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Peer Review of the Surface Impoundment Study Technical Plan for Human Health and Ecological Risk Assessment
General Comments and Recommendations by Ron SimsTechnical merits of the overall approach. The set of processes and the tiered approach proposed for screening the large number of surface impoundments in the U.S. is generally scientifically based, clearly defined, and based on reasonable assumptions. Addressing specific constituents, units, and pathways with regard to risks from surface impoundments is rigorous and comprehensive. The document is generally easy to read and understand, and the examples contribute strongly to the usefulness and application of the equations and information presented. The document is well organized, generally succinct, but sometimes editorially repetitive in the material before section 3.0 Phase II Risk Assessment.
Recommendations concerning the overall technical content and merit are made to increase the communication effectiveness of the materials, and are listed below.
1. Define or explain "Science Decision Rules." What is a "Decision Rule?" Are they benchmarks or standards? Do they relate to specific media? Are they "If... Then" statements?
2. Clarification is needed regarding material presented on pages 1-11 and 1-12. On page 1-11, section 1.2.2.1 it is stated that if the number of units of concern are small, the HWIR model will be used; conversely, if a large number of units are of concern, EPA will develop a number of "scenarios." On page 1-12, statements are made to the effect that for large numbers of units, the HWIR database will be used, and for a small number of units the 3MRA model will be used. There is an apparent confusion on the part of the reader that has more to do with communication than with an error in the procedures used. This should be clarified.
3. The identification of Management Goals, p. 2-39 and elsewhere, is very good to place within the context of risk assessment. Due to the limitations of resources and the often confusing implications of the nature of risk assessment, the statement of management goals gives meaning and application to the "process" of risk assessment.
4. The fact that no air concentration data were provided in the surveys is a real limitation for considering the air transport pathway. This is unfortunate. Air transport will be considered for VOCs. Caution is due with regard to the approach that "if you don't have the data, then model the situation."
5. p. 3-25. Provide a list of the specific constituents where no benchmarks are available for human health. As demonstrated in the report, ecological benchmarks vary depending on the target receptor.
6. Herptofauna, p. 2-39? Recommend using the word reptiles, as used in the other parts of the report, unless there is a specific reason for its use. And then there is the term "fisher" that's appropriate for everyone (reference p. 3-57).
7. Page 2-8, give reference for Exposure Factors Handbook, page bottom.
8. Page 2-47 QSARs missing words "structure activity relationships."
9. Page 3-7. Figure 3-3. I cannot find the number "1" in the Figure that is referenced and described in the footnote.
10. Page 3-42. Uncertainty Analysis. Reference is made to a methodology for HWIR analysis where there are no data for wildlife exposure factors, biological uptake factors, and/or toxicity benchmarks. How often is this methodology anticipated to be used? Because it is used for HWIR, how well does it apply to surface impoundment environments? How appropriate is the modification of HWIR for Surface Impoundments for ecological risk analysis? Explain this more clearly in the report.
11. Page 2-13. Time scale of 30 years appears reasonable for average exposure. Is this an assumption supported by census data?
12. Page 3-48/49. Figure 3-14. Leachate flow input parameter in clogged native soil layer. How is this determined? Survey data? Measurement? How determined?
13. Page 3-48/49. "Presumably aerobic biodegradation." Do data exist that indicate aerobic conditions in surface impoundments? Why is this assumed? In sediment compartment, is hydrolysis considered to be enhanced due to the presence of solid surfaces of the sediment? Hydrolysis is often enhanced in the presence of solid surfaces (Reference: Reactions and Movement of Organic Chemicals in Soils, Soil Science Society of America Publication Number 22, SSSA, Inc., Madison, WI, 1989., Chapter 4 "Adsorption and Reactions of Selected Organic Molecules on Clay Mineral Surface, and Chapter 9, Hydrolytic Transformations of Organic Chemicals in the Environment).
Also, acid or base catalyzed hydrolysis may play a dominant role in affecting the hydrolysis fate of organic chemicals in sediments and water in surface impoundments. Therefore, hydrolysis rates in surface impoundment environments may be very different in alkaline areas, for example the Northwest U.S. , versus more acidic areas in the Southeast U.S.
14. Page 3-48/49. "Mostly well mixed" would fit better for shallower surface impoundments and would fit more poorly for deeper surface impoundments.
Specific comments that address the areas that you identified:
1. Derivation of human health screening factors.
(i) methodology suitable for calculating screening factors. Human health risk screening factors are based on toxicity benchmarks for direct ingestion for drinking water, direct air inhalation, and direct soil ingestion at the surface impoundment (for the in-place closure scenario), and the methodology is a suitable one for initial screening using conservative assumptions of exposure to surface impoundments. The human receptor is essentially placed in the middle of the surface impoundment and the problem formulation with regard to risk is considered without dilution or attenuation. The methodology is suitable as those conditions presenting high risk will be more realistically evaluated at the next stage in the methodology.
(ii) other data sources readily available. This reviewer is not aware of other databases or data sources that are readily available that would provide credible benchmarks for the limited number of cases where there are no benchmarks available.
2. Derivation of ecological screening factors.
(i) whether list of representative species is suitable. The list of representative species is suitable for achieving a screening level assessment that will highlight possible ecological risks. There is a lack of benchmarks with regard to amphibians, bird, plants, soil invertebrates, and sediment associated biota as shown in Appendix C. However, Table 2-7 (p. 2-43) is comprehensive and appropriate. There is no need to considered endangered species or ultra-sensitive species since this is not a river or forest ecosystem situation.
(ii) whether there are serious gaps. There are no obvious serious gaps in the consideration of potential ecological risks that can be addressed by readily available data/benchmarks that are not identified in the report. Excellent coverage of readily available references are contained in the report.
3. Level of protectiveness.
The proposed screening approach appears to achieve the objectives of screening out constituents and units that present negligible (de minimis) risks. The methodology utilizes very conservative assumptions in the initial screening by placing the receptor at the source without attenuation or dilution. A source-release-exposure pathway-receptor analysis of the approach proposed, and the values for the risk screening factors and risk criteria are conservative in that specific constituents and units will be brought forward for further analysis that will be screened out under realistic conditions. Therefore, the screening approach achieves the objectives.
4. Approach for dealing with lack of information on chemical composition of wastewater in the impoundments or emissions.
(i) The calculation of sludge concentrations (page 2-16) using soil/sediment Kd (partition coefficients) may be too low. Kd includes both the inorganic part of the soil/sediment as well as organic part, and the organic fraction is generally less than 5% (by weight). If a calculational approach is to be used, I suggest using Koc (partitioning between organic carbon and water). Koc values are readily available, and to get Kd values for soils and sediments that are site specific, it is most often necessary to calculate a Kd value as: Kd = Koc (foc), where foc is the fraction of organic carbon in the soil or sediment. Otherwise, Kd values must be determined experimentally in the laboratory using the specific soil or sediment at the site. Therefore, I recommend using Kd = Koc (foc), where foc values are inexpensive, rapid, and site-specific. Since sludge consists of approximately 60% organic carbon, it may also be reasonable to use the Koc values from the literature as an estimate of the sludge concentration, or to use 0.6 of the Koc value. A conservative estimate would involve using the Koc value. The box in Figure 2-8 would then read:
Calculate sludge concentration as follows: water concentration X organic carbon/water partition coefficient (Koc).
(ii) The use of the other approaches described are reasonable when there is the described lack of information. Use of data from other impoundments at the facility are best. Next best is to use data from same industrial category. Modeling or back calculating to infer concentrations would be recommended if data from the facility or same industrial category are not available.
5. Approach for representing cumulative risk.
The approach described for representing cumulative risk on page 2-63 for each constituent, unit, and facility is reasonable. The facility risk is the cumulative risk for all constituents and impoundments. The constituent risk is the cumulative risk for all impoundments. The scoring proposed in Tables 2-11 through 2-13 are very reasonable, and provide a way to handle the information for prioritization.
Although a multiple-chemical exposure is realistic, since the synergistic or antagonistic effects of interactions of chemicals are not well known and cannot be modeled with confidence, the methodology presented in science-based and reasonable.
6. Modeling Approaches.
Due to the uncertainty regarding how many units and constituents will merit multi-media modeling, which is intensive and expensive, the proposed approach to modeling is very good. The IWAIR model is fast and easy to use, and is very satisfactory for preliminary screening purposes as described.
The IWEM Tier 1 model is quite satisfactory for the purposes stated. There may be difficulty locating a surrogate chemical for surface impoundments for DAFs with similar hydrolysis rate constants and similar retardation factors. QSARs can be used to predict solubility values that can then be used to predict Koc values, that can be used to calculate Kd, and therefore retardation values. The calculation of Kd (from which retardation is calculated), was addressed under item 4, lack of information. Hydrolysis rate constants are frequently affected by the pH value of the water. Reference is made to comment #13 under Technical merits of the overall approach.
The use of pre-defined representative scenarios for hydrogeologic conditions and watershed based on Monte-Carlo simulations is reasonable and has been conducted by the U.S.EPA for other situations, as described on page 2-29. The 3MRA model is comprehensive and appropriate for those sites with surface impoundments that present complicated risk assessments requiring further risk analysis.
How is groundwater - surface water interaction modeled? Contaminated groundwater, originating from surface impoundments, may enter surface water systems that provide the exposure pathway to the human or ecological receptor(s) described in section 3.0.