Presenter: Dave Bull, Golder Associates

Presentation title: Accounting for ‘the poisoner’s poison’: a soil guideline value for thallium

Thallium is a chemical element, symbol Tl, sitting in the fifth row of the Periodic Table between mercury and lead.  ‘The poisoner’s poison’, highly toxic to humans, it was more reputably used as a rat poison, insecticide, and ringworm treatment, but today has only a few specialist uses.  Tl can be emitted by metal smelters and coal-fired power plants, and is among the suite of elements associated with sulfide minerals.    Thallium has been detected in soils of the Moanataiari subdivision, Thames, Waikato, at levels of up to 29 mg/kg, associated with elevated arsenic and/or lead.  This is well above Tl’s typical natural abundance of approximately 1 mg/kg.  The health risk posed by such concentrations is not clear, as Tl is not on the Ministry for the Environment’s (MfE) list of priority contaminants, so there is no New Zealand Soil Contaminant Standard (SCS) for it.    This presentation will outline how a reference health standard for thallium can be obtained, and evaluate the critical exposure pathway, vegetable consumption. Given data on the site of interest, and on background soils, region-specific soil guideline values can then be calculated.     The results suggest that thallium might pose surprisingly significant risks as a soil contaminant in some New Zealand locations.

 Event: WasteMINZ Conference 2013

Date: Tuesday 22 October 213