Texas Administrative Code › Title 30 - ENVIRONMENTAL QUALITY › Part 1 - TEXAS COMMISSION ON ENVIRONMENTAL QUALITY › Chapter 350 - TEXAS RISK REDUCTION PROGRAM › Subchapter A - GENERAL INFORMATION › Section 350.4 - Definitions and Acronyms

Current through Reg. 49, No. 28; July 12, 2024
(a) Definitions.
(1) Affected property--The entire area (i.e.,
				  on-site and off-site; including all environmental media) which contains
				  releases of chemicals of concern at concentrations equal to or greater than the
				  assessment level applicable for residential land use and groundwater
				  classification.
(2) Alternate point
				  of exposure--A location other than the prescribed point of exposure where an
				  individual human or population will be assumed to have a reasonable potential
				  to come into contact with chemicals of concern based on property-specific
				  considerations.
(3) Assessment
				  level--A critical protective concentration level for a chemical of concern used
				  for affected property assessments where the human health protective
				  concentration level is established under a Tier 1 evaluation as described in
				  §
											350.75(b)
				  of this title (relating to Tiered Human Health Protective Concentration Level
				  Evaluation), except for the protective concentration level for the
				  soil-to-groundwater exposure pathway which may be established under Tier 1, 2,
				  or 3 as described in §
											350.75(i)(7)
				  of this title, and ecological protective concentration levels which are
				  developed, when necessary, under Tier 2 and/or 3 in accordance with
				  §350.77(c) and/or (d), respectively, of this title (relating to Ecological
				  Risk Assessment and Development of Ecological Protective Concentration
				  Levels).
(4) Attenuation action
				  level--The maximum concentration of a chemical of concern which can be present
				  at an attenuation monitoring point and not exceed the applicable critical
				  protective concentration level at the points of exposure over time.
(5) Attenuation monitoring point--A location
				  within the migration pathway of a chemical of concern which is used to verify
				  that the critical PCL will not be exceeded at the points of exposure.
(6) Background--A population of
				  concentrations characterized from samples in an environmental medium containing
				  a chemical of concern that is naturally occurring (i.e., the concentration is
				  not due to a release of chemicals of concern from human activities) or
				  anthropogenic (i.e., the presence of a chemical of concern in the environment
				  which is due to human activities, but is not the result of site-specific use or
				  release of waste or products, or industrial activity). Examples of
				  anthropogenic sources include non-site specific sources such as lead from
				  automobile emissions, arsenic from use of defoliants, and polynuclear aromatic
				  hydrocarbons resulting from combustion of hydrocarbons. There are some
				  commonalities regardless of the activity; specifically, the chemicals of
				  concern have resulted from the use of a product in its intended manner and may
				  be present at generally low levels over large areas (tens of square miles up to
				  hundreds of square miles). Background is required for use in a statistical
				  model appropriate for testing the hypothesis that the background area
				  characterized by these kinds of models has the same concentrations of the
				  chemical of concern as the affected property. The background area characterized
				  is as "close" as possible to the affected property, in either space or time, as
				  required.
(7) Bedrock--The solid
				  rock (i.e., consolidated, coherent, and relatively hard naturally formed
				  material that cannot normally be excavated by manual methods alone) that
				  underlies gravel, soil or other surficial material.
(8) Bioaccumulative chemical of concern--A
				  chemical of concern which has the tendency to accumulate in the tissues of an
				  organism as a result of food consumption or dietary exposure and/or direct
				  exposure (e.g., gills and epithelial tissue) to an environmental
				  medium.
(9) Carcinogen--A chemical
				  of concern which causes an increased incidence of benign or malignant
				  neoplasms, or substantially decreases the time to develop neoplasms, in animals
				  or humans (a chemical of concern can act as both a carcinogen and a
				  noncarcinogen).
(10) Carcinogenic
				  risk level--The probability of development of a neoplasm due to continuous
				  lifetime exposure to a single carcinogen acting through an individual or
				  combined exposure pathway.
(11)
				  Chemical of concern--Any chemical that has the potential to adversely affect
				  ecological or human receptors due to its concentration, distribution, and mode
				  of toxicity. Depending on the program area, chemicals of concern may include
				  the following: solid waste, industrial solid waste, municipal solid waste, and
				  hazardous waste as defined in the Texas Health and Safety Code, §
											361.003, as amended;
				  hazardous constituents as listed in 40 Code of Federal Regulations Part 261,
				  Appendix VIII, as amended; constituents on the groundwater monitoring list in
				  40 Code of Federal Regulations Part 264, Appendix IX, as amended; constituents
				  as listed in 40 Code of Federal Regulations Part 258 Appendices I and II, as
				  amended; pollutant as defined in Texas Water Code, §
											26.001, as amended;
				  hazardous substance as defined in the Texas Health and Safety Code, §
											361.003, as amended,
				  and Texas Water Code, §
											26.263, as amended;
				  other substances as defined in Texas Water Code, §
											26.039(a),
				  as amended; and daughter products of the aforementioned constituents.
(12) Closure--The act of permanently taking a
				  waste management unit or facility out of service.
(13) Commercial/industrial land use--Any real
				  property or portions of a property not used for human habitation or for other
				  purposes with a similar potential for human exposure as defined for residential
				  land. Examples of commercial/industrial land use include manufacturing;
				  industrial research and development; utilities; commercial warehouse
				  operations; lumber yards; retail gas stations; auto service stations; auto
				  dealerships; equipment repair and service stations; professional offices
				  (lawyers, architects, engineers, real estate, insurance, etc.); medical/dental
				  offices and clinics (not including hospitals); financial institutions; office
				  buildings; any retail business whose principal activity is the sale of food or
				  merchandise; personal service establishments (health clubs, barber/beauty
				  salons, mortuaries, photographic studios, etc.); churches (not including
				  churches providing day care or school services other than during normal worship
				  services); motels/hotels (not including those which allow residence);
				  agricultural lands; and portions of government-owned land (local, state, or
				  federal) that have commercial/industrial activities occurring. Land use
				  activities consistent with this classification have the North American
				  Industrial Classification System code numbers 11 - 21 inclusive; 22 except
				  22131; 23 - 56 inclusive; 61 except 61111, 61121, and 61131; 62 except 62211,
				  62221, 62231, 62311, 62322, 623311, 623312, 62399, and 62441; 71 except 71219;
				  72 except 721211 and 72131; 81 except 814; and 92 excluding 92214.
(14) Community--An assemblage of plant and
				  animal populations occupying the same habitat in which the various species
				  interact via spatial and trophic relationships (e.g., a desert community or a
				  pond community).
(15) Compensatory
				  ecological restoration--The creation of ecological services by or through
				  restoration or the setting aside of, preferably, a comparable type of habitat
				  as that which is impacted to offset residual ecological risk at an affected
				  property. A net environmental benefits analysis or similar evaluation of
				  ecological services may be used in the determination of the appropriate level
				  of compensation.
(16) Complete
				  exposure pathway--An exposure pathway where a human or ecological receptor is
				  exposed to a chemical of concern via an exposure route (e.g., incidental soil
				  ingestion, inhalation of volatiles and particulates, consumption of prey,
				  etc.).
(17) Construction zone--The
				  typical depth of construction within soil for an affected property considering
				  the planned or historical installation of subsurface utilities, foundations,
				  basements, or other such subsurface structures within the vicinity of the
				  affected property not to extend below the top of bedrock.
(18) Control--To apply physical or
				  institutional controls to prevent exposure to chemicals of concern. Control
				  measures must be combined with appropriate maintenance, monitoring, and any
				  necessary further response action to be protective of human health and the
				  environment.
(19) Critical
				  protective concentration level--The lowest protective concentration level for a
				  chemical of concern within a source medium determined from all of the
				  applicable human health exposure pathways as described in §
											350.71 of
				  this title (relating to General Requirements), and when necessary, protective
				  concentration levels for applicable ecological exposure pathways as required in
				  §
											350.77
				  of this title (relating to Ecological Risk Assessment and Development of
				  Ecological Protective Concentration Levels).
(20) Cumulative carcinogenic risk--The
				  aggregate risk due to exposure of an individual human receptor to multiple
				  carcinogens originating from a single affected property and acting through an
				  individual or combined exposure pathway.
(21) Decontaminate--Application or occurrence
				  of a permanent and irreversible treatment process to a waste or environmental
				  medium so that the threat of release of chemicals of concern at concentrations
				  above the critical protective concentration levels is eliminated.
(22) Deed notice--An instrument filed in the
				  real property records of the county where the affected property is located that
				  is intended to provide to owners, prospective buyers and others notice and
				  information regarding, but which does not, by itself, restrict use of the
				  affected property.
(23)
De
				  minimus
--The description of an area of affected property comprised of
				  one acre or less where the ecological risk is considered to be insignificant
				  because of the small extent of contamination, the absence of protected species,
				  the availability of similar unimpacted habitat nearby, and the lack of adjacent
				  sensitive environmental areas.
(24)
				  Ecological benchmark--A state standard, federal guideline, or other exposure
				  level for a chemical of concern in water, sediment, or soil that represents a
				  protective threshold from adverse ecological effects. An ecological benchmark
				  may also be a toxicity reference value that is established by the person based
				  on scientific studies in the literature.
(25) Ecological hazard index--The sum of
				  individual ecological hazard quotients of COCs within a class of compounds that
				  exert ecological effects which have the same toxicological mechanism or
				  endpoint (e.g., PAHs, PCBs).
(26)
				  Ecological hazard quotient--The ratio of an exposure level to a chemical of
				  concern to a toxicity value selected for the risk assessment for that chemical
				  of concern (e.g., a no observed adverse effects level).
(27) Ecological protective concentration
				  level--The concentration of a chemical of concern at the point of exposure
				  within an exposure medium (e.g., soil, sediment, groundwater, or surface water)
				  which is determined in accordance with §
											350.77(c)
				  or (d) of this title (relating to Ecological
				  Risk Assessment and Development of Ecological Protective Concentration Levels)
				  to be protective for ecological receptors. These concentration levels are
				  primarily intended to be protective for more mobile or wide-ranging ecological
				  receptors and, where appropriate, benthic invertebrate communities within the
				  waters in the state. These concentration levels are not intended to be directly
				  protective of receptors with limited mobility or range (e.g., plants, soil
				  invertebrates, and small rodents), particularly those residing within active
				  areas of a facility, unless these receptors are threatened/endangered species
				  or unless impacts to these receptors result in disruption of the ecosystem or
				  other unacceptable consequences for the more mobile or wide-ranging receptors
				  (e.g., impacts to an off-site grassland habitat eliminate rodents which causes
				  a desirable owl population to leave the area).
(28) Ecological risk assessment--The process
				  that evaluates the likelihood that adverse ecological effects may occur or are
				  occurring as a result of exposure to one or more stressors; however, as used in
				  this context, only chemical stressors (i.e., COCs) are evaluated.
(29) Ecological services--The physical,
				  chemical, or biological functions of natural resources that one natural
				  resource provides for another or to the public. Examples include provision of
				  food, protection from predation, and nesting habitat, among others.
(30) Ecological services analysis--A
				  measurement of the potential change in ecological services based on
				  considerations which may include, but are not limited to: the percent change in
				  ecological services at the affected property that are attributable to COCs
				  and/or potential response actions; the spatial extent of the affected property;
				  and the recovery period.
(31)
				  Environmental medium--A material found in the natural environment such as soil
				  (including non-waste fill materials), groundwater, air, surface water, and
				  sediments, or a mixture of such materials with liquids, sludges, gases, or
				  solids, including hazardous waste which is inseparable by simple mechanical
				  removal processes, and is made up primarily of natural environmental
				  material.
(32) Exclusion
				  criteria--Those conditions at an affected property which preclude the need to
				  establish a protective concentration level for an ecological exposure pathway
				  because the exposure pathway between the chemical of concern and the ecological
				  receptors is not complete or is insignificant.
(33) Exposure area--The smallest property
				  surface area within which it is believed that exposure to chemicals of concern
				  in soil or air by a receptor would be limited under reasonably anticipated
				  current or future use scenarios.
(34) Exposure medium--The environmental
				  medium or biologic tissue in which or by which exposure to chemicals of concern
				  by ecological or human receptors occurs.
(35) Exposure pathway--The course that a
				  chemical of concern takes from a source area to ecological or human receptors
				  and includes a source area, a point of exposure, and an exposure route (e.g.,
				  ingestion), as well as a transport mechanism if the point of exposure is
				  different from the source area.
(36) Facility--The installation associated
				  with the affected property where the release of chemicals of concern
				  occurred.
(37) Facility Operations
				  Area--One or more areas (lateral and vertical extent) of an operational
				  chemical or petroleum manufacturing plant with North American Industrial
				  Classification System code numbers 325 or 324, respectively, with a hazardous
				  waste permit or commission corrective action order within which response
				  actions to multiple releases of COCs can be consolidated for purposes of
				  compliance with this chapter on an area-wide basis by using interim or
				  permanent response actions. The lateral extent of the facility operations area
				  is limited to the contiguous area actively used for the development,
				  manufacture, process, transfer, storage, and management of chemical or refinery
				  products, hazardous materials, substances and wastes subject to Resource
				  Conservation and Recovery Act regulation, and includes ancillary components
				  such as, but not necessarily limited to, power plants and cooling
				  units.
(38) Feeding guilds--Groups
				  of ecological receptors used to represent the variety of species that may be
				  exposed to chemicals of concern at the affected property. The feeding guilds
				  are generally based on function within an ecosystem, potential for exposure,
				  and physiological and taxonomic similarity. Examples include carnivorous
				  mammals, carnivorous birds, and piscivorous birds.
(39) Functioning cap--A low permeability
				  layer or other approved cover meeting its design specifications to minimize
				  water infiltration and chemical of concern migration, and prevent ecological or
				  human receptor exposure to chemicals of concern, and whose design requirements
				  are routinely maintained.
(40)
				  Groundwater-bearing unit--A saturated geologic formation, group of formations,
				  or part of a formation which has a hydraulic conductivity equal to or greater
				  than 1 x 10-5 centimeters/second.
(41) Groundwater production zone--The
				  groundwater-bearing unit(s) which contributes water to a well. For example, if
				  a well penetrates four distinct groundwater-bearing units isolated by competent
				  aquitards, but the well is screened in only two of the units and has a
				  competent annular seal to isolate the other two units, then the groundwater
				  production zone consists of only the two units that contribute water to the
				  well.
(42) Groundwater protective
				  concentration level exceedence zone--A protective concentration level
				  exceedence zone within a groundwater-bearing unit.
(43) Hazard index--The sum of two or more
				  hazard quotients for multiple noncarcinogens originating from a single affected
				  property.
(44) Hazard quotient--The
				  ratio of the level of exposure of a noncarcinogen acting through an individual
				  or combined exposure pathway over a specified time period to a reference dose
				  for the noncarcinogen derived for a similar exposure period.
(45) Implementation Procedures--The most
				  current version of
Procedures to Implement the Texas Surface Water
				  Quality Standards,
as amended.
(46) Innocent Owner or Operator--Those
				  persons so designated in accordance with Texas Health and Safety Code, Chapter
				  361, Subchapter V, Immunity From Liability of Innocent Owner or Operator, as
				  amended.
(47) Institutional
				  control--A legal instrument placed in the property records in the form of a
				  deed notice, Voluntary Cleanup Program Certificate of Completion (VCP
				  Certificate of Completion), or restrictive covenant which indicates the
				  limitations on or the conditions governing use of the property which ensures
				  protection of human health and the environment or equivalent zoning and
				  governmental ordinances.
(48)
				  Judgmental sample--An investigative sample of an environmental medium which is
				  purposefully located based upon property-specific information.
(49) Laboratory Control Sample--A spiked
				  blank sample analyzed by the laboratory to assess laboratory ability to
				  successfully recover chemicals of concern from a control matrix.
(50) Landscaped area--An area of ornamental,
				  introduced, commercially installed, or manicured vegetation which is routinely
				  maintained.
(51) Long-term
				  effectiveness--The ability of a remedy to maintain the required level of
				  protection of human health and the environment over time.
(52) Lower explosive limit--The lowest
				  concentration of a vapor or gas in air that will produce a flash of fire when
				  an ignition source (heat, arc, or flame) is present.
(53) Method detection limit--The minimum
				  concentration of a substance that can be measured and reported with 99%
				  confidence that the analyte concentration is greater than zero and is
				  determined for each COC from the analysis of a sample of a given matrix type
				  containing the COC.
(54) Method
				  quantitation limit--The lowest non-zero concentration standard in the
				  laboratory's initial calibration curve and is based on the final volume of
				  extract (or sample) used by the laboratory.
(55) Monitored natural attenuation--The use
				  of natural attenuation within the context of a carefully controlled and
				  monitored response action to achieve protective concentration levels at the
				  point of exposure.
(56) Natural
				  attenuation--The reduction in mass or concentration of a chemical of concern
				  over time or distance from the source of a chemical of concern due to naturally
				  occurring physical, chemical, and biological processes, such as:
				  biodegradation, dispersion, dilution, adsorption, and volatilization.
(57) Natural attenuation factor--The
				  numerical value which represents the natural attenuation (i.e., reduction) in
				  chemical of concern concentrations during transport from the source area to the
				  point of exposure. The natural attenuation factor is the concentration at the
				  source area divided by the concentration at the point of exposure. The natural
				  attenuation factor is always greater than or equal to one for the purposes of
				  this rule.
(58) Natural Resource
				  Trustees--The federal agencies as designated by the President and the state
				  agencies as designated by the Governor pursuant to the National Contingency
				  Plan, Oil Pollution Act, and CERCLA §107(f)(2)(A) and (B) to act on behalf
				  of the public as trustees of natural resources (e.g., water, air, land,
				  wildlife). The Trustees include TCEQ, Texas Parks and Wildlife Department,
				  Texas General Land Office, National Oceanic and Atmospheric Administration, and
				  the Department of the Interior.
(59) Off-site property (off-site)--All
				  environmental media which is outside of the legal boundaries of the on-site
				  property.
(60) On-site property
				  (on-site)--All environmental media within the legal boundaries of a property
				  owned or leased by a person who has filed a self-implementation notice or a
				  response action plan for that property or who has become subject to such action
				  through one of the agency's program areas for that property.
(61) Permanence/permanent/permanently--The
				  property of a response action which is capable of enduring indefinitely without
				  posing the threat of any future release of chemicals of concern above the
				  critical protective concentration levels established for the
				  property.
(62) Person--An
				  individual, corporation, organization, government or governmental subdivision
				  or agency, business trust, partnership, association, or any other legal
				  entity.
(63) Physical barrier--Any
				  structure or system, natural or manmade, that prevents exposure or prevents
				  migration of chemicals of concern to the points of exposure.
(64) Physical control--A structure or
				  hydraulic containment action which prevents exposure to and/or migration of
				  chemicals of concern when combined with appropriate post-response action care
				  to protect human health and the environment. Examples of physical controls are
				  caps, slurry walls, sheet piling, hydraulic containment wells, and interceptor
				  trenches, but typically not fences.
(65) Plume management zone--The area of the
				  groundwater protective concentration level exceedence zone at the time of
				  response action plan submittal, plus any additional area allowed in accordance
				  with §
											350.33(f)(4)
				  of this title (relating to Remedy Standard B).
(66) Point of exposure--The location within
				  an environmental medium where a receptor will be assumed to have a reasonable
				  potential to come into contact with chemicals of concern. The point of exposure
				  may be a discrete point, plane, or an area within or beyond some
				  location.
(67) Prescribed points of
				  exposure--The prescribed on-site and off-site locations within an environmental
				  medium where an individual human or population will be assumed to come into
				  contact with chemicals of concern from an affected property.
(68) Protective concentration level--The
				  concentration of a chemical of concern which can remain within the source
				  medium and not result in levels which exceed the applicable human health
				  risk-based exposure limit or ecological protective concentration level at the
				  point of exposure for that exposure pathway.
(69) Protective concentration level
				  exceedence zone--The lateral and vertical extent of all wastes and
				  environmental media which contain chemicals of concern at concentrations
				  greater than the critical protective concentration level determined for that
				  medium, as well as, hazardous waste. A protective concentration level
				  exceedence zone can be thought of as the volume of waste and environmental
				  media which must be removed, decontaminated, and/or controlled in some fashion
				  to adequately protect human health and the environment.
(70) Reasonably anticipated to be completed
				  exposure pathway--A situation with a credible chance of occurrence in which an
				  ecological or human receptor may become exposed to a chemical of concern (i.e.,
				  complete exposure pathway) without consideration of circumstances which are
				  extreme or improbable based on property characteristics.
(71) Release--Any spilling, leaking, pumping,
				  pouring, emitting, emptying, discharging, injecting, escaping, leaching,
				  dumping, or disposing into the environment, with the exception of:
(A) A release that results in an exposure to
				  a person solely within a workplace, concerning a claim that the person may
				  assert against the person's employer;
(B) An emission from the engine exhaust of a
				  motor vehicle, rolling stock, aircraft, vessel, or pipeline pumping station
				  engine;
(C) A release of source,
				  by-product, or special nuclear material from a nuclear incident, as those terms
				  are defined by the Atomic Energy Act of 1954, as amended (42
				  U.S.C. §2011 et seq.), if the release is
				  subject to requirements concerning financial protection established by the
				  Nuclear Regulatory Commission under §170 of that Act;
(D) For the purposes of the environmental
				  response law §104, as amended, or other response action, a release of
				  source, by-product, or special nuclear material from a processing site
				  designated under §102(a)(1) or §302(a) of the Uranium Mill Tailings
				  Radiation Control Act of 1978 (42 U.S.C.
				  §7912 and §
												7942),
				  as amended; and
(E) The normal
				  application of fertilizer.
(72) Remediation--The act of eliminating or
				  reducing the concentration of chemicals of concern in environmental
				  media.
(73) Remove--To take waste
				  or environmental media away from the affected property to another location for
				  storage, processing or disposal in accordance with all applicable requirements.
				  Removal is an irreversible process that results in permanent risk reduction at
				  an affected property.
(74)
				  Residential land use--Property used for dwellings such as single family houses
				  and multi-family apartments, children's homes, nursing homes, and residential
				  portions of government-owned lands (local, state, or federal). Because of the
				  similarity of exposure potential and the sensitive nature of the potentially
				  exposed population, day care facilities, educational facilities, hospitals, and
				  parks (local, state or federal) shall also be considered residential.
(75) Response action--Any activity taken to
				  comply with these regulations to remove, decontaminate and/or control (i.e.,
				  physical controls and institutional controls) chemicals of concern in excess of
				  critical PCLs in environmental media, including actions taken in response to
				  releases to environmental media from a waste management unit before, during, or
				  after closure.
(76) Restrictive
				  covenant--An instrument filed in the real property records of the county where
				  the affected property is located which ensures that the restrictions will be
				  legally enforceable by the executive director when the person owning the
				  property is an innocent landowner.
(77) Risk-based exposure limit--The
				  concentration of a chemical of concern at the point of exposure within an
				  exposure medium (e.g., soil, sediment, vegetables, groundwater, surface water,
				  or air) which is protective for human health. Risk-based exposure limits are
				  the fundamental risk-based values which are initially determined and used in
				  the development of protective concentration levels. Risk-based exposure limits
				  do not account for cumulative effects from exposure to multiple chemicals of
				  concern, combined exposure pathways, and cross-media or lateral transport of
				  chemicals of concern within environmental media.
(78) Sample detection limit--The method
				  detection limit, as defined in this section, adjusted to reflect
				  sample-specific actions, such as dilution or use of smaller aliquot sizes than
				  prescribed in the analytical method, and to take into account sample
				  characteristics, sample preparation, and analytical adjustments. The term, as
				  used in this rule, is analogous to the sample-specific detection
				  limit.
(79) Sediment--Non-suspended
				  particulate material lying below surface waters such as bays, the ocean,
				  rivers, streams, lakes, ponds, or other similar surface water body (including
				  intermittent streams). Dredged sediments which have been removed from below
				  surface water bodies and placed on land shall be considered soils.
(80) Selected ecological receptors--Species
				  that are to be carried through the ecological risk assessment as
				  representatives of the different feeding guilds and communities that are being
				  evaluated. These species may not actually occur at the affected property, but
				  may be used to represent those within the feeding guild or community that may
				  feed on the affected property.
(81)
				  Sensitive environmental areas--Areas that provide unique and often protected
				  habitat for wildlife species. These areas are typically used during critical
				  life stages such as breeding, hatching, rearing of young, and overwintering.
				  Examples include critical habitat for threatened and endangered species,
				  wilderness areas, parks, and wildlife refuges.
(82) Soil protective concentration level
				  exceedence zone--A protective concentration level exceedence zone within the
				  surface soil or subsurface soil which may extend down to a groundwater-bearing
				  unit(s). These protective concentration level exceedence zones may also be
				  present below or between groundwater-bearing units.
(83) Source area--The volume of a chemical of
				  concern in environmental media (e.g., soil or groundwater) which is leaching,
				  dissolving or emitting chemicals of concern. Of primary regulatory concern are
				  the source areas that are leaching, dissolving or emitting chemicals of concern
				  at unprotective concentrations under natural conditions, and not in
				  consideration of any physical controls (e.g., slurry walls, caps), that will
				  result in protective concentrations being exceeded at the point of exposure.
				  The source area need not be the horizontal and vertical extent of the
				  protective concentration level exceedence zone when cross-media or lateral
				  chemical of concern transport is required for a point of exposure to be
				  reached. Generally, a source area is located in the vicinity of or below
				  primary release sources (e.g., tanks, pipelines, drums, lagoons, landfills,
				  etc.).
(84) Source medium--An
				  environmental medium containing chemicals of concern which must be removed,
				  decontaminated and/or controlled in order to protect human health and the
				  environment. The source medium may be the exposure medium for some exposure
				  pathways.
(85) Stressor--Any
				  physical, chemical, or biological entity that can induce an adverse response;
				  however, as used in this context, only chemical entities apply.
(86) Subsurface soil--For human health
				  exposure pathways, the portion of the soil zone between the base of surface
				  soil and the top of the groundwater-bearing unit(s). For ecological exposure
				  pathways, the portion of the soil zone between 0.5 feet and 5 feet in
				  depth.
(87) Surface cover--A layer
				  of artificially placed utility material (e.g., shell, gravel).
(88) Surface soil--For human health exposure
				  pathways, the soil zone extending from ground surface to 15 feet in depth for
				  residential land use and from ground surface to 5 feet in depth for
				  commercial/industrial land use; or to the top of the uppermost
				  groundwater-bearing unit or bedrock, whichever is less in depth. For ecological
				  exposure pathways, the soil zone extending from ground surface to 0.5 feet in
				  depth.
(89) Surface water--Any
				  water meeting the definition of surface water in the state as defined in §
											307.3
				  of this title (relating to Definitions and Abbreviations), as
				  amended.
(90) Toxicity reference
				  value--An exposure level from a valid scientific study that represents a
				  conservative threshold for adverse ecological effects.
(91) Waste control unit--A municipal or
				  industrial solid waste landfill, including those Resource Conservation and
				  Recovery Act regulated units closed as landfills, with a liner system (i.e.,
				  synthetic or clay) and an engineered cap, that have been closed pursuant to an
				  approved closure plan, previous regulations, or will be implemented pursuant to
				  an approved response action plan.