Download Soil Contamination in The City

ORGANIC CONTAMINATION OF SOIL ‘IN THE CITY’
Mark Stuckey, Environmental & Earth Sciences
ABSTRACT
This paper covers the topics of describing, analysing and managing soil (organic) compounds in
‘the city’. Soil description is undertaken in two steps: researching existing information (such as
soil landscape series sheets); and site characterisation studies (field investigation). Where a
potential for ground disturbances, which may have introduced contamination, exists, correlation
between natural background and current site conditions is imperative, as are field quality control
procedures. Laboratory analysis should be carried out for all potential contaminating
compounds, and undertaken with supervised quality assurance procedures. Additionally, testing
of soil conditions such as fertility and potential to adsorb, exchange or leach contamination
should be performed. Management of contamination is dependent upon the results of
assessment, and hence potential impacts on human health and the environment (of which
potential receptors include plants and ecological systems such as groundwater, rivers and
wetlands). Management considerations are dependent on planned site land-use, as well as current
risks both on and off-site. Where a ‘significant risk of harm’ is found to exist, remediation and
subsequent validation is required.
Key words: organic compounds, contamination, investigation, urban, guidelines, remediation,
validation, total concentration, availability, degradation.
INTRODUCTION
Soil contamination can be found throughout urban and regional areas, and is not just limited to
old industrial sites. The identification of soil contamination in the urban environment has
become a prominent issue in the community and is a local, state, and federal government
concern.
Soil contaminants can be broadly categorised as inorganic or organic compounds. Inorganic
contamination most often pertains to the heavy metal group, but can be any inorganic element
present at potentially hazardous concentrations. Organic contaminants can be both naturally
occurring or synthesised compounds, and have a wide variety of uses in all industries.
To properly manage soil contamination an understanding of the chemical and physical properties
of the soil is paramount. This will enable a proper assessment of the contaminant under varying
soil and water conditions, and as such an actual ‘hazard rating’ can be obtained.
In conjunction with an understanding of the soil contaminant behaviour, several prescriptive
guidelines are now available which assist the site manager in the assessment of the site. These
guidelines and their application will be discussed. The guidelines referred to in this paper have
been derived primarily from human health studies, but have also in the past been based purely on
aesthetic features such as colour and odour of soil.
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Legislation in New South Wales is centred on the 1997 implementation of the Contaminated
Land Management (CLM) Act. Underpinning this legislation is the critical issue of whether
contamination at a given site presents a “significant risk of harm to human health or some aspect
of the environment”. This question often determines the extent of remediation required, and
hence management requirements of soil in the city.
WHAT IS CONTAMINATION?
Contamination has been described in a number of ways, including: any chemical substance that is
above background levels (Victorian EPA, 1998); and a condition or state which represents or
potentially represents an adverse health or environmental impact. This impact may be due to the
presence of a potentially hazardous material (ANZECC/NHMRC, 1992).
This definition has been recently updated by the National Environment Protection Council
(NEPC). The Council has stated contamination as the concentration of hazardous substances in
soil, sediments, surface water or groundwater that are above background concentrations. In
addition, a site assessment indicates that the substances pose, or are likely to pose, an immediate
or long-term risk to human health and/or the environment (NEPC, 1998).
Using ‘background levels’ to establish the occurrence of contamination may be inaccurate,
especially in older urban regions throughout Australia. It is very difficult, even in agricultural
regions, to find land that is truly representative of ‘pristine’ soil conditions because of the
expanse of past anthropogenic affects. In urban regions, or areas with industrial sources of
contamination (eg. Broken Hill, Newcastle, Wollongong, Inner Sydney and Port Pirie), soil that
has not been affected by contamination is virtually impossible to identify (Beavington, 1973;
Cartwright et al., 1976; Environmental & Earth Sciences, 1993; Markus and McBratney, 1996).
Tiller (1991) stated that pristine soils probably do not exist except in an archaeological sense and
suggests that background levels in the soil may be considered in terms of time, place, elapsed
time since onset of pollution and distance from any known source. In heavily populated and/or
industrialised societies, the overlapping aureoles of contamination from various sources make it
nearly impossible to detect a pristine background situation (Tiller et al., 1987).
The limitations of using regional background values needs to be recognised, especially if they are
being used to denote contamination. To avoid this confusion a site should be defined as
contaminated on the basis of being a current or potential threat to the environment, such as
defined by the ANZECC/NHMRC (1992) guidelines. Such a definition will also avoid the
confusion experienced by the general public in relating contamination to a health or
environmental hazard. If background levels are used to define contamination then a site may be
considered contaminated even though it possesses no potential threat to health or environment.
This definition also relates to the use of generic soil criteria values (ie. the same contaminant
threshold value for every soil type and location).
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FORMS OF ORGANIC CONTAMINATION
Occurrence and fate of organic chemicals
Organic compounds contain carbon, usually in combination with hydrogen, oxygen, nitrogen or
sulfur. Organic compounds were first associated with only plants and animals, however many
compounds have now been synthesised. Currently, organic chemicals play a major and important
role in the world economy. Examples include glyphosate based herbicides such as Roundup,
which is used throughout rural and urban areas, and petroleum- and oil-based products.
Organic contaminants in soil result from the deliberate or accidental introduction of industrial
products or wastes. The most common are petroleum products (petrol, kerosene, jet and diesel
fuels, fuel and lubricating oils, as well as more refined solvents such as benzene, toluene and the
xylenes). Other contaminant types include synthetic chemicals such as solvents, pesticides
(including their chemical impurities such as dioxins), PCBs and explosives, in addition to byproducts of industrial processes (e.g. polynuclear aromatic hydrocarbons in coal tars and waste
oils).
Organic contaminants in the soil are subject, in principle, to the same processes of transformation
and mineralisation as natural organic materials. Their behaviour and fate in the soil is a function
of their chemical structure, which determines interactions with soil particulates, soil organic
matter, soil water, soil gas and soil biota. Many are naturally occurring compounds, or have
similar structures, and are readily degraded by soil micro-organisms.
Some of the ‘xenobiotic’ contaminants (those which do not occur naturally) are slower to enter
the carbon cycle through biodegradation because few micro-organisms have developed the
metabolic capability to utilise them. A few of the most common organic pollutants are discussed
below.
Biodegradation is also influenced strongly by the concentration of a given organic pollutant (or
pollutants) in the soil environment. Hence, larger concentrations of compounds will take longer
to break down and assimilate into the biosphere.
Aliphatic hydrocarbons
Hydrocarbons contain only carbon and hydrogen atoms, and are typified by the n-paraffins,
which have a linear arrangement of carbon atoms:
CH3-CH2-CH2-CH2-CH2-CH2-CH2-CH3
n-octane
These aliphatic hydrocarbons may also have branched structures, or contain unsaturated bonds:
CH3-CH2

CH3-CH2-CH-CH2-CH2-CH3
3-ethyl hexane
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The naphthalenes or cycloalkanes contain cyclic structures (3- or more carbons in a ring), but are
chemically similar to the aliphatics. All of these compounds are significant constituents of
petroleum products. In the case of the n-paraffins, the first four exist as gases at ambient
temperatures (methane to butane), the next in the series are liquids (C5-C16), while heptadecane
and above are solids. The more volatile can be rapidly lost from systems by evaporation to the
atmosphere (volatilisation), while for heavier compounds other mechanisms such as
biodegradation are most important. Aliphatic hydrocarbons are predominantly found in
petroleum and solvents and are usually reported in total petroleum hydrocarbon (TPH) analysis.
Aliphatic hydrocarbons may be related to contamination sources on the basis of chain length. For
example, compounds with a chain length of between C6-C9 are usually associated with fresh
petrol, C10-C14 chain lengths indicate petroleum (most commonly super, kerosene or turpentine),
C15-C28 are diesel and light oils, and C28-C40 heavy oils.
Halogenated aliphatic hydrocarbons
These are simple aliphatic hydrocarbons in which a substitution of a functional group has
occurred (e.g. halogens-chloride and bromide). Simple alkyl halides and polyhaloalkanes are
readily available and are used extensively as solvents. Polychloromethanes are produced
industrially by the chlorination of methane. Carbon tetrachloride (CTC) has been used in the past
for dry cleaning but was found to cause liver damage. Other uses are in anaesthetics (halothane:
CF3CHClBr). A number of partially fluorinated alkanes have been used as cooling fluids for
refrigeration systems and aerosol propellants (eg., CFCl3, freon 11, or trichlorofluoromethane).
Aromatic compounds
Benzene is the simplest aromatic compound, and is a ring of six carbon and six hydrogen atoms.
All aromatic compounds contain one or more benzene rings, or chemically similar ring structures.
The chemical behaviour of aromatic compounds originates from the unique properties of this ring
structure. These compounds are generally more toxic than other hydrocarbons and, in some
cases, are carcinogenic. The ring structure is stable although hydrogen atoms on the ring can be
readily replaced by a number of different functional groups, producing new compounds with new
chemical and physical properties.
Analysis of mono-ring aromatic hydrocarbons (MAHs) is usually restricted to benzene, toluene,
ethyl-benzene and xylene (known collectively as BTEX); a representation of these compounds is
presented below. All of these compounds are hazardous and can be constituents of commercial
solvents as well as intermediates of synthesis in the chemical and pharmaceutical industry. They
are also present in petroleum hydrocarbons, which are found associated with gas works and
service stations, etc.
Benzene
Toluene
CH 3
Ethyl-benzene
CH2CH3
Xylene
CH3
CH3
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Phenol
Phenol is the simplest example of a compound with an OH group attached to an aromatic ring
(chemical formula C6H6O). Phenol is used industrially in making several kinds of plastics in the
preparation of dyes. Literature reports that the compound is degraded under methanogenic
(reducing) conditions to CO2 and CH4, while it is quite soluble in water (80 g/l at 15°C).
Polycyclic aromatic hydrocarbons
Polycyclic aromatic hydrocarbons (PAHs) are compounds with two or more benzene rings fused
together. PAHs may be grouped into two classes: the bi-aryls; and the condensed benzenoid
hydrocarbons. The biaryls are compounds in which two rings are linked together by a single
bond. The condensed benzenoid compounds are characterised by two or more benzene rings
fused or superimposed together in such a way that each pair of rings shares two carbons.
The majority of contaminants described in the environmental industry are associated with the
condensed benzenoid hydrocarbons. There are a large number of PAH structures, including those
with alkyl substituents, and a few occur as major pollutants. The US Environmental Protection
Agency produced a list of 16 priority PAHs, which are used for indicators of contamination. The
simplest ring structures (such as naphthalene) are relatively volatile, but the majority are not.
Some are known carcinogens (eg., benzo[a]pyrene). Most are relatively insoluble in water with
naphthalene (a two ring structure) being the most soluble.
Naphthalene
Anthracene
Benzo[a]pyrene
Dibenzo[ah]
-anthracene
Pyrene
The solubility of benzo[a]pyrene has been reported as 3.8 µg/L (parts per billion) in water at
25°C, while pyrene is 130 µg/L, and naphthalene 25 mg/L (parts per million) at 15°C.
PAHs in soil are both naturally occurring and may also be derived from anthropogenic sources.
A natural source is production by plants, while contamination results from activities such as coal
gasification, petroleum refining, coke production, iron and steel founding and the combustion of
fossil fuels and other organic matter (Imray and Langley, 1998).
Organochlorine compounds
Chlorinated hydrocarbons have been used extensively for pesticides in the past and the addition
of one or more chlorine atoms to an organic structure has several effects on its behaviour, ie.,
reduced volatility, water solubility and chemical reactivity. Such compounds are therefore found
to be associated with market gardens, sheep and cattle dip sites, under concrete slabs (as termite
prevention), and sometimes around telegraph poles. Organochlorine compounds are more
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persistent, i.e. they are less susceptible to biodegradation than other organic compounds. In soil
many can be mineralised, but some are biotransformed or combined with soil organic matter, thus
reducing their availability.
Both anaerobic and aerobic processes affect the behaviour of organic compounds in the soil
environment. For example, DDT in soil has been shown to naturally degrade to its derivatives
DDD (via anaerobic pathways) and DDE (aerobic pathways). This has been demonstrated in
studies of organochlorine pesticide concentrations in soil from new residential developments in
Western Sydney which were previously used as market gardens (Stuckey and Vallely, 1998).
Poly-chlorinated biphenyls
Poly-chlorinated biphenyls (PCBs) are organochlorines, are thermally and chemically very stable,
and comprise a group of 209 discrete compounds. In Australia PCBs were mainly used in
electrical components such as insulators, heat transfers or hydraulic fluids. They are persistent in
the environment, accumulate in biological systems and biomagnify in the food chain (Imray and
Langley, 1998).
Organophosphates and carbamates
Organophosphates are organic compounds in which the functional group is phosphate.
Organophophorus compounds and carbamates have replaced the organochlorine type of
insecticides. They are usually less persistent in the soil, however can be toxic to mammals.
Carbamates are not often detectable in the soil except in current or very recent spraying events
because of their low persistence, and hence are not considered of significant importance with
respect to potential soil contamination.
DEGRADATION OF ORGANIC CONTAMINANTS
Degradation of organic compounds by bacteria and other micro-organisms is well known and
documented (Al-Awadhi, Al-Daher, ElNawawy, Balba, 1996). These organisms obtain energy
and building blocks for their growth from these compounds. As previously discussed, natural
degradation processes occur in the soil environment, including that of the most persistent
chlorinated compounds such as the bio-accumulative pesticide DDT.
Solubility is one of the major factors determining the degradation rates of organic compounds.
Hydrocarbon and many common soil contaminants are essentially insoluble in water, while
biodegradation tends to occur at the organic contaminant/water interface, so is strongly
influenced by sorption to soil particles.
du Plessis et al. (1994) found that biodegradation occurs on the surface of soil particles. In
unsaturated conditions in the soil, surface tension results in a film of water surrounding the soil
particles. Micro-organisms adhering to the soil particles are bathed in water with only a short
distance for oxygen to diffuse from the pore air to the soil particle. In the soil environment this is
the most ideal location for microbial growth. Hence the most rapid degradation does not
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originate from organic compounds adsorbed onto the particulate surface, but those dissolved in
water.
Hydrocarbon biodegradation requires the presence of molecular oxygen, and so oxygen
availability is one of the key rate determining factors. While single microbial strains have the
capacity to mineralise some hydrocarbons (to carbon dioxide), degradation of petroleum
contamination occurs more rapidly in the presence of mixed populations because of the wide
range of chemical structures present.
According to Duggan et al. (1994), surfactants can enhance the solubility of hydrocarbons. With
the formation of micelles and other surfactant structures, the effective solubility of an organic
compound in water can be increased. This should result in increased availability of hydrocarbons
to micro-organisms. Martens and Frankenberger (1995) found that when a surfactant was added
to PAH contaminated soil, the rate of chemical oxidation increased by between 32 and 84%.
Field trials undertaken by Environmental & Earth Sciences Pty Ltd (Sydney) seem to indicate
that this is the case when surfactants are used as an additive for bioremediation.
Sandy or excessively dry soil often repels water; surfactants improve the ‘wettability’ of soil,
allowing bacteria to adhere and grow around the soil surface and receive water and moisture.
Without the surfactant the water would be repelled from the soil surface, inhibiting bacterial
growth and thus biodegradation.
Meanders (1994) found that bio-emulsifiers increase the bioavailability of previously soil bound
petrol-product and further enhance bio-activity, and hence degradation of organic compounds.
BIOAVAILABILITY OF ORGANIC CONTAMINANTS
Contaminant availability is dependent on a number of soil chemical and physical properties
including soil pH, cation exchange capacity (CEC), organic matter and soil texture, as well as
iron, manganese and aluminium oxide content (Higginson, 1993). In addition, other factors such
as the age of contaminants in the soil, the relationship between soil constituents, conditions of the
gut of the consuming animal, etc. should be considered.
In general, contaminants will bind to lattice structures in clay or organic matrices within the mass
of soil biota, if sufficient adsorption sites are available (Higginson, 1993).. The binding energy
between organic compounds (as well as inorganic elements) and soil structures is generally seen
to increase with time, due to increased affinity between the two with age. Hence, increased
energy is thus required to remove these compounds from soil and organic matter surfaces.
Issues of bioavailability are of greater importance when dealing with inorganic contaminants
(particularly heavy metals), as organic compounds are more likely to naturally attenuate and
‘break down’ in the soil environment.
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ANALYSIS OF ORGANIC CONTAMINANTS
Gas chromatography (GC) is used for organic compound analysis and is capable of detecting
volatile and semi-volatile compounds. However, it cannot detect compounds that are nonvolatile, thermally labile or highly polar. The analytes may be broken into two main categories:
volatiles; and semi-volatiles. The volatiles are measured using a purge and trap technique (i.e.,
utilising head space) coupled with GC, whilst semi-volatiles are determined by extracting with an
organic solvent and analysing the extract by GC.
Within the GC technique there are two main methods utilised: those using mass spectrometry
(GC-MS); and those using flame ionisation detection (GC-FID). Mass spectrometry is preferred
as it is a universal detector of moderate sensitivity, which can also identify and confirm the
presence of all compounds traced. GC-FID is of comparable sensitivity to GC-MS but is much
cheaper to purchase and run. A simple GC-FID screen will provide the same chromatographic
peaks as a GC-MS scan but without the additional information of the MS (Trout, 1993).
Less common detectors are available to assist in the analysis of organic compounds, and include
electron capture detectors, hall electrolytic conductivity detectors, photo ionisation detectors and
flame photometric detectors.
LEGISLATION INVOLVED WITH ASSESSING CONTAMINATED SITES
It has been suggested that primary legislative responsibility for ensuring that suitable assessment
and management of contaminated sites is performed rests with the state and territory authorities,
however there must be a consistent approach to the assessment and management of such sites.
Environmental issues in NSW are controlled at a legislative level by the CLM Act (1997), the
most important facet of which is the “duty to report” responsibility for owners, occupiers or
investigators on all sites. The legislation aims to ensure that all sites in NSW are either proven
environmentally sound, or if contaminated, measures are taken to address the clean-up of any
contamination. The centre piece of this legislation is whether contamination presents a
“significant risk of harm to human health or some aspect of the environment.”
The NSW EPA has developed a number of specific guidelines based on the ANZECC/NHMRC
(1992) guidelines, and the site manager should be aware of such guidelines in developing any site
assessment. These guidelines are currently under review as it has been recognised that they are
not complete for all contaminants. However, the guidelines are still widely accepted and are a
very useful document in the assessment of potentially contaminated land.
Common guidelines used in the assessment and classification of contaminated soil include :
 NSW EPA (1994) Contaminated sites: guidelines for assessing service station sites;
 NSW EPA (1995) Contaminated sites: guidelines for the vertical mixing of soil on former
broad acre agricultural land;
 NSW EPA (1995) Assessment of orchard and market garden contamination. Contaminated
sites discussion paper;
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 NSW EPA (1995) Contaminated sites: sampling design guidelines;
 NSW EPA (1997) Contaminated sites: guidelines for consultants reporting on contaminated
sites;
 NSW EPA (1998) Contaminated sites: guidelines for the NSW site auditor scheme; and
 Imray and Langley (1998) Health-based soil investigation levels, National Environmental
Health Forum monograph series, Soil Series No.1, 2nd Ed.
Although these guidelines are publicly available and offer a prescriptive approach to the
assessment of potentially contaminated land, it should be stressed that they are not a substitute for
an understanding of chemical and physical interactions in the soil. Scientific justification of any
assessment is still a necessary and important part of any investigation.
Standards Australia has also developed standards for the sampling of soils, as a guide to the
investigation of potentially contaminated sites.
INVESTIGATION OF CONTAMINATED SITES
The National Environment Protection Council (NEPC) have developed a flow chart illustrating
the processes involved for current assessment of contaminated sites. This chart (Fig. 1) illustrates
the steps that should be followed during most investigations throughout Australia. A brief
description of these steps is provided below, however a more detailed overview can be found
within the ANZECC/NHMRC (1992) guidelines.
Initial (or preliminary) site investigation
The initial investigation should :
 identify all past and present potentially contaminating activities;
 identify potential contamination types;
 discuss the site conditions;
 provide a preliminary assessment of site contamination; and
 assess the need for further investigation.
The initial investigation mostly relies on a desktop study of the site, with minimal localised
sampling to confirm any suspicions. A detailed site history is important in this study, and
involves researching aerial photos, Council records and/or personnel information, and includes a
site ‘walk over’. With this information an assessment of the need for further investigation can be
made.
Detailed site investigation
The detailed investigation usually includes soil (and/or groundwater) sampling, and should
confirm and quantify issues discussed in the preliminary investigation, such as the type, extent
and level of contamination present. The detailed investigation must adhere to specific guidelines,
as it is on the information gained in this study that assessments are made in terms of contaminant
distribution, potential effects of contaminants on health and the environment, and potential for
off-site impacts.
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The Sampling design guidelines published by the NSW EPA (1995), offer advice on the
statistical aspects of sampling by providing guides to:
 recommended sampling patterns and sampling depth;
 the number of samples required to characterise a site; and
 statistical procedures for interpretation of sampling results.
Quality control and assurance programs are now part of any investigation into potentially
contaminated land, and affect the sampling and analytical techniques used. These are detailed
procedures and as such will not be further discussed. Further information can be gained from
Australian Standards, NSW EPA and the Draft national environment protection measure and
impact statement for the assessment of site contamination (NEPC, 1999). Again it must be stated
that a proper understanding of the science, in order to enable understanding of the sampling and
analysis methods required, is vital in being able to apply the prescriptive guidelines.
Initial Site Investigation
Application of Investigation Criteria
No further
action
No apparent
problem
Risk
Assessment/Management
Detailed Site Investigation
Application of Investigation Criteria
No further
action
No unacceptable
impacts detected
Risk
Assessment/Management
Decision
Site Management/Remediation
No further
action
Validation
Fig. 1. Assessment of contaminated sites
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Further
investigation
required
Monitoring
Health risk assessments and management decisions
Once it has been established that contamination levels on a site or over a given area are a
potential problem (as a result of conducting a detailed site assessment), a risk assessment is
required to determine whether these levels are a concern for human health and the environment.
This process assesses current site contaminant levels with respect to their potential to be
hazardous to the environment and health, in relation to the intended use, users and environment.
This pertains to site specific justification that the levels present on any given site are appropriate
for the intended use of the site.
The risk is determined on the basis of potential exposure routes to receptors, as well as the
toxicity of the contaminant(s) present. The level of toxicity is determined by comparing reported
concentrations to established threshold parameters (such as guideline levels and published health
risk assessment findings).
Site management and/or remediation
A number of options are available to the site manager if the site assessment determines that the
existing contamination is a risk to health or the environment. These options include managing
the site so that contamination will not impact on or off-site receptors, a process which requires
on-going future monitoring, together with submission of detailed plans to, and consultation with,
the appropriate regulatory bodies. This process may require changing the land-use to a less
sensitive application, so as to decrease risk to potential receptors.
Remediation will ‘fix’ the problem so the site may be desirable for the proposed land-use. A
number of remediation options are available, some of which may require further monitoring. The
remediation stage will always involve certain satellite responsibilities, such as community
consultation programs to ensure complete awareness of activities on the site, and occupational
health and safety (OH&S) for all planned site activities. These requirements, as well as those for
the physical treatment of the on-site contamination and subsequent validation following
remediation of that contamination, are to be reported as one document, which is known as a
remediation action plan (RAP).
Following remediation, a separate validation report is required which states whether remediation
has been successfully completed, and that the site is suitable for its intended use. Stringent
sampling strategies need to be followed, and appropriate and relevant guidelines referred to at all
stages of the study.
An extra step, which may be required by the local Council governing the region, is ‘the auditor
process’. This process involves an accredited independent scientist (the auditor) reviewing all
work undertaken. If satisfied with the process and conclusions drawn, the reports will be ‘signed
off’ by the auditor and accompanied by their certificate of acceptance for the work undertaken.
Ideally the auditor should be involved before the remediation process commences, in order to
avoid any problems.
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SOIL CRITERIA USED IN ASSESSMENT & REMEDIATION OF CONTAMINATION
The NEPC (1998) suggested that the appropriate use of investigation levels is an important
component in the study of contaminated sites. In particular, it is important to be able to select the
most appropriate criteria from a range that are based on the protection of health, ecology,
groundwater and aesthetics. These must then be considered in light of actual site conditions such
as soil type, land-use (past, present and future) and regional setting. Currently soil criteria levels
are mainly taken from Imray and Langley (1998) for human health concerns under differing landuse applications, and ANZECC/NHMRC (1992) for ecological considerations.
An investigation and a response level was first detailed in the ANZECC/NHMRC (1992)
guidelines. An ‘investigation level’ is the concentration of a contaminant above which further
appropriate investigation and evaluation will be required. A ‘response level’ is specific to a site
(and site assessment), and is the level at which some form of response is required to protect
human health and/or the environment, where a wide margin of safety is required.
The ANZECC/NHMRC (1992) guidelines were developed as a balance between the use of soil
criteria and site specific assessment. The soil criteria presented in the ANZECC/NHMRC (1992)
document are defined as ‘investigation levels’ in that they represent concentrations that, if
exceeded, need further investigation. They do not necessarily indicate that levels are hazardous,
nor are they default clean-up levels. However, due primarily to lack of scientific application, this
process has been ignored and the investigation levels have become generic clean-up levels
throughout Australia.
In conjunction with investigation levels being used as default ‘clean-up’ levels, the criteria is also
expressed as a total element or compound concentration in the soil. Expressing soil criteria on a
total concentration basis does not account for soil conditions, and hence does not represent the
changing availability of the element or compound. Availability, and as such the toxicology, of an
individual element can be changed in different soil types and conditions.
Establishing site specific or regional soil criteria is not an easy process, nor is it easily agreed
upon by auditors or governing bodies, which perhaps explains why the current system is in place.
Despite these shortcomings, as long as site investigators are aware of the current limitations in
using the available soil criteria, and the need for site specific levels, the current values are good
indicators of threshold concentrations, which if exceeded have the potential to adversely affect
receptors.
Some of the problems associated with using generic soil criteria and criteria based on total
concentrations in the soil have been presented. The development of new criteria by the NEPC,
which in effect reinforces the ANZECC/NHMRC (1992) guidelines, is encouraging as it
recognises the limitations of generic guidelines, and the need for further site specific assessment.
This aside, the question remains: what soil criteria is appropriate in any given investigation? This
ultimately depends upon the intended land-use and users of the site. In developing soil criteria
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the assessor must investigate health-based investigation levels, ecologically-based investigation
levels, aesthetic guidelines and structural (physical and chemical) guidelines.
In summary, health investigation levels currently used are those expressed by Imray and Langley
(1998), and have been derived for different human exposures to different land-uses. Ecological
investigation levels are in the future to be developed, most likely by the NEPC, on a regional
basis and land-use type. Presently, there are limited appropriate data available for the
establishment of such criteria, and current levels are determined on a site specific basis utilising
observations such as plant health, contaminant leachability and availability, and potential
groundwater impacts.
There are no numeric aesthetic guidelines available, however investigation observations should
note whether the soil is discoloured, malodorous, or of an abnormal consistency. Substances
such as phenols and sulfates have the potential to impact on man made structures (e.g. through
acid attack or gas generation), which may override health and environmental considerations.
CONCLUSION
Organic soil contamination in urban areas is a result of a wide variety of processes, and
encompasses a large group of compounds. The relationship between current soil conditions and
‘background’ soil conditions is often difficult to define given that ‘pristine’ or original soil
settings rarely exist today. While generic ‘background’ soil conditions are unlikely to exist in
any urban setting, whether or not a given site is ‘contaminated’ depends on the potential risk to
site occupiers and the surrounding environment. This risk is determined by a regulated scientific
approach, involving soil (and groundwater) characterisation. The impact of organic compounds
in soil should therefore be assessed in terms of their potential risk to site occupiers and the
environment.
Any given site can be assessed and classified as contaminated if soil on the site is determined,
through a carefully regulated study, to be hazardous to health or the environment. If
‘background’ levels are used to define contamination, then a site may be considered
contaminated even though it possess no potential threat to health or the environment.
The processes associated with (potentially) contaminated sites include site assessment (initial and
detailed), risk assessment, and if required remediation and validation. For further detailed
explanation the reader is referred to the appropriate state and federal guidelines. An
understanding of scientific principles with regard to soil interaction with organic (and inorganic)
compounds is vital in derivation and successful application of these criteria.
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REFERENCES
Al-Awadhi, N., Al-Daher, R., ElNawawy, A., Balba, M.T., 1996. Bioremediation of oilcontaminated soil in Kuwait. I. Landfarming to remediate oil-contaminated soil. Journal of
Soil Contamination, 5, 243-260.
Australian and New Zealand Environment and Conservation Council/National Health and
Medical Research Council (ANZECC/NHMRC), 1992. Australian and New Zealand
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