January 25, 2009 Follow-up questions to issues first raised in EPA's May 21, 2009 letter,

comment 1-23-6

1. There seems to be a disconnect between the previous (CRA-2004 PABC) FMT modeling results and the experimental investig ation of Nd(III) solubilities. Borkowski et al. (2008) assumed that the stable Nd(III) phase in the carbonate system would be NdOHCO3(s). In the CRA-2004 PABC modeling results, the stable +III phase was predicted to be Am(OH)3(s). I have looke d at a few of the CRA-2009 PABC FMT output files that you sent to me, and it appears that the stable +III phase predicted by the FMT model continues to Am(OH)3(s). In Figure 4- 8 of Borkowski et al (GWB brine with carbonates), it appears that they were able to get fairly good reproducibility of results from undersaturation (dissolution of NdOH CO3) and oversaturation (precipitation) experiments. There appears to have beenlimited to no characterization of the postexperiment solid phases. Although precisely wh at was done to characterize the solids after the experiments was unclear to me after reading the test plan and Borkowski et al. 2008 report, Marion Borkowski stated in our co nference call this week that there were

insufficient amounts of solids to characteri ze after the oversaturation (precipitation) experiments. As a result, the identity of the solid phases at the end of the experiments is not certain. The data in Figure 4-8 and the in terpretation by Borkowski et al 2008 seem to indicate that the stable phase was NdOHC O3. Consequently, FMT calculations predict that Am(III) solubility for WIPP PA is c ontrolled by Am(OH)3(s) but the experiments seem to indicate the solubility-controlli ng phase will be AmOHCO3. This raises the question of whether the FMT modeling is accura tely predicting the concentrations of Am(III) (and by analogy Pu(III)) in the WIPP brines for PA.

2. The observed differences in the concentr ations at similar pcH in GWB and ERDA-6 brine are attributed to borate complexati on. GWB has approximately 2.5 times the borate concentration of ERDA-6. The neodymium c oncentrations measured at pcH 8.7 in ERDA-6 brine (no carbonate) is approximately 5 x 10-8 M (Figure 4-10). In GWB brine at pcH 8.7 (Figure 4-9) is approximately 5 x10-7 M. What is the neodymium-borate speciation that would explain a ten-fold increase in neodymium concentrations with a

2.5-fold increase in borate concentration? If it is a one-to-one complex such as NdB(OH)4++, then one would e xpect that increasing the borate concentration by a factor of2.5 would increase the Nd(III) concentra tion by approximately the same factor.

3. The "shoulder" in the Nd concentration in ERDA-6 brine as a function of increasing pcH was attributed by Borkowski et al. 2008 to borate complexation. However, the reason for the decreasing Nd complexation as a function of pcH above about 9.75 was not

clear. There is no reason thatI know of for borate complexa tion to cease at higher pcH, and ifhydrolysis species such as Nd(OH)3(aq ) became more important at high pcH, these concentrations would be presen t in addition to the Nd-borate species concentrations. It is unclear whether a different solid might be controlling the Nd(III) so lubilities in the highpcH region. An understanding of the solubility in this pcH region is important because

in ERDA-6 the maximum concentration is obse rved at approximately the repository pcH predicted by FMT calculations (9.6-9.7). [Note: Clarificati on on this issue was requestedin a phone call with LANL staff, but it was not resolved.]

4. If the trend in Nd concentrations in GWB (Figure 4-8) is simply extrapolated up to pcH 9.4 (the pcH predicted by FMT modeling fo r GWB brine in the re pository), the Nd concentration could be as high as 4 x 10-5 M, which is much higher than the concentrations predicted for the CRA-2004 PA BC (GWB no ligands) of 2 x 10-7 M. In our discussion, LANL staff said that the so lubility at the pcH of 9.4 would be best represented by the solubility of Nd(III) in ERDA-6, because as pcH is increased, Mg precipitates and the compositi on of GWB approaches that of ERDA-6. However, if one compares the predicted Mg concentrati on of GWB at pcH 9.4 based on FMT modeling for the CRA-2004 PABC to the composition of ERDA-6, the Mg concentration in the higher-pcH GWB is still 0.55 M, which is 29 times the Mg concentration in ERDA-6. Even if one accepts that the solubility in ERDA-6 is a perfect way to estimate the concentration of Nd in pcH 9.4 GWB (1 x 10- 6 M or higher), then the concentration is still, at a minimum, 5 times what was pred icted by FMT (2 x 10-7 M). There is a lot of uncertainty in such a prediction, becaus e the higher-pcH GWB is not the same composition as ERDA-6 and the results with carbonate in ERDA-6 (Figure 4-11) were only approached from undersaturation in this pcH region. Consequently, the concentration of Nd(III) in pcH 9.4 GW B could be higher than 10-6 M.

5. When discussing possible effects of increas ed +III actinide solubilities on PA during our call, a LANL staff member said that he thought the increased lig ands concentrations predicted for the CRA-2009 PABC FMT calcula tions might have increased the +III

actinide solubili ties to the extent that americium was inventory limited. Although he did not think Pu would be inventory-limited, he suggested that high ED TA concentrations might result in sufficient EDTA complexati on that the effects of borate would be relatively unimportant. I have only looked at what I believe are the ERDA-6 results for the CRA-2009 PABC calculations. This output file (FMT_PABC09_E6_HMAG_ORGS_013.OUT) is for ERDA-6 brine, with organics, and was run on July 8 2009. The total concentra tion of Am(III) was pred icted to be 1.51 x 10- 6 M at pcH 9.7. This concentration does not dw arf the concentration observed in Figure 4-11 (about 1 x 10-6 M), so it does not a ppear that EDTA complexation of the +III actinides will be so high that the potential effects of borat e complexation described bythe results of Borkowski et al. 2008 can be neglected.

6. Figure 4-9 presents Nd(III) solubility data in GWB as a function of pcH in the absence of carbonate. At pcH values of approximately 7.3 and 8.0, the concentrations measured in experiments approached from undersaturati on are higher by approximately an order of magnitude than the concentrations m easured in experiments approached fromoversaturation. Does DOE have an explanation for this?

7. DOE's response to 1-23-6c dated Novemb er 25, 2009 statement that "For the III actinides, the complexation of borate effectivel y substitutes for the calculated effect of carbonate presently in the WIPP model." Is inconsistent with the modeling results from the CRA-2004 PABC. These results, both with a nd without ligands present in the brine, showed that aqueous americium-carbonate co mplexes constituted negligible proportionsof the total americium in solution in all four solution compositi ons (GWB and ERDA-6 with and without ligands). In addition, because there is an assumed excess of Am(OH)3(s) in the WIPP repository, "com petition" between borate and carbonate for americium in the system will not occur. Americium-borate aqueous complexes, if formed, will form in addition to americium-carbonate complexes. However, under WIPP conditions, current FMT modeling does not pr edict significant carbonate complexation of

the +III actinides.

8. DOE should perform FMT calculations to pr edict the concentrations of Nd(III) under the conditions of the Nd(III) solubility experiments in the NaCl, GWB, and ERDA-6 brines. These calculations would provide direct evidence of the extent of the differences between the experimental results and the FMT calculated solubilities, which will be useful even though experimental data are unavailable at higher pcH in GWB. For the carbonate experiments, it may be useful to perform calculatio ns using Am(OH)3(s) as the solubility-controlling solid as well as calculations using AmOHCO3(s) as the solubilty controlling solid, to indicate potential solution composition ch anges that might be caused by a change in the stable phase or mixed solid phase formation.

9. Another question is related to the carbonate concentrations in the experiments that they conducted. I understand that they spiked an initial carbonate con centration into the solutions and used "gas-ti ght polypropylene bottles" for the experiments. Did they

measure carbonate concentrations in solution at the end of any of the experiments to

confirm that there was no loss of CO2? It is my understanding that all of the experiments were performed in a CO2-free glove box, so ga in of CO2 should not have occurred. Do they assume that the amounts of NdOHCO3 pr ecipitated in their experiments were so

small that it did not affect the total carbonate in the solutions?