In a RA Tier 2 or Tier 3 assessment, you need
to conduct an extensive literature review to obtain up-to-date
Properties of the Chemical
The chemical and physical properties of a contaminant will determine
both exposure and subsequent toxicity. In a RA Tier 2, one should
determine such things as:
- in what form (or forms) the contaminant exists in the
- whether or not the contaminant is water soluble
- the rate of chemical transformation
- how persistent the contaminant is
- its mobility through the soil
- physical properties of the chemical (i.e. size).
Both chemical and physical properties can influence the exposure
route to the receptor, for example, small, water-soluble particles are
more likely to be uptaken by plants than large hydrophobic compounds.
These properties will determine how available the chemical is likely to
be to the person or receptor organism. If a chemical can be shown to have limited
bioavailability, exposure and hence the risk of toxicity are reduced.
Important chemical and physical properties for organic compounds and
- molecular weight
- specific density
- boiling point
- vapour pressure.
A wide range of publications provide representative chemical and
physical property values for CCA and BTEX. These include:
- New Zealand guideline documents (MfE 1997, MfE/MoH 1997, and MfE
1999) provide specific details on chemical and physical properties
of BTEX and metals
- Databases provided within risk assessment models include a wide
range of relevant values (eg RBCA, RISK)
- Material Safety Data Sheets (MSDS) for specific
- Chemical characteristics texts - for example Perry’s Chemical
Engineers’ Handbook, the Merck Index, Montgomery (1996) Groundwater
Chemicals Desk Reference; Howard et al (1991) Handbook
of Environmental Degradation Rates.
Where site-specific pH values are known, tables of representative
values (e.g. USEPA 1996) can be used to derive pH-specific values of Koc
and Kd for metals and organic compounds.
Linear isotherms and soil sorption coefficients (Koc) are
generally used to estimate sorption and retardation of specific
contaminants in the soil. Values of Koc have been determined
for a wide range of chemicals (refer Fetter 1999). By knowing the value
of Koc for a contaminant and the fraction of organic carbon
present in the aquifer, the distribution coefficient can be determined
by using the relationship:
Kd = Koc . foc
Values of Kd for selected metals (including copper,
chromium and arsenic) are provided in USEPA (1996), and pH-specific
values of Koc and Kd for selected metals and BTEX
are provided in USEPA (1998).
Properties of the
Environment Specific to Your Site
It is important to know the chemistry of the contaminant under the
conditions likely to be experienced at the particular site. For example,
pH can influence the form of many heavy metals, the presence of a high
level of organic matter will often result in a lowered bioavailability,
due to bonding with the contaminant. For soils in particular, particle
size, cation exchange capacity (CEC) and organic carbon content should
also be defined.
Direct measurement of soil and aquifer properties on a site-specific
basis provides the most accurate information for use in fate and
transport modelling. The following additional soil and groundwater
properties may be measured on a site-specific basis:
- bulk density
- particle density
- particle size distribution
- moisture content
- air- and water-filled porosity
- organic carbon content
- partitioning coefficient that are chemical specific, (involves trials
conducted on a laboratory scale)
- hydraulic conductivity of the aquifer (requires pump tests, slug
tests or similar aquifer tests conducted in the field)
- hydraulic gradient
- dissolved oxygen.
Hydraulic conductivity and hydraulic gradient are important in
assessing the fate of groundwater contamination. It should be noted that
some parameters, for example, particle size distribution and organic
carbon content, are relatively straightforward to measure. However, the
other parameters, for example air-filled porosity at a site may be very
For a discussion on the methodologies for measurement of the above
parameters, refer to Freeze & Cherry (1989), Domenico & Schwartz
(1990) or Fetter (1999).
Sorption tends to slow the transport velocity of contaminants
dissolved in groundwater. The coefficient of retardation, R, is used to
estimate the retarded contaminant velocity. Refer to USEPA (1996) for
further information on use of the coefficient of retardation to estimate
retarded contaminant velocity in groundwater. A useful example of
retardation calculations for BTEX compounds is presented in USEPA
Properties of the
As discussed above, chemical and physical properties of the
contaminant and the environmental material in which it is contained
(soil, water, sediment etc) will influence the availability of the
chemical to the receptor (bioavailability).
Bioavailability is also dependent upon the receptor organism
and the route of exposure. For a plant the route of exposure might be
via root uptake, whereas for a bird, the route of exposure may be via
food. Both receptors are being exposed to the same contaminant but by
two completely different uptake mechanisms. Once inside the organism the
rate of biological transformation, or metabolism, of the contaminant
will have a direct influence on its toxicity.
One also needs to consider the bioaccumulation potential of the contaminant, and whether or not
the contaminant is likely to be passed along the food chain and biomagnified.
of the Chemical
Chemicals that are not easily metabolised or excreted, and/or
partition into cell membranes and fat stores (such as the highly
lipophylic organochlorine compounds like DDT, for example) tend to
concentrate in organisms and have the propensity to biomagnify along the
food chain. This means that the compound can reach very high levels in
higher trophic levels of the food chain such as in birds and mammals.
Biomagnification can result in the order of up to 100,000-fold higher
than the ambient concentration of a chemical.
biomagnification are collectively called bioaccumulation.
The types of
biological and chemical parameters that influence the bioaccumulation
potential of a chemical are its lipophylicity, its rate of metabolism,
and its rate of excretion (or half-life) by the receptor organism.
octanol / water partition coefficient is often related to the
bioaccumulation potential for aquatic organisms. The assumptions are
that octanol has similar solubility properties to that of lipids in cell
walls and the uptake of a chemical into an animal is a result of
partitioning between the organism and the surrounding water.
Toxicity of the
A Tier 2 RA toxicity assessment requires a reassessment of the
toxicity data used to derive regulatory criteria or
benchmarks, as well as critical assessment of other relevant toxicity
data. A literature search for studies that quantify data derived from toxicity
tests and detail mechanisms of action is necessary to
evaluate the liklihood of toxic effects in different groups of
In an RA Tier 2 assessment, toxicity data must be very
carefully analysed in the context of the selected assessment end point
for the site and environmental media being considered.
It is very
important that the risk assessor is able to critically evaluate the quality
of the toxicity data. Toxicity data can vary according to where they are
sourced from. Data comparability is a major problem in environmental
toxicology since ‘standard methods’ are constantly changing and
being improved. This is expected for standard methods for soil toxicity,
which are still in their infancy.
It is recommended that a database of the literature values for the
contaminant(s) of concern be derived. The table should contain information
regarding the form of the chemical tested, the test species common and
taxonomic name, the test type, duration, conditions and measurement end
point/s, the ecotoxicity values, and the source of the data.
Click here for an example of a summary table of soil
toxicity data for CCA and BTEX collated from the literature.
The risk assessor may consider
these values in a Tier 2 RA for these compounds; however, it is
strongly recommended that only primary literature that has been
carefully reviewed by an ecotoxicologist is used to support a decision
Quality of data
When reviewing the toxicity literature, it is important to
select data from experiments that have used standardised procedures
or that the laboratory where the tests were conducted were accredited
for Good Laboratory Practice (GLP), as much as possible. Preference
should be given to:
- data generated from standardised proceedures that
- data generated from long-term chronic exposures as opposed to
short-term acute exposures
- data generated from toxicity tests using a series of
concentrations rather than single concentrations
- measurement end points that have a direct relevance to
assessment end points (eg. adverse effects on development, reproduction
- data generated from studies using a similar route of
exposure(s) to the likely one(s) at the site.
Critical evaluation of toxicity data usually requires specialist
skills in ecotoxicology. The following list can be used as a guide for
assessing toxicity data:
- Did the test follow GLP / quality control practices?
- Was the test appropriately controlled for?
- Was the control response < 10% of the maxium test response?
- Was the route of exposure relevant to your site?
- Was there an adequate number of exposure concentrations (minimum
- Was the replication appropriate (minimum number of replicates is
- Was the exposures reported as nominal or actual concentrations?
- Can the measured response be related to a significant ecological
- Was a concentration - toxic response relationship established?
- Was the appropriate statistical analysis used to determine the
measurement end point (i.e. parametric analysis to determine EC50's)
- Can a NOEC or LOEC be determined directly from the toxicity data?