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Toxicity Assessment - Advanced details

In a RA Tier 2 or Tier 3 assessment, you need to conduct an extensive literature review to obtain up-to-date information regarding: 

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 environmental media
  • 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 metals include:

  • molecular weight
  • specific density
  • solubility
  • 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)
  • USEPA (1996, 1998)
  • Material Safety Data Sheets (MSDS) for specific compounds
  • 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 difficult.

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 (1998).

Properties of the Receptor Organism(s)

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

Bioaccumulation Potential 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.

Bioconcentration and 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. 

The 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 contaminant(s)

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 receptors. 

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 regarding toxicity.

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 follow GLP
  • 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 and survival).
  • 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:

Test Conditions

  • 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 of 3)?
  • Was the replication appropriate (minimum number of replicates is 3)?
  • Was the exposures reported as nominal or actual concentrations?

Measured Response

  • Can the measured response be related to a significant ecological response?
  • 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?


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