Environmental regulators have revoked approvals for the import, manufacture and use of the weedkiller methylarsinic acid (MSMA).
But the Environmental Risk Management Authority (Erma) is allowing the arsenic-based, persistent herbicide to be used for one more year.
The chemical has been used to control paspalum and kikuyu in fine turf growing north of Taupo, with one product, AGPRO MSMA 600, used on turf.
Erma's decision-making committee said today risks associated with the persistency of arsenic and its potential toxicity outweighed the benefits of using MSMA.
"The committee considered that the revocation of further imports at a time when stocks were seasonally low, combined with one final season after many years of use would allow disposal and management of the risks in the most cost-effective way," said committee chairman Dr Max Suckling.
Computer modelling of the risk of accumulating arsenic in the environment has identified the potential for increased cancer rates in children who have played on treated areas and adjoining properties over a number of years.
Overseas research has shown repeated used can lead to increasing arsenic levels in soil and groundwater.
More than 10 percent of New Zealand groundwater currently exceeds Ministry of Health guidelines for arsenic content. Some of it is due to natural contamination from rock, but some researchers have expressed concern that adding arsenic sprays could top up the "background" levels enough to make more groundwater unsuitable for drinking.
Eco-toxicological assessment of the chemicals have identified acute risks to aquatic plants, non-target plants outside the treatment area and birds, and to humans in terms of eye irritation and low level acute toxicity. Erma said 11 submissions on a re-assessment of the chemical included 10 in support of approvals being revoked and one on the way in which its withdrawal would be carried out.
Erma revoked two approvals -- one for the active ingredient methylarsinic acid, and the other for the chemical's sodium salt concentrate, which is what is used in this country.
Erma today gazetted a notice giving users until May 31, 2010, to dispose of or use up existing stocks. In the European Union and Canada approvals for MSMA have already been withdrawn, and the United States is preparing to cancel re-registration of all arsenical pesticides.
