Sensitivity analysis of uranium speciation modeling in groundwater systems with a focus on mobility.

摘要

This research is designed to show the relative effects of inorganic chemical perturbations, as well as the effects of humic/fulvic acids, mineral and bacterial sorption and temperature (to a limited extent), with a focus on uranium solubility, by performing a sensitivity analysis using Geochemist's Workbench, a commercially available chemical equilibrium software package.;Four groundwater systems of interest were selected upon which to perform the sensitivity analysis: the Yucca Mountain (YM) J-13 well water system (Harrar, et al., 1990), the mean well water system from the Simpsonville, SC (SSC) area (Woodruff, 2002), the Savannah River National Laboratory (SRNL) F-Area groundwater system, and Idaho National Engineering and Environmental Laboratory (INEEL) Snake River aquifer system (Ayaz, et al., 2000). The results of the sensitivity analysis are presented in terms of metrics called the uranium ratio (UR) and the sensitivity index (SIUR), defined as follows: UR=Uaqmol Utotmol and SIUR=URmin -URmaxURbaseline Where: URmin=UR at the minimum constraint concentration evaluated URmax=UR at the maximum constraint concentration evaluated URbaseline=UR at the baseline for the system The discussion of the results of the sensitivity analysis focuses on the relative effect varying a given constraint has on the system, defined as follows: significant or major effects (SIUR ≥ 1), moderate effects (0.1 ≤ SIUR 1), minor effects (0.01 ≤ SIUR 0.1%), no apparent effects (SIUR 0.01%).;The sensitivity analysis of the YM solubility controlled system indicated that aqueous carbonate concentrations dictated UR to a large extent. Phosphate and strontium, both of which were not included in the basis, were indicated to be of potential significance to uranium solubility in this system. Temperature also seems to have a strong role in uranium solubility in this system. Interestingly, pH seemed to have little effect on the UR. The presence, concentration and composition of the organic acid simulant appeared to be of little concern.;The solubility controlled SSC system precipitated Soddyite and Quartz in the baseline case at equilibrium indicating oversaturation of silicon and uranium in the natural groundwater. Similarly to the YM system, carbonate seemed to dominate the uranium solubility of the system, but silicon and temperature also had significant effects. The presence, concentration and composition of organic acids in this system could effect aqueous uranium concentrations by as much as 20--30%.;The SRNL system was modeled under both solubility and sorption controlled assumptions. The solubility controlled SRNL system was impressively unresponsive to physical, inorganic and organic constraint variations across the tested range, giving strong evidence that this system is indeed sorption controlled. Both pH and total uranium concentration exerted strong effects in the sorption controlled system, illustrating the importance of availability of sorption sites relative to the amount of uranium in solution. Other constraints, surprisingly including carbonate, had relatively little effect. The presence, concentration and composition of organic acids had little effect. Bacterial sorption was shown to have a significant potential to affect the aqueous uranium concentration in this system, especially as the total uranium in the system was reduced.;The sorption controlled INEEL system showed much more sensitivity than did the SRNL system, and seemed to be significantly (> 10%) effected by the variation of HCO3-, pH, pe, Na+, Ca2+, SO42-, Cl-, and Mg2+ in both the mineral-only and mineral-bacterial cases. The UR in this system was not shown to be sensitive to the presence, concentration or composition of organic acids. The mineral-only and mineral-bacterial sorption controlled systems behaved nearly identically. (Abstract shortened by UMI.)