Bright Smile Garden

Thames-Coromandel District Council has engaged Tonkin & Taylor Ltd (T&T) to undertake an assessment of ground contamination at the Bright Smile Garden, Bella Street, Thames.


Land subsidence occurred in the northeastern corner of the Bright Smile Garden in March 2011

The subsidence was investigated by the Council's contractors and a report was prepared which included options for further investigation and stabilisation of the void.

The history of the site was investigated as part of the 2011 geotechnical investigation. Available information shows that the site was involved in mining from the mid-1800s until the early 1900s. Land subsidence is known to have occurred at the site in the past.

The geotechnical investigation indicated that there is at least 2 m of fill material in the area of the subsidence. This material was described as comprising topsoil overlying soft to firm clayey silt with some cobbles/boulders and tree roots. The fill material is considered to be processed mine tailings, and/or unprocessed waste rock.

Further geotechnical investigations of the collapse were undertaken in February 2012. As a result, various remedial options were presented consisting of preparation of the void area followed by backfill with engineered or non-engineered fill.

Limited chemical testing of soil samples was undertaken in November 2011, and the topsoil and fill material in the vicinity of the subsidence were found to contain arsenic concentrations in excess of the new National Environmental Standards (NES) for residential and commercial land use.

The results of the sampling raised questions regarding contamination potential throughout the remainder of the site. In particular, to assess the produce ingestion pathway given the site's community use, through sampling of garden soils and garden produce as discussed below.

Project Scope

The objective of this investigation is to further assess ground contamination across the property, with particular emphasis on the community garden areas, and to review the results with respect to the NES requirements as they pertain to subsidence and contamination remediation management options. The work scope to achieve this objective has included:

  • Review of the NES with regard to appropriate guidelines for the current land use scenario and discussion regarding NES derivation and appropriate site-specific criteria.
  • Review of New Zealand literature regarding the uptake of arsenic by plants to determine the most appropriate plants to sample for contamination.
  • Collection of near surface soil samples from the garden areas.
  • Collection of fruit and vegetable samples from the garden.
  • Analysis of soil samples for arsenic and lead.
  • Analysis of fruit and vegetable samples for arsenic and lead.
  • Review of previous soil and vegetable data collected by the lease holder and provided by
  • TCDC.
  • Review of NZ food standards and derivation of produce acceptance criteria using NES
  • methodology.

The results

The fruit and vegetables tested for arsenic were lower than NZ's adopted food criteria.

The fruit and vegetables tested for lead were lower than NZ's adopted food criteria, except for some of the herbs that were tested. It seems that the herbs absorbed the lead more readily.

Brightsmile health comments from the DHB

The Waikato District Health Board have released the following comments after their analysis of the Bright Smile Garden report.

The results of testing of produce from the Bright Smile Garden have been reviewed and considered by Population Health, Waikato District Health Board, and by the Ministry of Health.  The Ministry of Agriculture now has responsibility for the safety of all foods, including non-commercially produced foods, and accordingly the results have been referred to their food safety section for formal comment.

We note that Thames-Coromandel District Council anticipated using the findings from the Bright Smile Garden produce as guidance for produce grown on Moanataiari soils, but the implications of any interpretation of these results will be much wider, and may potentially affect all food production.  A formal response from the Ministry of Agriculture will therefore take some time.

The results from Bright Smile Garden produce are reassuring in that there is only one exceedence of the Australasian Food Standard, for lead, which was found in herbs.  Herbs are less of a concern because they are seldom eaten in large quantities. 

However, the only recognised standard for arsenic in food is for cereals, and its applicability to other plant foods is unknown.  The food tested all met the derived standard used by Tonkin and Taylor, and the stricter Biogrow standard.

There is also no information available on seasonal variation, and the impact of different soil types on uptake of arsenic in plant foods. 

For these reasons, interim advice only can be given.  This was sought in particular in relation to the feijoa crop which was ripening, and interim advice has been given that consumption of seasonal fruit, including tomatoes, is unlikely to create a health risk provided the current health advice provided to Moanataiari residents is followed. 

This is available via the Ministry of Health’s booklet “Arsenic and Health”.

It is reasonable to follow the same approach in respect of other locally grown produce if it is seasonal and does not normally constitute a major proportion of an adult diet.

Root crops and other vegetables which are typically eaten all year (e.g. potatoes, carrots, cabbage) could potentially create a higher risk and the advice not to eat these vegetables must stand until the Ministry of Agriculture is able to comment.

This advice is particularly important for pregnant women and young children.

People working in Bright Smile Garden must also continue to follow the health advice provided for Moanataiari residents. 

More copies of the booklet can be provided for distribution to the garden users.

Dell Hood
Medical Officer of Health

Read the full report

Read the full report by downloading the pdf from the 'download box' on the right-hand side of this page.