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Phase 2 – site specific investigation Summary of results

Phase 2 – site specific investigation Summary of results

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Phase 2 – site specific investigation Summary of results

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  1. Phase 2 – site specific investigationSummary of results Tonkin & Taylor Ltd 2 July 2012

  2. Summary of investigation • 206 residential properties and 1 ECC • 6 residential properties not included in the investigation • 544 investigation locations – hand augers • Approx 30 sample locations per ha • 1668 soil samples • All samples analysed with the XRF • 409 samples (25%) analysed at the lab

  3. Subsurface materials • East of Tararu Road • Fill placed in late 1800s • Discontinuous layers of silty sand - mullock • Minimal evidence of processed mine tailings or industrial waste • East of Moanataiari Street • Fill placed between 1870s – 1940s • Discontinuous layers of silt, sand and clay - mullock • Minimal evidence of processed mine tailings or industrial waste • West of Moanataiari Street • Fill placed in 1960s • Discontinuous layers of silt, sand and clay - local quarried rock • Minimal evidence of industrial waste

  4. Subsurface materials • Groundwater • Present at 0.8 - 1.2 below ground level • Slightly above sea level • Fill depth • Natural ground encountered at some locations in the east (<1m fill depth) • 0.95 and 1.6m deep in groundwater wells • Cap in eastern area • No visible difference in surface material

  5. Results Summary of laboratory results

  6. Key contaminants • Arsenic • Most concentrations exceed 20 mg/kg • Consistently high in east • Consistently lower in west • Good fit with site history

  7. Key contaminants • Lead • Present in isolated clusters only • Mostly affecting eastern area • Clusters can be linked to site history

  8. Key contaminants • Thallium • Mostly affecting eastern area • Associated with elevated arsenic

  9. Certainty • Heterogeneity of soils • Testing (XRF and laboratory) • Receptor behaviour (now and in the future) • Toxicology (eg bioavailability)

  10. Certainty • Conservative • Ministry for the Environment Guidance • Number of samples • Careful logging of soils • Field and laboratory quality assurance and control • Staged approach

  11. Certainty Laboratory = 1000 mg/kg XRF = 400mg/kg

  12. Certainty Laboratory = 1000 mg/kg XRF = 400 mg/kg Human Health Guideline = 100 mg/kg

  13. Certainty Laboratory = 550 mg/kg XRF = 500 mg/kg

  14. Certainty Laboratory = 550 mg/kg Human Health Guideline = 525 mg/kg XRF = 500 mg/kg

  15. Ground model