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Appendix B: Definitions

Biologically Active Soil – In the context of a PVI evaluation means that the subsurface soil environment will support populations of microorganisms that are present in sufficient quantities to aerobically degrade PHC vapors before they intrude into a receptor. Effective aerobic biodegradation of petroleum hydrocarbons depends on the soil having sufficient oxygen and enough soil water to provide a habitat for adequate populations of active microorganisms. Soil that is too dry will not support microbial life. The soil will not be too dry for bacteria if the depth to the water table is less than 300 feet, or if the soil around the receptor supports the growth of plants characteristic of temperate climates. See USEPA 2015a.

Conceptual Site Model or CSM – An integration of site information and interpretations pertaining to the physical, chemical, transport, and receptor characteristics present

Direct Volatilization – A subset of the VIAP and is the process by which vapors of volatile hazardous substance directly volatilize or migrate into a structure’s breathing space without migrating through the vadose zone resulting in a potential unacceptable exposure risk to human health

Facility – As defined in Sec. 20101

Hazardous substance – As defined in Sec. 20101(1)(x)

Interpolated – to estimate values of (data or a function) between two known values

Institutional control or IC – As defined in Sec. 20121(8) and R 299.1(q). NOTE: See also “Land or resource use restrictions” in Sec. 20121 and R 299.2(a)

Mobile NAPL – As defined in Sec. 20101(1)(ee) via Sec. 21302

Multiple Lines of Evidence or MLE – Field data, modeling, and other pertinent site information from multiple sources used to assess a specific relationship or conclusion. Typically, this approach is used to support a conclusion because the analysis may not be fully supported on its own.

NAPL body – As defined in EGLE, 2023

NAPL – As defined in Sec. 20101(1)(ff) via Sec. 21303(a)

Petroleum – As defined in Sec. 20101(1)(x)(iv) in "Hazardous substance" via Sec. 21303

Petroleum Hydrocarbons or PHC – Hydrocarbons derived from petroleum and present in various refined products of petroleum (such as automotive gasoline, diesel fuel, lubricating oils).

Petroleum Release – See Petroleum and Release

Petroleum Vapor Intrusion or PVI – See Petroleum and Vapor Intrusion

Release – As defined in Sec 20101(1)(pp)

Response Activity – As defined in Sec. 20101(1)(vv)

Soil Vapor – a term used to describe vapor in soil (see definition below)

Soil Gas – Gas found in the air space between soil particles or components

Total Petroleum Hydrocarbons or TPH – As defined in EGLE, 2023

Vapor – is a broad term used in this document to describe the vapor forming chemicals in gas that has migrated from any subsurface vapor source.

Vapor Intrusion or VI The migration of potentially hazardous vapors from any subsurface vapor source through the vadose zone and into a building or structure.

Vapor Source – The presence of volatile hazardous substance associated with soil, groundwater, or NAPL that may form vapors.

Vertical Separation Distance – Is the thickness of clean, biologically active soil between the highest vertical extent of a vapor source and the lowest point of an overlying building. This lowest point could be a building basement floor, foundation, or crawlspace surface. It is also used to describe the separation distance from a sampling location at depth beneath a structure.

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