Trace Elements in Sediment in Firth of Thames

Arsenic, cadmium, copper, mercury, lead and zinc are enriched in sediments of the lower eastern coast of Firth of Thames, relative to concentrations present before Polynesian and European colonisation, and reference concentrations in sediments from Raglan Harbour. Concentrations of the other five elements measured in some or all samples (chromium, nickel, aluminium, iron and lithium) are more typical of those observed in harbour sediments in other areas. Relative to its expected background concentration, the most highly enriched element is mercury. Mercury concentrations are on average about seven times higher in Firth of Thames sediments than those of reference sites. In mass terms, the most highly enriched element is zinc. Firth of Thames sediments contain about 10 mg/kg more zinc than reference sites. One area (Kuranui Bay) shows localised metal contamination, which looks likely to have been from an urban industrial source. Apart from this area, the influence of local sources appears minor. Generally the data suggests that elevated arsenic, copper, lead, cadmium and zinc concentrations along the lower eastern coast of the Firth of Thames are likely to be dominated by larger-scale sources, most likely enhanced weathering and erosion following land clearance, the impact of distant historic mining operations (tailings) and agricultural inputs (zinc and cadmium). A fourth possible source identified in this work(for mercury) is dissolved organic matter entering the Firth of Thames from wetlands and peatlands of the Hauraki Plains. Further work is needed to confirm this hypothesis. Purpose: This dataset includes trace element sampling undertaken in the lower Firth of Thames by Environment Waikato in 2004, and a more detailed study undertaken by URS New Zealand Ltd (URS) in June 2005.

Table of contents from review report: Executive summary 1 Introduction 1.1 Potential sources of the elements 1.1.1 Weathering and erosion 1.1.2 Past mining 1.1.3 Agricultural and horticultural inputs 1.1.4 Urban inputs 1.2 Previous data 2 Study methodology 2.1 Sampling objectives 2.2 Sampling locations 2.2.1 General 2.2.2 History of the sample locations 2.3 Sample types 2.3.1 Manual grab samples 2.3.2 Vertical profiles 2.3.3 Grab sampling 2.4 Analysis and analytical quality assurance 2.5 Use of ANZECC sediment quality guidelines 3 Results and discussion 3.1 Raw results and summary statistics 3.2 Comparison with sediment quality guidelines 3.2.1 Comparison to the ISQG-Low 3.2.2 Comparison to the ISQG-High 3.2.3 Key findings 3.3 Comparison between the Firth of Thames and other areas 3.3.1 Comparison to coastal sediments in Raglan Harbour 3.3.2 Comparison of this survey’s data with historical data 3.3.3 Key findings 3.4 Small scale variability 3.4.1 Assessment 3.4.2 Key finding 3.5 Comparisons between sites in the Firth of Thames 3.5.1 Statistical analysis and data normalisation 3.5.2 Local historical mining sites versus control sites 3.5.3 Seaward versus landward 3.5.4 Urban areas versus control sites 3.5.5 Agriculturally proximate versus control sites 3.5.6 Key findings 3.6 Correlations and spatial trends 3.6.1 Approach and correlation matrix 3.6.2 Zinc, cadmium, and lead 3.6.3 Arsenic and copper 3.6.4 Chromium and nickel 3.6.5 Mercury 3.6.6 Principal Components Analysis 3.6.7 Key findings 4 Summary and recommendations 4.1 Summary 4.1.1 Relative enrichments 4.1.2 Comparison to guidelines 4.1.3 Local and general sources 4.1.4 Probable dominant sources by element 4.2 Recommendations

Initial EW samples taken from Kaiaua, Miranda, Thames Airfield, Tararu and Te Puru. URS collected 78 sediment samples from the following 11 locations: Kuranui Bay, Piako River mouth, Tararu, Tapu, Te Mata, Te Puru, Thames mudflats, Thames urban area, Thornton Bay, Waihou River mouth, and Waiomu.

2004-2005

All sediment samples were analysed for arsenic, cadmium, chromium, copper, lead, mercury, nickel and zinc. Samples collected from selected sites also underwent analysis for grain size, lithium, iron and aluminium, to aid in interpreting the results. Statistical tests were applied to ascertain if there were significant differences between sites.

Data quality: Good - details of standards and quality control reported in Kim (2007)

Data available in .pdf tables as appendices in Kim (2007) report. Digital Format: Contact Environment Waikato for raw data.

Freely available in report form. Sensitivity/Confidentiality: No confidentiality.

Complete, but report contains recommendations for 5 yearly sampling of sediments.