Report: TR 2005/51

Author: Nick Kim

Abstract

Policy position and issue

A central objective of the Land and Soil section of the Waikato Regional Policy Statement is that the range of existing and foreseeable uses of the soil resource should not be reduced as a result of the contamination of soils.  This rests alongside a complementary objective that versatility and productive capacity of the region’s soil resources should be maintained. 

An estimated 8.3 tonnes of the heavy metal cadmium is currently applied to Waikato soils each year, with the largest single source being superphosphate fertiliser.  Cadmium is a naturally occurring, toxic, non-essential and biologically cumulative heavy metal.  Concentrations of cadmium in Waikato soils have been gradually increasing since the advent of topdressing. 

There are three means by which existing and foreseeable uses of the productive soil resource may be lost as a result of cadmium accumulation in soils.  These are:

1. Future inability to subdivide the land for residential or rural-residential purposes without some form of site assessment and/or remediation
2. Possible market access restrictions for produce
3. Non-compliance with food standards for crops grown on a property because of soil contamination. 

Loss of soil resource capacity can be defined as points at which one or more of these outcomes are realised. 

In terms of the first and second outcomes, the most readily quantifiable point for soil resource loss is 1 mg/kg for total soil cadmium.  This is both the current recommended limit for cadmium in agricultural soils, and a default human health protection limit for Waikato properties being subdivided to residential or rural-residential land. The recommended agricultural soil cadmium limit is set partly with respect to current and anticipated expectations of New Zealand’s international trading partners. 

The third outcome, non-compliance with food standards, relates mainly to particular types of horticultural and arable crops.  The exact point at which soil cadmium has become high enough to cause food standards to be exceeded can be difficult to predetermine, as it depends on crop and soil conditions.  However, this outcome has been observed to occur at soil cadmium concentrations below the current recommended agricultural guideline of 1 mg/kg.  For this reason, the agricultural soils guideline for cadmium can not be used as a proxy for food compliance.

Current status of Waikato’s productive soils

Based on recent sampling, it is estimated that perhaps 11% of Waikato’s pastoral soils and 17% of Waikato’s horticultural soils already exceed 1 mg/kg soil cadmium.  For horticultural soils, this would represent approximately 1775 hectares of land. For pastoral soils (sheep, beef and dairy land), this would represent about 157000 hectares. Within the pastoral soils sample set, all soil samples that have so far exceeded the 1 mg/kg agricultural guideline have been from dairy farms. 

On average, Waikato’s productive pastoral, horticultural and arable surface soils now contain five times more cadmium than they began with, and are two-thirds of the way to the 1 mg/kg threshold. 

Loading calculations confirm that the dominant source of this cadmium is superphosphate fertiliser, which contains cadmium as an impurity.

Projections for pastoral and horticultural soils

At current cadmium loading rates, the next significant point at which the 1 mg/kg cadmium guideline will be reached over wide land areas is expected for dairy farms, which cover about 25% of the Waikato region (about 623000 hectares). Conservatively, the average cadmium concentration in Waikato dairy soils is projected to reach the recommended guideline in under 16 years. 

In total, productive pastoral soils (mainly sheep, beef and dairy farms) comprise 57% of Waikato’s land area (about 1,430,000 hectares).  At current loading rates, the average cadmium concentration over all pastoral surface soils in the Waikato region is expected to reach the 1 mg/kg guideline in under 40 years.

In horticulture, the most rapid rate of accumulation occurs in soils under potato crops, where the estimated period before these soils average 1 mg/kg cadmium is 13 years.   The Waikato is now New Zealand’s second largest potato growing region.

It is important to note that these estimates are based on averages: on specific properties the guideline has been reached, or will be reached in a shorter time. 

Implications for current and foreseeable uses of the soil resource

A current impact associated with the presence of cadmium in drystock farming is the rejection of offal meat.  As a result of its high cadmium content, offal from animals older than 2.5 years is not permitted to be sold for human consumption.  To the sheep and beef industries, this represents a significant lost income stream. 

In recent years, the New Zealand food standard for cadmium in key crops has dropped by a factor of ten to 0.1 mg/kg, and recommended guidelines for cadmium in New Zealand agricultural and residential soils have dropped by a factor of three to 1 mg/kg.

In relation to the revised soil guidelines, two specific impacts associated with surface soils exceeding 1 mg/kg cadmium are the inability to subdivide land for residential (or rural residential) use without some form of assessment or remediation, and possible hindrances to market access.  Although milk and muscle meat make a minimal contribution to human dietary intakes of cadmium, the agricultural soil guideline is set partly with reference to expectations of international trading partners.  Such guidelines may take on a more significant trade role in the future. 

As a result of the tightening of food standards, cadmium accumulation in agricultural surface soils is likely to be causing progressive loss of soil resource capacity through the food non-compliance mechanism.  This loss of soil resource capacity relates to the proportion of soils that become unsuitable for (current or future) production of certain horticultural or arable crops at any point in time, based on whether crops grown in these soils could continue to meet the food standard for cadmium.

Overall, ongoing cadmium accumulation has the potential to reduce the range of foreseeable uses of approximately 58% of the Waikato region’s total land area in the short-to-medium term (between 10-60 years depending on land use), covering pastoral agriculture (primarily dairy, beef and sheep farming), arable cropping and horticulture. 

This proportion may further increase in response to land being converted from plantation forests to dairy farms.  Dynamic changes to land use underscore a regulatory need to manage cadmium inputs in all areas. Land that is forested today may be pastoral tomorrow, and land that is pastoral today may be horticultural, or residential, tomorrow.

Comparison of estimates

Average projections estimated in this report for dairy soils, and soils under potatoes, are consistent with estimates reported by the New Zealand fertiliser industry, when differences in survey years and national versus regional coverage are accounted for. 

The New Zealand fertiliser industry has also reported that preventing further accumulation in New Zealand soils would require an 80% reduction in the cadmium content of superphosphate fertiliser, to approximately 24 mg Cd/kg P₂O₅

• This estimate is consistent with a recent opinion delivered by the European Commission’s Scientific Committee on Toxicity, Ecotoxicity and the Environment (SCTEE), that cadmium accumulation in soil should not be significant at fertiliser concentrations less than 20 mg Cd/kg P₂O₅.
• A figure reported for US superphosphate fertiliser, where cadmium accumulation has not been significant, is 16 mg Cd/kg P₂O₅.

The current voluntary limit for cadmium in New Zealand phosphate fertilisers is 122 mg Cd/kg P₂O₅.

Human exposure

Exposure of the general population to cadmium is mainly through food.  Of all contaminants in the diet, cadmium is the one that comes closest to its provisional tolerable weekly intake (PTWI).  Evidence exists that the current food standard is routinely being exceeded in some New Zealand crops, particularly some varieties of wheat. 

In order to provide a reliable benchmark for future work, a survey was carried out of cadmium concentrations in some key commercial produce sold in the Waikato region.   The average cadmium concentration in potatoes purchased in the Waikato region in 2004 was 25.4 µg/kg, which is one quarter of the food standard, and analytically indistinguishable from the figure for potatoes from the latest New Zealand Total Diet Survey (25.8 µg/kg). 

Based on an analysis of the sample distribution, it is estimated that approximately 1.5% of potatoes purchased in the Waikato region were likely to exceed the food standard in 2004.

Off-site effects

In terms of diffuse discharges to the wider environment, the main potential issue identified is the gradual accumulation of cadmium in stream, river,  lake and coastal sediments of agricultural catchments. However, further work is required to assess whether cadmium accumulation in rural streams, rivers, and coastal receiving areas is sufficient to pose a risk to ecological receptors in the medium term.

Management approaches

The area of technical management and policy options that might be applied to avoid, remedy or mitigate cadmium accumulation in agricultural soils is substantial enough to require a separate investigation and report in its own right.  Production of such a report (or reports) at national level is suggested as a recommendation of this work.

Further summary information

A more detailed summary can be found in Section 9 of this report.

Cadmium Accumulation in Waikato Soils [PDF, 1.6 MB]