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Moanataiari Project: Update # 28: 20ppm assumptions explained and Bright Smile Garden update

08 May 2012

At last week's community forum, the residential standard for arsenic was discussed (20 parts per million). Members of the forum requested a plain-English explanation regarding some of the underlying assumptions that were made to formulate the national environmental standard.

20 parts per million assumptions explained

It is important to note that it is government policy that the acceptable risk for compounds such as arsenic is one additional cancer in 100,000 people.

So here goes...

The standard assumes:

  • That all of the arsenic is absorbed by the body if the soil is eaten (100% bioavailability)
  • A person's weight at 70kgs
  • That 10% of produce (fruit and veggies) consumed are grown at home
  • That the person will live at the site for 35 years (i.e. at the same address)
  • A life expectancy of 75 yrs 
  • A person spends 350 days a year at the site (residential exposure frequency)


Bright Smile Garden results

In early 2012, council contractors working on a land subsidence project at the Bright Smile Garden in Thames discovered elevated levels of arsenic and lead in the soil in some places where fruit and vegetables were grown. As a result of this discovery, fruit and vegetables at the garden were also tested to see if any of the contaminants were transferred into the fruit or vegetables as they were growing in the soil.

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