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  Environment » Environmental Information » Environmental indicators » Coasts: monitoring and reporting » Pollutants in sediments » Methods - how we monitor

Methods - how we monitor

Where and how we collect the data

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.

So far we have monitored the southern Firth of Thames (five sites), Raglan (Whaingaroa) Harbour (five sites), Aotea Harbour (22 sites), Kawhia Harbour (23 sites), Port Waikato (19 sites) and Tairua Harbour (29 sites).

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.

Data history

So far, estuarine sediments have been analysed in 2003, 2008 and 2010.

Table 2: Monitored estuaries

Estuary Year monitored
Firth of Thames 2003, 2008
Raglan (Whaingaroa) Harbour 2003, 2008
Aotea Harbour 2008
Kawhia Harbour 2008
Port Waikato 2008
Tairua Harbour 2010

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 (Whaingaroa) 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.

How this indicator is compiled

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

      Trace element or organic compound ISQG-Low
      (mg/kg dry weight of sediment)
      ISQG-High
      (mg/kg dry weight of sediment)
      Antimony 2 25
      Arsenic 20 70
      Cadmium 1.5 10
      Chromium 80 370
      Copper 65 270
      Lead 50 220
      Mercury 0.15 1
      Nickel 21 52
      Silver 1 3.7
      Zinc 200 410
      Total DDT 0.0016 0.046
      Dieldrin 0.00002 0.008
      Total PAHs 4 4.5

       

      Limitations

      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. This data is currently being collected around the country, for example by NIWA (external link).

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