John Bascietto
USDOE

Carlos Martins
HAZMED

Peter Tong
Woodward-Clyde

The Environmental Protection Agency’s (EPA) guidance on preparation of Comprehensive Environmental Response, Compensation, and Liability Act (CERCLA) scoping documents requires the development of a Site Conceptual Exposure Model (SCEM) as a planning tool for public health and ecological risk assessments. Corrective actions under the Resource, Conservation and Recovery Act (RCRA) also require risk assessments to determine potential impacts on public health and the environment. To streamline this process, the Department of Energy (DOE) Office of Environmental Policy and Assistance, RCRA/CERCLA Division (EH-413), developed the SCEM Builder, a user-friendly computer application, that helps environmental restoration program managers prepare SCEMs.

The Department of Energy (DOE), Office of Environment, Safety and Health, Office of Environmental and Policy Assistance (EH-413) provides policy, guidance, and technical assistance on hazardous and mixed-waste issues and environmental restoration activities at DOE facilities. Environmental risks from chemical and radiological contaminants do not occur in the absence of complete exposure pathways. Complete pathways occur when site-related contaminants are released and transported into the environment, and receptors are exposed via ingestion, inhalation, or direct dermal contact. The SCEM helps to identify the potential for this type of release and exposure. SCEMs were first recommended by EPA for use in the assessment and selection of site remedies [1]. Subsequently, SCEM use in other project phases has been encouraged by various guidance documents [2,3,4]. Since the understanding of site conditions improves with additional information (as the project progresses from discovery to close out), the use of a site-specific, constantly-updated SCEM is invaluable tool to project decision-making. SCEMs help decision-makers focus on issues that are of critical importance, and ignore issues of negligible concern.

SCEMs are used in various project phases to identify complete, incomplete, or insignificant exposure pathways. At the project planning phase, when site data are limited, SCEMs contain a high degree of uncertainty. As uncertainties are reduced and the site is better characterized, the SCEM should be modified to better reflect actual site conditions; the purpose of a SCEM is not just to provide an intellectual exercise, but rather provide the project team with a tool to support project decisions at each of the various project phases. In addition to their value in site decision-making, SCEMs are a powerful communication tool. SCEMs help the extended project team exchange information with stakeholders and regulatory agencies, and identify their concerns.

SCEMs are used as planning tools during the environmental site investigation scoping phase to allocate the finite financial and personnel resources that are needed to address data gaps. Project managers also use SCEMs to help identify sources of contamination, release mechanisms, exposure pathways, and human or ecological receptors. In addition, SCEMs include a visual presentation of site conditions and provide a narrative description of the assumptions used in the model. Project managers can use the information in SCEMs to develop risk assessment data quality objectives (DQOs) and prioritize field sampling activities, thereby reducing the uncertainties associated with risk characterization. SCEMs also provide sufficient information for the development of early response actions that address exposure pathways that are considered complete, and that pose an imminent risk to public health.

The SCEM Builder is a user-friendly computer graphics tool that considerably shortens the time required to generate SCEM diagrams and associated documentation. Once a SCEM has been constructed, any of the variables can be easily and quickly modified. The SCEM Builder can then generate a range of SCEMs that reflect various "what-if" scenarios, and allow for site data assumptions and uncertainties to be clearly stated in the individual SCEMs. Figure 1 shows the SCEM Builder Main Window.

The SCEM Builder was developed using Microsoft Visual Basic software, and runs on either Windows 3.11 or Windows 95 operating systems. The SCEM Builder was designed to perform efficiently on an IBM compatible computer system with a 486 processor running at the clock speed of 25 MHz, and with 4 megabytes of random access memory (RAM). A Microsoft Windows 3.1 or higher operating system should be installed. The SCEM Builder requires approximately 3 megabytes of disk space for storage. A "mouse" input device is useful, but not necessary, to operate the program.

The SCEM Builder consists of:

• Diagram Builder - The Diagram Builder enables the user to quickly construct or modify the SCEM variables, which include the primary, secondary, or tertiary source of contamination; the receptor; the release mechanism; and the exposure mechanism. A SCEM Builder diagram consists of the following objects (in this context, an object refers to a box or a line in the SCEM Builder diagram) :

Notes Feature - The Notes Feature allows the user to document details regarding each diagram element and any assumptions. Each object on the diagram can have an associated note. For example, a note can describe the findings on the site, any assumptions, descriptions of the sources and pathways, etc.

• Help System - The Help System consists of both Program Help, which relates to operation of the program, and SCEM Help, which relates to technical issues or concerns associated with environmental restoration sites. Program Help provides information and guidance on the operation of the program. It explains, for example, how to cut and paste an object, how to print, and how to save or load a diagram. SCEM Help provides information on default environmental factors built into the program, such as descriptions of contaminant sources commonly found at CERCLA sites. SCEM Help also has a table of contents from which the user can select topics of interest.
• Diagram Information Screen - The Diagram Information Screen provides specific information about the diagram, such as title, file saving information, and creation date. This screen also includes a notes area for inputting additional of information.

Prior to development of the SCEM Builder, SCEMs were usually drawn manually by drafting technicians in an office setting. Modifications could not be made in the field where new site conditions are encountered, or in a meeting where "what if" scenarios are discussed or debated among decision-makers. Hand-drawn SCEMs are essentially "static" models that become obsolete quickly. Their use in site decision-making is often minor for this reason. The SCEM Builder addresses these inadequacies by automating labor-intensive drafting requirements, and by providing ways for the SCEM to be modified quickly "on the fly," on a laptop computer. It also encourages program managers and the expanded project team members to work together to postulate or conceptualize the exposure model, or modify the SCEM when additional characterization data are available.

As a powerful and flexible tool in site investigation and remediation, the SCEM Builder:

• helps to scope data needs, following the principles of data quality objectives, by identifying complete or potentially complete pathways and the associated media where exposure could occur;
• allows potentially incomplete pathways to be identified for verification of incompleteness or elimination from further data collection and evaluation;
• enables the restoration program manager to update SCEMs throughout the project phases as new data are acquired that more accurately represent current site conditions in order to effectively communicate risks with regulators and stakeholders, and to receive their input or concerns about risks;
• supports field decisions when expedited or rapid site characterization techniques are employed by identifying additional data needs quickly and systematically (this is particularly important when deviations from anticipated site conditions are encountered);
• provides rationale for, or supports performance of, a screening or streamlined risk evaluation as a basis for implementing early actions, or selecting preferred or presumptive remedies or corrective measures to mitigate risks;
• helps identify potentially complete pathways and risks for implementation of removal actions, interim measures, and remediation activities;
• provides exposure information, in combination with risk information, for setting priorities or ranking sites for remedial investigations/remedial actions or corrective actions; and

In summary, the SCEM Builder is a valuable tool for building SCEMs, which are used to (a) define the scope of a data collection effort to meet the project phase decision requirements, (b) support early actions (removal actions or early remedial actions under CERCLA and interim measures or stabilization initiatives under RCRA), (c) differentiate efficiency or efficacy of potential remedial alternatives or corrective measures, (d) ascertain the need for control measures for constituents, waste streams, or discharge effluents generated by the selected remedial action or corrective measure, and (e) support development of contingency plans under Streamlined Approach for Environmental Restoration (SAFER) as unacceptable deviations from expected conditions are uncovered during implementation of remedial action or corrective measures.

The SCEM Builder software and User Manual is posted on the DOE/EH-413 Home Page, http://www.eh.doe.gov/oepa/tools. Although minimal system training is required, EH-41 plans to provide training workshops on the use of the SCEM Builder. The presentation at the Waste Management ‘98 Symposia will include a demonstration of the SCEM Builder.

1. USEPA, Guidance for Conducting Remedial Investigations and Feasibility Studies Under CERCLA,. 1988
2. USEP, Guidance for Data Quality Objectives. 1993
3. USDOE, DOE's RI/FS Process, Elements, and Techniques Guidance. 1993
4. USDOE, Phased Response/Early Actions Guidance. 1995

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