Environmental Risk Assessment Reports - Chapter 26

CHAPTER 26 Remediation Risk Assessment William Phillips CONTENTS I. Introduction.................................................................................................497 II. Technical Review of Remediation Risk Assessment.................................498 A. Evaluation of Remedial Alternatives............................................501 B. Residual Risk................................................................................501 III. Conclusion..................................................................................................502 References...................................................................................................503 I. INTRODUCTION Remediation risk assessments calculate chemical specific numerical performance standards which must be achieved for a regulatory agency to find that medium or site acceptably clean. They are conducted to determine how much of one or more environmental contaminants must be removed from a site or medium to achieve acceptable risk levels sufficient to protect human and environmental health. Cleanup decisions are not solely based on calculated risk levels and acceptable risk criteria. Risk managers are not captives of risk findings and must weigh several factors to arrive at a final equitable cleanup decision. For example, when formulating a cleanup decision for contaminated wetland sediments, risk managers must weigh reduced ecological risks against habitat destruction. Risk managers must balance the costs and benefits of chemical risk reduction and not rely solely on achieving a numerical performance standard. Site or medium remediation efforts are often hampered by technological and financial constraints. Currently available technologies may not be able to reduce site contamination levels to regulatory agency numerical performance standards. As a remediation technology reaches its removal limits, its ability to efficiently and 497 © 2001 by CRC Press LLC 498 A PRACTICAL GUIDE TO ENVIRONMENTAL RISK ASSESSMENT REPORTS economically remove contaminants decreases. At the limits of remediation technol-ogy, each increment of pollutant removal increases cleanup costs steeply. For exam-ple, removal of a substance to 10 ppm could cost $1 million, 5 ppm, $10 million, and 1 ppm, $100 million. In an age of limited financial resources, risk managers must balance the need to achieve calculated acceptable risk levels with real economic and technological constraints. Remediation risk assessments allow decision makers to balance public health concerns and cleanup goals with technological feasibility and cost-effective reme-dies. Such analyses support the selection of innovative technologies which can result in lower remedial costs, yet provide equivalent protection of human health and the environment. Remediation risk analyses can clearly define aspects of the costs and benefits to aid in decision making and can also potentially reduce financial liabilities associated with the cleanup. Considering responsible party cleanup costs at a Super-fund site can range from $20–100 million, it is essential that rigorous remediation risk assessment be conducted and used effectively by risk managers in the remedy identification process. II. TECHNICAL REVIEW OF REMEDIATION RISK ASSESSMENT Remediation risk assessment practices in the U.S. have been driven by CERCLA and RCRA requirements. While a particular jurisdiction may have unique environ-mental laws and regulations, the basic processes and procedures are probably derived from these two national environmental statutes. The exact processes, procedures, and levels of regulatory flexibility can vary within and between government agencies. These differences are important because they provide an opportunity for the reme-diation risk assessor to negotiate risk assessment workplans and remedies that can be more economical while not reducing the level of human or environmental health protection offered by a site cleanup remedy. Three important uses of risk assessment in environmental cleanup are: selection of numerical cleanup criteria, evaluating short and long term risks when using a cleanup technology, and residual risks following cleanup. While the discussion of these three items will be general in nature, examples from Superfund and RCRA will be used to illustrate key points. Numerical cleanup criteria are typically derived for each environmental medium of interest. Called “preliminary remediation goals” under Superfund or “action levels” under RCRA, the cleanup criteria provide the remedial design staff with targets to use during the evaluation of alternative cleanup technologies. Chemical-specific cleanup criteria are concentrations based on applicable or relevant and appropriate requirements (ARARs) mandated by CERCLA, such as maximum con-taminant levels promulgated under the Safe Drinking Water Act or Ambient Water Quality Criteria promulgated under the Clean Water Act, and risk-based calculations that set concentration limits using toxicity values under specific default exposure conditions. ARARs are rarely available for all chemicals and media of concern. When available, they provide a quick and convenient frame of reference for estab-lishing the scope of site cleanup. © 2001 by CRC Press LLC REMEDIATION RISK ASSESSMENT 499 Guidance for calculating risk-based cleanup criteria is available from the CERCLA and RCRA programs. EPA publications provide general guidance, default exposure scenarios, and mathematical algorithms for developing risk-based preliminary remediation goals for the Superfund program. State regulatory agen-cies must be contacted for guidance regarding cleanup criteria for state-regulated sites or voluntary cleanup programs. Cleanup criteria can be generic or site specific. Government developed generic risk-based cleanup criteria use a series of conservative assumptions that may not be appropriate for an individual site and are used as default cleanup values. U.S. EPA’s development of national soil screening levels for compounds frequently detected at Superfund sites are typical of generic criteria. Generic criteria, ostensibly developed to shorten the time and resources needed to develop cleanup levels, use highly conservative methods to generate very conservative cleanup concentrations that may be technologically impossible to achieve and financially crushing. Inappropriate use of generic cleanup criteria as immutable standards is a common problem encountered by remediation risk assessors. In contrast, site specific solutions are tailored to the unique site conditions and can better reflect site risks and result in more equitable site contamination remedies. For most sites, the environmental media requiring direct remediation (and devel-opment of cleanup criteria) are groundwater, soil, and sediment. Federal and state agencies have developed algorithms to back calculate environmental cleanup con-centrations based on a desired risk level and generic or site specific scenarios. Agencies use target risk levels for cleanup purposes. Target carcinogenic and non-carcinogenic risks vary within and between government agencies and must be obtained from an appropriate responsible governmental unit. This hypothetical level of protectiveness is the typical point of departure for evaluating remedial technolo-gies. Target risks for the final remedy may change, but are expected to achieve residual risks (cumulative across all exposure pathways) that lie within a government agency’s acceptable risk range or equal to or less than a specific risk criteria. The availability of generic risk assessment-based cleanup concentrations varies with media and locality. Since most waste sites and facilities involve contaminated groundwater, it is one medium where ARARs are most likely to be available. Cleanup criteria are often based on MCLs. When ARARs are not available or appropriate, site specific values can be generated. The key to developing a credible site specific cleanup value is using a high quality equation and input values. Since there are many possible input values that can be used in such equations (e.g., the amount of drinking water consumed per day for an adult or child), selection of equation input variables is extremely important. It is the job of the remediation risk assessment consultant to ensure that appropriate equations and input values are used to generate a credible cleanup concentration. ARARs for surface water can be ambient water quality criteria developed under the Clean Water Act. For surface water bodies that are designated for drinking water supply, MCLs have been used as criteria. The potential for exposure through fishing should also be assessed. There is considerable inconsistency from site to site in the development of cleanup criteria for surface water. A practical consideration is to focus on source control in other media to prevent continuing discharges to surface © 2001 by CRC Press LLC 500 A PRACTICAL GUIDE TO ENVIRONMENTAL RISK ASSESSMENT REPORTS water. Once the source is controlled, concentrations in surface water rapidly attenuate to nondetectable concentrations. Cleanup criteria for soil have historically been risk-based criteria based on default future land use assumptions. Residential or commercial/industrial development sce-narios are used to identify target receptors (residential child, residential adult, or adult worker) and the assumed frequency of exposure. For sites with volatile organic compounds (VOCs) in soil, the conservative default assumptions in the vapor release models make the inhalation exposure pathway the overriding determinant for soil cleanup criteria. Due to the excessive conservatism in the EPA’s choice of vapor release models, site-specific modeling of volatile emissions may support less con-servative cleanup criteria for soil. Soil cleanup criteria often consider the potential for cross-media transfer of contaminants. Contaminated soils can be a source of groundwater contamination, and fate and transfer models have been developed to quantify the potential for leaching of soil contaminants to groundwater aquifers. Soil-to-groundwater fate and transport models estimate cleanup criteria using the physical and chemical characteristics of the soil, the properties of the chemicals of concern, and the hydrogeological characteristics of the site. Generic cleanup criteria derived to protect groundwater resources are typically more conservative than criteria based on direct exposure to soil. It is usually advantageous to collect site-specific data, rather than to use default assumptions about the site characteristics. Many of the models and default exposure assumptions favored by regulatory agencies in the development of cleanup values have not been validated. As a result, input numbers and final cleanup levels should be amenable to negotiations based on sound science. Remediation risk assessors bear the burden of proof for demonstrating that generic criteria or default assumptions are inappropriate for their particular site. Considerable effort and money often must be expended to meet that burden of proof, especially when negotiating with inexperienced regulators. However, the money spent on site characterization is usually a fraction of what must be spent on an overly conservative cleanup. Software programs are available from vendors and can simplify calculations of cleanup criteria. Software programs are generally required for detailed statistical manipulation of monitoring data (e.g., kriging) or to perform Monte Carlo simula-tions of the distribution of input variables, exposures, and associated risks. Proba-bilistic methods like kriging and Monte Carlo simulations invariably provide a more accurate representation of potential risks than do deterministic methods. In selected situations, it may also be necessary to evaluate indirect exposures that can occur through the aquatic and terrestrial food chains. Indirect exposures through the aquatic food chain can occur when contaminated sediments or surface water occur in sport fishing areas. Terrestrial food chain exposures are typically assessed when incinerators can enable downgradient dispersion of contaminants. However, for most sites, exposures through the food chain are unlikely. Remember that the cleanup criteria are only preliminary and may change during the implementation of the cleanup. Site constraints such as fractured bedrock and the practical limits of remedial technologies make it difficult to achieve ideal criteria based on considerations of hypothetical exposure pathways. © 2001 by CRC Press LLC REMEDIATION RISK ASSESSMENT 501 A. Evaluation of Remedial Alternatives For most site cleanups, a number of potential technologies are evaluated for effective-ness, implementability, and cost. Those waste management options that are ineffective or too costly are eliminated from further consideration. Typical questions project managers should ask about the health risks posed by remedial technologies include: · Which technologies are capable of achieving cleanup criteria in each environmental medium? · Which alternative will not address the exposure pathways identified in the baseline risk assessment? · Are the expected short-term risks or residual risks significantly different between alternatives? · For each technology, what are the major uncertainties affecting implementation and performance? · Are there other risk-based benefits, such as shorter time to completion, that are presented by particular technologies? · Is there a need for engineering controls to mitigate risks during installation of a technology? If so, are the controls available and are they reliable? · Will operating a specific technology create new chemicals of concern (e.g., prod-ucts of incomplete combustion from incinerators) or new significant exposure pathways for the surrounding community? Are there appropriate engineering con-trols to mitigate the risks? · Are containment technologies that leave contaminants at the site being used? If so, five-year reviews must be considered. Programs such as Superfund and RCRA impose specific evaluation criteria and a labor-intensive selection process. In practical terms, technologies are selected because they accomplish the common sense considerations of cleaning up spills, controlling the source of the contaminants, and effectively managing the wastes that are generated by the cleanup activities. Considerations of protectiveness generally involve evaluating the short-term and long-term human health risks. Short term risks occur during implementation of the remedy or installation of a technology and include worker exposure to fugitive dusts or VOCs during soil excavation, or poten-tial injury due to physical hazards, heat stress, and precarious work environments. Proper use of emergency response plans, engineering controls, work practices, and personal protective equipment can modify the magnitude of potential risks to work-ers. People who live and work in the vicinity of the site may also receive short-term exposures from, for example, fugitive emissions, emissions from an onsite air strip-per or incinerator, runoff of water and sediments or leaching of water, and rupture of vessels containing treatment chemicals. Long-term risks are associated with a remedial alternative and involve evaluating permanence or protectiveness of the technology over time. B. Residual Risk Risks are associated with in situ treatment-based remedies such as bioremediation or soil vapor extraction, where total removal of a contaminant is not technically © 2001 by CRC Press LLC ... - tailieumienphi.vn