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Preparing a Site Conceptual Model

Typical Site Management Problems:

Site complexities Complicated hydrogeology Multiple contaminants of concern (COCs) Multiple receptors/pathways

Multiple phases of investigation and remediation Deliverables that are not stand-alone documents Changes in consultants Changes in regulatory oversight Case load

Common Outcome:

An abundance of dataLack of clarity concerning the major site

issues and how to move the site toward closure

Suggestion:

Direct the Responsible Party (RP) to complete a Site Conceptual Model

Site Conceptual Model (SCM):

EPA: A representation of site conditions developed using readily available (existing) data that illustrates the relationship between contaminants, retention/transport media, and receptors.

EPA. November 2000. Using the Conceptual Site Model to Select Performance Standards and Develop Data Quality Objectives in the CAS.

Purpose:

Organize information already known about the site

Help identify additional information that must be obtained

Suggest when site characterization is complete—i.e., if the SCM is not likely to significantly change upon collection of additional information, the existing data are adequate

SCM Presentation:

The SCM is generally documented by written descriptions and supported by maps, geological cross-sections, tables, diagrams and other illustrations.

EPA. November 2000. Using the Conceptual Site Model to Select Performance Standards and Develop Data Quality Objectives in the CAS.

SCM Support:

1) Local and regional plan view maps showing location of sources, extent of contamination (i.e., an interpretive drawing—not a plot of laboratory results), direction and rate of groundwater flow, and locations of receptors. “Receptors” include, but are not limited to, all supply wells within 2000 feet of the source area.

Arulanantham, R. December 2000. Assessment and Management of MtBE Impacted Sites.

SCM Support:

2) Cross-section maps showing subsurface geologic features, depth to groundwater, man-made conduits, monitoring well construction, and an interpretive drawing of the vertical extent of soil contamination (i.e., an interpretive drawing—not a plot of laboratory results)


SCM Support:

3) Exposure evaluation flowchart (similar to Figure 2 in ASTM’s Standard Guide for Risk-Based Corrective Action Applied at Petroleum Release Sites)


SCM Support:

4) Plots of chemical concentrations vs. time (e.g., if groundwater monitoring is being conducted, plots should be shown for each monitoring well which has had detectable levels of contaminants)


MW-6 Chemical Concentrations versus Time

0

50000

100000

150000

200000

250000

2/18/99 5/26/99 8/13/99 12/3/99 3/20/00 5/25/00 7/31/00 10/27/00 2/2/01 4/12/01 9/18/01

Date

Chem

ical

Con

cent

ratio

ns (p

pb)

0

2

4

6

8

10

12

14G

roundwater Elevation (feet above M

SL)TPHg

Benzene

Groundwater

SCM Support:

5) Plots of chemical concentrations vs. distance from the source


TPHg Concentrations versus Distance

0

20000

40000

60000

80000

100000

120000

140000

160000

180000

0 20 40 60 80 100 120 140Distance (feet)

TPH

g C

once

ntra

tion

(ppb

)

MW-1

RW-1MW-5

MW-7 MW-13

SCM Support:

6) Summary tables of chemical concentrations in different media


SCM Support:

7) Boring and well logs (including construction/screening)


Summary:

SCMs are a “big picture” look at a site contamination problem

SCMs may help you through some common site management problems and clarify major site issues

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