EVALUATION OF THE EFFECTS OF DECARBONATION, REDUCTANT ADDITION AND LIME ADDITION ON GROUNDWATER RESTORATION AT URANIUM IN-SITU RECOVERY SITES IN WYOMING AND NEBRASKA
At the Wyoming site, recirculated groundwater has been treated by ion exchange and reverse osmosis followed by forced-air decarbonation prior to reinjection. Use of forced air decarbonation was compared to use of a vacuum system that did not add dissolved O2 to the injectate, and to no decarbonation before reinjection. Modeling results indicated that dissolved oxygen introduction during use of the forced air system could significantly increase uranium concentrations in the aquifer, although addition of a reductant to the injectate or the presence of reducing phases in the aquifer are likely to limit or reverse this effect.
Evaluation of the effects of using Na2S as a reductant compared to H2S at both the Nebraska and Wyoming sites indicated that use of Na2S is likely to be beneficial because of predicted increased pH and reduced carbonate concentrations caused by calcite precipitation in the aquifer. Because Na2S is generally safer and easier to handle than H2S, use of Na2S as a reductant during groundwater restoration may be preferable, provided the presence of swelling clays does not cause decreased aquifer permeability.
The results of calculations carried out to evaluate the effects of lime addition to injectate at the Nebraska site indicate that lime increases the pH and calcium concentration, promoting the precipitation of calcite and reducing carbonate concentrations. These effects led to lower predicted uranium concentrations in the groundwater.