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Tāmaki Makaurau / Auckland marine sediment contaminant monitoring: data report for 2022. Summary

Hamish Allen
Auckland Council Research and Evaluation Unit, RIMU
Publication date:  

Download on the right. A summary of Auckland Council technical report, Tāmaki Makaurau / Auckland marine sediment contaminant monitoring: data report for 2022, TR2023/15.



Executive summary from Tāmaki Makaurau / Auckland marine sediment contaminant monitoring: data report for 2022

Contaminants such as copper, lead, zinc, arsenic, and mercury, can accumulate in the sediments of our harbours, estuaries, and beaches. They can originate from a range of different activities and land uses including vehicle tyre and brake wear, industrial discharges, and the breakdown of some building materials. When it rains, these pollutants are washed into our stormwater networks and waterways, ending up in our marine environment. The build-up of these contaminants can affect ecological health, by reducing the abundance and/or diversity of animals living in the sediment. This can have harmful effects on the natural functioning of these ecosystems and result in degraded communities that are dominated by the remaining few species that are tolerant of higher contaminant levels. Understanding the distribution and level of chemical contaminants in marine sediments provides a useful marker of land use impacts on aquatic receiving environments and ecosystem health.

This document describes the monitoring undertaken at 40 sites in October and November 2022 as part of Auckland Council’s Regional Sediment Contaminant Monitoring Programme (RSCMP). Sites were located in the Upper Waitematā Harbour, Tāmaki Estuary, Mahurangi Harbour, the East Coast Bays (including Okura Estuary), and Tāmaki Strait.

The report provides:

  • an overview of the RSCMP monitoring programme
  • description of the sampling undertaken in 2022
  • the sediment contaminant (metals) and particle size distribution (PSD) results obtained for the 2022 samples
  • a summary of contaminant (metals) state and changes over time in state
  • quality assurance (QA) assessments undertaken to verify the data were acceptable for the purposes of the RSCMP.

Changes in state refer to a change relative to Environmental Response Criteria (ERC) threshold levels only. More sensitive trend analysis (statistical analysis of the monitoring data to obtain the magnitude and direction of change over time) can be found in Mills and Allen (2021).

Samples used for sediment chemistry analysis were processed and analysed for the following metals: copper, lead, zinc, arsenic (a metalloid species), mercury, and at six sites in the Mahurangi Harbour, cadmium. Total recoverable metals, on the <500μm fraction, were analysed. One composite sample from each site was also analysed for PSD.

The quality assurance data analysis indicated that overall, the metals and PSD data obtained in 2022 are largely within acceptance criteria and considered suitable for use in the RSCMP.

Contaminant state is compared with sediment quality guidelines, including the Australian and New Zealand Guidelines for fresh and marine water quality (ANZG), the Auckland Council Environmental Response Criteria (ERC), and the Threshold Effects Level / Probable Effects Level (TEL/PEL). See section 3.1 for more detail on the sediment quality guidelines used in the RSCMP.

Results from sampling undertaken in 2022 showed a wide range of sediment contamination. Most of the sites sampled (30 out of 40; 75%) were assessed in the conservative ERC-green category. Fewer sites trigger the higher ANZG amber thresholds (only three sites for zinc and two for mercury; 88% are in the ANZG green category). Encouragingly, no sites sampled in 2022 triggered the ANZG red threshold for any metal.

The spatial pattern of contamination remains consistent with previous monitoring, with rural locations recording low levels of metals, while elevated concentrations were observed in the upper reaches of the Tāmaki Estuary and (to a lesser degree) in the Upper Waitematā Harbour.

In the Tāmaki Estuary, zinc remains a key contaminant of concern, exceeding ERC sediment quality guidelines at seven of nine sites. At several of these sites, levels of copper and mercury are also elevated. Previous monitoring has found that higher contaminant concentrations are most prevalent in catchments with intensive industrial and urban areas, particularly where there is a long history of this type of land use. The pressures associated with these land uses have cumulatively had a negative impact on sediment quality, particularly in the upper reaches of the Tāmaki Estuary, where the sheltered, low energy environment tends to accumulate fine sediment and can have a high proportion of mud and metals. Sites in the lower reaches have lower metal and mud content, likely as a reflection of these sites’ location in sandier substrate, exposed to higher wave and tidal energy.

Despite the surrounding catchment containing large rural areas, in the Upper Waitematā Harbour several sites trigger conservative copper and mercury thresholds (the ERC and TEL/PEL, respectively). These levels are higher than expected, and the area has a long history of elevated copper, with sites observed above the ERC amber threshold since monitoring began at Paremoremo and Lucas Upper in 1998. The cause or causes of this are unknown, however it is possible that largely historic copper-based pesticide and herbicide use in the surrounding catchment has been a contributing factor.

Cadmium was included in the suite of analytes for sampling conducted in the Mahurangi Harbour. Cadmium has the potential to be elevated in marine sediments of rural areas, because it is an unavoidable contaminant present in phosphate fertiliser. Overall, low levels of cadmium were observed in Mahurangi, with two sites recording concentrations below lab detection limits, and the remaining four sites at concentrations well below guideline thresholds. Results of the other metals tested in Mahurangi were also low, with no sites triggering any of the applied threshold guidelines.

In general, ERC contaminant status (for metals copper, lead and zinc) has remained relatively stable over time at most sites sampled in 2022. Two sites (Awaruku Stream and Lucas Upper) both changed from ERC amber to ERC green for the metal copper. Further monitoring at these sites will be required to determine if these changes remain, or if concentrations are fluctuating above and below guideline values.

Auckland Council technical report, TR2023/15

December 2023