Further description:-  Conceptual site model 

Glossary Entry
A representation which sets out the critical pollutant linkages of concern for a particular land 
contamination problem. It crystallises understanding of what needs to be done to achieve risk management,
and from this point appropriate remediation techniques for those risk management goals can be chosen
Site Investigation: Conceptual Site Model

Site Investigation: Conceptual Site Model

 

1.     Summary

 

The Conceptual site Model (CSM) is one of the primary planning tools that can be used to support the decision making process managing contaminated land and groundwater on a large scale. The CSM organizes available information about a site in a clear and transparent structure and facilitate the identification of data and information gaps. Once the CSM is established, additionally needed data can be gathered and integrated in the CSM, followed by a revision of the CSM and a refinement of decision goals, if required. Thus, the CSM matures and enables an improved understanding of the site characteristics, such as contamination status, receptor profiles, etc., and the re-adjustment of decision criteria.

 

2.     Characterization of the Conceptual Site Model approach

 

Following information needs to be integrated in a CSM:

 

  • Site contamination concerns by graphical or written representations (or "conceptualizations"),
  • How contaminations got at the site,
  • Whether or not contaminations are migrating or degrading ,
  • How variable concentrations are across the site,
  • What receptors might be exposed,
  • What risk-reduction strategies are most feasible.

 

Once the CSM is established, it can be used as a basis to:

 

  • Support the development of a framework for conducting and scoping a site investigation of the site or cleanup action that takes into account the future land use,
  • Elaborate a detailed description of the site and its setting that is used to form hypotheses about the release and ultimate fate of contamination at the site,
  • Identify sources of contamination at the site, potential chemicals of concern, and the media (soil, groundwater, surface water, structures) affected,
  • Quantify how contaminants may be migrating from the sources, and the media and pathways through which migration and exposure of potential human or environmental receptors could occur (including possible air releases),
  • Evaluate of potential or preferred cleanup options,
  • Develop site-specific sampling designs and procedures for sample collection and analysis,
  • Estimate site conditions that may lead to unacceptable risks and warrant further study.

 

By integrating permanently data and information to a CSM in a step-wise fashion, it achieves an increasing level of complexity. During the life cycle of a CSM following maturity levels will be achieved:

 

Maturity Level I

 

Preliminary Conceptual

Site Model

(little or no site-specific data available)

 

 

 

 

  • Maturity Level I – Key Decisions

 

1.   Is there a potential threat to human health and the environment?

2.   Which chemicals from what media pose a potential risk under the land use scenario?

 

 

Maturity Level II, III

 

Quantitative Risk Assessment Based on a Mature CSM

 

Design and Implement a Dynamic Work Plan to Verify/Define the Preliminary CSM

 

 

 

 

  • Maturity Level II, III – Key Decisions

 

1.   Does a risk exist above tolerable levels based on default criteria?

2.   What action level would be acceptable based on default risk criteria?

3.   Based on realistic exposure and response scenarios, what site cleanup goals or action levels are required?

 

At maturity level III a CSM should give reliable answers to following questions:

 

  • What is the proposed reuse or current use of the site?
  • Is the proposed reuse of the site politically, economically, and socially viable?
  • What media are impacted and by what type of contamination?
  • Are there any potentially complete receptors pathways networks present at the site?
  • What exposure point concentration might represent a potential risk?
  • Do the exposure assumptions used in the risk assessment match the reuse scenario?
  • What method reporting limits are needed to assure the delineation of potential hot spots?
  • What are some of the available remedies for the site?
  • What are the potential human exposure pathways?

 

It should be noted that a CSM:

 

  • May not be limited to soil and groundwater contamination,
  • Need to consider all potential exposures and receptors, e.g. human health, ecological receptors.

 

3.     Data Acquisition and Data Sources

 

In order to build-up a complete, full-matured CSM data and information are required from different areas or disciplines:

 

  • Archeological/Historical Use,

Sources: archives, historical databases,

  • Physiography: region with similar geologic structures and climate,

Sources: geological state offices, planning divisions of communities and cities,

  • Climatic data: hydrologic budget, fauna, flora, and land use, precipitation rates, air temperature, and prevailing wind speed and direction,

Sources: worldwide weather stations (www.worldclimate.com/worldclimate, meteorological offices),

  • Geology: types of geologic materials, structural geologic features, depositional environments, and geomorphology,

Sources: geological state offices,

  • Hydrogeology:

-           Aquifer characteristics:

a.         Type (examples: unconfined, confined, or semi-confined),

b.         Characteristics (examples: hydraulic conductivity, transmissivity, storitivity)

c.         Geology (materials and structure),

-           Hydrologic budget:

                        a.         Recharge rates (examples: precipitation, artificial recharge),

b.         Discharge rates (examples: evaporation, transpiration, groundwater pumping),

-           Groundwater flow:

                        a.         Hydraulic gradient (examples: groundwater elevations, flow direction),

                        b.         Flow velocity (travel time),

                        c.         Boundary conditions (examples: Dirichlet, Neumann),

Sources: water agencies, geological state offices, research institutes.

 

4.     CSM Models and Tools

 

  • Spatial Analysis Decision Assistance (SADA): A software program that integrates visualization, geospatial analysis, statistical analysis, human health and ecological risk assessment, cost-effective analysis, sampling design, and decision analysis (www.tiem.utk.edu/~sada/)
  • U.S. EPA Field Environmental Decision Support (FIELDS): An ArcView® extension that combines geographic information systems (GIS), global positioning systems (GPS), database analysis, and imaging technologies to identify, assess, communicate, and help solve environmental problems; includes modules for human health and ecological risk assessment, sampling design, remediation design (www.epa.gov/region5fields/)
  • U.S. DOE Pacific Northwest National Laboratory Visual Sampling Plan (VSP): VSP provides statistical solutions for sampling design to decide where samples should be collected and how many are needed (http://dqo.pnl.gov/vsp)
  • Center for Subsurface Modeling support (CSMoS): CSMoS provides public domain groundwater and vadose zone modeling software (www.epa.gov/ahaazvuc/csmos.html)
  • Center for Exposure Assessment Modeling (CEAM): CEAM provides proven predictive exposure assessment techniques for aquatic, terrestrial, and multimedia pathway for organic chemicals and metals (www.epa.gov/ceampubl/)
  • Army Risk Assessment Modeling System (ARAMS): ARAMS is a computer-based, modeling- and database-driven analysis system for estimating human and ecological health impacts and risks associated with MRCs and other potential contaminants of concern. ARAMS is based on a widely accepted risk paradigm that integrates exposure and effects assessments to characterize risk (http://www.wes.army.mil/el/arams/arams.html)
  • OnSite OnLine Tools for Site Assessment (OnSite): The OnSite set of online tools for site assessment contains calculators for formulas, models, unit conversion factors and scientific demonstrations to assess the impacts from ground water contaminants (http://www.epa.gov/athens/onsite/)

 

5.     Literature

 

American Society for Testing and Materials (1998):

Standard Guide for Accelerated Site Characterization for Confirmed or Suspected Petroleum Releases. West Conshohocken. E 1912-98.

 

American Society for Testing and Materials (1998):

Standard Practice for Expedited Site Characterization of Vadose Zone and Groundwater Contamination at Hazardous Waste Contaminated Sites. West Conshohocken. D 6235-98.

 

American Society for Testing and Materials (2000):

Standard Practice for Environmental Site Assessment. West Conshohocken. E 1527-00.

 

U.S. Environmental Protection Agency (1998):

Innovations in Site Characterization Case Study: Hanscom Air Force Base, Operable Unit 1 (Sites 1, 2, and 3). Office of Solid Waste and Emergency Response. Washington D.C. EPA/542/R-98/006.

 

U.S. Environmental Protection Agency (2001):

Road Map to Understanding Innovative Technology Options for Brownfields Investigation and Cleanup, Third Edition. Office of Solid Waste and Emergency Response. Washington D.C. EPA/542/B-01/001.

 

U.S. Environmental Protection Agency (2003):

Using the Triad Approach to Streamline Brownfields Site Assessment and Clean-up. Brownfields Technology Primer Serie. Office of solid Waste and Emergency Response. Washington D.C. EPA/542/B-03/002.

 

 

 

Authors
Martin Bittens
UFZ Centre for Environmental Research, Germany

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