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Appendix C: Proposed Framework for Deriving Soil Contaminant Values

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Stage 1: Human toxicology and health effects

Toxicological information about each substance or group of substances is collated from jurisdictions recognised by the Ministry of Health, including:

a) tolerable daily intakes (TDIs), mean daily intakes (MDIs), no observed adverse effects levels (NOAELs) and lowest observed adverse effect levels (LOAELs), and index doses for non-threshold carcinogens

b) risk information, including uncertainties relating to quantitative estimates, relevant to estimating the toxic effects of these substances arising from their presence in soil.

At the end of this step, the toxicological criterion appropriate to each substance or group of substances is identified.

Stage 2: Generic exposure pathways and receptors

The consideration of exposure pathways includes:

a) estimating the levels of contaminants in the media (air, water, groundwater, food, etc) that potentially convey the contaminants from soil to people (the “receptor”)

b) identifying the typical physical characteristics of New Zealanders (area of skin, weight, air breathed, food and water ingested, etc) that collectively determine a standard exposure model in association with the exposure pathways; this involves making a number of judgements about (for example):

  • the groups of people (children, adults, workers, etc) who may potentially be exposed, as well as the group considered the most sensitive to the toxic effect of contaminants

  • the time periods over which exposure occurs

  • the age-related ingestion and inhalation rates

  • how much produce people consume from their own home gardens.

These assumptions are then combined with the findings from stage 1 to estimate provisional soil guideline values for specific contaminants.

Stage 3: Practical and policy considerations

This stage “reality tests” or “bench marks” the provisional soil guideline value to take account of New Zealand’s physical environment. For example, a relevant factor is the background levels of contaminants commonly found in New Zealand soils, such as copper, arsenic and PAHs.2

The costs and benefits of implementing the guideline values also need to be assessed. If a specific soil contaminant proves too costly or difficult to implement, a supportive approach to address the risk posed by this contaminant may need to be explored.

The policy and legal contexts are also critical. It may not be appropriate to derive soil guideline values for some land uses. For example, it has been argued that hazardous substances in agricultural soils are more appropriately managed under our food safety regime rather than under the RMA.

The outcome of this consideration is the RMA-based policy decision that defines that a certain level of the contaminant in soil has or is reasonably likely to have significant adverse effect. A major influence on this decision is the toxicity of the substance.

Stage 4: Decisions on individual sites

On a site-specific basis there may be scope to adjust some of the generic assumptions (within specified limits) to derive a site-specific soil guideline value if a valid case can be made.

In this stage, generic guideline values are applied at an individual site level to determine whether the site meets the RMA definition of contaminated land. Generic assumptions may not be relevant to all sites. Good practice requires assessors to check that the circumstances of their site match the circumstances and assumptions in generic guidance.

However, collecting data from sites can be complex, and difficult to understand and interpret. Increased data collection can add substantial costs and is not always cost effective in providing further answers. The appropriate use of guideline values is intended to simplify decisions and provide a cost -effective approach.

Stage 5: Action and outcome

If a site contains hazardous substances at levels that exceed soil guideline values, then the site requires action to make it safe. There are two types of actions that can be applied to contaminated land:

  • remediation - removing or reducing soil contaminants to levels below guideline values

  • management - instituting a regime to ensure that people do not come into contact with the hazardous substances present (a management regime may include actions such as laying down physical barriers, providing signage, and restricting site access or site use).

If a site contains hazardous substances that are below the guideline values, then the site is not contaminated land (for the purpose of protecting human health) and no further action is required.

Note: this assessment considers only the human health effects of soil contaminants. The land may still be “contaminated” under the RMA definition of contaminated land based on other environmental effects.

2 Polycyclic aromatic hydrocarbons.

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