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Phase 2 – site specific investigation Summary of results

This report provides a summary of the findings from a site-specific investigation conducted by Tonkin & Taylor Ltd. The investigation included analysis of 206 residential properties, 1 ECC, and 544 investigation locations using hand augers. The report highlights the presence of subsurface materials, groundwater levels, and key contaminants such as arsenic, lead, and thallium. The results emphasize the importance of careful soil logging, quality assurance, and a staged approach for future actions. The report includes guidelines for human health and certainty levels for laboratory and XRF testing.

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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

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