Natural and anthropogenic processes affecting radon releases during mining and early stage reclamation activities, Pinenut uranium mine, Arizona, USA

Highlights

• Physical disturbance of the ore pile did not cause increases in Rn_air concentrations during the monitoring period.

• Relative size of ore pile displays positive trend with mean Rn_air in publicly accessible areas near the ore pile.

• Elevated Rn_air concentrations were usually associated with low wind speeds in publicly accessible areas near the ore pile.

• Venting of the underground mine workings did not result in elevated Rn_air in publicly accessible areas near the mine vent.

Abstract

Radon (Rn_air) was monitored in open air in publicly accessible areas surrounding the Pinenut uranium (U) mine during mining and reclamation activities in 2015–16 to address concerns about mining related effects to areas surrounding Grand Canyon National Park (GCNP) in Arizona, USA. During July 2015, Rn_air concentrations associated with the ore storage pile monitoring site were larger than those at the mine vent monitoring site and likely resulted from the relatively large amount of ore stored on site during this period. Higher wind velocities at the ore pile monitoring site generally resulted in lower Rn_air concentrations; however, wind velocity did not appear to be an important factor in controlling Rn_air concentrations at the mine vent monitoring site. Physical disturbances of the ore pile by heavy equipment did not coincide with elevated Rn_air concentrations at the ore storage pile or mine vent monitoring sites. The relative size of the ore storage pile showed a positive trend with the daily mean Rn_air concentration measured at the ore pile monitoring site. Principal component analysis (PCA) was applied to the ore pile and mine vent multivariate data sets for simultaneous comparison of all measured variables during 230 days of the study period. A significant positive coefficient for Rn_air was associated with a significant negative coefficient for wind speed for principal component (PC) 2_{ore pile}. Significant, positive PC2_{mine vent} coefficients included Rn_air, wind direction, and relative ore pile size indicating that Rn_air variations at the mine vent monitoring site may be affected by Rn sourced from the ore pile. The ore pile is located about 200 m south of the mine vent Rn monitor with the prevalent wind direction coming from the south. All data generated during the field study and laboratory verification tests were published by Naftz et al. (2018) and are available online at: https://doi.org/10.5066/F79Z946T.

Introduction

Radon (Rn) is a naturally occurring and radioactive noble gas (Cothern, 1987). Uranium (U) mining activities can be associated with elevated Rn concentrations measured in samples of indoor and outdoor air (Rn_air) (Vandenhove et al., 2006; Somlai et al., 2006; Fernandes et al., 2006). Sources of Rn_air associated with mining activities include ore storage areas, ore crushing and grinding, ore processing, yellowcake production, and tailings impoundments (Hartley et al., 1985). The isotope ²²²Rn is produced by the alpha decay of radium (²²⁶Ra) in the U decay series and has a half-life of 3.82 days (Sakoda et al., 2011). Inhalation of the short-lived daughters of Rn are known to cause lung cancer (Kumar et al., 2003). Recent epidemiological studies combining seven residential case-control studies in North America provided direct evidence of an association of residential radon and lung cancer risk, consistent with previous studies focusing on underground miners (Krewski et al., 2005; Schubauer-Berigan et al., 2009). As an inert gas, Rn mobility in interconnected pore spaces can be affected by ²²⁶Ra distribution, grain size, moisture content, and temperature (Sakoda et al., 2011).

Previous studies have monitored the concentration of Rn_air in active, abandoned, and reclaimed U mines. The World Health Organization recommends a reference level for Rn_air of 2.7 picoCuries per liter (pCi/L) for indoor air compared with the average baseline Rn_air concentration in outdoor air of 0.4 pCi/L (World Health Organization, 2009). High Rn_air concentrations (mean = 18 pCi/L) have been measured in dwellings near the surface projection of a tunnel associated with a closed underground U mine in Hungary (Somlai et al., 2006). An AlphaGUARD instrument was used for continuous monitoring of Rn_air concentrations around waste rock piles associated with legacy U mines in Japan (Furuta et al., 2002), where elevated Rn_air concentrations were observed at selected monitoring locations. The calculated effective dose was less than the public effective dose limit of 1 millisievert per year (Furuta et al., 2002). Substantial diurnal variations in Rn concentration have been observed from uncapped U tailings material during controlled field experiments (Schubert and Schulz, 2002). The observed variation in Rn_air was likely associated with the diurnal inversion of the soil/air temperature gradient that resulted in convective soil gas migration (Schubert and Schulz, 2002). High frequency monitoring of Rn_air concentrations in capped U tailings in central Portugal documented large (~2.7 × 10⁷ pCi/L) daily variations in Rn_air below the surface cap (Barbosa et al., 2015). Diurnal variations in Rn_air were also observed below the surface cap and were associated with daily oscillations in atmospheric pressure (Barbosa et al., 2015).

Discovery of high-grade U ore in breccia pipe deposits led to U extraction on patented claims within Grand Canyon National Park (GCNP) from 1956 to 1969 (Alpine, 2010). The GCNP is recognized as one of the most treasured landscapes on our planet and was designated as the 17th U.S. National Park in 1919 (Anderson, 2000). Increased U mining claim activity from 2004 to 2008 in northern Arizona generated concerns about potential U-mining related effects to natural, cultural, and social resources in GCNP (U.S. Department of Interior, 2012). In 2012, the U.S. Secretary of the Interior withdrew 404,000 ha of Federal land near GCNP into north, east, and south parcels (Fig. 1) to protect the park and the associated watershed from potentially adverse effects of mineral exploration and development (U.S. Department of the Interior, 2012). A key factor in the 2012 withdrawal decision was the limited amount of scientific data available to assess potential mineral extraction effects to GCNP and surrounding areas (U.S. Department of the Interior, 2012). One area of unknown potential impact(s) was off-site migration of Rn_air from ore storage piles and underground mine vents to publicly accessible areas during U extraction and subsequent reclamation activities. Because the 20-year withdrawal was subject to existing mining rights, four previously approved U mines were allowed to start/continue to extract U ore during the 20-year withdrawal period (U.S. Department of the Interior, 2012).

Our study objective was to utilize continuous onsite Rn monitors in combination with high frequency time-lapse photography and meteorological data to better understand human vs. natural processes controlling Rn_air concentrations in publicly accessible areas adjacent to an operating U mine. The Pinenut mine (Fig. 1) was selected as our study site and is one of the four approved mines that could legally operate during the 20-year withdrawal period. Our study covered the time period from March 2015 through May 2016 and included the final stages of ore extraction and on-site storage through the early phases of off-site ore removal and mine reclamation.