Department of Health Seal

TGM for the Implementation of the Hawai'i State Contingency Plan
Section 3.3
CONCEPTUAL SITE MODELS

3.3 CONCEPTUAL SITE MODELS

Figure 3-6. Pictorial Conceptual Site Model Depiction of common environmental hazards associated with contaminated soil and groundwater as well as potential exposure pathways for human and ecological receptors.

The CSM prepared during the first step of the systematic planning is a comprehensive representation of site environmental conditions with respect to recognized or potential environmental hazards. CSMs are also a necessary starting point for preparation of an Environmental Hazard Evaluation. The CSM is presented in a series of figures that depict current and future site conditions in three dimensions, with textual explanations of the figures, as needed. There are a number of ways to present a CSM. Figure 3-6 is a pictorial depiction of environmental hazards associated with contaminated soil and groundwater (see also Figure 3-5). Exposure pathways to human and ecological receptors are also indicated (e.g., incidental ingestion, dermal absorption, and inhalation). These types of depictions are useful for those not well versed in Environmental Hazard Evaluation or human health and ecological risk assessment. Additional examples of CSMs are presented in Section 13.

3.3.1 SUMMARIZE KNOWN SITE CONDITIONS

The first step in the preparation of a CSM is to summarize current site conditions. At the most basic level, this includes a summary of the known or suspected extent and magnitude of soil and groundwater contamination. In addition, site conditions such as land use, groundwater use, potential onsite and offsite receptors, exposure or isolation of contaminated soil, etc., are identified, as are specific environmental hazards that may be posed by the identified contamination. The CSM is continually updated as the site investigation proceeds and site conditions are better understood.

3.3.2 SCREEN FOR POTENTIAL ENVIRONMENTAL HAZARDS

A basic understanding of potential environmental hazards in terms of the environmental fate and transport of COPCs targeted for a site is important for development of a CSM and subsequent stages of an investigation. As discussed in Subsection 3.4, the designation of DUs is intricately tied to the type of environmental hazard(s) posed by the COPC. Common environmental hazards associated with contaminated soil and groundwater (Figures 3-5 and 3-6) include:

Contaminated Soil:

  • Direct/indirect exposure to impacted soil (ingestion, dermal absorption, inhalation of vapors and dust in outdoor air);
  • Emission of subsurface vapors to building interiors;
  • Leaching and contamination of groundwater resources;
  • Impacts to terrestrial habitats (terrestrial ecotoxicity);
  • Gross contamination conditions (explosive subsurface vapor conditions, odors, general resource degradation, etc.);

Contaminated Groundwater:

  • Contamination of drinking water resources (toxicity, taste and odors);
  • Emission of subsurface vapors to building interiors;
  • Discharges of contaminated groundwater to surface water aquatic habitats (aquatic ecotoxicity or gross contamination conditions);
  • Gross contamination conditions (generation of explosive vapors from free product, odors, sheens, general resource degradation, etc.).

Additional environmental hazards also may require evaluation on a site-by-site basis (e.g., uptake of contaminants in produce, runoff of contaminated soil into surface water bodies, etc.). A more detailed discussion of these and other potential environmental hazards is provided later in this Section and in Section 13 (Environmental Hazard Evaluation), as well as the HEER Office document Evaluation of Environmental Hazards at Sites with Contaminated Soil and Groundwater (HDOH 2016).

These environmental hazards form the basis of the default CSM used to develop the HDOH Tier 1 EALs. The applicability of each hazard for a given COPC should be reviewed on a site-by-site basis, depending on the nature of the contaminant (e.g., volatile vs. nonvolatile) and site characteristics (e.g., presence or absence of significant ecological habitat). For example, potential environmental hazards flagged based on a comparison of site data to HDOH EALs may in fact not be present under current site conditions but could pose a threat under future conditions (e.g., potential vapor intrusion hazards identified but no buildings currently on site; refer to Section 13 and HDOH 2016)

3.3.3 DEFAULT CONCEPTUAL SITE MODELS

Located within 150m of surface water body or sensitive aquatic habitat? Groundwater is a current or potential drinking water resource Groundwater is NOT a current or potential drinking water resource
Yes Default CSM A Default CSM C
No Default CSM B Default CSM D

Figure 3-7. Four Default Conceptual Site Models Provided in the HDOH Tier 1 EALs

The HDOH Tier 1 EALs are based on an assumption that the environmental hazards noted above (as applicable to the specific contaminant) could exist at a site given adequately high contaminant concentrations within targeted DU areas and the absence of engineered or institutional controls (see Section 13). Four default CSMs or site scenarios are provided for in the Tier 1 EALs, depending on groundwater utility and location of a subject site with respect to nearby surface water bodies and aquatic habitats (Figure 3-7; HDOH 2016):

The default site scenarios reflect the basic CSMs used to develop and compile the Tier 1 EALs and serve as the starting point for preparation of an Environmental Hazard Evaluation (see Section 13). Only surface water bodies that are hydraulically connected to groundwater are considered to be potentially threatened by contaminated groundwater. This could include streams, drainage ways, or even leaky storm sewers that lead to a surface water body. Given their direct and potential rapid link to aquatic habitats, storm sewers in direct hydraulic connection with contaminated groundwater are considered to represent a "surface water body" for initial screening purposes. Measuring the piping invert in relation to the groundwater table at high-high tides and the presence or absence of free product at the discharge point of a storm drain can help determine if it serves as a pathway to a surface water body.

Data for a site are screened against Tier 1 EALs for the default CSMs most appropriate to the subject site. Preparation of a more site-specific CSM is not required, but may be useful or even necessary for sites with extensive contamination and/or significant public interest. One of the four default CSMs should, however, serve as the starting point for more site-specific CSMs. The default CSMs can also be depicted in a more classical "risk assessment" format, as presented for the default CSM in Figure 3-8.

Figure 3-8. Default Conceptual Site Model Default Conceptual Site Model used to develop Tier 1 EALs for sites that overlie a source of drinking water and are within 150m of a surface water body; assumes impacted soil exposed at surface.

Primary Sources Primary Release Mechanism Secondary Sources 1Potential Environmental Hazards 2Hazard Present Under Current or Future Site Conditions? Comments
Current
Future
ASTs, USTs, pipelines, drums, disposal areas, etc. Spills, leaks, improper disposal Soil 3Risk to Human Health Direct Exposure YES  
YES
Vapor Intrusion into Buildings YES  
YES
4Risk to Terrestrial Ecological Habitats YES  
YES
5Leaching YES  
YES
6Gross Contamination YES  
YES
Groundwater 7Risk to Human Health Direct Exposure YES  
YES
Vapor Intrusion into Buildings YES  
YES
8Risk to Aquatic Ecological Habitats YES  
YES
9Gross Contamination YES  
YES

Notes (Figure 3-8):

  1. Refer to Section 13 for discussion of specific environmental hazards. Tier 1, default conceptual site model can be modified on a site-by-site basis as needed.
  2. All noted hazards assumed present or potentially present under current or future site conditions. Exposure pathways assumed complete for toxicity-related hazards.
  3. Human health hazards include direct exposure to contaminated soil or vapors & dust from soil as well as the intrusion of vapors into overlying buildings.
  4. Assumes a significant terrestrial, ecological habitat is impacted by the contamination with resulting toxicity to flora and fauna.
  5. Assumes potential leaching of contaminants from soil and impacts to underlying groundwater.
  6. Gross contamination hazards for soil include potential explosive hazards, odors, interference with construction work (e.g., soil reuse and disposal) and related concerns.
  7. Human health hazards based on ingestion of contaminated groundwater as well as exposure via dermal absorption and vapors during showering and other water use.
  8. Assumes discharge of contaminated groundwater into an aquatic habitat. Contaminants in groundwater screened using chronic, aquatic toxicity action levels for sites <150m from a surface water body (acute toxicity action levels applied if >150m from surface water body).
  9. Gross contamination hazards for groundwater include potential taste & odors concerns for drinking water, presence of free product, explosive hazards, odors, sheens, interference with construction work (e.g., dewatering) and other related concerns.

3.3.4 ADVANCED CONCEPTUAL SITE MODELS

The default CSMs used to develop the HDOH Tier 1 EALs are intentionally designed to be very conservative. The appropriate default CSM should be used to initially screen sites and, where appropriate, clear the site for unrestricted land use with minimal additional effort.

Site-specific CSMs can be prepared by modifying the default CSMs to more closely evaluate potential environmental hazards under current and anticipated future site conditions, as needed. A more detailed CSM is generally warranted at sites where cleanup costs could be significant, or at sites where long-term management of contaminated soil or groundwater will be required. A closer evaluation of current and future risks to human or ecological receptors will be particularly important. These types of CSMs will typically identify sources of contaminant releases, types of contaminated media, migration pathways, exposure pathways, and human and/or ecological receptors.

Figure 3-9. Expanded Conceptual Site Model

Primary Sources Primary Release Mechanism Secondary Sources 1PotentialEnvironmental Hazards 2Hazard Present Under Current or Future Site Conditions?
UST Spills, leaks, improper disposal Soil 3Risk to Human Health Exposure Type Secondary Release Mechanism Exposure Route Receptors
On-Site Workers Offsite Residents Construction Workers
Current Current Current
Future Future Future
Direct Exposure none Ingestion *No *No Yes
*No *No Yes
Dermal *No *No Yes
*No *No Yes
Dust/ Vapors Inhalation *No *No Yes
*No *No Yes
4Vapor Intrusion into Buildings Vapors Inhalation Yes No  na  
Yes No
5Risk to Terrestrial Ecological Habitats Current -No (no habitat or receptors)
Future – No (no habitat or receptors)
6Leaching Current -Yes (soil in contact with groundwater)
Future - Yes
7Gross Contamination Current - Yes (potential explosive vapors)
Future – Yes (potential explosive vapors)
Ground-water 8Risk to Human Health Exposure Type Secondary Release Mechanism Exposure Route Receptors
On-Site Workers Offsite Residents Construction Workers
Current Current Current
Future Future Future
Direct Exposure none Ingestion No No Yes
No No Yes
Dermal No No Yes
No No Yes
Inhalation No No Yes
No No Yes
4Vapor Intrusion into Buildings Vapors Inhalation; Yes No na
Yes No
9Risk to Aquatic Ecological Habitats Current - *No (monitoring shows plume not migrating)
Future - *No (monitoring shows plume not migrating)
10Gross Contamination Current - Yes (free product present)
Future – Yes (while free product present)

Notes (Figure 3-9):

  1. Summary of default environmental hazards to be initially evaluated at all contaminated sites.
  2. Hazard evaluation results based on assumption that contaminated soil is capped with pavement and contaminated groundwater is not migrating (naturally or via storm sewers, dewatering, etc.). *Long-term management of contamination must be addressed in an Environmental Hazard Management Plan in the absence of cleanup.
  3. Exposure pathways for daily workers not complete *provided site remains paved. Potential exposure of construction workers during future subsurface activities.
  4. Recommend collection of soil gas data to further evaluate potential explosive hazards and vapor intrusion hazards.
  5. No significant terrestrial, ecological habitat located on site or threatened by contamination.
  6. Assumes contaminated soil is in direct contact with groundwater. Used to support collection of groundwater data for further evaluation.
  7. Recommend remediation of gross contamination at a minimum to reduce vapor concerns.
  8. Assumes groundwater is not used as a water supply and monitoring indicates that plume is not likely to migrate offsite under natural conditions.
  9. Threat to aquatic habitats assumed insignificant *provided plume is not allowed to migrate offsite. Contaminants screened using acute, aquatic toxicity action levels.
  10. Recommend removal of free product to extent practicable to reduce vapor concerns and continued source of contaminants to groundwater.

Figure 3-9 presents a more site-specific CSM for a hypothetical commercial/industrial site contaminated with petroleum. The CSM includes the following site assumptions:

  • Contamination is restricted to the site boundaries;
  • Area of contaminated soil is paved;
  • Underlying groundwater is not a current or potential source of drinking water;
  • Site is located more than 150m from the nearest surface water body.

A "Yes" in a cell under "Receptors" indicates that the noted exposure route is complete or potentially complete. This is important information for development of short-term or long-term response actions to address human health or ecological risk concerns.

The example CSM documents that the ingestion, dermal absorption and inhalation pathways for direct exposure to the contaminated soil are incomplete for daily on-site workers. Although the inhalation pathway could in theory still be complete, the presence of the pavement can reasonably be assumed to make this pathway insignificant. For construction workers, however, all of the direct-exposure pathways are considered complete because their work may involve removing pavement and disturbing contaminated soil.

The example CSM also indicates that the pathway for leaching of contaminants from soil and contamination of groundwater is complete, because contaminated soil is in direct contact with groundwater, even though the area is assumed to be capped with pavement. This is used to support the collection of groundwater data to more directly evaluate impacts and potential concerns. Removal of pavement could also exacerbate leaching and groundwater contamination due to infiltrating rain or irrigation water. This could require the maintenance of an impermeable cap over the contaminated soil under a long-term management plan prepared for the site (discussed below).

It is important to note that environmental response actions must identify and address all environmental hazards posed by a release, based on both current use and reasonably expected future use scenarios. In many cases, based on the current use of the site and the presence of existing engineered controls, an Environmental Hazard Evaluation might conclude that no current hazards exist (e.g., sites currently used for commercial purposes, with contaminated soil covered by existing buildings and pavements and no vapor intrusion concerns). Contaminant concentrations at the site could, however, indicate a potential hazard under land use scenarios that could lead to completed exposure pathways (e.g., redevelopment for residential use with open areas of exposed soil).

The more detailed CSM may be used to support a conclusion that contaminated soil and groundwater does not pose unacceptable environmental hazards under current site conditions. Depending on site conditions and planned uses, active remediation to eliminate future environmental hazards under any potential land use condition could be recommended or required. If active remediation is not practicable due to site conditions and/or financial constraints, the assumptions used in the CSM to support an absence of potential hazard under current site conditions can be used to develop an Environmental Hazard Management Plan (EHMP; Section 18; see also HDOH 2016). In the example, the EHMP would require that the area of contaminated soil remain capped, that a health and safety plan and soil and groundwater management measures be developed prior to any subsurface construction activities at the site, and that the need for long-term monitoring of groundwater be further evaluated. Actions related to restricted-use site closure, and the preparation of Environmental Hazard Management Plans, are discussed in more detail in Section 19.

A basic understanding of contaminant migration pathways and exposure pathways is necessary to formulate a CSM and guide site investigation and response actions, including preparation of an EHMP. Preparing and submitting a formal, detailed CSM, however, is generally only required at sites where significant contamination exists and cleanup and/or Environmental Hazard Management Plan activities are anticipated to take more than a year to complete.

Additional information on the development of CSMs is available in USEPA’s Guidance for Conducting Remedial Investigations and Feasibility Studies Under CERCLA (USEPA, 1988) and USEPA’s Data Quality Objectives Process for Hazardous Waste Site Investigations (USEPA, 2000). Note that examples of CSMs in these guidance documents often focus on human health or ecological risk assessment concerns and may not consider other potential environmental hazards, including leaching and potential contamination of groundwater (refer to Figure 3-5; see also Section 13). As discussed in Sections 2, 3 and 13, assessments of risk to human and ecological receptors are important parts of a more comprehensive Environmental Hazard Evaluation. Site-specific human health and ecological risk assessments do not replace Environmental Hazard Evaluations, however, and it is important to ensure that all potential hazards at a site are adequately evaluated.

3.3.5 MAINTAINING AND UPDATING THE CONCEPTUAL SITE MODEL

The CSM should be maintained and updated as needed throughout the life of the site activities. As appropriate based on additional site information, refine the CSM to more accurately identify known or suspected sources of contamination, types and concentrations of contaminants detected at the site, potentially contaminated media, potential environmental hazards, potential exposure and migration pathways, potential human and environmental receptors, and related information.

Information that should be used to maintain and continuously update the CSM includes (along with other relevant information):

  • Additional soil, soil vapor or groundwater data;
  • Location of existing monitoring wells and past soil borings;
  • Soil contamination summary figures with areas above EALs highlighted (preferably based on decision unit and Multi Increment sample data);
  • Groundwater contamination summary figures with areas above EALs highlighted;
  • Soil gas survey summary figures with areas above EALs highlighted;
  • Direction of groundwater flow, depth to groundwater;
  • Cross sections that depict the site stratigraphy as well as the lateral and vertical extent of contamination; etc.
  • Identification of existing buildings, structures, infrastructure changes that might affect subsurface conditions or preferential pathways (e.g., addition of underground piping), roads, surface water bodies, neighboring property operations and land uses, geographical features, etc.;
  • Review of sources, exposure pathways, and potential receptors (see example in Figure 3-8);
  • Advanced evaluations of specific environmental hazards.

Significant changes to the CSM may necessitate updates to decision statements (Step 5 of systematic planning), the sampling and analysis plan (Step 6 of systematic planning) and/or the Environmental Hazard Management Plan (see Section 19).