Monday, March 11, 2019

Harnessing the Power of AEM to Develop Quantitative Conceptual Site Models

Since the late 1980s, the development and use of a Conceptual Site Model (CSM) has been described in regulatory guidance and the technical literature as a sound platform for developing a qualitative (narrative and pictorial) description of contamination sources and groundwater flow conditions at a contaminated site. CSMs have typically been used for identifying data needs; for performing very preliminary receptor identification; and for providing a qualitative basis for decision making regarding site cleanup planning and implementation.

Linkage between the CSM and quantitative analysis of key aquifer and contaminant processes, however, is typically either not discussed, or mentioned briefly in the context of risk assessment.  To remedy this issue,  the development and use of a Quantitative Conceptual Site Model (QCSM) is warranted, as it provides a functional tool for project leaders to support data collection, conceptual model testing, receptor impact analysis, and remedy evaluation, selection and design.

What is a QCSM?  Whereas a traditional CSM is typically defined as a written and/or illustrative representation of the various processes that control the transport, migration, and potential impacts to receptors via soil, air, groundwater, surface water, and/or sediments (and is typically qualitative in nature), a QCSM consists of merging quantitative analysis results with the framework of a sound conceptual site model to form a proper basis for high level decision making.  Although there are many quantitative tools that can be utilized to facilitate QCSM development, groundwater models, and AEM models in particular, provide a fast, efficient tool that can be utilized to better understand site conditions, guide site investigations, and evaluate remedial options.

When describing CSM development, regulatory and technical guidance is typically vague with regard to the application of computer models, and if and when it is discussed, modeling typically appears to be an ancillary, not integral process.  In contrast, the QCSM approach integrates modeling into the decision-making process, explicitly calling for the use of groundwater (and soil zone) flow and transport models to develop a quantitative representation of the flow system, and provide answers to problems that are primarily hydraulic in nature ( e.g. capture zone, mounding, etc.) and/or related to the transport of contaminants (e.g., evaluation of potential receptors, monitored natural attenuation evaluations, etc.).

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As an example of developing and applying a QCSM, the approach was effectively utilized at an industrial site in New Jersey, where it was important to understand how both regional pumping influences and site-specific features may affect groundwater flow conditions at the local scale, which in turn was important to investigate the nature and extent of groundwater at the site. Accordingly, a groundwater flow model was developed using AnAqSim, which allowed for the computationally efficient incorporation of large-area regional flow features (which have an effect on flow patterns at the site-scale), while still allowing for detailed analysis of flow conditions at the local scale.  The AnAqSim model was constructed in the early phases of investigations at the site, and once calibrated to site conditions, was integrated into the decision-making process.  Specifically, the model was used by the project team to better understand:
  • Groundwater flow conditions in the complex, multi-layer aquifer beneath the Site;
  • Groundwater-surface water interactions;
  • The effects of pumping from regional public supply wells and site production wells;
  • Hydraulic communication between hydrostratigraphic units at the site; and
  • Potential contaminant migration pathways.

Once this quantitative information was conveyed, it facilitated a discussion between the analysis team and the field investigation Project Manager that led to quickly identifying data and information gaps, and effectively planning additional investigation activities to address those data gaps.  Information collected from additional investigation activities (e.g., groundwater and surface water elevations) was incorporated back into the model, which allowed for an improved model calibration, and a higher degree of confidence not only in the model, but for the QCSM of the site and surrounding area.

As this example illustrates, a QCSM can be integrated and utilized at every stage of the project, and provides a strong clear basis for higher level decision making in a manner that is scientifically supported and transparent.

To learn more about the QCSM approach, download McLane Environmental’s recent White Paper entitled “A Quantitative Conceptual Site Model Approach for Environmental and Engineering Decision MakingHERE

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