glass Geltman and LeClair 2018).

8 Conclusion

Naturally occurring radioactive materials are ubiquitous in the environment and pose a health risk to the public and radiation worker alike. Exposures to 220Rn and 222Rn gas in air pose the highest risk to public health from NORM, making up 42% of the public radiation dose worldwide. Modifications made to the environment, such as mining and drilling for natural resources, and the extraction of drinking water can liberate and concentrate radionuclides from their confined deposits. Controlling the dispersal of NORM from human activities is important in controlling potential exposure and reducing overall cancer risk.