A Conceptual Site Model (CSM) represents the physical, chemical, and biological processes that influence the transport, migration, and resulting impact of environmental contamination on human and ecological receptors. The CSM can take many forms. It could be as simple as a qualitative graphical representation, or the condition of the site and magnitude of risk could demand something as complex as a three-dimensional, numerical fate and transport model linking all the processes that may affect contaminant distribution. The CSM may also incorporate potential uses of the site. Regardless of its scope, a CSM is dynamic and always subject to update, revision, and reinterpretation based upon new data and information collected from the contaminated site. It is an invaluable tool for developing management strategies, evaluating risks, designing remedies, and identifying uncertainties, as well as communicating those factors to stakeholders including regulatory agencies and the public.
Elements of a CSM Vary Based on Site
A CSM is designed in response to a specific site, thus the elements incorporated will vary. For example, a CSM developed for a site with a leaking underground gasoline storage tank will often include operational history of the facility, physical location of the tank(s), location of associated piping, anecdotal and recorded information regarding spills, and records of missing inventory. The CSM will also consider the geologic and hydrogeologic setting within property boundaries and possibly beyond to the surrounding area. Sampling and observational history may be recorded to determine the amount, form, and distribution of contaminants, as well as the design, operation, and performance of any previous or ongoing remedial activity. Additionally, the CSM may include the history of surrounding properties to determine if other potential contamination sources exist, such as a previous service station or maintenance facility.
The CSM is not static after initial development. Rather, it can be reassessed throughout its implementation. For example, additional data may be collected, such as an additional groundwater sampling event or sampling of a monitoring well. Additional data collected may reveal that modifications are needed to ensure the CSM remains effective.
Application of a CSM
Once formulated, the CSM can identify data gaps for additional investigation. It will aid in understanding the relevant risks, which may include discharge to a surface water body, affect a drinking water source, or create vapor intrusion for nearby residences. The CSM is also critical in developing remedial objectives and design. Because remedial objectives are often developed from risk-based criteria, an accurate CSM provides a basis to justify alternative cleanup levels, technical impracticability waivers, and groundwater reclassification. The CSM may also help identify remedial objectives that are more appropriate than default maximum contaminant levels or drinking water standards. A CSM provides an understanding of the contaminants present, their concentrations, phase distribution, and factors affecting their fate and transport, which are important to producing an effective remedial design. An incomplete or inaccurate CSM (such as higher than expected contaminant mass, inaccurate plume delineation, additional or incompletely delineated source areas, or offsite plumes contributing to onsite contaminants) can contribute to ineffective remedies.
In a free whitepaper entitled Expedited Exit and Remedy Optimization Strategies for Contaminated Sites, I further explore Woodard & Curran’s use of a CSM to evaluate site conditions, identify contaminant fate and transport, risks to human health and the environment, develop appropriate cleanup objectives, and effective contaminated site management strategies for our clients.