Understanding the Radioactive Ingrowth and Decay of Naturally Occurring Radioactive Materials in the Environment: An Analysis of Produced Fluids from the Marcellus Shale

Background The economic value of unconventional natural gas resources has stimulated rapid globalization of horizontal drilling and hydraulic fracturing. However, natural radioactivity found in the large volumes of “produced fluids” generated by these technologies is emerging as an international environmental health concern. Current assessments of the radioactivity concentration in liquid wastes focus on a single element—radium. However, the use of radium alone to predict radioactivity concentrations can greatly underestimate total levels. Objective We investigated the contribution to radioactivity concentrations from naturally occurring radioactive materials (NORM), including uranium, thorium, actinium, radium, lead, bismuth, and polonium isotopes, to the total radioactivity of hydraulic fracturing wastes. Methods For this study we used established methods and developed new methods designed to quantitate NORM of public health concern that may be enriched in complex brines from hydraulic fracturing wastes. Specifically, we examined the use of high-purity germanium gamma spectrometry and isotope dilution alpha spectrometry to quantitate NORM. Results We observed that radium decay products were initially absent from produced fluids due to differences in solubility. However, in systems closed to the release of gaseous radon, our model predicted that decay products will begin to ingrow immediately and (under these closed-system conditions) can contribute to an increase in the total radioactivity for more than 100 years. Conclusions Accurate predictions of radioactivity concentrations are critical for estimating doses to potentially exposed individuals and the surrounding environment. These predictions must include an understanding of the geochemistry, decay properties, and ingrowth kinetics of radium and its decay product radionuclides. Citation Nelson AW, Eitrheim ES, Knight AW, May D, Mehrhoff MA, Shannon R, Litman R, Burnett WC, Forbes TZ, Schultz MK. 2015. Understanding the radioactive ingrowth and decay of naturally occurring radioactive materials in the environment: an analysis of produced fluids from the Marcellus Shale. Environ Health Perspect 123:689–696; http://dx.doi.org/10.1289/ehp.1408855


Polonium-210 ingrowth
The long-lived 238 U (t 1/2 = 4.5 x 10 9 years) in the formation supports the activity of 226 Ra, which then supports 210 Pb (t 1/2 = 22.3 years) and 210 Po. Thus, these elements should be present (and of equal activity) in the Marcellus Shale. Given that 226 Ra was 670 Bq/L, we expected similar activities of 226 Ra decay products. Yet, when we directly measured 210 Pb by gamma spectrometry, its activity was below the critical level (Currie Limit, 14 Bq/L). We acknowledge this critical level may be unacceptably high for many applications. The high critical level is due to several reasons, first, the relatively high activity of Ra isotopes and decay products create a large Compton scatter, which buries the low energy peak of 210 Pb (46 keV, 4%). Emission data for 210 Pb were extracted from the NuDat 2 Database [National Nuclear Data Center (NNDC) 2013]. Secondly, the high levels of ions in produced fluids attenuate gamma emissions thereby further reducing counting efficiency of the low intensity peak (Landsberger et al. 2013).
Importantly, 210 Pb was not in secular equilibrium with its parent, 226 Ra, which led us to investigate the levels of 210 Po (the final radioactive species in the 238 U decay series). Similarly, experiments indicated 210 Po was not in secular equilibrium with either 226 Ra or 210 Pb. When we measured 210 Po levels ~2 months later, we noticed levels had increased approximately 450%.
This ingrowth, follows the theoretical Bateman equation with the assumption that all decay products of 226 Ra are initially absent (Figure 2A).
Yet, over time the levels of 228 Th steadily increased. In this sample, 228 Th is supported by 228 Ra and its ingrowth can be modeled on a transient equilibrium model ( Figure 2B). Given that Th is generally insoluble in environmental waters (Kumar et al. 2013) and in produced fluids Ra is soluble, the transient equilibrium model of 228 Ra/ 228 Th has the potential serve as a 'forensic' tool to determine when samples removed from the Marcellus Shale (for up to ~ 10 years (Schmidt and Cochran 2010)). The 228 Ra/ 228 Th system provides key advantages over other tools including (1) the chemical disequilibrium introduced by the poor solubility of Th (2) the relative ease of measuring 228 Th and 228 Ra (via 228 Ac) and (3) the relatively short half-live of 228 Th (t 1/2 = 1.9 years) that allows for a chronometer that may be used within a matter of weeks.

Uranium absent
In addition to Ra decay products, we investigated levels of 238 U, 235 U, and 234 U in the produced fluids. Although U is often analyzed by mass spectrometry, the method we developed provides the advantage of simultaneous determinations of multiple U, Po, and Th isotopes. Given the high level of 226 Ra ( 238 U decay product), we were surprised to find levels of U isotopes less than 5 mBq/L (n=4), which is nearly 5-log lower than the activity of 226 Ra (Figure 2). There are very few peer-reviewed reports of U activities in produced fluids; however, the notably lower levels of U compared to 226 Ra is similar to data from the PA Department of Environmental Protection (Barbot et al. 2013). Our analysis indicates there is a slight enrichment of 234 U compared to 238 U ( 234 U/ 238 U = 2.3), which is common in groundwater and indicative of daughter recoil (Osmond et al. 1983).