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Reassessment RI/FS Baseline Modeling Report Peer Review 3
Charge for Peer Review 3
This is the third in a series of four peer reviews being conducted on scientific work products prepared for the Reassessment Remedial Investigation and Feasibility Study (Reassessment) for the Hudson River PCBs site. Previous peer reviews were conducted on the modeling approach and the Data Evaluation and Interpretation Report and Low Resolution Sediment Coring Report. Subsequent to this peer review the Human Health and Ecological Risk Assessments will be peer reviewed.
Members of this peer review are asked to determine whether the baseline modeling effort presented in the Revised Baseline Modeling Report (Revised BMR) is credible and whether the conclusions of the Revised BMR are valid. The reviewers are asked to determine whether the modeling work is technically adequate, competently performed, properly documented, satisfies established quality requirements, and yields scientifically credible conclusions. The peer reviewers are not being asked whether they would have conducted the work in a similar manner. In addition, the reviewers are asked to determine whether the models and the associated findings are appropriate to help answer the following three principal study questions that EPA will consider in its decision-making process for the site:
1. When will PCB levels in fish meet human health and ecological risk criteria under continued No Action? (1)
2. Can remedies other than No Action significantly shorten the time required to achieve acceptable risk levels? (2)
3. Could a flood scour sediments, exposing and redistributing buried contamination?
The following documents will be provided to the peer reviewers:
Primary
Revised Baseline Modeling Report (Jan. 2000)
Responsiveness Summary to the Baseline Modeling Report (Jan. 2000)
Reference
Baseline Modeling Report (May 1999)
QEA/GE - PCBs in the Upper Hudson River (May 1999, amended July 1999)
Suggested charge questions from the public (Dec. 1999)
Hudson River Reassessment Database (August 1998)
Executive Summaries for other EPA Reassessment Reports
Peer Review Reports from first two peer reviews
The peer reviewers should base their assessments primarily on the Revised BMR, and on EPA's Responsiveness Summary for the Baseline Modeling Report, in which EPA responded to significant public comments received by the Agency on the May 1999 Baseline Modeling Report. These two documents are currently in preparation, and will be issued to the peer reviewers by the end of January 2000. The reference documents listed above are being provided to the reviewers as background information, and may be read at the discretion of the reviewers, as time allows, although the reviewers are not being asked to conduct a review of any of the background information. It should be noted that the Revised BMR to be issued in January 2000 will supercede the May 1999 Baseline Modeling Report.
For additional background information, please visit USEPA's web site on the Hudson River PCBs site, www.epa.gov/hudson.
Specific Questions
Fate and Transport (HUDTOX)
1. The HUDTOX model links components describing the mass balance of water, sediment, and PCBs in the Upper Hudson. Are the process representations of these three components compatible with one another, and appropriate and sufficient to help address the principal study questions?
2. The HUDTOX representation of the solids mass balance is derived from several sources, including long-term monitoring of tributary solids loads, short-term solids studies and the results of GE/QEA's SEDZL model. The finding of the solids balance for the Thompson Island Pool is that this reach is net depositional from 1977 to 1997. This finding has also been assumed to apply to the reaches below the Thompson Island Dam. Is this assumption reasonable? Are the burial rates utilized appropriate and supported by the data? Is the solids balance for the Upper Hudson sufficiently constrained for the purposes of the Reassessment?
3. HUDTOX represents the Upper Hudson River by segments of approximately 1000 meters in length in the Thompson Island Pool, and by segments averaging over 4000 meters (ranging from 1087 to 6597 meters) below the Thompson Island Dam. Is this spatial resolution appropriate given the available data? How does the spatial resolution of the model affect the quality of model predictions?
4. Is the model calibration adequate? Does the model do a reasonable job in reproducing the data during the hindcast (calibration) runs? Are the calibration targets appropriate for the purposes of the study?
5. HUDTOX employs an empirical sediment/water transfer coefficient to account for PCBs loads that are otherwise not addressed by any of the mechanisms in the model. Is the approach taken reasonable for model calibration? Comment on how this affects the uncertainty of forecast simulations, given that almost half of the PCB load to the water column may be attributable to this empirical coefficient.
6. Are there factors not explicitly accounted for (e.g., bank erosion, scour by ice or other debris, temperature gradients between the water column and sediments, etc.) that have the potential to change conclusions drawn from the models?
7. Using the model in a forecast mode requires a number of assumptions regarding future flows, sediment loads, and upstream boundary concentrations of PCBs. Are the assumptions for the forecast reasonable? Is the construct of the hydrograph for forecast predictions reasonable? Should such a hydrograph include larger events?
8. The 70-year model forecasts show substantial increases in PCB concentrations in surface sediments (top 4 cm) after several decades at some locations. These in turn lead to temporary increases in water-column PCB concentrations. The increases are due to relatively small amounts of predicted annual scour in specific model segments, and it is believed that these represent a real potential for scour to uncover peak PCB concentrations that are located from 4 to 10 cm below the initial sediment-water interface. Is this a reasonable conclusion in a system that is considered net depositional? After observing these results, the magnitude of the increases was reduced by using the 1991 GE sediment data for initial conditions for forecast runs. Is this appropriate? How do the peaks affect the ability of the models to help answer the Reassessment study questions?
9. The timing of the long-term model response is dependent upon the rate of net deposition in cohesive and non-cohesive sediments, the rate and depth of vertical mixing in the cohesive and non-cohesive sediments and the empirical sediment-water exchange rate coefficient. Are these rates and coefficients sufficiently constrained for the purposes of the Reassessment?
10. The HUDTOX model uses three-phase equilibrium partitioning to describe the environmental behavior of PCBs. Is this representation appropriate? (Note that in a previous peer review on the Data Evaluation and Interpretation Report and the Low Resolution Sediment Coring Report, the panel found that the data are insufficient to adequately estimate three-phase partition coefficients.)
11. HUDTOX considers the Thompson Island Pool to be net depositional, which suggests that burial would sequester PCBs in the sediment. However, the geochemical investigations in the Low Resolution Sediment Coring Report (LRC) found that there was redistribution of PCBs out of the most highly contaminated areas (PCB inventories generally greater than 10 g/m2) in the Thompson Island Pool. Comment on whether these results suggest an inherent conflict between the modeling and the LRC conclusions, or whether the differences are attributable to the respective spatial scales of the two analyses.
12. The model forecasts that a 100-year flood event will not have a major impact on the long-term trends in PCB exposure concentrations in the Upper Hudson. Is this conclusion adequately supported by the modeling?
Bioaccumulation Models
1. Does the FISHRAND model capture important processes to reasonably predict long term trends in fish body burdens in response to changes in sediment and water exposure concentrations? Are the assumptions of input distributions incorporated in the FISHRAND model reasonable? Are the spatial and temporal scales adequate to help address the principal study questions?
2. Was the FISHRAND calibration procedure appropriately conducted? Are the calibration targets appropriate to the purposes of the study?
3. In addition to providing results for FISHRAND, the Revised BMR provides results for two simpler analyses of bioaccumulation (a bivariate BAF model and an empirical probabilistic food chain model). Do the results of these models support or conflict with the FISHRAND results? Would any discrepancies among the three models suggest that there may be potential problems with the FISHRAND results, or inversely, that the more mechanistic model is taking into account variables that the empirical models do not?
4. Sediment exposure was estimated assuming that fish spend 75% of the time exposed to cohesive sediment and 25% to non-cohesive sediment for the duration of the hindcasting period. The FISHRAND model was calibrated by optimizing three key parameters and assuming the sediment and water exposure concentrations as given, rather than calibrating the model on the basis of what sediment averaging would have been required to optimize the fit between predicted and observed. Is the estimate of sediment exposures reasonable?
5. The FISHRAND model focuses on the fish populations of interest (e.g., adult largemouth bass, juvenile pumpkinseed, etc.) which encompass several age-classes but for which key assumptions are the same (e.g., all largemouth bass above a certain age will display the same foraging behavior). This was done primarily because it reflects the fish data available for the site. Is this a reasonable approach?
General Questions
1. What is the level of temporal accuracy that can be achieved by the models in predicting the time required for average tissue concentrations in a given species and river reach to recover to a specified value?
2. How well have the uncertainties in the models been addressed? How important are the model uncertainties to the ability of the models to help answer the principal study questions? How important are the model uncertainties to the use of model outputs as inputs to the human health and ecological risk assessments?
3. It is easy to get caught up with modeling details and miss the overall message of the models. Do you believe that the report appropriately captures the "big picture" from the information synthesized and generated by the models?
4. Please provide any other comments or concerns with the Revised Baseline Modeling Report not covered by the charge questions, above.
Recommendations
Based on your review of the information provided, please identify and submit an explanation of your overall recommendation for each (separately) the fate and transport and bioaccumulation models.
1. Acceptable as is
2. Acceptable with minor revision (as indicated)
3. Acceptable with major revision (as outlined)
4. Not acceptable (under any circumstance)
(1) Appropriate levels to meet human health and ecological risk criteria will be evaluated in the upcoming Feasibility Study.
(2) The Revised BMR represents a baseline modeling effort, and therefore does not include an evaluation of potential remedial scenarios. The modeling work presented in the Revised BMR will be used to develop potential remedial options in the Feasibility Study for the Reassessment.