What are “Soil Guideline Values” and which should I use?
Jo Cavanagh
Soil guideline value (SGV) is a generic term used to collectively describe different criteria that typically identify concentrations of substances such as nutrients and contaminants in order to manage soil quality.
SGVs can include ‘soil target values’ such as for Olsen phosphorus, and for pH, as desirable concentrations, i.e. targets to achieve. However, SGVs are commonly used to refer to contaminant concentrations. In this context, SGVs are typically used to trigger further investigation (e.g. soil remediation or disposal) to protect a receptor from negative effects. Receptors include people, biota (e.g. soil microbes, invertebrates, plants, animals), and water. This article focusses on SGVs developed for managing contaminants.
Protection of human health
SGVs for the protection of human health are based on toxicological intake values combined with exposure to that substance (e.g. via soil ingestion, ingestion of home-grown produce), which differs under different land uses. In New Zealand, protection of human health is covered under the National Environmental Standard for Assessing and Managing Soil Contaminants for the protection of human health (NES). Specific SGVs, called Soil Contaminant Standards (SCSs(health)), have been developed for 7 inorganic contaminants and 6 organic contaminants for five generic land-use scenarios, utilising standardised receptors and exposure parameters. A summary of the SCSs(health) derived for inorganic substances for different land uses is shown in Table 1.
Table 1. Summary of soil contaminant standards – SCSs(health) – for inorganic substances (mg/kg)
| Land-use scenario | Arsenic | Boron | Cadmium (pH 5) | Chromium (VI) | Copper | Inorganic lead | Inorganic mercury |
|---|---|---|---|---|---|---|---|
| Rural residential / lifestyle block 25% produce1 | 17 | NL | 0.8 | 290 | NL | 160 | 200 |
| Residential 10% produce1 | 20 | NL | 3 | 460 | NL | 210 | 310 |
| High-density residential | 45 | NL | 230 | 1,500 | NL | 500 | 1,000 |
| Recreation | 80 | NL | 400 | 2,700 | NL | 880 | 1,800 |
| Commercial / industrial outdoor worker | 70 | NL | 1,300 | 6,300 | NL | 3,300 | 4,200 |
1 ‘produce’ refers to the proportion of home-grown produce assumed be consumed by people living on site. A higher proportion of home-grown produce leads to lower SCSs(health).
Primary production land is generally exempt from being considered under the NES, except for land surrounding residences or land on which hazardous activities may have occurred. A Hazardous Activities and Industries List (HAIL) was developed by the Ministry for the Environment to help identify such activities and includes livestock dips and locations, including spray sheds, market gardens, and orchards, where persistent pesticides may have been used or stored. Conversion of primary production land to residential land use, e.g. lifestyle blocks, may also require assessment under the NES, and remedial activities have been required on some properties to meet the rural residential SCS for cadmium.
Protection of ecological receptors
SGVs developed to protect terrestrial biota provide a means for assessing the potential environmental impact of contaminants, and are called Eco-SGVs. An article in Soil Horizons Issue 26 provides details of the development of Eco-SGVs, and this work has been updated to incorporate recent international developments.
Briefly, Eco-SGVs were developed for 8 inorganic contaminants including cadmium, copper, and zinc, and 3 organic contaminants. Toxicity data for individual contaminants were collated and analysed. Where enough data were available, species sensitivity distributions (SSDs) were used, enabling the selection of different levels of protection for different land uses. The ‘added-risk’ approach was used to derive Eco-SGVs for trace elements where the contaminant limit developed by analysis of toxicity data is added to the background concentration. Background concentrations are naturally occurring levels and were determined from the underlying geology. The predicted background concentrations and further information are available at https://lris.scinfo.org.nz/layer/48470-pbc-predicted-background-soil-concentrations-new-zealand/.
Eco-SGVs are intended to help with the management of contaminated land and the protection of soil quality, with the specific applications outlined in Table 2.
Table 2. Proposed application of Eco-SGVs for each land-use category
| Land-use category | Contaminated land management | Protection of soil quality |
|---|---|---|
| Commercial /Industrial | Inform remediation standards1 –specifically the quality of any soil imported onto site Trigger further site investigation, including off-site effects, in the event of significant exceedance2 | NA |
| Residential and recreational areas | As above Identification of contaminated land | Consent limits for application of wastes (e.g. biosolids, cleanfill, managed fill) to land Regional council State of the Environment monitoring |
| Agriculture | As above3 | As above |
| Non-food production land | As above3 | As above |
| Ecologically sensitive areas | As above3 | As above |
1 noting that Eco-SGVs for copper and zinc, in particular, should not automatically be applied as remediation standards – the effect of excavation and disposal of soil should be considered relative to the effect of actively managing the land to reduce concentrations over time.
2 >2 times the Eco-SGV over an area of 25 m2
3 Typically for small areas of contamination such as sheep dips, spray sheds.
NA – not applicable
Soil cadmium SGVs
For agricultural soils, management of cadmium (Cd), introduced as a contaminant in superphosphate fertilisers, should also be considered. The 2011 national cadmium management strategy includes the Tiered Fertiliser Management System (TFMS) as a tool to manage soil Cd accumulation. The TFMS has five tiers and four trigger values (SGVs), based on increasing soil Cd concentrations, which result in different levels of action primarily related to fertiliser application. The first trigger value of 0.6 mg/kg is based on 99th percentile of background soil Cd concentrations. Above 1.8 mg/kg no further accumulation of Cd is allowed without a site-specific risk assessment. However, for food crop production, cadmium concentration in the food product is the best indicator to determine whether management is required, as non-compliance with food standards may occur at any tier level, depending on the crop grown and other soil properties, e.g. pH, which may enhance plant uptake.
Which SGV is relevant?
Which SGV is relevant primarily depends on the land use and the receptor or activity (e.g. crop production). Where amendments derived from organic waste are applied, or waste is being applied to land (e.g. cleanfills), then leaching to groundwater can also be a key consideration and different criteria can apply.
For further reading see:
Protection of human health
https://www.mfe.govt.nz/land/nes-assessing-and-managing-contaminants-soil-protect-human-health/about-nes
Protection of ecological receptors
Cavanagh JE 2019. Updated User Guide: Background soil concentrations and soil guideline values for the protection of ecological receptors (Eco-SGVs) – Consultation draft. Landcare Research Report for Envirolink Tools Grant C09X1402.
Available at: http://www.envirolink.govt.nz/envirolink-tools/land-and-soil-tools/
Cadmium
Cadmium management strategy: https://www.mpi.govt.nz/dmsdocument/10073-cadmium-and-new-zealand-agriculture-and-horticulture-a-strategy-for-long-term-risk-management