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Pollutants in sediments

Why we monitor pollutants in sediments

Estuaries are some of the most at-risk coastal ecosystems in the Waikato region. Many of the pressures affecting these coastal ecosystems are focused on inter-tidal sand and mud flats, which occupy significant areas of the region's estuaries. For example, 30 to 50 per cent of the area of eastern Coromandel estuaries consist of sand and mudflats.

This indicator monitors levels of the following trace elements and organic compounds in some of our region's estuarine sediments:

Photo of feeding snapper

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

All of these, apart from organochlorines and PAHs, are trace elements, which have both natural and man-made sources. Most of these elements are found in small amounts in the earth's crust. When volcanic rocks containing these elements are weathered and erode in the catchment, trace elements enter the marine sediments and naturally occur there. For example, in some areas of the Coromandel Peninsula, these elements are naturally enriched due to presence of sulphide mineralization.
At low levels, various trace elements are not harmful and are even required by some living organisms. For example, many enzymes in the human body need and contain zinc and chromium to allow the body to process sugar, protein and fat. Trace element levels can be increased as a result of input from mining, urban and industrial processes.

The organic compounds measured (organochlorines and PAHs) are man-made persistent organic pollutants, and enter the sediments through runoff from catchments in which these compounds have been used. Organochlorines are synthetic organic chemicals with chlorine in their structure. They have been used in pesticides and insecticides and DDT was mixed with fertiliser. Consequently, most pastoral soil in New Zealand contains these compounds.

PAHs are released during combustion processes. They are sourced both naturally (from volcanoes) and from human activities, such as vehicle emissions (main source), home heating (mainly wood burning but also coal), and forest fires.
Adverse (or toxic) effects can arise when levels of these trace elements and organic compounds are too high. Interim sediment quality guidelines (ISQGs) have been developed to show whether levels of these trace elements and organic compounds are elevated. As awareness of the adverse affects of using these trace elements and organic compounds has increased, their use has been reduced. Both organochlorines and PAHs are now banned in New Zealand and levels of these compounds (where detected) should be decreasing with time.

Pressures on estuaries increase as population numbers grow and developments increase both in catchments and coastal areas. Inter-tidal flats and their associated communities are highly susceptible to changes in land use and other activities in catchments. Mining, urban and industrial activities can cause input of trace elements and organic compounds into estuaries which may be stored in the sediments:

  • Mining volcanic rocks and following runoff from mine tailings release antimony, copper, mercury, silver, zinc and arsenic, a proportion of which can find their way to marine sediments.
  • Alloys used in materials such as batteries, solder, sheet and pipe metal, bearings and castings can contain antimony, cadmium, chromium, copper, nickel, silver and arsenic. Use of such products causes release of small quantities of each element.>
  • Combustion of lead-rich petrol released lead, and coal-burning releases antimony, mercury and nickel. (Leaded petrol was withdrawn from the New Zealand market in 1996.) Some of this is carried by stormwater runoff into estuary waters where they can be deposited into the sediments.
  • Paints based on copper (copper-based antifouling paint on boat hulls) leach into estuaries.
  • Cadmium, chromium, copper, lead, mercury, nickel, silver, and zinc can also be input with sewage effluent.
  • Agricultural run-off - agrichemicals such as fertilisers and pesticides are sources of some elements including zinc, cadmium and copper.

The sediments then become a potential source of trace elements and organic compounds to animals and plants both in the sediment and the overlying water.
The natural release of trace elements and organic compounds from sediments depends upon the relative levels of the trace elements and organic compounds in the sediment and the water. Trace elements and organic compounds in the sediments can each cause specific problems for animals and plants in estuaries, which may cause flow-on effects in humans.

What's happening?

A number of catchment activities can supply trace elements and organic compounds to estuaries. Waikato Regional Council has monitored the levels of trace elements and organic compounds in sediments of the inter-tidal sand and mudflats in six estuaries: the Firth of Thames, Raglan (Whaingaroa) Harbour, Aotea Harbour, Kawhia Harbour, Port Waikato and Tairua Harbour.

In 2003, Waikato Regional Council commenced measuring trace elements and organic pollutants in the sediments of the southern Firth of Thames and Raglan (Whaingaroa) Harbour. During 2008, sampling in these estuaries was repeated and further sampling was carried out in Aotea Harbour, Kawhia Harbour and Port Waikato. In 2010 we sampled the sediments of Tairua Harbour. Sediment samples are analysed for a range of trace elements and organic compounds. Trace elements that we tested for include antimony, arsenic, cadmium, chromium, copper, lead, mercury, nickel, silver, and zinc. Organic compounds include organochlorine pesticides (including DDT and dieldrin) and polycylic aromatic hydrocarbons (PAHs).

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.

>> Find out more about these data and trends

More information

When this indicator is updated

This indicator is updated when results become available.

Contact at Waikato Regional Council

Coastal Ecologist - Science and Strategy Directorate