Aquifer restoration after ISL mining includes injection of hydrogen sulfide gas into groundwater
Posted February 6, 2008
According to a January 2007 report from the U.S. Nuclear Regulatory Commission, recent aquifer restoration efforts at in situ leach uranium mines in Wyoming and Nebraska have included the use of hydrogen sulfide gas. The highly toxic gas is injected into the groundwater aquifer in an effort to create chemically-reducing conditions and thus cause the uranium and other heavy metals mobilized by the ISL process to precipitate out of solution. The reducing effect is not permanent, as the NRC report explains:
The data from Schmidt (1989) suggest that the mined zone remained oxic during the first year of groundwater restoration that included groundwater sweep and reverse osmosis treatment. Only the injection of hydrogen sulfide into the system at the end of one year of treatment (at 0.5 g/liter and total of 113 kg per well) returned the mined zone of the aquifer to reducing conditions (Schmidt, 1989). The hydrogen sulfide was added to the RO (reverse osmosis) permeate during the last two months of the RO treatment phase. Several other ISL facilities have also indicated that hydrogen sulfide gas injection is needed as a part of the groundwater restoration process (Rio Algom, 2001; Crow Butte Resources, 2000; Altair Resources, 1988; and USNRC, 1997).
The effects of the hydrogen sulfide injection into the aquifer were significant at the Ruth ISL for several months (Schmidt, 1989). Hydrogen sulfide gas was injected for six weeks at an average concentration of 500 mg/liter. At the end of the hydrogen sulfide gas injection, the pH in the aquifer had dropped from 8.6 to 6.3 (Fig. 11), sulfate concentrations had risen from 28 mg/liter to 91 mg/liter, and the dissolved uranium, selenium, arsenic, and vanadium concentrations decreased markedly (one order of magnitude or more).
After the hydrogen sulfide injection was completed, the recirculation of groundwater/RO permeate was ceased and the aquifer was allowed to stabilize, with monthly groundwater sampling conducted for one year (Schmidt, 1989). The sampling results during the groundwater stabilization period suggest that the reducing conditions may have not been maintained for the entire year. Dissolved iron and manganese concentrations increased during the first 5 months and then abruptly began to decline. As this abrupt decline began, dissolved uranium, arsenic, and radium began to increase. Vanadium concentrations declined; selenium concentrations were not given. Elevated uranium and vanadium (Table 6) were still observed after groundwater restoration was completed.
Elevated concentrations of iron and manganese were also noted in the postrestoration groundwater sampling at the Highland A-wellfield and Crow Butte Mine Unit 1 (Tables 3 and 5). However, these elements do not usually pose major water quality issues at their post-restoration concentrations and generally indicate that reducing conditions may be present, which can be advantageous, as described above. More problematic are the elevated concentrations (above baseline) of arsenic, selenium, radium, uranium (Highland A wellfield, Table 3) and of molybdenum, radium, uranium, and vanadium (Crow Butte Mine Unit 1, Tables 4 and 5) after extensive groundwater restoration activities. The long-term trends in the concentrations of these elements are important in establishing whether the groundwater restoration activities have been adequate to ensure the stability of the aquifer water quality and the class of use required by regulatory authorities. The industry experience at the Highland A-wellfield (PRI, 2004) indicates that a long period (5 years) for the groundwater stabilization phase may sometimes be needed and that long-term monitoring (13 years) may be required to ensure that the concentrations of uranium, arsenic, selenium, and radium have stabilized at satisfactory levels. One reason for this may be that a rebound in concentrations is observed during groundwater recirculation, or after is completed, due to mixing and diffusion of water from lower permeability zones into regions with higher permeability (PRI, 2004, pg. 7). However, another reason a rebound concentrations could occur is if the system becomes increasingly oxidized over time, as will be demonstrated with modeling in the following section.
Injection of hydrogen sulfide gas into an aquifer results in a temporary decrease in groundwater concentrations of uranium and other heavy metals, but levels may rise after injection ceases.
The Material Safety Data Sheet for hydrogen sulfide describes the human health risks of exposure to various concentrations. Groundwater concentrations would likely be relatively low.
Hydrogen sulfide reacts with enzymes in the bloodstream and inhibits cellular respiration resulting in pulmonary paralysis, sudden collapse and death. Continuous exposure to low (15-50 ppm) concentrations will generally cause irritation to mucous membranes, and may also cause headache, dizziness or nausea. Higher concentrations (200-300 ppm) may result in respiratory arrest leading to coma or unconsciousness. Exposures for more than 30 minutes at concentrations greater than 700 ppm have been fatal.
Low concentrations will generally cause irritation to the conjunctiva. Repeated exposure to low concentrations is reported to cause conjunctivitis, photo phobia, corneal bullae, tearing, pain and blurred vision.
Consideration of Geochemical Issues in Groundwater Restoration at Uranium In-Situ Leach Mining Facilities (NUREG/CR-6870) - U.S. Nuclear Regulatory Commission and U.S. Geological Survey - January 2007 (pdf)
Technologically Enhanced Naturally Occurring Radioactive Materials From Uranium Mining - Appendix III. Occupational and Public Risks Associated with In-Situ Leaching