Science for Decision Making on Uranium Mining in Arizona

Study Themes

The USGS has identified 19 tasks designed to better characterize the impact of uranium and other trace elements on regional water resources, native flora and fauna, and cultural and recreational uses. These tasks will help meet the future informational needs of Federal, State, local, and tribal entities and when possible, will leverage expertise from each of the participating agencies. Tasks are grouped by general themes, which follow a chronological order in terms of exploration and mining.

Full descriptions of each theme and tasks are available in the Grand Canyon Science Plan (GCSP). Task priorities and descriptions are available on the Tasks page.

1 - Improve Understanding of the Genesis of Ore

The genesis of ore deposits in breccia pipes is poorly understood and the number of mineralized breccia pipes in the Grand Canyon region is currently unknown. Comparing the age, geochemistry, minerology, and other geologic characteristics of the breccia pipe uranium ores and nearby sandstone-hosted uranium ore deposits will help determine if these mineralizing events are related in time and space. Determining the relation, if one exists, expands our ability to predict areas most likely to host yet-to-be discovered uranium deposits in the region. Applying generic models to estimate the number of breccia pipe ore deposits results in large uncertainty in uranium resource estimates. Uncertainty of resource estimates in the region can be minimized by improving our geologic and hydrologic understanding of the origin, distribution, and geochemistry of ore deposits.

To decrease uncertainty associated with the genesis of ore deposits in breccia pipes, we propose to:

  • Evaluate the distribution and stratigraphic positions of uranium ore-bearing breccia pipes across the Grand Canyon Region to consider possible geologic controls on their formation and ultimate distribution ( Task 1 )
  • Estimate the number of buried and concealed mineralized breccia pipes using existing geologic maps and research findings, and to cooperatively use geophysical, drilling, and exploration data ( Task 2 )
  • Evaluate the role regional hydrology had on breccia pipe mineralization (hundreds of millions of years ago) by reconstructing the geologic and paleohydrologic setting that existed during ore formation ( Task 3 )
  • Evaluate the mineralogy, chemistry, isotopic character, and age of breccia pipe ore deposits ( Task 4 )

2 - Characterization of Transport Pathways of Mining Related Contaminants in Water, Soil, Sediment and Biota

Uranium and other chemical constituents associated with breccia pipe deposits occur naturally in soil, sediment, groundwater, and surface water from the weathering and leaching of ore deposits. Mining of breccia pipes has the potential to enhance release of chemical constituents (such as uranium and associated trace elements) into the environment, which can then expose soil, water and biological resources to mining-related contaminants. Determining the relative contribution of radiation and chemical constituents in water and biological resources from mining activities is problematic and challenging without an understanding of background concentrations. The potential for increased exposure resulting from mining activities has not been well characterized in the Grand Canyon region because regional/background concentrations of uranium and other trace elements in water, sediment, soil, and biota are largely unknown and there is a general lack of understanding regarding the local and regional groundwater flow systems. Water can move through breccia pipes into deeper aquifers naturally, but water movement is not well characterized once a pipe is opened for mining. In addition, relative contributions of uranium and other trace metals introduced into the environment from natural sources compared to sources enhanced by anthropogenic activities (e.g., mining) is poorly understood. Determining how mining-related chemical constituents move through the environment will allow future mining operations to be designed to minimize exposure and potential impacts to water and biological resources.

To improve characterization of exposure to mining-related contaminants, we propose to:

  • Determine regional/background concentrations of uranium and other trace elements and radiation levels in water, sediment, soil, and biota (vegetation, invertebrates, amphibians, reptiles, birds, and small mammals) ( Task 5a , Task 5b , Task 5c )
  • Characterize the water chemistry associated with a breccia pipe mine and a naturally exposed mineralized ore body, which will be considered the reference standard for comparison of mine impacts ( Task 6 )
  • Conduct habitat and species surveys (including threatened and endangered species, NPS species of management concern, BLM species of concern, and USFS species of concern) in and adjacent to the Canyon Mine and EZ Mine site before mining starts, during active mining, and after mine remediation to determine the degree that biota occur near and are attracted to mining activities ( Task 7a )
  • Characterize concentrations of uranium and other trace elements and radiation levels in water, sediment, soil, and biota (including Federal trust resources) in and adjacent to the Canyon Mine and EZ Mine site before mining starts, during active mining, and after mine remediation. Other mine sites will also be sampled to represent environmental conditions during different phases of the mine life cycle including the Arizona 1 Mine (2014-2015 after active mining), the Pinenut Mine (2014-2015 during active mining), and the Kanab North Mine (various times representing after mining, during and after remediation). These additional sites were sampled because the halting nature of uranium mining (driven by commodity price fluctuations and mining company business decisions) does not guarantee that the Canyon Mine or the EZ Mine will progress through the complete mine life cycle during the 15-year duration of USGS studies. Sampling before, during, and after precipitation events will also occur. ( Task 7b , Task 7c )
  • Analyze deeper horizons of soils and sediment deposits in washes to characterize the vertical distribution of uranium and other trace elements to provide information on the soil profile to depths of burrowing animals that may potentially be affected by mine operations ( Task 7d )
  • Continue to evaluate and refine the isotopic Uranium Activity Ratio (UAR) analysis in water, and ore samples from historical and active mines ( Task 8a , Task 8b )
  • Drill and develop observation wells into the regional aquifer at strategic locations within the withdrawal area (a minimum of two on the north parcel and one on the south parcel). Sample perched aquifer. ( Task 9 )
  • Update the Northern Arizona Regional Groundwater Flow Model (NARGFM) so that it can be used as a transport model to help evaluate the local and regional groundwater flow for the past several decades as it affects the transport of solutes and different elements from source areas (natural or mined ore bodies) to discharge areas (springs or the Colorado River) ( Task 10 )

3 - Determine Pathways of Exposure

Although breccia pipe mines generally have a small footprint, the scale of environmental risk is not necessarily directly proportional to the footprint, particularly when exposure pathways are considered at landscape scales and ecosystem levels of organization. Mining and extraction of uranium ore from breccia pipe deposits can result in transport of chemical constituents via dissolution, seeps/springs, erosion (wind, runoff and floods) and deposition, and biotic uptake. Pertinent ecological exposure pathways from this secondary contamination include ingestion, inhalation, absorption, biotic uptake, and cell membrane mediated uptake. Prioritization of these exposure pathways is critical to understand where mitigation measures could be applied to minimize exposure and therefore potential risk to radiological and chemical constituents.

To prioritize exposure pathways associated with mining-related contaminants, we propose to:

  • Characterize the dispersion characteristics of uranium and trace elements associated with uranium mining and the subsequent risk to biota by installing dust collectors in the vicinity of ore bodies, mines, and at sites unaffected by uranium ore contamination ( Task 11 )
  • Monitor surface waters to determine the mobility and flux of uranium and other trace elements in drainages with historical and current mining ( Task 12 )
  • Evaluate scenarios of impact from potential overflowing of wastewater containment ponds or disturbance of waste rock resulting from precipitation or flash flood events that overwhelm a mine site and subsequent risk to biota ( Task 13 )
  • Determine the fluxes and storage of uranium and other trace elements in Lake Mead sediments ( Task 14 )

4 - Determine Biological Effects of Exposure to Contaminants

By prioritizing the relative importance of various exposure pathways identified above, the biological effects of exposure can be examined to determine if mining-related contaminants or activities are adversely affecting Federal trust resources. There is very little chemical contamination data related to uranium mining for biota in the Grand Canyon region. Many species in the region have specialized life history strategies that allow them to survive in the arid environment but that may also increase their exposure to contaminants. For example, some reptiles, amphibians, birds, and mammals spend significant amounts of time in burrows, where they may inhale, ingest, or absorb uranium and other contaminants while digging, eating, preening, and hibernating. As such, existing toxicity effect thresholds developed using common laboratory test organisms may have limited applicability given the unique physiology and behaviors of species inhabiting the Grand Canyon region. Therefore, laboratory studies with native species or appropriate surrogates are warranted to reduce uncertainty associated with the biological effects of exposure.

  • Quantify the risk to native flora and fauna caused by chemical or radiation releases linked to uranium mining ( Task 15 )
  • Develop radiation and chemical effect thresholds for uranium for biota inhabiting mining areas of the Grand Canyon region ( Task 16a , Task 16b )
  • Characterize the risks to biota chronically exposed to uranium such as genetic effects on species populations, density, ecosystem dynamics, and biodiversity ( Task 16c )
  • Evaluate the effects (synergistic, additive, antagonistic) of chemical exposure to uranium and other metals, semi-metals, and other toxic compounds common in breccia pipes including arsenic, selenium, copper, nickel, lead, and zinc ( Task 17 )
  • Evaluate the risk of radiation exposure for trust resources that spend prolonged periods of time (e.g. hibernation, avoiding heat of the day) in the subterranean environment ( Task 18 )
  • Determine the sensitivity of native wildlife receptors to chemical and radiation exposure encountered in the field. Because of the lack of radionuclide toxicity data for most biological receptors, controlled chronic laboratory exposure to environmentally relevant concentrations of uranium and radiation will be conducted on native biota and common toxicity test organisms. Species selection will be based on results of toxicity threshold studies described previously ( Task 19 )