Terrestrial ecological risk analysis via dietary exposure at uranium mine sites in the Grand Canyon watershed (Arizona, USA)
Introduction
The United States was a leading producer of uranium (U) from the mid-to late 20th century, with U primarily used for nuclear power production (OECD 2006). When U price declined in the late 1970s, the lower grade deposits of U ore present in the United States could not compete with higher-grade deposits in Australia and Canada. Uranium production in the United States consequently dropped significantly (USEIA 2019). However, energy independence and energy dominance of domestic mineral resources has been emphasized within the United States in recent years. In 2018, the U.S. Department of the Interior published a list of mineral commodities that are considered critical to economic and national security (Federal Register 2018). Uranium was included on the list of minerals being identified as critical for commercial and residential energy production, U.S. defense applications, medical device technologies, and energy generation for space vehicles and satellites (Fortier et al., 2018). Evaluating the potential impacts associated with alternative energy sources and their extraction may be useful to estimate life cycle costs (both financial and environmental) for various types of power generation. The environmental impacts of uranium extraction are not always well quantified and may vary among ecological settings.
The Grand Canyon in northern Arizona was established as one of the first national parks in 1919 and added as a World Heritage Site in 1979. The unique geologic formations that contributed to these designations also contain naturally occurring radioactive material. Some areas have unique features called breccia pipes; the breccia pipes contain some of the highest-grade U ore in the United States (>0.5% U3O8; Alpine 2010). As a result, U extraction from breccia pipe deposits has occurred for decades within the Grand Canyon watershed (Alpine 2010). The proximity of these mines to the Grand Canyon has increased the scrutiny of mining operations, and outcomes from ecological risk analysis are of interest to a broad group of stakeholders (Fig. 1). Breccia pipe mines located on Federal lands require natural resource managers to balance the protection of public and natural resources with sustainable production of fuel minerals. With uranium prices remaining relatively low for these past several years, industry stakeholders may use the analysis for planning environmental mitigation and/or remediation strategies. Despite the relatively small footprints of these breccia pipe uranium mining operations (generally <20 acres; Alpine 2010), the environmental consequences of U extraction have been a concern because the Grand Canyon and surrounding areas host cultural and economic resources that are important to tribes, ranchers, and recreational users. Tribal nations and environmental groups have expressed concern over potential mining impacts to cultural properties/resources and natural resources. Private citizens, such as landowners and recreational users, may have questions related to their property protection or activity safety. The mere concept of U mining can elicit perceptions of risk associated with radioactivity that may not be supported by empirical data. Such perceptions ignore the complex nature of these mining sites, including the presence of natural sources of radioactivity, chemical as well as radiological risks, contaminants of concern list that needs to extend beyond U, and the potential bioaccumulation of elements and radionuclides into the local food web (Hinck et al., 2017; Bern et al., 2019; Cleveland et al. 2019, 2021; Van Gosen et al., 2020).
Several ecological exposure pathways, including direct contact, inhalation, ingestion, and dietary uptake, have been identified at breccia pipe mines (Hinck et al., 2014). These mines often have small footprints (<20 acres), limited production life spans (5–7 years), restricted site access (i.e., fencing), and no on-site milling (only ore storage; Alpine 2010). These characteristics generally reduce ecological exposure potentials and thus risk scenarios (Hinck et al. 2014, 2017; Cleveland et al., 2021). However, other factors need to be considered. For example, the complex groundwater hydrology in this region contributes to uncertainty about how mining activities affect local aquifer systems and consequently the biota using seeps and springs as water sources (Jones et al., 2018; Tobin et al., 2018). Mining-related elements (e.g., U, thallium (Tl), lead (Pb), nickel (Ni), copper (Cu), arsenic (As)) can be transported off-site via aeolian deposition, resulting in exposure and risk to biota inhabiting or foraging in areas near mining operations (Hinck et al., 2017; Walton-Day et al., 2019). Chemical and radiological exposure of wildlife have been documented in various U mining stages within the region (Cleveland et al. 2019, 2021; Hinck et al., 2017; Minter et al., 2019). While radiation risks to rodents were determined to be minimal (Minter et al., 2019), chemical risks from mining-related elements have not been quantified.
Cleveland et al. (2019; 2021) were the first to report that active production of U ore resulted in elevated elemental concentrations in some local biota at breccia pipe mines in northern Arizona compared to non-mineralized reference or pre-mining sites, indicating chemical uptake and exposure from the local environment. Cleveland et al. (2021) went on to note that their data would be useful for site-specific ecological risk analysis that could contribute to future decisions regarding mineral extraction in the region. Our objective was to conduct a screening level ecological risk analysis based on exposure to mining-related elements via diets and incidental soil ingestion to provide context to the chemical characterization and exposures at breccia pipe mines reported by Cleveland et al. (2019; 2021). Although these breccia pipe deposits are known for their U ore, U was not expected to drive wildlife risk based on existing ecotoxicological literature. Most organisms that have been studied (e.g., birds, small mammals) have less sensitivity to the chemical toxicity of uranium (Alpine 2010); other co-occurring elements like As and Cu likely drive ecological risk based on receptor sensitivity and mode of toxicity. Moreover, most of the elemental constituents present in breccia pipe deposits generally do not bioaccumulate or biomagnify. Therefore, we hypothesized that U would not be the main risk driver at breccia pipe mines and that the greatest risk would be for biota at lower trophic levels (herbivores and omnivores).
Section snippets
Study areas
Five sites were located in the Grand Canyon watershed in northern Arizona (Fig. 2) and represented the pre-production, active production, and post-production phases of the breccia pipe uranium mining lifecycle as well as a non-mineralize reference area. This arid area generally has sparse vegetation and is dominated by sagebrush and grasslands (Mann and Duniway 2020); areas around all study sites are open to grazing and managed by the U.S. Bureau of Land Management or U.S. Forest Service. The
Dietary exposure dose models
Hazard quotients were generally low for all model types; models indicated no risk at the non-mineralized reference site except for Cu in the Western Bluebird model (Table 3; Supplemental Table 3). Hazard quotients were <1 for all elemental constituents and sites in the carnivore model (coyote; HQs<0.19; Table 3; Supplemental Table 3). Herbivore models of jackrabbit (HQs<0.54), mule deer (HQs<0.20), and elk (HQs<0.05) had similar patterns. These results indicate minimal to no risk for carnivores
Discussion
Uptake of and exposure to elemental constituents associated with breccia pipe mines has been documented in local biota (e.g., As, Cu, Mo, U; Cleveland et al., 2021; Hinck et al., 2017). Ecological risk analyses at breccia pipes in northern Arizona have been absent or very limited, primarily because of the lack of empirical data available for these types of mines (Liz Schuppert/US Forest Service and Rody Cox/US Bureau of Land Management, written communications August 11, 2018). Species-specific
Conclusions
Results from this study help define the ecological risks of U, which is an important consideration as an identified critical element for energy production. Overall, we found that the risk to the terrestrial food chain from mining-related metals at breccia pipe U mines in the Grand Canyon watershed was low based on our limited data and conservative ingestion models. As hypothesized, U was not the driver of ecological risk. Arsenic, Cd, Cu, and Zn were of concern for biota consuming
Author contribution
Jo Ellen Hinck: Conceptualization, Methodology, Formal analysis, Investigation, Resources, Validation, Writing – original draft, Writing – review & editing; Danielle Cleveland: Methodology, Formal analysis, Data curation, Writing – review & editing; Bradley E. Sample: Methodology, Resources, Validation, Writing – review & editing
Data availability
Supplemental information for this manuscript is available on the https://doi.org/10.1016/j.chemosphere.2020.129049. Metadata and digital datasets are also available, per USGS Data Management Policy, at https://doi.org/10.5066/P94OVQO9 (active and post-production; non-mining reference) and https://doi.org/10.5066/F7QF8R16 (pre-mining).
Declaration of competing interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Acknowledgements
We sincerely thank staff from USGS (J. Arms, W. Brumbaugh, E. Cheng, G. Linder, V. Melton, H. Smith, N. Thompson, R. Tillitt, E. Valdez, M. Walther), US Bureau of Land Management (L. Christian, R. Cox), US Forest Service (Liz Schuppert), and Bethel College (F. Mendez-Harclerode) for assistance with sample collection, processing, and analyses. David Walters and Jeff Steevens provided helpful review comments to earlier versions of this manuscript. Funding was provided by the USGS Contaminant
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