Pollutants in sediments

The graphs below compare the likelihood of toxic effects from trace elements on sediment-dwelling organisms among investigated estuaries. The likelihood of toxic effects is determined by comparing trace element levels measured in sediments to the relevant ANZECC interim sediment quality guideline (ISQG) values.

Regional comparison of trace elements in sediments in 2018

Figure 1: Regional overview comparing the geometric mean of trace elements sampled in 2018 from Firth of Thames, Raglan (Whāingaroa) Harbour, Port Waikato and Tairua Harbour. *Where more than 40% of samples at a site were below the detection limit.

What do these results show?

• The risk of toxic effects from trace elements on sediment-dwelling organisms is generally low in all sampled estuaries.
• At Port Waikato mercury and silver were not detected; however, arsenic was equal to the low-moderate threshold, indicating a potential risk to sediment dwelling organisms. The arsenic found at Port Waikato is likely from geothermal sources. Trace element levels for Port Waikato displayed in Figure 1 represent a single sediment sample. Trace metal concentrations are similar to those in 2008, when 19 samples were collected throughout the estuary.
• The Firth of Thames contained the highest concentrations of several trace elements, notably lead, zinc and mercury; but overall represented a low risk of toxic effects on sediment-dwelling organisms.
• Organic compounds (total DDT, dieldrin, and total PAHs) were not analysed in 2018. These have been sampled previously and found to be very low, often below the detection limit of the analytical method.
• The risk to aquatic ecosystems caused by trace elements in Raglan (Whāingaroa) Harbour and Tairua Harbour is particularly low.

Trace element levels in sediments in the Firth of Thames in 2018

Figure 2: Firth of Thames sediment trace element levels sampled in 2018 from sites Kaiaua, Miranda, Gun Club and Kuranui Bay. *Antimony at Miranda was below the detection limit of <0.04 mg/kg dry weight.

Sampling sites in the Firth of Thames. Pie charts illustrate the overall risk of toxic effects on organisms at each site. Satellite image source: Google Earth.

What do these results show?

• Higher levels of trace elements were found at the eastern monitoring sites (Gun Club and Kuranui Bay) than at the western sites (Kaiaua and Miranda).
• Concentrations of antimony, cadmium, chromium, copper, lead, nickel and silver indicated low risk of toxic effects on organisms in the sediment.
• A moderate likelihood of toxic effects was revealed at Kuranui Bay for arsenic, mercury and zinc, and Gun Club showed moderate likelihood of toxic effects for mercury. Arsenic concentrations at Kuranui Bay were bordering between low and moderate toxic effects on benthic fauna (101% of ISQG‑Low).
• The risk to sediment-dwelling organisms caused by levels of trace elements in the Firth of Thames was generally low. Levels of arsenic, zinc, and particularly mercury were found to exist at levels 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 levels of mercury at Gun Club 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). 

Comparison of results from 2003 to 2018

Figure 3: Comparison of the geometric mean from all sites sampled in Firth of Thames from 2003 to 2018. Te Puru was not sampled in 2018, so data related to Te Puru was removed from 2003 to 2013 samplings for comparison. For that reason, plots relating to the Firth of Thames may differ from previous plots.

What does this comparison show?

• The mean levels of most trace elements remained similar over time (2003 to 2018).
• Mercury showed the largest variation and decreased from 2008 to 2018 by 20%, largely driven by Kuranui Bay.
• Please note that conclusions about trends cannot be drawn from this data set since there are only four 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. 

Trace element levels in sediments of Raglan (Whāingaroa) Harbour in 2018

Figure 4: Raglan (Whāingaroa) Harbour sediment trace element levels sampled in 2018 from sites Ponganui Creek, Whatitirinui Island, Te Puna Point, Okete Bay and Haroto Bay. *Samples below the detection limit have not been plotted (<0.02 mg/kg dry weight for mercury and silver).

Sampling sites in Raglan (Whāingaroa) Harbour. Pie charts illustrate the overall risk of toxic effects on organisms at each site. Satellite image source: Google Earth.

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.
• Previous monitoring (2008) found no DDT or dieldrin at any of the monitoring sites and PAHs were only detectable in low concentrations at Ponganui Creek.
• Mercury was not detected at Ponganui Creek and Te Puna point, and silver was not detected at all monitoring sites.
• From the results obtained during this sediment sampling, it appears that the risk to sediment-dwelling organisms from trace elements in Raglan (Whāingaroa) Harbour is low. However, it should be noted that there is an insufficient number of samples to adequately spatially characterise Raglan (Whāingaroa) 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 to 2018

Figure 5: Comparison of the geometric mean from all sites sampled in Raglan (Whāingaroa) Harbour from 2003 to 2018. *Where more than 40% of samples at a site were below the detection limit (<0.02 mg/kg dry weight for silver).

What does this comparison show?

• The mean level of most trace elements in Raglan (Whāingaroa) Harbour were similar across the years of sampling.
• Organic compounds (total DDT, dieldrin, and total PAHs) were not analysed in 2018 and so comparisons are not possible for these contaminants. These have been sampled previously and found to be very low, often below the detection limit of the analytical method.
• Silver was not detected in 2008 and 2018.
• Conclusions about trends can not be drawn from this data yet given the limited number of sample points. 

Trace element levels in sediments of Tairua Harbour in 2018

Figure 6: Tairua Harbour sediment trace element levels sampled in 2018 from sites Pepe, Manaia, Pauanui, Gumdigger and Oturu. *Samples below the detection limit have not been plotted (<0.02 mg/kg dry weight for mercury and silver).

Sampling sites in Tairua Harbour. Pie charts illustrate the overall risk of toxic effects on organisms at each site. Satellite image source: Google Earth.

What do these results show?

• Trace elements indicated low likelihoods of toxic effect on sediment-dwelling organisms at all sampling sites in Tairua Harbour. Therefore, the risk to aquatic ecosystems caused by concentrations of trace elements in Tairua Harbour is low.
• Mercury was only detected at Gumdigger and Oturu, and silver at Pepe.
• These recent results fit with the findings from 2010 when 29 samples were collected from across the upper middle and lower harbour. In 2010, all trace elements were also low with arsenic being the highest relative to the guide at 36% of ISQG-Low. 

Trace element levels in sediments of Whitianga Harbour in 2014

Figure 7: Whitianga Harbour sediment trace element and organic compound levels sampled in 2014 from 12 sites spanning upper to lower harbour regions. *Samples below the detection limit have not been plotted (detection limits are <0.02 and <0.07 mg/kg dry weight for silver and PAHs, respectively).

Sampling sites in Whitianga Harbour. Pie charts illustrate the overall risk of toxic effects on organisms at each site. Satellite image source: Google Earth.

What do these results show?

• Most trace elements indicated a low likelihood of toxic effects across the 12 sites.
• Arsenic showed a moderate likelihood of toxic effects for sediment-dwelling organisms at Central NE Arm. Arsenic levels were raised at most sites, and above 70% of the ISQG-Low at 11 of 12 sites except for Waipakupaku Creek.
• Mercury levels were also raised across sites, on average 48% of the ISQG-Low threshold and exceeded this threshold at Owhero Bay.
• Although still representing a low likelihood of toxic effects, the Central NE arm site also showed the greatest levels of antimony, chromium, copper and nickel.
• The organic compound DDT showed a moderate likelihood of toxic effects for sediment-dwelling organisms at all 12 sites. Albert Street South was the only site to detect PAHs, which was 8% of the ISGQ-Low.

The maps on this page show where our sampling sites were located in the estuaries. On each map sampling sites are represented by pie charts. These pie charts illustrate the overall risk of toxic effects on organisms at this site by indicating the percentage of trace elements and organic compounds measured at this site that fall into the categories:

• low likelihood of toxic effects (green),
• moderate likelihood of toxic effects (orange), and
• high likelihood of toxic effects (red).

Scroll down or click on the estuary names below to view the estuary maps

• Firth of Thames
• Raglan (Whaingaroa) Harbour
• Aotea Harbour
• Kawhia Harbour
• Port Waikato

Sediment samples collected at some sites were combined for analysis. In these instances, the location of the pie chart represents approximately the centre of the original sampling sites.

Sediment samples collected at some sites were combined for analysis. In these instances, the location of the pie chart represents approximately the centre of the original sampling sites.

Monitoring sites

We monitor sediment trace element and organic compound levels in a number of estuaries in our region. Estuaries are chosen based on public interest as well as their exposure to trace elements and organic compounds from the catchment.

For this indicator update we have presented the most recent results from the southern Firth of Thames, Raglan (Whāingaroa) Harbour, Tairua Harbour, Whitianga Harbour, and Port Waikato. We also have historic data for Aotea and Kawhia Harbour. 

Monitoring sites are distributed throughout the main part of each estuary, and are representative of the area in which they are located. They are in the mid-intertidal areas of non-vegetated sand and mudflats.

Monitoring frequency

Trace elements and organic compounds are known to accumulate slowly in sediments. For this reason we do not monitor estuaries at a regular frequency. Instead, the frequency of monitoring is determined by the risk of toxic effects on sediment-dwelling organisms revealed in the monitoring results as well as the risk of pollution from activities in the catchment.

So far, estuarine sediments have been analysed in 2003, 2008, 2010, 2013, 2014 and 2018. This indicator presents the most recent results from 2013 to 2018.

Table 1: Monitored estuaries

Measurement technique

Surface sediments are sampled to a depth of 2 cm. In some estuaries, single samples are analysed whereas at others several samples are combined to bulked samples. In the Firth of Thames and Raglan (Whāingaroa) Harbour, samping is carried out at the permanent sampling sites of the Regional Estuary Monitoring Programme (REMP).

Sediment samples are stored and maintained by freezing them in the laboratory. Sediments are tested by Hill Laboratories for organic carbon levels, ten trace elements, five organic compounds and twelve polycylic aromatic hydrocarbons (PAHs) for monitoring. We report on all the trace elements, two of the most important organochlorines (total DDT and dieldrin) and PAHs:

• antimony
• cadmium
• chromium
• copper
• lead
• mercury
• nickel
• silver
• zinc
• arsenic
• organochlorines (total DDT and dieldrin)
• total PAHs

Quality Control Procedures

The sampling protocol is statistically designed to ensure collection of a high quality data set. Hills Laboratories are an IANZ (International Accreditation New Zealand) certified laboratory.

Data storage

The data is currently stored in Excel spreadsheets. Please contact Waikato Regional Council if you like to obtain the raw data used to calculate this indicator.

This indicator displays the likelihood of toxic effects from trace elements and organic compounds on sediment-dwelling organisms. 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.

1. The level of trace element and organic compound at each sampling site is expressed as a percentage of the ISQG-Low and ISQG-High values using the formulas:
        Trace element or normalised organic compound level/ ISQG-Low * 100, and
        Trace element or normalised organic compound level/ ISQG-High * 100.
2. For each trace element and organic compound at each sampling site the likelihood of toxic effects on sediment-dwelling organisms is rated:
        Low if the level of trace element or organic compound is less than the ISQG-Low,
        Moderate if the level of trace element or organic compound is greater than the ISQG-Low but less than the ISQG-High,
        High if the level of trace element or organic compound is greater than the ISQG-High.
3. All results are scaled so that they can be graphed accurately on one scale.
4. For each estuary these ratings are graphed (y-axis) against all the trace elements and organic compounds (x-axis).
5. For each estuary the geometric mean of all sampling sites is calculated for each trace element and organic compound to provide an average likelihood of toxic effects on sediment-dwelling organisms. These values are plotted in one graph providing a comparison of estuaries. 

Guidelines and standards

Trace element and organic compound levels are compared to the guidelines derived by the Australian and New Zealand Environment and Conservation Council (ANZECC).

For each trace element and organic compound, ANZECC has derived a low interim sediment quality guideline value (ISQG-Low) and a high interim sediment quality guideline value (ISQG-High). The ISQGs relate to the toxic effects of trace elements and organic compounds on sediment-dwelling organisms.

The ISQG-Low value is the level below which adverse effects are very unlikely (low likelihood of toxic effects). As such, it is not a level that is cause for concern but simply the trigger point indicating the need for further investigation. The ISQG-High value is a level at which adverse effects are expected in half of the exposed organisms. Concentrations above the ISQG-High value are interpreted as being reasonably likely to cause significant adverse effects on aquatic organisms (high likelihood of toxic effects). Between the ISQG-Low and ISQG-High values the effects of trace elements and organic compounds are unknown. Therefore they are thought to pose a moderate level of risk to sediment-dwelling organisms (moderate likelihood of toxic effects).

Table 2: ANZECC's ISQG-Low and ISQG-High values for each trace element and organic compound

In the surveys conducted for this indicator only the top 2 cm of the sediment are sampled. It is possible that trace element and organic compound levels are greater in deeper sediment layers. This is particularly relevant at locations where concentrations might have been elevated because of past human activities. In these instances, sediments containing elevated historic concentrations of trace elements and/or organic compounds might have been covered by more recent sediment.

Trace elements exist in a number of forms, some of which are bioavailable (the degree to which the chemical is available for physiological reactions) and some of which are not. Some chemical forms are more problematic than others. Laboratory tests determining the levels of each element do not distinguish between chemical forms and only measure the element as a whole. Therefore, only a portion of the level of the element in the sediment that is measured may be problematic and the indicator potentially makes overestimates the level of contaminants.

The ANZECC ISQGs used in this indicator are limited due to the data on which they are based. Because of the lack of New Zealand data available, ANZECC ISQGs are:

• based primarily on a single, large biological-effects dataset of North American sediment data; and
• supplemented by data collected at specific sites and predictions based on 'equilibrium partitioning.'

To remedy this and make the guidelines more accurate for this country, more data is needed to determine the effects of trace elements and organic compounds specifically in New Zealand.