Sanders, and three co-authors, reported in 2016 that sandstone-hosted roll-front uranium ore deposits originate when U(VI) dissolved in groundwater is reduced and precipitated as insoluble U(IV) minerals. Groundwater redox geochemistry, aqueous complexation, and solute migration are important in leaching uranium from source rocks and transporting it in low concentrations to a chemical redox interface where it is deposited in an ore zone typically containing the uranium minerals uraninite, pitchblende, and/or coffinite; various iron sulfides; native selenium; clays; and calcite. In situ recovery (ISR) of uranium ores is a process of contacting the uranium-mineral deposit with leaching and oxidizing (lixiviant) fluids via injection of the lixiviant into wells drilled into the subsurface aquifer that hosts uranium ore, while other extraction wells pump the dissolved uranium after dissolution of the uranium minerals.
Environmental concerns during and after ISR include water quality degradation from:
1) potential excursions of leaching solutions away from the injection zone into down-gradient, underlying, or overlying aquifers;
2) potential migration of uranium and its decay products (e.g., Ra, Rn, Pb); and,
3) potential mobilization and migration of redox-sensitive trace metals (e.g., Fe, Mn, Mo, Se, V), metalloids (e.g., As), and anions (e.g., sulfate).
This review describes the geochemical processes that control roll-front uranium transport and fate in groundwater systems, identifies potential aquifer vulnerabilities to ISR operations, identifies data gaps in mitigating these vulnerabilities, and discusses the hydrogeological characterization involved in developing a monitoring program.
Environmental concerns during and after ISR include potential migration of leaching solutions away from the injection zone into down-dip, underlying, or overlying aquifers. Uranium, its decay products, and a number of redox-sensitive trace metals (e.g., Fe, Mn, Mo, Se, V), metalloids (As), and anions (sulfate) of environmental concern may migrate in the groundwater.
Monitoring wells are located up- and down-gradient of the production area to determine if contaminant migration occurs. Monitoring strategies could include installation of monitoring wells down-gradient of the boundary of the ISR site; installation of monitoring wells in aquifers above and below the ore body and ISR zone; multi-level or nested monitoring wells; capture zone evaluation of nearby groundwater supply pumping wells; and installation of up-gradient wells outside the boundary of the capture zone, as well as within it; and proper sampling techniques for the potential contaminants related to uranium ISR sites, to ensure that the contaminants and geochemical parameters are representative of the aquifer.
Potential aquifer vulnerabilities to in situ recovery operations include undetected or mis-characterized faults that may result in contaminant migration along the faults; the presence of numerous sandstone, siltstone, and mudstone lenses that can influence the distribution of lixiviant as well as uranium- and ISR-related contaminants; undetected preferential flow paths that could lead to contaminant migration in unexpected directions; the numerous boreholes in an ISR well field that may allow vertical flow along a borehole into overlying or underlying permeable units; and the presence of nearby groundwater supply pumping wells that may lead to capture zones that intersect the exempt ISR area or areas having down-gradient migration of ISR-related contaminants.[Subtle Bias Note: No ISL operation in the U.S. has ever experienced an excursion of lixiviant fluids or other types of fluids impacting drinking water in use. The above discusses a problem that does not exist, becasue of the overwhelming reducing conditions that prevail away from ISL operations. This precipitates all known metals of interest. However, to meet landowner concerns, current regulations require monitoring wells be installed and sampled to insure that no excursions can occur.]
Data gaps in an existing conceptual site model may include any of these potential vulnerabilities [aka vulnerabilities that have have never been reported or occurred]. Poorly understood geochemistry in the areas down gradient of an ISR site may lead to inaccurate estimates of contaminant concentrations and velocities. Research related to remediation of uranium-contaminated groundwater takes advantage of biotic (sulfate-reducing bacteria) and abiotic processes (reductive precipitation and co-precipitation) to reduce uranium concentrations to low levels, and the literature indicates that in situ remediation has been successfully demonstrated in the field in some cases for metal(loid)-contaminated groundwater (including uranium) using both biotic and abiotic pathways [But none have ever been reported in the U.S.] MNA assessments should include:
i) demonstration of contaminant sequestration mechanisms;
ii) estimates of attenuation rates;
iii) estimates of the attenuation capacity of aquifer solids; and,
iv) evaluation of potential reversibility issues.
Suggestions for future work to better understand the subsurface geochemistry related to uranium roll-front deposits, and to develop effective post-ISR monitoring programs, include:
- Conducting modern multi-element analyses of samples from traverses through roll-front ores using ICP-OES and ICP-MS techniques, microscopy, and new detailed spectroscopic studies of aquifer minerals and their trace metal associations. This will allow for an improved understanding of potential
contaminant mobilization and attenuation processes. - Drilling of cores post-leaching and post-aquifer restoration for comparison to pre-mining cores to better understand leach-ore gangue mineral reactions, particularly reactions involving groundwater and potential contaminants of concern.
- Evaluation of in situ remediation techniques to improve postmining aquifer restoration efforts.
Original URL: https://www.i2mconsultants.com/downloads/corp/Potential_aquifer_vulnerability_in_regio.pdf
Managed & Built by I2M Consulting, LLC | © 2010-
