Conceptual site model

Collecting and analyzing existing data. Existing data (Table 16.1) are collected and analyzed to develop a conceptual site model that can be used to assess both the nature and the extent of contamination and to identify potential exposure pathways and potential human health or environmental receptors. 

Developing a conceptual site model. An example of this is presented in Figure 16.4.12... 

Developing preliminary remedial action alternatives. This involves initiating limited field investigations if available data are inadequate to develop a conceptual site model and adequately scope the project, and identifying preliminary remedial action objectives and likely response actions for the specific project. 

FIGURE 16.4 Example of a conceptual site model. (From U.S. EPA, Guidance for Conducting Remedial Investigations and Feasibility Studies under CERCLA, EPA/540/G-89/004, U.S. EPA, Washington, October 1988.)... 

Data analysis should focus on the development or refinement of the conceptual site model by analyzing data on source characteristics, the nature and extent of contamination, the contaminants transport pathways and fate, and the effects on human health and the environment. All field activities, sample management and tracking, and document control and inventory should be well managed and documented to ensure their quality, validity, and consistency. 

A pumping test may be performed to evaluate hydrogeologic characteristics of the aquifer. Derived data can be applied to computer models or at least included in the conceptual site model. 

Finding 3. Closure of any chemical agent disposal facility necessitates the identification of potential exposure pathways and environmental receptors in an initial conceptual site model. Closure of JACADS has been complicated because this was not done early and the end use will not be determined until much of the closure planning has been completed. 

Island would likely become a national wildlife refuge. The cleanup standards, which EPA would establish for each end use scenario, differ in terms of receptors and exposures to inhabitants, which should be defined in an appropriate risk assessment. Receptors are plants, biota, animals, and humans that are exposed to a contaminant of concern. The risk assessment should assess the risks to both human health and ecological receptors, because they may require different end states. PMCD assigned the U.S. Army Center for Health Promotion and Preventive Medicine (USACHPPM) to prepare the Conceptual Site Model (CSM) for JACADS closure and to perform the risk assessment. 

Environmental Protection Agency. 2000. EPA Region IX, letter to James Bacon, Program Manager for Chemical Demilitarization, Febmary 22. Re Comments on the Conceptual Site Model for JACADS Closure Risk Assessment. EPA ID TTO-570-090-001. San Francisco, Calif. Environmental Protection Agency. 

U.S. Army. 2000e. Conceptual Site Model and Assessment Methodology for the Human Health Risk-Based RCRA Permit Closure of the JACADS Facility, Johnston Island, Johnston Atoll, North Pacific (Ocean). Prepared by United States Army Center for Health Promotion and Preventive Medicine for the Project Manager for Chemical Stockpile Disposal, June 27. Report No. 39-EJ-8929-99. Aberdeen Proving Ground, Md. U.S. Army Program Managerfor Chemical Demilitarization. 

Develop one or more conceptual site models of the site from that existing data... 

A conceptual site model is useful in helping to determine the type of environmental samples that is required. A conceptual model emphasizes the type and extent of the contamination, defines the pathways for contaminant migration, and identifies potential receptors (e.g., well users, surface water bodies, and food and feed material) (US EPA, 2002). 

The foundation for systematic project planning is the conceptual site model (CSM). Under Triad, a CSM is developed for each site, based on all available historical and current information to estimate where contamination might be located how much is there and how it varies across the site possible fate and migration of the contamination exposure risks associated with the site and contaminants and means of mitigating exposure risk. Once the project team develops an initial CSM, it identifies data gaps and seeks additional information to resolve those gaps. Throughout the project, the team updates the CSM to reflect new data and identify additional needs for data collection. This iterative process continues until the site project team is confident in its decisions about actions that need to be taken to address the site contamination. 

As mentioned earlier, an important element in the Triad approach is the conceptual site model (CSM). Early in the project, EPA and the project consultant developed a detailed preliminary CSM based on data from previous investigations. The CSM indicated that potential threats to human health and the environment at this site were limited to direct contact with contaminated surface soil and sediment that contained PCBs, petroleum-related contaminants, and arsenic. The resulting CSM is shown in Fig. 14.2. 

Figure 14.2 Conceptual Site Model (CSM), Cos Cob power plant (US EPA, 2005a).

To pose a risk, a constituent must be present in the environment at a concentration high enough to cause a toxic effect if exposure occurs. Complete pathways must exist, and exposure to a constituent at that concentration must occur. Without the potential for exposure to a constituent, there is no risk therefore, the driving force behind any remediation would not be the protection of human health. If no complete exposure pathways exist now or are not anticipated to exist in the foreseeable future at a site, the RBCA process technically can be terminated at this point as there is no risk. One example of this concept is the lack of exposure to impacted subsurface soil beneath a building that houses an operating business. However, it is often difficult to determine the future use of a site, and it may be necessary to evaluate potential risks associated with hypothetical future exposure. A key to the Tier 1 evaluation is the development of a conceptual site model that evaluates the possible exposure pathways for the site under current and hypothetical future conditions. 

ASTM (1995) Standard guide for developing conceptual site models for contaminated sites, American Society for Testing and Materials. West Conshohocken. PA. USA, E, pp. 1689-95. 

An effective conceptual site model is essential for the identification of risk-based and non-risk remedial action objectives. The conceptual model should identify source areas complete, potentially complete, and incomplete exposure pathways and receptors. In addition, the conceptual model should identify contaminants, affected environmental media, and specific areas within the affected environmental media to be addressed by the selected remedial action. Although a conceptual model should be developed prior to initiation of the risk-based remedy selection process, the conceptual model should be updated as needed during the remedy selection process to reflect any changes in the understanding of the site. ASTM (2003), Standard Guide for Developing Conceptual Site Models for Contaminated Sites, provides guidance for the development of an effective conceptual model. 

For each potential remedy identified for evaluation, the effectiveness must be evaluated in order to determine whether the remedy is capable of achieving all of the remedial action objectives. The evaluation of effectiveness for each remedial action should be made within the context of the conceptual site model (i.e., the contaminants, physical and hydrogeologic conditions, and other site-specific factors affecting technology effectiveness). A number of resources are available to assist with the evaluation of technical effectiveness of potential remedial actions ... 

ASTM, 2003, E 1689-95 (2003)el, Standard Guide for Developing Conceptual Site Models for Contaminated Sites, http //www.astm.org. 

These documents also suggest general lines of inquiry. One of these documents (Chapter 11) was written at the early stages of the cleanup effort when little was known. Nevertheless, it illustrates how broad concerns can be framed to create a study framework by the regulator. This and other documents can provide checkhsts for regulators, residents, and stakeholders to ensure that all necessary considerations are taken into account and all necessary work is performed. Documents hke the Conceptual Site Model in Chapter 17 are actually encouraged by the EPA. 

District of Columbia believed that the bulk of the work still needed to be completed, as stated in their original recommendations. The District s view on the need for a complete site investigation with a study of environmental contamination, health impacts, and a search of the entire area for individual UXO and buried stockpiles is also confirmed by the proposed Range Rule. It is also confirmed by the tremendous amount of ordnance and chemical bottles found subsequent to the District s original conceptual site model. 

This chapter presents a superb example of the new mandate from the EPA for a Conceptual Site Model. Many experienced state regulators may never have written one, and it is hoped that this will provide an example of the completeness that is advisable. The format recommended by the EPA must be altered slightly, due to the uniqueness of the site. In most sites, there will be data gaps or considerations that do not need to be addressed (i.e.. Civil War forts may lack aerial photographs but are likely to have artists sketches). 

This uncertainty has resulted in a patchwork of remedial projects. Although each project has been beneficial in finding and removing hazards or at least increasing the knowledge base about, waste and inefficiency have been major by-products due to the lack of a comprehensive plan. For example, most properties were sampled for arsenic. Now a repeat mobilization will have to be conducted to sample for a small handful of other toxic metals and constituents, which could have been sampled for in the first event. It is in an effort to minimize such future duplication of effort and data gaps that the District of Columbia has tackled the enormity of a comprehensive planning document with this Conceptual Site Model. 

Adjusting the EPA s Triad System Conceptual Site Model... 

Recently, the ERA has set forth an approach for environmental remediation called, the Triad System. A key step in this is the development of a Conceptual Site Model. According to the Triad System, the Conceptual Site Model includes the following ... 

The FPA has also experimented with conceptual site models for UXO. 

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