Conceptual Site Model

A CSM (Conceptual Site Model) is a written and illustrative description of the release site (based on all known environmental and site information) and is the primary communication tool utilized between all release stakeholders. A CSM is required in all reports submitted to OPS. CSM development is a dynamic process that continually incorporates new site information, beginning from release discovery through release closure.

A thoroughly developed CSM should identify the following:

  • Contaminant concerns and remedial objectives associated with each contaminant phase of the release
  • Next steps to be taken
  • Data gaps

OPS strongly recommends incorporating the Triad Approach early into the CSM development process. This approach identifies systematic project planning, real-time measurement technologies and dynamic work strategies as pillars that lead to reducing project uncertainty.

The following figure depicts the components of a CSM and reporting due dates. 

SCR CAP Process Timeline

Site characterization efforts lead to the development of an initial CSM. The CSM is continuously refined as corrective action efforts are implemented and new project data is gathered. It is important to know where your project is within the CSM, but it is also important to understand that this dynamic process is not linear and that new information may lead to reassessing work that has already been completed. Developing, refining and understanding the CSM will lead to targeted risk reduction and reduced project time and costs. The following sections identify the components of the CSM.

The CSM should be thoroughly defined upon completion of site characterization activities. All data associated with the release event should be summarized in the CSM narrative and depicted in figures (plan and cross-sectional view) and tables, as appropriate, to build a basis for data sharing and release understanding.

Release Discovery

The CSM should summarize the understanding of the release and identify the sources, causes and repairs made to the storage tank system. The CSM should also clearly identify the chemicals of concern, if the release was chronic or acute, the estimated duration of the release and if the release is potentially ongoing. Unknown information should be identified as data gaps.

Vertical and Horizontal Release Extent

The CSM should summarize the vertical and horizontal extent of all chemicals of concern in the subsurface. Thorough definition and understanding of the contaminant distribution during site characterization will lead to the development of appropriate remedial objectives, targeted treatment areas, significant cost savings and shorter project cleanup times.      

Site/Release Understanding

A historic understanding of the release site is critical for reducing data gaps and developing the CSM. Previous petroleum storage tank systems and configurations should be presented on-site figures and evaluated for data gap understandings. Previously documented releases and remedial actions associated with a facility should also be summarized in the CSM narrative and evaluated relative to the current release event.   

Hydrogeologic Understanding

Hydrogeologic conditions should be well understood and incorporated into the CSM as they relate to contaminant distribution and transport. A thorough understanding of hydrogeologic conditions can lead to an enhanced understanding of distribution and migration pathways, which can lead to the development of appropriate and targeted corrective actions. Contaminant mass transport and mass storage areas should be identified.

Point of Exposure Identification

All points of exposure should be identified early on during release discovery and summarized within the CSM. Ultimately, actual or potential impact to a point of exposure drives risk-based corrective action decisions.

Data evaluation should happen in concurrence with data gathering and should result in the collection of additional data until data evaluation objectives are met. 

Present the following data evaluation components in the CSM upon completion of site characterization activities, summarize them in the CSM narrative and depict them in plan and cross-sectional figures and tables as appropriate. Abate acute health and safety risks immediately until the risk has been adequately reduced.

Point of Exposure/Exposure Pathway Evaluation

Identify all impacted or potentially impacted points of exposure during the initial CSM development. Additionally, evaluate and identify all exposure pathways as either complete or incomplete during this process. Points of exposure and exposure pathway evaluation are major factors when considering whether a release poses risk or if a release event may be closed.

SSTL Calculations

SSTLs (site-specific target levels) must be calculated for on-site locations that exceed the Tier I risk-based screening levels and form a footprint for remedial objectives. SSTLs should be established for source areas, mid-plume and the distal end of the plume to identify potential treatment areas.

Contaminant Mass Estimates

Complete mass estimates for all identified potential treatment areas, as they are the criteria for evaluating remedial applications and establishing performance milestones. OPS understands that mass calculations are difficult to precisely determine. Order of magnitude approximations (e.g., 500 pounds or 5,000 pounds TPH) or mass estimate ranges (e.g., between 10,000 pounds and 20,000 pounds) are appropriate for understanding the nature and magnitude of the release. Separate mass estimates for different potential treatment areas are appropriate.

Data Gap Identification

Data gap identification is a critical part of the data evaluation process, but not all data gaps are the same. As such, qualify data gaps as either significant gaps that require the collection of additional information to properly develop the CSM or minor data gaps where additional data collection will not likely result in an enhanced site understanding. Data gaps can be identified at any time during data collection activities or data evaluation activities. Ideally, data gaps will be identified during data collection activities (e.g., real-time data measurement) such that field decisions can be made to address the gap.

Overall, data gathered during the site characterization phase should lead to a well-developed CSM such that the aforementioned data evaluation components are thoroughly understood. 

Active Remediation Determination

Evaluate the need for active remediation once data gathering objectives and the data evaluation objectives have been completed. Clearly identify an active remediation evaluation within the CSM. Corrective Action development should occur if it is determined that active remediation is necessary to reduce the risk associated with the release.

Utilize the corrective action process to select and implement the most technically and economically feasible remedial methods to reach the remedial objectives identified for the release site. Critical components of the CAP (corrective action plan) development process as they relate to the CSM are described below. These components should be summarized in the CSM narrative and depicted in plan and cross-sectional figures and tables as appropriate.

Identify Contaminant Concerns

The initial step of CAP development is to identify contaminant concerns associated with each phase of the petroleum release. The CSM should clearly identify these concerns based on the results of the SCR.

Clearly Define Remedial Objectives and Targeted Treatment Areas

Clearly state the remedial objectives in the CSM and plainly identify the targeted treatment area for the objective.  Clearly defining the remedial objectives and treatment areas highlights the understanding of the risk associated with the release and allows all stakeholders to understand the extent and purpose of the corrective actions.

Identify Critical Data Needs for the Selected Remedy

Follow the remedial selection process to select appropriate remedial technologies based on the site understanding and other remedial evaluation factors. Identify and gather critical data needs to confirm the selection of the cleanup approach and enhance the full-scale design of the system to ensure that it meets the remedial objectives.

Identify Performance Metrics and Establish Milestones

Identify remedial system performance metrics and performance milestones in the CAP CSM development phase. These metrics and milestones should be the basis for evaluating the success, progress or failure of the selected remedy, as well as for the corrective action progress reporting frequency.

Identify Environmental Data Needs

Identify environmental data needs (monitoring well data, additional spatial groundwater data and soil confirmation/evaluation data) as they relate to the remedial objectives, targeted treatment areas and performance milestones. Specify chemicals of concern and other analytical needs. The location and frequency of data collection should be specified and relevant to the remedial objectives and performance milestones.

Corrective action should be implemented consistent with the approved CAP (corrective action plan). Any deviations from the approved CAP should be documented. The following components should be incorporated into the CSM as the implemented corrective action progresses toward meeting the remedial objectives and, ultimately, toward site closure.

Evaluate New Data

The performance metric and environmental data identified in the CAP phase should be incorporated into the existing CSM. These data should be compared to the expectations identified in the CAP development phase.

Determine Whether Performance Milestones Were Met

Post-implementation evaluation reports should identify whether the remedial action is performing as expected. If the implementation is not performing as expected, optimization efforts should be identified and implemented.

Determine Whether Remedial Objectives Were Met

Ideally, a well-defined CSM will lead to well-defined remedial objectives, selection of the right remedial approach, appropriate performance metrics and milestones and the collection of critical data to evaluate the progress of the site cleanup. The CSM should be updated to incorporate a review of the remedial objectives as corrective action progresses. Data gaps are likely, and redevelopment of the remedial objectives should be re-evaluated if the objective is not met.

Data gaps should be identified in the CSM during this evaluation phase as soon as they become apparent. Data gaps may be related to the corrective action (e.g., it did not perform as expected) or to the remedial objective (e.g., the corrective action performance expectations were met, but the remedial objective was not met).

No Further Action evaluations should occur during both the site characterization and the corrective action data evaluation phases. The CSM narrative should clearly identify what the cleanup goals are and what the targeted tiered closure goal is. For many sites, corrective action is not necessary, as the release may already be considered of low risk to human health and the environment; however, it is also true that many sites will go through multiple corrective action efforts to appropriately reduce the associated risk. Understanding and following the CSM process significantly increases the understanding of the release, leads to targeted remedial efforts and reduces the time and costs associated with a release event.