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Toxicity Assessment

What is a contaminated site?

Who does what?

Why is it important?

What is risk management?

Risk assessment fundamentals

Risk assessment methods

Limitations of risk assessment

What are RA tiers?


Problem Identification

Receptor Characterisation

Exposure Assessment

Toxicity Assessment


Terrestrial Benchmark Values

Aquatic Benchmark Values

Risk Characterisation

RM Decisions





Toxicity Assessment establishes the relationship between the contaminant/s of concern and the receptor.  It asks the question 'What does the contaminant do to the receptor?' The objective of Toxicity Assessment (also known as hazard assessment) is to determine 

'What potential adverse effects might the contaminants of concern cause and at what concentration?' 

Regardless of whether you are concerned with the effects of a group of contaminants or a specific contaminant, you should be able to demonstrate a link between the contaminant/s and human health effect or a biological response .   

Adverse biological responses to contaminants vary in scale both within and between individuals, populations and communities and over time scales ranging from seconds to decades or centuries. The following indicates the kind of responses that might be expected at each scale for ecological receptors:

  • neuro-endocrine, e.g. DNA mutation, modification of hormone activity
  • physiological, e.g. oxygen consumption, photosynthesis
  • integrators, e.g. morphology, growth, survival, behaviour, reproductive potential
  • integrators, e.g. population averages, population health measures
  • dynamics, e.g. recruitment, age-class survival, reproductive success
  • production, e.g. population size or structure
  • production, e.g. primary production, decomposition, respiration
  • dynamics, e.g. colonisation, species loss, succession
  • ecosystem function e.g. nutrient flow, energy flow.

For more information on each of these aspects, we recommend reading Section 6.4 of Environment Canada’s ERA Framework (1994).

Toxicity assessment will necessarily involve a degree of extrapolation between the information provided in the literature and the actual situation occurring at the site of concern. This extrapolation may introduce sources of error and uncertainty to toxicity estimates. Error and uncertainty can significantly affect the validity of models to which these estimates contribute, or the appropriateness of risk management decisions made on the basis of toxicity estimates. For discussions on the uncertainty associated with extrapolation, refer to Section 6.6 of Environment Canada’s ERA Framework (1994) and Section including Text Boxes 4-19, 4-20 and 4-21 in USEPA (1998).

Ecotoxicity Terms

To fully understand how toxicity assessment in RA is conducted, it is important to familiarise yourself with the following basic terms and concepts:  

And for ecological receptors:

Tier 1 – Toxicity Assessment tasks

Tier 1 Toxicity Assessments are screening level assessments. In the Tier 1 Toxicity Assessment, chemicals of potential concern are identified if they are found at concentrations greater than a regulatory value. 

These regulatory values (which are also referred to as acceptance criteria, environmental standards, or environmental quality criteria but referred to generically in this web site as guideline values) are numeric values designed to help users assess whether the contaminant is present in sufficient concentration to pose a potential risk to humans or a specific part of the environment. They are generally conservative. 

NOTE: The terms 'criteria', 'guidelines' and 'standards; are often used interchangeably and confusingly, and one author's 'guidelines' may be another's 'criteria'. Additionally, documents presenting 'recommendations' (i.e. guidelines) sometimes use the term 'criteria' in their title. See the Glossary for definitions of criteria, guideline and standard as used in this web site.

Exceedance of these values does not necessarily mean the chemical poses an actual or significant risk, but rather it signals that a higher level of Risk Assessment may be required to further examine the potential for harm to human health or the environment. 

As stated in the ANZECC 2000 Water Quality Guidelines, "for some environmental values the guideline number provided may be an adequate guide to quality (e.g. for recreation or drinking). For other specific environmental values the guideline can be just a starting point to trigger an investigation to develop more appropriate guidelines. 

Invariably, the process of refining these guidelines to local conditions will result in numbers for toxicants at least, that are less conservative and hence less constraining on surrounding activities.

It is good practice to check the assumptions under which the guideline values are most relevant to ensure that they are similar to the actual situation at your site. 

Most guideline documents outline the assumptions and methods used in deriving the guideline values (e.g. aggregating toxicity test data for individual species or from the most sensitive species), or refer to the specific research or regulatory authority recommended guideline values that are considered to be appropriate. As a result, the guideline values will be more appropriate in some circumstances and less appropriate in others, and the document should outline the relevant circumstances limiting the use of the guideline values. 

For example, a guideline value may assume that workers on an industrial site located near a landfill may be exposed to landfill gas emissions for only 8 hours per day for 5 days per week. However, on your site, the workers may work in shifts of 10-12 hours over a 6 day week. This effectively increases their exposure time at the site by more than 50%, which means that a guideline value for landfill gas emissions may not be sufficiently protective of the health of the workers. 

You need to take into account a variety of factors when using overseas values such as, for example, differing soil properties and rainfall conditions, and the differing sensitivities of New Zealand species.

This information may be found in the text accompanying the guideline values, and may include detailed information on the data or testing from which the values were derived, assumptions used, any uncertainty values applied, and guidance on how to apply the values. These should be noted to ensure that the assumptions and data used match the specific circumstances of your site so that the guideline values are valid in relation to the results you obtain from Site Investigations.

Follow this link for an explanation of toxicity values.

Background Concentrations 

Another important consideration is the function of background concentrations. 'Background' is defined as the level of contaminants found in the vicinity of a locality, but away from a specific activity or site. In other words, they are an indication of the concentrations of contaminants typically found in the area and put the concentrations of contaminants found at your site into the context of the surrounding environment. In the great majority of cases the concentrations shown in background samples will not represent "pristine" concentrations. The background samples are likely to have a direct bearing on the risk management decisions made, particularly in relation to clean-up goals.

Two regional council documents give background levels of metals in soils in specific regions:

  • Auckland Regional Council, 2001. Background concentrations of inorganic elements in soils from the Auckland region. ARC Technical Publication No 153. 68p. This looks at arsenic, barium, boron, cadmium, chromium, cobalt, copper, magnesium, manganese, mercury, phosphorus, potassium, nickel, nitrogen, sulphur, tin, vanadium and zinc
  • Percival HJ, Webb TH, and Speir TW, 1996. Assessment of background concentrations of selcted determinands in Canterbury soils. Canterbury Regional Council Report No U96/39. 38p. This looks at barium, cadmium, chromium, cobalt, copper, lead, nickel, sulphur and zinc.

Although not intended as such, some data on background soil quality for a range of New Zealand soils is presented in the paper by Sheppard et al. on contaminant mobility, and ARC have assessed background contaminant concentrations in soils in the Auckland region, Auckland Regional Council Working Report 96 (2002), Auckland Regional Council Technical Publication 153 (2001). 

Because the natural chemical composition of surface and groundwater is determined by the hydrochemistry of underlying strata, site specific conditions, and catchment land uses, background water quality should be determined on a site-by-site basis.

Environmental Quality Criteria


For surface water samples, contaminant concentrations are generally compared to their applicable water quality guideline (WQG) for protection of aquatic organisms. Because releases from contaminated sites are often continuous and long-term, concentrations are compared directly with the chronic WQG, when available. If, however, the majority of the exposure is from intermittent events (such as with stormwater), then the concentrations may be compared with acute guideline values. Groundwater concentrations are also screened against WQGs. However, given the dilution expected during migration and upon discharge of groundwater to surface water, a screening value of 10 times the WQG would be recommended for this assessment. If available, suitable site-specific dilution factors should be used. Maximum contaminant levels applicable to drinking water sources must also be considered in making these assessments if a pathway to people is likely.

A number of approaches have been applied for derivation of acute and chronic water quality guidelines (WQG) and criteria for marine and freshwaters. These methods aggregate data for individual species toxicity tests to generate ‘community’ sensitivity distributions from which guideline values are derived.

Click here for a description of the ANZECC/ARMCANZ water quality guidelines. These should be used wherever possible.


Sediment quality guidelines have been derived from the biological effects database compiled by NOAA (Long et al., 1995). The chemical concentrations were sorted, and the lower-10 percentile and median concentrations were identified along with an apparent effects threshold. The lower-10 percentile data are identified as Effects Range-Low (ER-L), and the median as Effects Range-Median (ER-M). Acceptance criteria for these data are described in the ANZECC 2000 guidelines (ANZECC / ARMCANZ 2000), and in the Canadian protocol (CCME 1995). In the Canadian guideline both threshold effects level (TEL), and probable effects level (PEL) are given. Derivation and guidance for their usage are also presented in CCME (1995).


Soil quality values generally used in New Zealand include the ‘environmental investigation soil quality guidelines’ given in the ANZECC guideline (ANZECC, 1992b), as well as the ‘health-based acceptance criteria’ of specified chemicals contained within industry-specific guidelines for contaminated gasworks sites (MfE, 1997), timber treatment chemicals (MfE/MoH, 1997) and petroleum hydrocarbon contaminated sites (MfE, 1999). These values are generic, intended as an initial measure to determine the significance of contamination. The Canadian soil quality guidelines are also often referred to (CCME, 1997). In the ANZECC guidelines, where an environmental investigation level has not been nominated for a specific chemical, reference is made to the Dutch B guidelines that define ‘risk limits’ for specific chemicals. Click here for other sources of soil quality guidelines

Environmental benchmark values

Benchmark values are sometimes applied where regulatory guidelines or standards to protect ecological receptors are not available (Sample et al., 1998). Benchmarks used in a risk assessment are based on the ‘assessment end points’ the risk assessor / risk manager has selected, according to the ecological receptors of concern.

Benchmarks should not be used as remedial goals nor should they be interpreted as concentrations that pose significant risks. They should not be applied in the risk assessment process without a reasonable understanding of their development and limitations. For example, the Effects Range-Low (ERL) values for sediment (Long et al 1995) are calculated as the lower 10th percentile concentration of the available sediment toxicity data that had been screened and designated toxic by the original investigators. These are not LC10 values and are not to be used interchangeably.

Detailed information is available on benchmark values for aquatic biota (Suter & Tsao, 1996), sediment-associated biota (Jones et al., 1997), plants (Efroymson et al., 1997a), soil invertebrates and soil microbial processes (Efroymson et al., 1997b) and wildlife (Sample et al., 1996). Refer to the ORNL website for more information. 

The drawback to applying generic values such as benchmark values or environmental quality criteria is that:

  • there is often a low confidence associated with the value due to the paucity of appropriate environmental and toxicity data
  • each value has often been calculated using different methods, and based on different toxicity data
  • these values are guidance values only and do not account for site-specific differences.

Follow links for tables of environmental quality criteria or benchmark values for CCA and BTEX in soil and receiving waters

Dealing with mixtures of contaminants

When applying generic values for toxicity assessments it is important to have an understanding of how mixtures of chemicals can interact, and how exposure to chemical mixtures can affect toxicity. It is usually assumed that the toxicity of chemical mixtures will be additive (additive toxicity). However sometimes toxicity may be substantially greater than (synergistic toxicity) or less than (antagonistic toxicity), the simple addition of individual toxicities. In general, where compounds have a similar site of impact and mechanism of action, their combined toxicity will be additive. In cases where a compound may interfere with the toxicokinetics of another, for example, by inhibiting its detoxification pathway, potentiation of toxicity (synergism) can occur. Or, a compound may inhibit the activation pathway of another, thus diminishing the toxicity of the individual compound (antagonism).

This link takes you to an example to demonstrate what is required at this level in the ERA. Follow link for the XYZ Enterprises example at ERA Tier 1.
Introductory risk assessment information on the next RA stage: Risk characterisation 
  Advanced risk assessment information on Toxicity Assessment


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