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Results - data and trends

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Regional overview

The graph below compares the likelihood of toxic effects from trace elements and organic compounds on sediment-dwelling organisms among all investigated estuaries. The likelihood of toxic effects is determined by comparing trace element and organic compound levels measured in sediments to the relevant ANZECC interim sediment quality guideline (ISQG) values. More detail on how this indicator is compiled can be found on the Technical Information page.

*where more than 20% of samples at a site were below the detection limit the geometric mean could not be calculated

Results for Waikato estuaries

In summary:

  • The risk of toxic effects from trace elements and organic compounds on sediment-dwelling organisms is generally low in all sampled estuaries.
  • In most samples, organic compounds (total DDT, dieldrin and total PAHs) were below the detection limits. Dieldrin was not detected in any samples. DDT was detected at one sampling site in Port Waikato. In 2003 DDT was also measured at sampling sites in the Firth of Thames and Raglan (Whaingaroa) Harbour but in 2008 no DDT could be detected in these estuaries. Total PAHs were detected at some sampling sites in all estuaries.
  • The Firth of Thames contained the highest concentrations of several trace elements.
  • Sediments sampled in Port Waikato contained concentrations of arsenic that indicate a moderate risk of toxic effects on sediment-dwelling organisms. The arsenic found at Port Waikato is likely from geothermal sources.
  • Sediments sampled in the Firth of Thames contained concentrations of mercury that indicate a moderate risk of toxic effects on sediment-dwelling organisms. Mercury in the Firth of Thames may originate from a combination of natural geochemical processes, historic mining operations and the drainage of wetlands on the Hauraki Plains. Mercury may present a problem for biota because it biomagnifies and bioaccumulates.
  • It appears that the risk to aquatic ecosystems caused by trace elements and/or organic compounds detected in Raglan Harbour, Aotea Harbour, Kawhia Harbour and Tairua Harbour is particularly low.

The Firth of Thames

Trace element and organic compound levels in sediments in the Firth of Thames in 2008


*samples below the detection limit have not been plotted

Click here to view a map of sampling sites in the Firth of Thames. The map also illustrates the overall risk of toxic effects on organisms at each site.

What do these results show? 

  • In general, higher levels of trace elements were found at the eastern monitoring sites (Thames, Kuranui Bay and Te Puru) than at the western sites (Kaiaua, Miranda). Measured levels of antimony, cadmium, chromium, copper, lead, nickel and silver indicated low risk of toxic effects on organisms in the sediment. However, a moderate likelihood of toxic effects was revealed at Te Puru due to elevated levels of arsenic and at Thames due to elevated levels of mercury. At Kuranui Bay arsenic and mercury  levels indicated a moderate likelihood of toxic effects and the concentration of mercury reached the threshold to the high risk category.
  • No DDT or dieldrin was detected at the Firth of Thames monitoring sites in 2008.
  • Low concentrations of PAHs were detected at the monitoring sites at Thames and Kuranui Bay.
  • From the results obtained during this survey, it appears that the risk to sediment-dwelling organisms caused by concentrations of trace elements and/or organic compounds in the Firth of Thames is generally low. However, concentrations of arsenic, zinc, and particularly mercury were found to exist at concentrations that may cause adverse effects to the aquatic ecosystem.

Mercury at Thames and Kuranui Bay:

  • It is likely that the main cause of the elevated concentrations of mercury at Thames and Kuranui Bay is the mine tailings and industrial fill that were used as a part of a historic land reclamation (the Moanataiari reclamation), located in the area between the two sampling points.
  • Natural geochemical processes, such as weathering of catchment rock containing trace elements, are also potential sources of trace elements in the area. For example, a cinnabar (mercuric sulphide) deposit is situated in the Kauaeranga Valley, which could be transported to Thames via the Kauaeranga River.
  • In addition, it is thought that some of this mercury may originate from the drainage of the wetlands and peatlands on the Hauraki Plains (delivered via the Piako River).

For a more detailed technical assessment of trace elements in Firth of Thames sediments, please refer to our technical report, TR 2007/08 Trace Elements in Sediments of the Lower Eastern Coast of the Firth of Thames.

Comparison of results from 2003 and 2008

*where more than 20% of samples at a site were below the detection limit the geometric mean could not be calculated

What does this comparison show?

  • The mean levels of most trace elements were similar in 2003 and 2008.
  • No dieldrin was detected in the Firth of Thames in either year.
  • Low levels of DDT were detected at three sites in the estuary in 2003 but not in 2008.
  • Low levels of PAHs were measured at three sites in 2003 and at two sites in 2008.
  • Please note that conclusions about trends cannot be drawn from this data set since there are only two data points. Further, the variations observed are unlikely to represent real changes, because they are within the ranges expected when sampling sediment from a slightly different location in a given area.

Raglan (Whaingaroa) Harbour

Trace element and organic compound levels in sediments of Raglan (Whaingaroa) Harbour in 2008


 *samples below the detection limit have not been plotted

Click here to view a map of sampling sites in Raglan (Whaingaroa) Harbour. The map also illustrates the overall risk of toxic effects on organisms at each site.

What do these results show? 

  • All trace elements indicated a low likelihood of toxic effects at all five sites.
  • The levels of each trace element were similar at each monitoring site.
  • No DDT or dieldrin was detected at any of the monitoring sites.
  • PAHs was detected at low concentration at Ponganui Creek.
  • From the results obtained during this sediment sampling, it appears that the risk to sediment-dwelling organisms from trace elements and/or organic compounds in Raglan (Whaingaroa) Harbour is low. However, it should be noted that there is an insufficient number of samples to adequately spatially characterise Raglan Harbour, so it is possible that there are areas of localised contamination. No samples were collected from around the township itself.

Comparison of results from 2003 and 2008

*where more than 20% of samples at a site were below the detection limit the geometric mean could not be calculated

What does this comparison show?

  • The means levels of most trace elements in Raglan (Whaingaroa) Harbour were similar in 2003 and 2008.
  • No dieldrin was detected in the harbour in either year.
  • Low levels of DDT were detected at three sites in the harbour in 2003 but not in 2008.
  • Low levels of PAHs were measured at three sites in 2003 and at one site in 2008.
  • Please note that conclusions about trends cannot be drawn from this data set since there are only two data points.

Aotea Harbour

Trace element and organic compound levels in sediments in Aotea Harbour in 2008

* samples below the detection limit have not been plotted
^ organic compounds were not measured at sites 1, 2 and 3

Click here to view a map of sampling sites in Aotea Harbour. The map also illustrates the overall risk of toxic effects on organisms at each site.

What do these results show? 

  • Trace elements indicated low likelihoods of toxic effect on sediment-dwelling organisms at all sampling sites.
  • The levels of each trace element were similar at each site.
  • No DDT or dieldrin was detected at any site.
  • Low concentrations of PAHs were detected in the combined sample from sites 10, 12 and 13.
  • From the results obtained during this survey, it appears that the risk to aquatic ecosystems caused by concentrations of trace elements and/or organic compounds in Aotea Harbour is low.

Kawhia Harbour

Trace element and organic compound levels in sediments in Kawhia Harbour in 2008

*samples below the detection limit have not been plotted

Click here to view a map of sampling sites in Kawhia Harbour. The map also illustrates the overall risk of toxic effects on organisms at each site.

What do these results show?

  • Levels of most trace elements at most of the sampling sites indicate a low likelihood of toxic effects on sediment-dwelling organisms.
  • The combined sediment sample from sites one, two and three marginally exceeded the threshold of moderate toxicity risk. The elevated arsenic levels may arise from a geothermal ‘hotspot’ or arsenic may be naturally elevated in the area. It is unlikely that the elevated arsenic is caused by human activity.
  • DDT was detected in the combined sediment sample from sites 10, 11 and 12 at a level that marginally indicates a moderate likelihood of toxic effects.
  • No dieldrin was detected in Kawhia Harbour.
  • Low levels of PAHs were detected at several of the sampling sites in Kawhia Harbour.
  • It appears, from the results of this survey, that the risk to aquatic ecosystems caused by concentrations of most trace elements and organic compounds in Kawhia Harbour is low. However, concentrations of arsenic and DDT detected in some areas of the estuary fall within the ranges where adverse effects might be expected for some organisms. 

Port Waikato

In Port Waikato trace elements and organic compounds were measured at 19 sites. For the purpose of displaying results we have divided the estuary into two parts and created two separate graphs. The first graph shows results from the lower estuary and the second graph shows results from the upper estuary as shown in this map:

Trace element and organic compound levels in sediments of the lower Port Waikato in 2008

*samples below the detection limit have not been plotted

Trace element and organic compound levels in sediments of the upper Port Waikato in 2008

*samples below the detection limit have not been plotted

Click here to view a map of sampling sites in Port Waikator. The map also illustrates the overall risk of toxic effects on organisms at each site.

What do these results show?

  • Levels of most trace elements at the sampling sites in Port Waikato indicate a low likelihood of toxic effects on sediment-dwelling organisms.
  • The exception was arsenic which exceeded (or was equal to) the threshold indicating moderate risk to organisms at all but one of the monitoring sites. Arsenic did not reach the high risk threshold. Arsenic is known to be elevated within the Waikato River due to geothermal inputs (both natural and man-made). This is the likely source of the arsenic in sediments in Port Waikato. 
  • No DDT was detected at any of the Port Waikato monitoring sites.
  • Dieldrin was detected at one monitoring site (site 8), at a concentration that indicated a moderate risk on sediment-dwelling organisms. The measured value was only marginally above the detection limit. This illustrates that the threshold value represents a conservative assessments of the risk of dieldrin to organisms.
  • Low levels of PAHs were detected at several sampling sites at Port Waikato. 
  • It appears, from the results of this survey, that the risk to aquatic ecosystems caused by concentrations of most of the trace elements and organic compounds detected at Port Waikato is low. However, concentrations of arsenic and dieldrin were detected in the estuary within the range where adverse effects might be expected for some organisms.

Tairua Harbour

In Tairua Harbour trace elements and organic compounds were measured at 29 sites. For the purpose of displaying results we have divided the estuary into three parts and created three separate graphs. The first graph shows results from the lower estuary, the second graph shows results from the middle estuary and the third graph show results from the upper estuary as shown in this map:

Trace element and organic compound levels in sediments of the lower Tairua Harbour in 2010

*samples below the detection limit have not been plotted

Trace element and organic compound levels in sediments of the middle Tairua Harbour in 2010

*samples below the detection limit have not been plotted

Trace element and organic compound levels in sediments of the upper Tairua Harbour in 2010

*samples below the detection limit have not been plotted

What do these results show?

  • Trace elements indicated low likelihoods of toxic effect on sediment-dwelling organisms at all sampling sites in Tairua Harbour.
  • No DDT or dieldrin was detected at any site.
  • Low concentrations of PAHs were detected at site 27.
  • From the results obtained during this survey, it appears that the risk to aquatic ecosystems caused by concentrations of trace elements and/or organic compounds in Tairua Harbour is low.