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Decision Tree for Contaminated Sediment Site Strategy Development Master Overview
Summary of Approach Developing an effective strategy for managing contaminated sediment sites requires the integration of a variety of factors, including, but not necessarily limited to, the following:
Most Important Questions to be Answered in the Decision Process
While there are many other questions that must be answered, these are the ones that are especially important. These questions will be highlighted throughout the framework.
Overview of Decision Tree Framework The Decision Tree tool encompasses four basic sub-elements: Initial Evaluation and Early Decision, Source Control, Site Evaluation and Risk Assessment, and Feasibility Study and Remedy Selection. The inter-relationships between these four elements can be clearly seen with reference to this Decision Framework Diagram. When finished viewing the diagram, return to this point by clicking your browser's "Back" button. The decision tree provides for early actions, where appropriate, to address more imminent adverse effects or to undertake actions that can be readily conceived and implemented without significant site evaluation. It also provides for early recognition and/or elimination of important ongoing external sources of contamination. The decision tree ultimately rests on the synthesis of a prospective conceptual model of the contaminated sediment site. This model allows an examination of historic data trends, a prediction of future changes, and an understanding of existing and potential risks to human health and relevant environmental receptors. Site-specific remedial action objectives (RAOs) are developed based on the temporal conceptual model. Because potential remedial technologies are evaluated in accordance with the RAOs to examine effectiveness and screen for implementability and order-of-magnitude costs, RAOs are the building blocks of a site-specific management strategy. It is important to note that the decision tree should be used in an iterative
manner to arrive at a long-term sediment management strategy. In many cases, it
is more effective to undertake remedial actions within specific portions of a
site and evaluate their impact on the system rather than to defer action until
the completion of a full-scale feasibility study. Such iterations are usually
accompanied by a period of monitoring, which serves to calibrate the temporal
conceptual model and enhance its reliability as a predictive tool. Specific Elements of the Decision Tree Tool
Many contaminated sediment sites can benefit from undertaking an initial problem analysis in anticipation that 1) early actions may be suggested on the basis of a qualitative assessment, thereby obviating the need for an in-depth assessment of the site; or 2) interim actions that will ultimately support a long-term remedial action may be suggested. In both cases, the opportunity may be provided to undertake actions in the form of a pre-emptive strike that will
The need for early actions is generally triggered by relatively low action thresholds; this is necessarily the case as there are generally little empirical data available and even baseline conceptual model development has not occurred. It is important to critically review potential early actions to ensure that they do not actually exacerbate an existing problem. An example of potentially negative results from ill-advised early actions might be a decision to undertake dredging of areas of elevated contamination, without fully evaluating the potential for a net increase in the areal extent of contamination within the bioavailable surface sediments. A review of potential early actions may also highlight data needs that can be resolved in a very focused manner within a short time frame, thus enabling the decision-maker to gain better comfort that the action proposed will have the desired effects. In most cases, early action will be insufficient to constitute permanent remedy and a Site Evaluation will be necessary. The decision-maker should remain aware of opportunities to evaluate early actions throughout the site characterization process. The Early Decision element can be entered at several points in the overall process of driving toward a long-term strategy. Are there simple actions that can quickly reduce the apparent problem?
Another element that may proceed in tandem with Early Decision is that of evaluation of potential ongoing external sources to the contaminated sediment site. Ultimately, the site conceptual model will need to assess the relative significance of all ongoing sources, both those external to the site (e.g., process discharges, non point-source ground water discharge to surface water) and those within the site (e.g., body of contaminated sediments). However, an approximate mass balance should be undertaken as one of the first site characterization efforts in an attempt to identify and prioritize sources and sinks of COCs within the system. As such, the mass balance serves as a fundamental building block of the baseline conceptual model. Further, it aids in assessing the significance of each of the potential sources and permits 1) an assessment of the potential value of source control measures; and 2) early definition of potential data limitations that will need to be resolved in site characterization. In addition, significant lack of closure in the mass balance may suggest sources that have not yet been identified. Once potential external sources have been identified and generally prioritized, a decision can be made as to whether early action is appropriate. The Ongoing Sources element of the Decision Tool provides for a qualitative assessment of the significance of each of the sources, followed by an abbreviated site investigation and then development of a range of potential source control options. Similar to the discussion of Early Actions within the Early Decision element, if readily implemented and cost-effective, a decision may be made to undertake a source control action prior to further site characterization. The likelihood that such action may exacerbate the current problem is minimized by virtue of the fact that these actions are all external to the contaminated sediment site; however, this issue should be addressed for waterborne actions such as removal of upstream contaminated sediments. Are ongoing, external sources significant? If no source control actions are identified, the Decision Tree tool directs the decision-maker to proceed into the Site Evaluation element.
This element of the Decision Tree tool is analogous to the Remedial Investigation under CERCLA or the RCRA Facility Investigation and much of the guidance developed for these programs is pertinent here. Of particular interest will be EPA guidance relative to development of Data Quality Objectives (DQOs) which are critical to an effective sampling strategy. In addition, the literature contains numerous reference texts that will assist in the selection of effective sampling designs and techniques. In concert with the planning of the site assessment activity, it is critical that a baseline conceptual model be developed based on existing data, approximate mass balances performed as part of the source identification element, the Initial Assessment, and any other pertinent facts regarding the subject site. The conceptual model will assist in definition of data limitations that should be incorporated into the initial Site Evaluation program. Site characterization should be viewed as an iterative or phased process. Doing so will aid in ensuring that data collection does not "overdrive" site understanding (as encompassed in the conceptual model) and will result in a more concise, cost-effective, and interpretable data base. What are the real risks - is there truly a problem? Conceptual models range in complexity from qualitative assessments to full-quantitative site models. The technical paper on Effective Decision-Making Models for Evaluating Sediment Management Options provides a detailed summary of the range of conceptual models available and their applicability to various types of contaminated sediment sites. This Paper also provides a stepwise procedure for the development of both baseline and prospective (temporal) conceptual models. When will risks become acceptable via natural
recovery?
Again, the comparative evaluation of alternatives and selection of remedies at contaminated sediment sites parallel those for CERCLA or RCRA sites. While ample guidance is provided regarding this activity under CERCLA, contaminated sediment sites represent a unique environmental setting. All portions of the site may not be amenable to the same remedy. For example, capping may be appropriate in some areas, natural recovery in others, and dredging of areas of elevated areas of sediment contamination appropriate in still others. The CERCLA FS guidance includes an initial screening step whereby technologies and process options are compared against each other based on the parameters of effectiveness, implementability, and order-of-magnitude costs. Those technologies and representative process options which pass this screening step are then assembled into alternatives which undergo a detailed comparative evaluation. CERCLA has traditionally advanced the following nine factors as the basis for this comparison:
Overall protection of the public health and environment and compliance with ARARs have generally been held to be threshold factors (i.e., any alternative not satisfying both of these criteria is screened out of further consideration). Compliance with ARARs that are intended to govern siting of remedial actions obviously cannot be achieved if remedial actions are to be taken within sensitive environments; however, actions can be taken to mitigate impacts and to ensure that such impacts are of the shortest possible duration. Location-specific ARARs that discourage impacts to sensitive wetland or riverine/estuarine environments may need to be addressed when confined aquatic disposal facilities (CADFs) or other near-shore disposal sites are considered. Action-specific ARARs may come into play in situations where upland disposal of dredged sediment or other off-site treatment/disposal or reuse options are considered. There are few chemical-specific ARARS that are directly derived for sediments. Due to the uniqueness of each contaminated sediment site, these chemical-specific ARARS should always be reviewed in the context of a site-specific risk assessment. Short-term effectiveness should be viewed in the context of implementation risk, especially for intrusive actions such as dredging or installation of engineered caps. Long-term effectiveness and permanence may be inferred, but seldom defined with any precision. The prospective (temporal) conceptual model may be used to assist in making comparative evaluations in this regard that take into consideration site-specific conditions as much as is practicable. In reality, the long-term effectiveness of many of the actions that leave residual contamination in place to be managed through some form of cover or engineered capping system can only be fully tested through an effective monitoring program. Such predictive shortcomings should not dissuade the decision-maker from employing these remedial alternatives, but do argue in favor of effective monitoring programs. Reduction of toxicity, mobility, or volume is difficult to achieve at contaminated sediment sites where contaminants are often in intimate contact with biota or, at a minimum, with the surface water migration pathway. In addition, it is important to note that reduction of contaminant volume may have little impact on biologically available contaminants due to the location of contaminants within a sediment profile prior to taking action, the importance of contaminant speciation to the question of biological uptake and retention, and the potential for certain highly bioaccumluative contaminants to concentrate to unacceptable levels in receptors even at very low sediment concentrations. Can active remediation significantly accelerate
achievement of acceptable risk? Finally, following remedy implementation, it will be important to undertake an effective monitoring program to ensure that RAOs were actually met; to permit a re-establishment of RAOs based on the practical limitations of the selected technologies; or to permit a revisitation of the remedy selection in the event that the selected technologies prove to be ineffective. Importance of Communications
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