Contents
SCEM Builder

 HELP System

The following Help Topics are available:

SCEM Introduction

Contaminant Sources

Release Mechanisms

Exposure Pathways

Receptors


Introduction

The Office of Environmental Policy and Assistance (EH-41) provides policy guidance and technical assistance on hazardous and mixed waste issues and environmental restoration at Department of Energy (DOE) facilities.  CERCLA and RCRA programs require risk assessments to be conducted at many DOE sites to determine potential impacts to public health and the environment.  In order to develop risk assessment data quality objectives (DQOs), observational approach decisions, and contaminant sampling design specifications, the environmental restoration project team frequently uses a Site Conceptual Exposure Model (SCEM).

The SCEM includes a visual presentation of site exposure conditions that relates the source of contamination to human and ecological receptors.  Based on the SCEM, a data collection strategy is developed to prioritize field sampling activities and reduce uncertainty in risk characterization (e.g., contaminant release/transport mechanisms, receptor profiles, etc.).  The SCEM also provides sufficient information to develop a strategy for early response actions to address exposure pathways that are considered complete and may pose an imminent risk to public health.

DOE environmental restoration program managers (ERPMs) need the ability to develop SCEMs that promptly generate a range of "what-if" scenarios to respond to questions regarding site exposure conditions and data collection programs.  The SCEM Builder provides a friendly graphical environment that considerably shortens the time required to generate SCEM diagrams and associated documentation.


Contaminant Sources

Definition

Contaminated Ground Water or Plume

Cooling Towers and Their Discharges

Drip Pad or Sludge Drying Bed

Fire Training Area

Firing Range

French Drain or Disposal Pit

Lagoon/Pond/Surface Impoundment

Laboratory/Sanitary Sewers and Wastewater Treatment Plants

Landfarm

Landfill

Map Tube Vault and Footing Drain

Oil/Water Separator

Residual Soils and Sediment After Physical Removal

Storage Pad for Waste Drums or Containers

Waste Pile/Open Disposal Area


Contaminated Ground Water or Plume

An area in the ground water aquifer or saturated zone in subsurface soils containing ground water contaminants that may be dissolved or exist as separate phase products (i.e., Light Non-Aqueous Phase Liquids [LNAPL] or Dense Non-Aqueous Phase Liquids [DNAPL]).

Possible secondary sources:

•	Fractured bedrock that has trapped DNAPL

•	Conduits (e.g., utility lines or underground piping/sewers) that have trapped LNAPL

•	Lake, river, stream, bay, harbor, or coastal waters that receive the ground water plume discharges

•	Lake, river, stream, and marine sediment located at the embankment soil/surface water interface that receives the ground water plume discharges


Cooling Towers and Their Discharges
Cooling Towers and Their Discharges -

- Building structures, debris and residuals, and the associated swales and drainage ditches that are currently receiving or have received cooling tower discharges.

Possible secondary sources:

•	Surface and subsurface soils within the contaminant source area

•	Surface soil immediately downgradient from the source area including soil and sediment in swales, drainage ditches and runoff areas, if any

•	Air containing airborne particulates or dust originating from the source area


Drip Pad or Sludge Drying Bed

An above ground area, including enclosures or berms, sumps, and overflow channels, used to drain liquids off wastewater treatment plant sludges, laboratory tank bottoms, or semisolids.  The pad or bed may be constructed of concrete, asphalt, and/or graveled or soil

Possible secondary sources:

•	Surface and subsurface soils within the contaminant source area

•	Surface soil immediately downgradient from the source area, including soil and sediment in swales, drainage ditches, and runoff areas, if any

•	Air containing airborne particulates/dust or gaseous materials originating from the source area


Firing Range

An aboveground area that encompasses razed or existing buildings (indoor pistol ranges) and/or open area areas used by security personnel for target practice.  Unexploded or spent ammunition containing lead and copper are typical wastes associated with firing ranges.

Possible secondary sources:

•	Surface and subsurface soils (for a demolished indoor range) within the contaminant source area (the entire range area)

•	Surface soil immediately downgradient from the source area, including soil and sediment in swales, drainage ditches, and runoff areas, if any


Fire Training Area

An aboveground area that may contain existing or razed mock-up used to train fire fighting personnel on fire control techniques.  A fire training area is typically an outdoor facility that may not be readily identifiable if the area has no engineering structures (e.g., concrete or gravel pads) and has not been used for a long time.

Possible secondary sources:

•	Surface and subsurface soils within the contaminant source area

•	Surface soil immediately downgradient from the source area, including soil and sediment in swales, drainage ditches, and runoff areas, if any

•	Air containing airborne particulates/dust originating from the source area


French Drain or Disposal Pit

A below ground structure or excavation that may or may not contain engineered piping sections/extensions.  It may contain gravel, stones, or other aggregates to allow seepage of liquid waste or the disposal of semi-liquids.  This source area may have been filled in with soil and may not be visually identifiable on the ground surface.  Historical aerial photos may show these areas barren or devoid of vegetation or ground cover.

Possible secondary sources:

•	Surface and subsurface soils within the contaminant source area, including those of the lateral sections or extensions

•	Surface soil immediately downgradient from the source area, including soil and sediment in swales, drainage ditches, and runoff areas, if any

•	Air containing airborne particulates/dust or gaseous materials originating from the source area

•	Infiltration seeps at downgradient locations that may or may not be mixed with surface water or shallow groundwater.


Laboratory/Sanitary Sewers and Wastewater Treatment Plants

An area that includes existing or razed structures associated with the treatment of laboratory and/or sanitary wastewaters.  Many of these structures may be buried or abandoned in place, broken, and contain sludges that are radioactive or that could be classified as hazardous or mixed wastes.

Possible secondary sources:

•	Subsurface soils within the contaminant source area

•	Conduits (i.e., including the abandoned in place underground treatment plant components, sewer main, manholes, and lateral sections or extensions) that were part of the sewer systems

•	Surface soil (at razed structures or subsurface soil that has been turned over due to demolition and excavation) and surface soil located immediately downgradient from the source area, including soil and sediment in swales, drainage ditches, and runoff areas, if any

•	Air containing airborne particulates/dust originating from the source area


Landfarm

An aboveground area or open space on the ground surface where solid waste such as sludges or contaminated soil or sediment are placed, dumped, or discharged, and commingled with soil by mechanical means (e.g., bulldozer or tractor equipped with soil mixing discs.  Abandoned landfarms or landfarms that have not been used for a long time may not be visually identifiable.

Possible secondary sources:

•	Surface and subsurface soils within the contaminant source area

•	Surface soil immediately downgradient from the source area, including soil and sediment in swales, drainage ditches, and runoff areas, if any

•	Air containing airborne particulates/dust or gaseous materials originating from the source area


Landfill

A below ground area filled with waste materials, debris, sludges, or contaminated soils.  This source is typically abandoned or not operational, uncontrolled, or not designed according to EPA's minimum technology requirements for a land-based disposal unit, and may include the entire physical boundary of the landfill or a portion thereof (e.g., old cells).  This source may be covered by soil or an artificial barrier, or may contain exposed contaminated structures, drums, or contaminated surface soil.

Possible secondary sources:

•	Subsurface soils that are commingled or in contact with the waste within the landfill

•	Surface soil or exposed subsurface soil that is commingled or in contact with the waste within the landfill and surface soil located immediately downgradient from the source area, including soil and sediment in swales, drainage ditches, and runoff areas, if any

•	Air containing airborne particulates/dust or gaseous materials vented from the landfill or originating through the landfill cover

•	Infiltration seeps at downgradient locations that may or may not be mixed with surface water or shallow groundwater


Lagoon/Pond/Surface Impoundment

A below ground or partially aboveground structure or excavation area containing waste (solid and liquid), a mixture of waste and runoff/precipitation, or their residuals.  It may include natural or engineered physical structures such as pipes, swales, and embankments.  Equalization ponds for receiving process water and process water/stormwater combined flow are in this category.

Possible secondary sources:

•	Subsurface soils that are commingled or in contact with the waste within the lagoon/surface impoundment

•	Surface soil or exposed subsurface soil that is commingled or in contact with the waste within the surface impoundment and surface soil located immediately downgradient from the impoundment area, including soil and sediment in swales, drainage ditches, and runoff areas, if any

•	Air containing airborne particulates/dust or gaseous materials originating from the impoundment or its contaminated dikes or embankments

•	Infiltration seeps at downgradient locations that may or may not be mixed with surface water or shallow groundwater


Map Tube Vault and Footing Drain

A below ground vault with drainage footing tile systems to divert subsurface water.  The vault has multiple compartments (tubes) that can be used to store high level or transuranic wastes that may also contain hazardous substances.  Due to maintenance or construction problems, it is not uncommon to find that the vault's structures have been degraded

Possible secondary sources:

•	Surface and subsurface soils within the contaminant source area, including the subsurface soil at the footing drain

•	Surface soil immediately downgradient from the source area, including soil and sediment in swales, drainage ditches, and runoff areas, if any

•	Air containing airborne particulates/dust originating from the source area


Oil/Water Separator

A below ground concrete structure or tank-like device used to trap or separate oil from process water or fluids containing oil as a free-phase product.  It may also contain sludges or residues.  Many oil/water separators were abandoned in place, cracked, or had their aboveground structures partially demolished.

Possible secondary sources:

•	Surface and subsurface soils within the contaminant source area, including the subsurface soil surrounding the inlet and discharge pipes

•	Surface soil immediately downgradient from the source area including soil and sediment in swales, drainage ditches, and runoff areas, if any

•	Air containing airborne particulates/dust and gaseous materials originating from the source area


Residual Soils and Sediment After Physical Removal

Surface soil, subsurface soil, or sediments remaining in an area that has undergone physical removal actions (e.g., demolition of a contaminated building).  Pursuant to a removal action, physical structures, such as receptacles, tanks, containers, and buildings, and visually contaminated soils, sediments or aggregates may have been removed; however, the residual media may still be contaminated and could be a source for potential exposure pathways.

Possible secondary sources:

•	Groundwater in the vadose and saturated zones

•	Infiltration seeps at downgradient locations that may or may not have been mixed with surface water or shallow groundwater


Storage Pad for Waste Drums or Containers

A concrete, asphalt, or graveled razed building foundation, or simply a clear open area on the ground.  Typically, debris, abandoned equipment or articles, and waste containers of different sizes and shapes are stored on these pads.

Possible secondary sources:

•	Surface and subsurface soils (if the pad has poor integrity) within the contaminant source area

•	Surface soil immediately downgradient from the source area, including soil and sediment in swales, drainage ditches, and runoff areas, if any


Waste Pile/Open Disposal Area

An above groundarea or a depression on the ground surface where scrap metal, solid waste, drums (with or without their contents), and other contaminated aggregates or media are placed, discharged, or abandoned.  The waste pile may or may not have an engineered barrier such as concrete or asphalt pads between the waste materials and the soil.  Typically, the waste pile is not covered by a roof, although sometimes canvas or plastic tarp may be used to cover the waste materials either partially or entirely.

Possible secondary sources:

•	Surface and subsurface soils (if the pile has no engineering barrier or if the barrier has poor integrity) within the contaminant source area

•	Surface soil immediately downgradient from the source area, including soil and sediment in swales, drainage ditches, and runoff areas, if any

•	Air containing airborne particulates/dust and gaseous materials originating from the source area


Release Mechanisms

Definition

Chemical Transformation

Digging and Burrowing

Dredging and Tracking

Force of Gravity

Hydraulic Pressure

Leaching

Runoff

Solubility

Tidal Action

Uptake

Volatilization

Wind Erosion


Chemical Transformation

This release mechanism modifies the effectiveness of other release mechanisms.  During chemical transformation, a hazardous substance (chemical or radionuclide) changes its physical and chemical characteristics, making the hazardous substance more or less amenable to other release mechanisms.  An example of chemical transformation is the conversion of lead to lead chloride or lead sulfate in acidic soil.  The resulting lead compounds are more soluble than metallic lead, which could be subject to leaching action of infiltration.  Other examples include the hydrolysis of many chemicals with water, the methylation of mercury, radiological decay, and photolysis.


Digging and Burrowing

Similar to dredging and tracking, this release mechanism refers to human or animal activities that cause the hazardous substance and/or their constituents to be exposed and subject to other release mechanisms.  Construction activities that entail soil or sediment excavation, and burrowing actions by small mammals are examples of this release mechanism.


Dredging and Tracking

This release mechanism is caused by the frictional force posed by the act of dredging or tracking (vehicular or foot traffic).  Under this release mechanism, the hazardous substances and/or their constituents are released from the matrix surfaces (solid, semisolid, or liquid) into air in the form of particulates or fugitive air emissions, or into water in the form of solubilized chemicals or suspended solids.  Dredging or tracking actions also serve as migration pathways that transport the released hazardous substances and their constituents to other media or locations.


Force of Gravity

This release mechanism is most applicable to sources that have inadequate containment systems or poor integrity.  Hazardous substances and their constituents may leak from the unit (e.g., tank, container, surface impoundment, waste pile, highly contaminated soil, waste disposal area or cell) to the soils adjacent to the unit or source.  Under this release mechanism, the mass of the hazardous substances is pulled by gravity toward the subsurface strata through the path of least resistance.


Hydraulic Pressure

This release mechanism refers to the water pressure that is being applied to the source which causes the hazardous substances and their constituents to leak out or to be released from the source or structures/barriers.  A typical example is the leaching of hazardous waste or hazardous constituents through the liner of a surface impoundment or landfill.


Leaching

This release mechanism refers to the movement of soluble chemicals via infiltration into surface soils.  As a result of precipitation, runoff from storm, and/or groundwater recharge, the infiltration leaching action removes the hazardous substances and their constituents from the source (soils) to another medium (e.g., perched groundwater and aquifer).  The release mechanism could be viewed as the combined mechanisms of gravitational force, hydraulic pressure, and solubility.  Leaching also serves as a migration pathway that transports the released hazardous substances and their constituents to other media or locations.


Runoff

This release mechanism refers to the physical force, posed by surface water moving in a downstream direction, that removes the hazardous substances and their constituents from the source and transports them to another medium.  Runoff also serves as a migration pathway that transports released hazardous substances and their constituents to other media or locations.  The effectiveness of this release mechanism is typically related to other release mechanisms, such as solubility and chemical transformation, that may be occurring.  Runoff action is applicable to inland areas with measurable topographic relief, and is a predominant cause of release of hazardous substances in contaminated soils along swales, channels, river banks, or groundwater recharge areas.  Flooding or overland flow may be considered a runoff mechanism on a larger scale.


Solubility

This release mechanism refers to the forming of chemical bonds within the solvent or water matrices.  Polar compounds form ionic bonds with the water molecules, acids, or bases.  Non-polar compounds form weak intermolecular bonds with van der Waals forces in common solvents like kerosene, jet fuels, hexane, or isopropyl alcohol.  Release stops when the solubility or saturation limit is reached within the solubilizing agent.


Tidal Action

Like runoff, this release mechanism refers to the physical force posed by the movement of surface water caused by gravitational force from the moon (diurnal) or storms (episodic).  Tidal action can act as both a release and a transport mechanism, removing the hazardous substances and their constituents from one medium to another, as well as from one location to another.  Tidal action is applicable to large inland lakes, and marine and coastal areas.


Uptake

This release mechanism refers to physical and/or chemical means occurring at the barrier between an organism and the contaminated media.  Under this release mechanism, the hazardous substances and/or their constituents are released from the matrix (food, soil, sediment, and surface water) by physical means (e.g., molecular diffusion), biochemical means (e.g., active transport), or chemical means (stomach acids), or a combination of the above.  The biochemical processes following ingestion and plant translocation mechanism at the root zone are examples of uptake release mechanism.


Volatilization

This release mechanism is dependent on the chemical characteristics of the hazardous substance (i.e., molecular weight, vapor pressure, Henry's Law constant, boiling point, etc.), ambient temperature, and wind velocity or air movement, where appropriate.  Under this release mechanism, the hazardous substances and/or their constituents are released from the matrix surfaces (solid or liquid) into air (pores in soil or the ambient air).  This release mechanism is most applicable to volatile organic chemicals and,to a lesser extent, semivolatile organic chemicals.


Wind Erosion

This release mechanism refers to the frictional force, posed by air movement near the earth surface, that removes the hazardous substances and their constituents from the source to air.  Under this release mechanism, the hazardous substances and/or their constituents are released from the matrix surface into air.  This release mechanism is most applicable to metals and semivolatile organic chemicals.


Exposure Pathways

Definition

Dermal Contact

Ingestion

Inhalation


Inhalation

This exposure pathway refers to the exposure to released hazardous substances and their constituents at the point of exposure (location where the exposure medium [air] is present) via the inhalation route.

The released substances and constituents present in air may be inhaled if the receptor is located downwind from the source.


Ingestion

This exposure pathway refers to the exposure to released hazardous substances and their constituents at the point of exposure (i.e., location where exposure media, [e.g., soil, sediment, surface water, ground water, or biota] are present) via the ingestion route.

The released substances and constituents present in groundwater or surface water may be ingested if the ground water is potable and provides sufficient yield, and the receptor well is located downgradient from the source and is hydraulically connected to the contaminated plume.  The released substances and constituents may be present in biota if the contaminant or chemical is bioaccumulative (e.g., PCBs, dioxins/furans, PAHs, and certain metals [e.g., antimony, arsenic, barium, beryllium, cadmium, chromium, lead, nickel, mercury, selenium, silver, and thallium) and  is attractive to ecological species due to availability of food, and harborage/shelter.


Dermal Contact

This exposure pathway refers to the exposure to released hazardous substances and their constituents at the point of exposure (i.e., location where exposure media [e.g., soil, sediment, surface water, groundwater, or biota] are present) via the dermal route.  The migration pathways are identical to the ingestion pathways.


Receptors

Definition

Aquatic

Occupational

Public

Terrestrial

Trespasser/Non-Residential Visitors


Public

Public receptors include humans who live or work near the site (off-site residents and workers).

Public receptors may be exposed to airborne contaminants (gaseous or fugitive dust emissions/particulates) if they are located downwind from the site;  or to contaminants in surface water or groundwater, if these waters are used and if contamination is feasible or has occurred, and if the flow directions are toward these receptors.  Public receptors may also be exposed to contaminants via ingestion of contaminated food/species collected or hunted.


Occupational

Occupational receptors include persons who live or work on site.  They may be office workers, groundkeepers, or construction workers.  They may also be on-site industrial workers or visitors.  Depending on their activities and where they are located relative to the source area, these humans may be exposed via airborne contaminants (gaseous [including indoor air] or fugitive dust emissions/particulates), incidental soil and sediment ingestion, and dermal contact.  If they use on-site wells or surface water, they may be exposed to the contaminants if such media have been contaminated.  If food chain species have been impacted, these persons may be impacted if such food or species are collected or hunted and ingested, although this is highly unlikely in most occupational settings.


Trespasser/Non-Residential Visitors

These receptors include humans who live off site and come in contact with the site at a frequency less than on-site workers or residents.  They include scholar/student visitors, contract personnel, and delivery personnel.  The relevancy of these receptor categories will depend on the exposure area and the anticipated activities of the receptor groups.  Depending on their activities and where they trespass or visit the site and the source area, these receptors could be exposed via airborne contaminants (gaseous emissions or fugitive dust emissions/particulates), incidental soil and sediment ingestion, or dermal contact.  If they wade or swim, they may be exposed to the contaminants in on-site sediment or surface water.  If food chain species have been impacted, these receptors may be impacted if such food/species are collected or hunted and ingested.


Terrestrial

These receptors include wild, domestic, farm, and game animals.  These animals may be subject to exposure from airborne contaminants (gaseous or fugitive dust emissions/ particulates) if the ambient air is contaminated.  Moreover, if these receptors use contaminated surface or groundwater or are provided with such water for consumption, they may be exposed.  If these species ingest food crops or other species that bioaccumulate site contaminants, these terrestrial species may also be impacted.  On-site burrowing animals may be impacted or exposed via incidental soil ingestion and dermal contact routes of exposure.


Aquatic

These receptors include both free-swimming and benthic organisms.  These species could be exposed to contaminated surface water and sediment if such contaminants have been released or transported to the location where these species are located.  Groundwater infiltration (intersecting surface water), runoff carrying contaminated soil or sediment, or surface water could cause these species to be exposed.  Bottom feeders or prey fish may ingest contaminated benthic invertebrates or other aquatic species if the site contaminants are bioaccumulative and have been transported from the site to surface water bodies.


Definition of Receptor

The human or ecological species that is potentially exposed to contaminants at the source area or other locations where contaminants were transported because of release mechanisms.  A receptor may be subject to several exposure pathways.  Receptor characterization includes determining locations of potential receptors, activity patterns, and the presence of subpopulations.  This characterization is also conducted for future land uses.

Many receptor types or species could be exposed to the hazardous substances or radionuclides released and transported from the source to the exposure media.  According to Risk Assessment Guidance for Superfund (RAGS), only significant receptors are to be evaluated for exposure pathway completeness.  These receptors should be those who have the highest degree of exposure (high-end exposure), based on consideration of the exposure patterns (frequency, duration, and contact rate).  In the case of ecological receptors, any identified threatened or endangered species should also be considered.


Definition of Exposure Pathway

This is a term used to describe the manner by which hazardous substances or radionuclides released from the source and transported to the exposure media  (e.g., soil, sediment, groundwater, indoor air, ambient air, surface water, and food/biota) are available to the organism (humans or ecological receptors) for absorption at the barrier surfaces


Definition of Release Mechanism
Definition of Release Mechanisms

The manner by which hazardous substances and their constituents (chemicals or radionuclides) are released or made to leave their sources or matrices of sources.  Under CERCLA and RCRA, a potential area of concern or unit would not meet the definition of a release site or an SWMU unless a release of hazardous waste or hazardous constituents has occurred.

The release mechanism can be physical, chemical, thermal, or radiological.  When a release mechanism is presented to the sources, transfer of the hazardous substances and their constituents to another medium or media is likely to occur, given the time and intensity of the release mechanisms.  The physical and chemical nature of the hazardous substances or radionuclides in the source and its immediate environment (e.g., acidity, alkalinity, and sorptive properties of the receiving media) may influence the effectiveness of the release mechanism.

Several release mechanisms can be working simultaneously at a source to facilitate the release of hazardous substances or radionuclides.  Unless the source is continuously being recontaminated, release from a source will likely result in lowering the original contaminant level(s).


Definition of Contaminant Source
Definition of Contaminant Sources

An area where hazardous substances (including waste and waste constituents) and/or radionuclides are located and/or media where these substances or radionuclides have been deposited, placed, released, or disposed.  This is a general term describing a site, a solid waste management unit (SWMU), or an area of contamination or concern (AOC) for which cleanup decisions are applicable.  It describes the physical features or boundaries that are potentially contaminated with hazardous substances or radionuclides attributable to current or past operations at the area.

Contaminant sources may be grouped as primary, secondary or tertiary.  The differences among these groups are subtle, particularly those differences between secondary and tertiary sources.  All of these source groups include contaminated media (e.g., soils, sediment, surface water, groundwater, and air).  In addition to contaminated media, the primary source may include containment devices (such as drums, tanks, lagoons) and physical structures, piping, or appurtenances containing the bulk or residuals of hazardous substances, hazardous wastes or hazardous constituents, and radionuclides.

Contamination of secondary and tertiary sources results from releases from the primary source, intermedia transfer, and or transport mechanisms. Contaminated biota could be secondary or tertiary sources for a primary source that has chemicals or radionuclides that are bioaccumulative and amenable to uptake by terrestrial or aquatic species.