ISO 21365:2019

INTERNATIONAL ISO
STANDARD 21365
First edition
2019-10
Soil quality — Conceptual site models
for potentially contaminated sites
Qualité du sol — Schémas conceptuels de sites pour les sites
potentiellement pollués
Reference number
©
ISO 2019
© ISO 2019
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ii © ISO 2019 – All rights reserved

Contents Page
Foreword .iv
Introduction .v
1 Scope . 1
2 Normative references . 1
3 Terms and definitions . 1
4 Basics . 3
4.1 Structure of this document . 3
4.2 Principles . 4
4.3 Scope of a conceptual site model . 5
4.4 Representing and communicating a conceptual site model . 5
5 Development of a conceptual site model . 6
5.1 Step wise procedure . 6
5.2 Step 1: Defining overall objectives and the boundaries (both spatial and temporal) . 8
5.3 Step 2: Identifying the known and potential contaminant(s) and characterizing the
source . 8
5.4 Step 3: Identifying and characterizing each known and potential contaminated medium . 9
5.5 Step 4: Identifying potential migration pathways .10
5.6 Step 5: Identifying potential receptors, exposure pathways and points of exposure .13
5.7 Step 6: Identifying possible foreseeable events .15
6 Conceptual site model development during site investigation .15
6.1 General .15
6.2 Preliminary investigation .16
6.3 Exploratory investigation .17
6.4 Detailed investigation .17
6.5 Supplementary Investigation .17
7 Conceptual site model for remediation and mitigation measures .18
8 Conceptual site model for construction works .18
9 Data collection and Quality Assurance .19
Annex A (informative) Expressions and illustrations of conceptual site models for
potentially contaminated sites .20
Annex B (informative) Scope of phases of investigation (based on ISO 18400-203) .33
Annex C (informative) Investigations phases and links with conceptual site model (adapted
from ISO 18400-202 and 18400-203) .35
Annex D (informative) Review during and after Remediation and Construction works .37
Bibliography .39
Foreword
ISO (the International Organization for Standardization) is a worldwide federation of national standards
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electrotechnical standardization.
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described in the ISO/IEC Directives, Part 1. In particular the different approval criteria needed for the
different types of ISO documents should be noted. This document was drafted in accordance with the
editorial rules of the ISO/IEC Directives, Part 2 (see www .iso .org/ directives).
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URL: www .iso .org/ iso/ foreword .html.
This document was prepared by Technical Committee ISO/TC 190, Soil quality, Subcommittee
SC 7, Impact assessment.
Any feedback or questions on this document should be directed to the user’s national standards body. A
complete listing of these bodies can be found at www .iso .org/ members .html.
iv © ISO 2019 – All rights reserved

Introduction
This document provides a definition of the conceptual site model (CSM) for contaminated sites
consistent with other ISO standards related to contaminated land. It refers to ISO accepted terminology
and generally accepted understanding of a CSM. Links with the ISO 18400 series of standards (Soil
quality — Sampling) are made. It is applicable to the management of potentially contaminated sites,
sites that are known to be contaminated, and also to land with naturally elevated concentrations of
potentially harmful substances.
It provides general guidance on the application of CSMs, how they are developed and how they can
evolve, with respect to all media, for example, air, surface water, sediments, soil, groundwater, soil gas,
biota, subsoil, including buildings and other artefacts.
The CSM is a synthesis of all relevant information about a potentially contaminated site with
interpretation as necessary and recognition of uncertainties. The description relies on the concept, of
“source-migration pathway-receptor linkages” (sometimes termed « contaminant linkages ») that are,
or might be, present.
The investigation of land potentially affected by contamination is usually performed using observations
and measurements made on-site as well as by taking samples for laboratory analysis and testing. Soil
and groundwater characteristics include a wide span of features, such as chemical and mineralogical
composition, soil texture, the concentrations, amounts and distribution of contaminants and soil
components. For practical and economic reasons, these investigations cannot cover the total volume of
interest, and the on-site measurements and especially the sampling have to be limited to certain points
or small areas/volumes.
Spatially limited investigations will give the best possible information if they are be planned thoroughly.
The questions: “what are we looking for, and what can we expect?” are essential for developing an
investigation programme that is efficient and fit for purpose. The best way to start the planning of the
investigation is to formulate a CSM, based on a thorough preliminary investigation (desk study and site
reconnaissance in accordance with ISO 18400-202) prior to any intrusive investigation.
Therefore, a CSM is a synthesis of information about the site together with some interpretation,
assumptions, and hypotheses. By testing the assumptions and hypotheses, intrusive investigations
can concentrate on the essential questions and data gaps, and can be planned and carried out more
efficiently. Depending on the results of the intrusive investigation, the CSM can be developed further.
It can become more detailed, more reliable, and often also modified or corrected, and step by step can
lose its conceptual character, although remaining a model.
In the context of potentially contaminated land, a CSM is a tool that can be developed for the planning of
an investigation, for undertaking a risk assessment, and for planning remediation and aftercare of a site.
It can also be used for construction or other engineering works that are planned for after remediation.
A CSM can be used when conducting environmental audits and “due diligence” exercises. The degree of
detail needed for the CSM can depend on the objectives of any of these tasks, and the nature, current
use and possible development of the site.
When preparing a CSM the terms that are being used should be carefully defined because terms might
not be understood to have the same meaning by people with different backgrounds and experience. In
addition, CSMs are intended to be of use to those without a technical background.
NOTE This document follows the established convention for documents published by ISO Technical
Committee 190 (TC 190) in distinguishing between “contaminant” (“substance or agent present in an
environmental medium as a result of human activity – see 3.2 in this document) and pollutant (“substance
or agent present in the soil (or groundwater) which, due to its properties, amount or concentration, causes
adverse impacts on soil functions” - see ISO 11074:2015, 3.4.18). Hence, “contamination” and “pollution” are not
considered to be the same thing. However, it is recognised that this distinction is not always made at “official”
level in all jurisdictions. Even in those jurisdictions where it is recognised, it might be for some purposes but not
others and the definitions of “contamination” and “pollution” used in legislation and regulations for different
purposes can differ. In addition, the use of the terms is not necessarily consistent between and even within
guidance documents produced by government and professional bodies.
INTERNATIONAL STANDARD ISO 21365:2019(E)
Soil quality — Conceptual site models for potentially
contaminated sites
1 Scope
This document provides guidance on developing and using conceptual site models (CSMs) through
the various phases of investigation, remediation (if required), and any subsequent construction or
engineering works.
It describes what CSMs are, what they are used for and what their constituents are. It stresses the need
for an iterative and dynamic approach to CSM development.
This document is intended to be used by all those involved in developing CSMs and by those who rely
on using them such as regulators, landowners, developers, and the public (and other relevant parties).
Ideally, this includes representatives from all phases of the investigative and remedial processes, for
example, preliminary assessment, detailed investigation, baseline human health and environmental
risk assessments, and feasibility study, and, any subsequent construction or engineering work.
NOTE 1 This document is applicable whenever the presence of “potentially harmful” or “hazardous”
substances are present irrespective of whether they are naturally occurring or present due to human activity
(i.e. are “contaminants”).
NOTE 2 Although most of the principles described for developing CSMs in this document can apply to other
domains, such as groundwater resources management, the present document is specifically written for the
management of potentially contaminated sites or known contaminated sites.
2 Normative references
The following documents are referred to in the text in such a way that some or all of their content
constitutes requirements of this document. For dated references, only the edition cited applies. For
undated references, the latest edition of the referenced document (including any amendments) applies.
ISO 11074, Soil quality — Vocabulary
3 Terms and definitions
For the purposes of this document, the terms and definitions given in ISO 11074 and the following apply.
ISO and IEC maintain terminological databases for use in standardization at the following addresses:
— ISO Online browsing platform: available at https:// www .iso .org/ obp
— IEC Electropedia: available at http:// www .electropedia .org/
3.1
anthropogenic ground
deposits which have accumulated through human activity
1)
[SOURCE: ISO 11074:2015/DAmd 1:2019 ]
1) Under preparation. Stage at the time of publication: ISO 11074:2015/DAmd 1:2019.
3.2
conceptual site model
synthesis of all information about a potentially contaminated site relevant to the task in hand with
interpretation as necessary and recognition of uncertainties
3.3
contaminant
substance or agent present in an environmental medium (3.4) as a result of human activity
Note 1 to entry: There is no assumption in this definition that harm results from the presence of the contaminant.
Note 2 to entry: ISO 11074:2015, 3.4.6 defines “contaminant” as “substance or agent present in soil as a result of
human activity”.
3.4
environmental medium
soil, underlying material, sediments, surface water, groundwater, soil gas, and air that can contain
contaminants (3.2)
3.5
exposure pathway
path, route or other means, a contaminant (3.3) or hazardous substances from a particular source takes
to a receptor (3.7)
Note 1 to entry: Each exposure pathway links a source to a receptor.
[SOURCE: ISO 11074:2015, 5.2.12 modified]
3.6
fill
anthropogenic ground in which the material has been selected, placed and compacted in accordance
with an engineering specification
[SOURCE: ISO 11074:2015/DAmd 1:2019]
3.7
made ground
anthropogenic ground comprising material placed without engineering control and/or manufactured
by man in some way, such as through crushing or washing, or arising from an industrial process
[SOURCE: ISO 11074:2015/DAmd 1:2019]
3.8
migration pathway
means by which contaminants (3.3) or hazardous substances from a particular source of contamination
can spread or distribute
Note 1 to entry: A migration pathway does not necessarily link to a receptor.
3.9
pollutant
substance or agent present in an environmental medium (3.3), which, due to its properties, amount or
concentration, causes adverse impacts on an environmental medium
Note 1 to entry: ISO 11074:2015, 3.4.18 defines “pollutant” as “substance or agent present in the soil (or
groundwater) which, due to its properties, amount or concentration, causes adverse impacts on soil functions”
2 © ISO 2019 – All rights reserved

3.10
receptor
defined entity that is vulnerable to the adverse effect(s) of a hazardous substance or agent
Note 1 to entry: Receptors might include persons (e.g. trespassers, current and intended users, construction
workers), other organisms or complete ecosystems, environmental media or artificial construction.
[SOURCE: ISO 11074:2015, 3.3.29, modified, note added]
3.11
source
place from which a contaminant (3.2) or hazardous agent is released
Note 1 to entry: ISO 11074:2015, 3.3.35 defines “source” as “place from which a substance or agent is released
giving rise to potential exposure to one or more receptor” (3.7).
4 Basics
4.1 Structure of this document
The structure of this document is shown in Figure 1.
Figure 1 — Content of present document and interactions between the descriptive clauses
4.2 Principles
The conceptual site model (CSM) is a synthesis of all information about a potentially contaminated site
or a site known to be contaminated, relevant to the task in hand with interpretation as necessary and
recognition of uncertainties.
CSMs are important aids for the development of site-specific investigation programmes, the
undertaking of site risk assessments, remediation design, follow up post remediation, and if necessary
for subsequent construction on those sites that have been managed to deal with contamination. It is
developed following a step-by-step approach (see Clause 5). Its preparation requires judgement by the
person(s) preparing and developing the model.
Once, developed, a CSM should comprise all relevant information, including:
— past and present uses (see ISO 18400-202);
— intended future uses, included where known existing configuration and or future buildings/
infrastructure (e.g. basements, crawlspace under the floor of a building);
— the geological, geomorphological, hydrogeological and hydrological settings, soil, sediments, and
air (indoor air and the atmosphere) of the site and surrounding area;
— the properties of the potential contaminants (e.g. volatility solubility, toxicity) and their sources,
including distribution of contamination [e.g. plume of contaminant(s)], potential migration pathways
(natural and anthropogenic features such as sewer lines) and transport mechanisms;
— potential receptors of the contamination;
— possibilities of new exposure pathways and new receptors associated with the construction and
completion of a new development;
— foreseeable events [e.g. potentials for flooding (rivers, sea, groundwater), rising groundwater or
seawater levels, extreme weather conditions, change of use, etc.].
The aim of a CSM can be, as appropriate, to:
— present the characteristics of the site;
— identify uncertainties and data gaps and act as a basis for designing further investigations and
assessments;
— provides a basis for planning remediation and mitigation measures;
— provide a systematic review of where risks might potentially occur by summarizing possible direct
and indirect exposure pathways;
— facilitate as a communication tool, the overall management of potentially contaminated site, e.g. to
help in the decision-making process of experts in designing and planning all required actions;
— enable experts from all disciplines, clients, members of the public and regulators to communicate
effectively with one another about issues concerning a site and facilitate the decision-making
process.
CSM development should start as early in the site investigation process as possible. It should be
an iterative process of refinement in which the uncertainties are recognized and reduced as more
information becomes available (see Clause 6). The CSM can evolve as the results of investigations
become available and remediation strategies are formed. Refinement of the CSM should continue
through any remediation works and protective mitigation measures (see Clause 7). The planned use of
a site following any remediation cannot always be known when the CSM is first developed. The CSM is
likely to require review and possible extension following construction works and additional protective
mitigation measures as these confirm expected site conditions or reveal new information (see Clause 8).
4 © ISO 2019 – All rights reserved

Eventually, the CSM should take into account all measures whose implementation can determine the final
acceptability of the project, i.e. the total compatibility of site conditions and current or planned uses.
4.3 Scope of a conceptual site model
The complexity of a CSM should be consistent with the complexity of the site and available data and the
purpose for which it is developed.
The formulation of CSM should make it possible to determine the linkages between:
— the potential sources of hazardous substances (see 5.3);
— the potential migration pathways, including the various transport mechanisms in each medium and
their characteristics (see 5.4 and 5.5);
— the existing and/or future receptors that must be protected (see 5.6).
A CSM should:
— be developed for a defined purpose;
— be no more complex and detailed than required by the task in hand;
— identify uncertainties in the available information and in the conclusions.
A CSM should be prepared taking into account:
— the objectives of the investigation, or the purpose of the remediation (if required) and the purpose
of any subsequent construction or engineering works;
— the reason(s) for preparing the CSM, e.g. to aid risk assessment, help communicate with interested
parties, to plan remedial measures, or any subsequent construction or engineering works;
— uncertainties in the available data and other information.
The concerns of environmental risk assessment are different from those of human-health risk
assessment. These differences are usually sufficient to warrant separate descriptions and
representations of the CSM in the human health and environmental risk assessment reports. There can
be elements of the CSM that are common to both representations. However, the risk assessors should
develop these together to ensure consistency.
4.4 Representing and communicating a conceptual site model
The development of a CSM helps integrate technical information from various sources so it can be used
to communicate effectively.
A fully developed CSM can be seen as a mental construct of all the gathered information (see the six
steps in Clause 6). The CSM or one or several aspects of the CSM can be expressed or presented using
one or more of the following approaches/representations:
— a text description of the site and all relevant features and processes;
— one or more maps of the site;
— one or more tabular or matrix description;
— one or more drawings or other diagrammatic illustration;
— a series of hypotheses to which qualitative probabilities can be attached.
For example, a diagram can be used to illustrate important examined questions on a potentially
contaminated site. It can also help to identify and formulate what the risks are, i.e. understanding what
the sources of contaminants, the migration pathways and receptors are. This allows an understanding
of a site in a simple and schematic way.
Examples of the various representations of CSM are provided in Annex A.
5 Development of a conceptual site model
5.1 Step wise procedure
Six main steps have been identified for the development of a realistic and complete conceptual site
model (CSM) of a potentially contaminated site (see Figure 2). The six steps are:
1) defining the overall objectives and boundaries (both spatial and temporal);
2) identifying the known and potential contaminant(s) and characterizing the source;
3) identifying and characterizing each known and potential contaminated medium;
4) identifying potential migration pathways;
5) identifying receptors, exposure pathways and the points of exposure;
6) identifying possible foreseeable events.
6 © ISO 2019 – All rights reserved

Figure 2 — Diagram illustrating the 6 steps to develop a conceptual site model (Adapted from
[20]
French National guidance on developing conceptual site model )
The CSM should be reviewed by re-considering the six steps again after each phase of investigation and
regularly throughout the risk assessment process. Changes should be made based on the additional
information that has become available. A note should be made of whether the new information has
decreased or increased the model uncertainties. Further investigation should be considered if the
uncertainties are too great to permit realistic risk assessment(s) to be carried out.
All the relevant information that has been gathered should be taken into consideration, whether it
concerns the sources of contaminants, the migration pathways or the receptors to be protected. The
analysis of conditions for media of concern (e.g. geochemistry, redox conditions) helps determine the
magnitude and extent of migration pathways and help to identify the potential exposure points.
NOTE Depending on the phase of the site investigation, there are different degrees of uncertainty as
to whether contaminants are present and which media are contaminated. To avoid repetition in the text,
“contaminants”, “contaminated medium” or “contaminated site” refer to both potential contaminants, or
potentially contaminated medium or potentially contaminated sites and to confirmed contaminants or confirmed
contaminated medium or confirmed contaminated sites, as appropriate.
5.2 Step 1: Defining overall objectives and the boundaries (both spatial and temporal)
The overall objectives of the CSM should first be defined. The CSM boundaries, both spatial and
temporal, should then be agreed and thereafter clearly communicated through the CSM(s).
All underlying models and assumptions should be valid within the time frame which the CSM is valid
for. The changes that are likely to occur during the period of interest should be identified, for example,
changes in land use, changes due to erosion, or sea-level change. If changes are likely, different versions
of the CSM could be needed to cover/embrace varying conditions over time.
Attention might need to be paid to legal or other context in which the studies are to be carried out.
The defined spatial boundary should correspond to the area of the site being considered initially
(e.g. site’s premises, installation boundaries etc.), and if appropriate, be reviewed and encompasses a
much larger area to include, for example, all the downstream areas that are affected by contaminants
spreading from the site (e.g. between step 2 and step 3 in Figure 2).
A unique CSM should be developed for the site under consideration, unless there are multiple sites in
proximity to one another such that it is not possible to determine the individual source or sources of
contamination. Sites may be aggregated in that case. An overall CSM should then be developed for the
aggregate.
Considerations should be given to whether the site should be divided into separate sub-areas, or zones
for which separate CSMs is needed within the overall CSM. The separate sub-areas can be based on
differing topography, ground conditions; contamination situation; and past, present and future uses
(zoning of sites – see ISO 18400-104). Care should be taken that focus on separate zones does not
obscure the overall picture and that the potential interactions between the zones are not overlooked in
the separate CSMs.
Several “conceptual models” could also be required to cover different aspects of an investigation, for
example, a biogeochemical model to cover contaminant partitioning and availability, a model of the
migration pathways, a model of food chain transport or of human exposure. These types of sub-model
are then be integrated into the overall CSM.
It is important that the development of the contamination situation with time is considered. This gives
us information about whether contaminant levels have increased or decreased in the past and whether
they are expected to increase or decrease in the future.
NOTE An example is the case of organic contaminants that degrade either biologically or chemically, where
it is important to estimate the likely extent of degradation that has already taken place and that might take place
in the future. Another example is the passage of a plume of contaminants, and the situation of the site relative
to the passage of the plume: The question can be asked as whether the site is at the breakthrough front of the
plume, in the middle of the plume or at the end of the plume.
5.3 Step 2: Identifying the known and potential contaminant(s) and characterizing
the source
This step involves identifying the contaminant(s) that are or might be present at the site, and in its
surroundings. A first identification is usually based on the assembled information from the preliminary
investigation (desk study and site reconnaissance) prepared (see ISO 18400-202). Subsequent
investigations can then be carried out to confirm the occurrence of contaminants (see 6.4). Each
identified contaminant must then be related to a source. Primary and secondary sources should be
distinguished (see the example in Figure 3).
8 © ISO 2019 – All rights reserved

Figure 3 — Example illustrating primary and secondary sources of contaminant released from
a spill (e.g. in the case of a fuel spill)
Each source should as far as possible be characterized by the following:
— its origin(s), location(s), depths, extent and relationship to the property boundaries, and accurately
shown on a site map or other drawings (including a scale and direction indicator);
— its amount, the volume of the medium which is contaminated, the times of initiation, duration and
rate of contaminant release from the source;
— the chemical, biological, physical, toxicological properties of the contaminant(s) that are present in
the source and its concentrations in the environmental medium;
— the partitioning of contaminants between the contaminated media (for example between soil,
groundwater, soil gases, free phase, surface water, sediments, biota and air).
At this step, the model should propose hypotheses on how and why the contaminant was released from
the source(s). If no contaminants are found, a CSM should still be developed, documenting the reasons
why contamination is not believed to be present.
Background values at local and regional scale should be sought for all contaminants found at the site.
The model should include sufficient information to distinguish contamination attributable to the
source(s) under consideration from naturally occurring or nearby anthropogenic sources (e.g. of diffuse
contamination).
NOTE 1 This information also helps establish the extent to which contamination exceeds background.
Guidance on how to establish background concentrations in soils can be found in ISO 19258.
NOTE 2 Guidance on why particular substances can be of concern and the receptors to which they could be
hazardous is provided in ISO 18400-202, Annex A. Examples of associations of important contaminants with
industrial uses of land are also provided.
5.4 Step 3: Identifying and characterizing each known and potential contaminated
medium
This step involves identifying each known contaminated media or suspected contaminated media
i.e. all environmental media or artefacts that would be contaminated by releases from the sources
identified in Step 2. Contaminants could be present, for example in soil, sediments, groundwater,
surface water, soil gas, air, and biota. Contaminants can also impact buildings and other constructions,
e.g. concrete foundations can become saturated with oil. Potentially contaminated media should be
identified based on the information gathered during a preliminary investigation (desk top study and
site reconnaissance) (see ISO 18400-202). The model should be reviewed when measurements become
available regarding the presence of contaminants in the different media.
Only the relevant media should be considered for developing the CSM. The properties of the media
that can influence contaminant behaviour should be determined. For example, pH, redox potential, or
permeability, can influence contaminant forms, transport and toxicity.
The media that are not included in the model, however, should be identified as such and the reasons for
omitting them from the CSM should be given.
5.5 Step 4: Identifying potential migration pathways
For each contaminated medium identified, contaminant migration pathways should be identified and
represented and described in the CSM. The aim is to identify potential points of exposure for people or
the environment, thus allowing identification of contaminant receptors.
When developing the CSM, the description of migration pathways can include:
— the mechanism of contaminant release from primary or secondary sources and the type of transport
process (see examples in Table 2);
— the medium in which the contaminant is transported (if the relevant environmental receptors are
not located at the source);
— the transport processes operating in the contaminated medium and the important factors which
determine the rates of transport.
Modelling might be required to characterize migration pathways. The quality and reliability of the
results of a contaminant transport modelling study depends on the data that have been used to develop
the conceptual model and to construct and refine the mathematical model. If the data are inadequate,
the model results are unreliable. The data used should be site-specific and should characterize the site
and area being modelled. For example, the final model should contain sufficient information to support
the development of current and future exposure scenarios.
A diagram could be used to illustrate the migration pathways for all source types at a site (See Examples
in Annex A: A1.5; A16; A4.1; A4.2). Table 2 illustrates examples of migration pathways.
10 © ISO 2019 – All rights reserved

Table 2 — Examples of migration pathways
Contaminated Possible migration pathways and mechanism of contaminant release
medium
Soil Plant uptake
Leaching by infiltrating water to the groundwater
Suspension of dust
Particle transport by surface runoff
Groundwater Unsaturated zone to the saturated zone by infiltration and percolation
Groundwater transport through aquifers by diffusion, advection, dispersion, capillary
transport
Groundwater transport through trenches (for water, sewage, electricity etc.) and other con-
structions
Volatilisation to soil air
Solution of free-phase contaminants into the groundwater
Surface water Transport of dissolved and particulate contaminants with storm runoff in the storm water
drainage system
Transport of dissolved and particulate contaminants by flowing water in rivers and lakes
Sedimentation of contaminants which have become associated with suspended particles
Ingestion of particles by biota/uptake by or sorption to aquatic plants
Sediments Bed load transport
Resuspension of particles
Desorption of contaminants from sediment to water
Plant uptake from sediments
Sediment ingestion by animals
Air Gaseous phase transport though soil- air to the soil surface
Transport to indoor air from soil air through building foundations
Transport of soil gas through trenches (for water, sewage, electricity etc.) and other con-
structions
Wind transport of dust and of vapours
Free phase Gravity driven transport through soils and rocks
contaminant
Solution in groundwater, and groundwater transport
Evaporation to gaseous phase followed by transport through soil air
Biotic transport Uptake of contaminants by plants
Sorption of contaminants to plant or animal surfaces
Uptake of contaminants by animals from water or pore water
Uptake of contaminants by animals as a result of food ingestion
Uptake of contaminants by animals as a result of the direct ingestion of contaminated soil or
sediment
[20]
NOTE 1  Adapted from French National guidance on developing conceptual site model .
NOTE 2 The identification of migration pathways relies on a good understanding of the physico-chemical
properties of the contaminant, which determine its behaviour in the environment being investigated. The
physico-chemical properties of the contaminant determine the distribution of the contaminant between
different environmental media and the transport processes operating. Examples of important physico-chemical
properties are the contaminant’s affinity for lipids, solubility in water, density, complex-formation and sorption
behaviour.
NOTE 3 The identification of migration pathways and transport processes relies also on a good understanding
of the underlying geology and hydrology, on the properties of the environmental medium, the results of site
investigations, and information relative to the physicochemical and behavioural characteristics of the substances
under consideration (for example, the acidity and alkalinity of the environment, the oxygen status, presence of
organic matter, clays or metal oxides). Once in contact with a medium, contaminants are affected by a number
of physical or reactive geochemical and biological processes that can retard, concentrate, immobilize, liberate,
degrade or otherwise transform the contaminants. Guidance on these parameters is given in ISO 15800 and
ISO 15175.
Soil should be considered as a medium and possible migration pathway as human or environmental
receptors could come into direct or indirect contact with contaminated soils. Exposure of human,
environmental receptors and other receptors at different soil depths should be considered.
Groundwater is not only a potential contaminated medium, but also a possible migration pathway when
identified contaminants have already come or could come into contact with the groundwater. When
considering groundwater transport, the movement through the unsaturated zone and the saturated
zone should be considered. Transport in aquifers can be important and the presence of down gradient
springs and wells (irrigation or drinking water) should be considered. In addition, the presence of
caves and fractures or other preferred flow paths should be considered as possible migration routes.
Understanding the presence of artesian conditions is also important.
NOTE 4 Examples of other fate and transport phenomena for consideration include infiltration, percolation,
hydrodynamic dispersion, interphase transfers of contaminants, and retardation.
Surface water and sediments should be considered as media and possible migration pathways when a
body of water (river, lake, drainage ditch, etc.) is in direct contact with, or is contaminated by a source
or contaminated area. Contaminated groundwater can discharge contaminants to surface water and
surface runoff or storm water can transport contaminants from land to surface water. Also, under arid
conditions ephemeral drainage can convey contaminants to downstream points of exposure.
Air should be considered as a medium and possible migration pathway when contaminants are in
media capable of releasing gases or particulate matter to the air (for example surface soil, subsurface
soil, surface water). Contaminated air can be found in the pore space of soils, in construction artefacts
(for example in drainage and water pipes), in indoor spaces and outdoors. Transport of air between
these different compartments could be an important migration pathway, for example the penetration
of soil air into buildings. Diffusion and convection are import transport processes for contaminants in
the vapour-phase, whereas particulates are transported only by air flows. Transport of soil air is highly
dependent on the soil water conditions. The migration of contaminants from air to other media should
be considered, for example, deposition of airborne particulates onto surface waters and soil.
Free-phase organic contaminants that are lighter than water (LNAPLs – light non-aqueous liquids) can
form layers on surface waters and groundwater and migrate in this form over large distances. They can
migrate upwards into the unsaturated zone due to capillary action and dissolve in the water in which
they are in contact.
Free-phase organic contaminants that are denser than water (DNAPLs – dense non-aqueous liquids) are
transported under gravity in free-phase until they reach an impermeable layer. Gravitational transport
is often not in the direction of groundwater transport. DNAPLs can be found in pools over impermeable
layers in the soil or at the surface of impermeable bedrock. Alternatively, free phase DNAPLs can
penetrate into fractures in the rock. Further transport of free-phase contaminants can occur after
dissolution in water with groundwater flows or after evaporation to soil air with pore-air transport.
In the soil, free-phase contaminants (both light NAPLs and DNAPLs) can be retained by adsorption to
the solid phase, particularly to organic matter and also be left in the pore spaces of soils.
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