Linear isotherms and soil sorption coefficients (K_oc) are generally used to estimate sorption and retardation of specific contaminants in the soil. Values of K_oc have been determined for a wide range of chemicals (refer Fetter 1999). By knowing the value of K_oc for a contaminant and the fraction of organic carbon present in the aquifer, the distribution coefficient can be determined by using the relationship:

K_d = K_oc . f_oc

Values of K_d for selected metals (including copper, chromium and arsenic) are provided in USEPA (1996), and pH-specific values of K_oc and K_d 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:

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. 

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.

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.

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:

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

Measured Response