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R. Michael Turnipseed, Director Allen Biaggi, Administrator (775) 687-4670 TDD 687-4678
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STATE OF NEVADA KENNY C. GUINN Governor 
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Waste Management Corrective Actions Federal Facilities Facsimile 687-6396
Air Quality |
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Division of Environmental Protection 333 W. Nye Lane, Room 138 Carson City, Nevada 89706-0851 |
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Ms. Runore C. Wycoff Director, Environmental Restoration Division U. S. Department of Energy Nevada Operations Office P. O. Box 98518 Las Vegas, NV 89193-8518 RE: Review of Interim Work Products Related to the Underground Test Area (UGTA) Source Term Evaluations On Pahute Mesa, Nevada Test Site (NTS) Dear Ms. Wycoff: The Nevada Division of Environmental Protection (NDEP) has reviewed the following two Interim Work Products related to the source term evaluations on Pahute Mesa:
The reports serve as Interim Work Products for the Pahute Mesa (CAU 101 and 102) Corrective Action Investigation. The following are NDEP's comments and concerns with regard to these documents and the hydrologic source term phase of the investigation. 1) Both reports present examples of how to construct complex models with limited field data. All through the Tybo/Benham report similar phrases appear such as: "...data regarding ... does not exist..." "... no hydraulic data from..." , "... confidence in the uniqueness...is not great...", etc. Limited data availability is also mentioned in the Cheshire report. NDEP remains concerned that data-limited modeling efforts will result in unsatisfactory predictions. 2) The modeling approach of involving site-scale models nested inside a larger sub-regional model appears to provide an effective strategy when data for boundary conditions are limited. This approach was used in the Tybo/Benham work. The Cheshire work involved the use of what were initially considered competing modeling techniques (incremental streamline and particle simulations), but in retrospect evolved into complimentary numerical methods. Understanding the processes at work in the system through the innovative and flexible use of models (utilized as tools) is an approach that NDEP encourages NNSA/NV to continue. NDEP also encourages NNSA/NV to consider applying this approach of multiple nested models to other modeling work in the UGTA program. 3) The release mechanisms considered in both studies appear to be modeled using very conservative approaches. The methods will automatically indicate that migration is occurring, whether or not migration is actually taking place. The melt glass equations assume that radionuclide release will occur. NDEP questions if this is representative of real world conditions and is concerned that an overly conservative approach may not always be justifiable when all the ramifications of that approach are considered. For example, throughout the modeling process there are many points at which a decision is made which affects how conservative of an approach is taken. The decision may involve selecting a conservative array of values for a parameter versus more realistic, non-conservative values. In most cases, the more conservative path is deemed justifiable and is subsequently chosen. A multiplicative effect may begin to skew the simulations, which results in conservative predictions of contaminant transport. Future regulatory decisions, such as the placement of boundaries and monitoring wells, will be made on the basis of model predictions. Gross over prediction or under prediction of the contaminant boundary are both unacceptable. Highly conservative predictions may lead to the establishment of incorrect contaminant boundaries and subsequent validation monitoring wells being placed far down gradient of areas affected by contamination. NDEP intends to see that in each CAU some down gradient monitoring wells are placed in an area where contamination should be encountered. If CAU investigations consistently place monitoring wells where no contamination is found, additional wells may be required. This may in turn lead to a reexamination of the entire characterization strategy. The Monte Carlo simulation strategy attempts to include all reasonable parameter ranges. However, conservative assumptions made early in the development of the model may overshadow the representativeness of the parameter ranges that are selected. NDEP needs to be assured that these models produce realistic predictions of contaminant migration. Conservative predictions for the contaminant boundary must be balanced with possible non-conservative scenarios. Decisions relating to regulatory boundary selection, "proof of concept" monitoring, and long-term sentinel well site selection, must be based on realistic predictions of radionuclide migration. 4) One finding of significance involves recognition of the utility of fully coupled thermal-hydrologic-chemical models for the UGTA Project. This specialized and highly involved modeling approach may be necessary to defensibly simulate significant influences on flow in all CAUs. The simulation of thermal effects over a wide area, generated from individual shots through time, would be a very complex undertaking. However, this work has shown that non-isothermal flow appears to be a real phenomenon, and its effect on radionuclide transport is significant. 5) The Tybo/Benham report references the Drellack & Prothro, '97 model quite extensively. NDEP does not believe this work was ever formalized into a report and it is not referenced in the Pahute Mesa CAIP. The Drellack & Prothro,'97 model is reportedly the most complete hydrogeologic model put together at that time for Pahute Mesa, but was not submitted to NDEP. The Tybo/Benham work is grounded largely on the hydrogeologic interpretations in this model. Confidence in the Drellack & Prothro,'97 model has significant influence over NDEP's acceptance of this work. NDEP is concerned that new interpretations developed in more recent studies on the hydrogeologic framework would cause the Tybo/Benham work to be reassessed or updated. 6) A major element that is missing from both documents is a discussion on how quality assurance is being handled. NDEP recognizes that quality control may be ongoing; however, this should be addressed in the documents. For example, both documents move directly from describing the objective and purpose of the investigation to discussions and analysis of the modeling. Missing was a discussion which detailed how and why certain modeling codes were selected and how they did not meet the needs of the investigators. 7) The current pre-decisional drafts only discuss in general terms the level of confidence that these evaluations provide. The final versions of these documents must clearly state at what level of confidence these evaluations are presented. It is understood that quantification of uncertainty may be difficult to assess due to the nature of the work. But it is this quantification that may ultimately prove the value and usefulness of the work. 8) What course will NNSA/NV now pursue as a result of these two studies? How will these studies be incorporated into the overall Pahute Mesa Hydrologic Source Term definition? 9) Finally, these efforts are evaluating complicated processes for release and transport of radionuclides. These evaluations, when completed, will provide plausible simulations of transport potential. As presented these documents may leave an impression that other alternative failure or migration mechanisms are not possible. The conclusions in both documents need to address the possibility of other plausible interpretations. NDEP finds both of these interim work product submittals, with the above noted qualifications, acceptable and beneficial to the UGTA corrective action investigations. As stated earlier, the integration of this research into the Pahute Mesa Corrective Action Investigation and CAU-scale model development process will be of particular interest in the coming months. If you have questions on this matter, please contact Eric Noack at (775)-687-4670 ext. 3032, or me at (775) 687-4670 ext. 3039. PJL/KKB/REN/SJ/CJG/js cc: Patti Hall, NNSA/NV-ERD Ken Hoar, NNSA/NV-ERD Wayne Griffin, BN/DTRA Frank Di Sanza, NNSA/NV-WMD Robert Bangerter, NNSA/NV-ERD Les Winfield, NNSA/NV-ERD |
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