Sediments assessment scheme

Background

An initial qualitative condition assessment of the 970 estuaries on the seaboard of Australia was completed during the National Land and Water Resources Audit based on observable human impact to catchments and waterways. The next stage will be to gather environmental data for modified estuaries. Since 1996, The Environmental Geology Group at the University of Sydney has been undertaking a systematic contaminant study of sediments in estuaries, lagoons and coastal lakes of New South Wales (NSW). Sediments in 30 of the largest and most contaminated 130 estuaries in NSW have been investigated. Strict conformity in field and laboratory techniques have resulted in a large, comparable, GIS-based data set of over 4000 estuarine sediments analysed for a suite of metals. A smaller number of other toxicants (e.g. organochlorine pesticide, polycyclic aromatic hydrocarbon, polybiphenol) and nutrient analyses have also been undertaken in the more important waterbodies. The most contaminated parts of estuaries in the Sydney region have been tested for sediment toxicity using a raft of ecotoxicological tools.

Sediments are being used in this assessment in preference to water and the scheme adopted in this regional investigation is outlined below.

Use of Sediments in Marine Environmental Assessments

Sediments are being used to monitor these coastal environments because sediments faithfully record and time-integrate the environmental status of an aquatic system. Contaminant concentrations are high in sediments, and thus they are easily, cheaply and accurately analysed. Sediments can be an important secondary source of pollutants, and because they integrate contaminants over time, sediments provide useful spatial and temporal information. Sediment quality influences the nature of overlying and interstitial waters through physical, chemical and biological processes. Because sediments play a major role in the transport and storage of contaminants, they are important in identification of contaminant sources and determining dispersion pathways. Sediments also provide an important habitat for animals and are a food source for many species. Sediment quality thus determines, to a large degree, biodiversity and ecological health in aquatic systems, and they are economically attractive in environmental assessment of coastal environments.

Previous Approach to Describe Contaminant Status of Estuaries

Until recently, the contaminant status of an estuary was described by basic statistical parameters, e.g. mean, maximum and minimum concentrations. To make meaningful comparisons between these estuaries, it has been necessary to size-normalise the data to minimise the confounding introduced by variable grain size, as well as to keep field, laboratory and analytical techniques consistent. However, these parameters provide little information that is useful to environmental managers and strategic planners in the governance of these estuaries.

New Approach to Describe Estuarine Health

The new approach being used at Sydney University is to:

• assess sediment quality;
• determine severity of impact, and
• identify contaminant sources and establish dispersion pathways.

To make this information available, it is necessary to provide both total sediment and normalised contaminant data.

Assessment of Sediment Quality

Sediment quality is the ability of sediment to support a healthy benthic population. The Australian and New Zealand Environmental and Conservation Council (ANZECC) recently adopted sediment quality guidelines (ANZECC/ARMCANZ [1]) largely based on a scheme developed by the U. S. National Oceanic and Atmospheric Administration (NOAA) [2,3]. This scheme provides two values, ERL and ERM, which delineate three concentration ranges for a particular chemical. The concentrations below ERL values represent a minimal-effects range, which is intended to estimate conditions where biological effects would be rarely observed. Concentrations equal to, or greater than ERL, but below ERM, represent a range within which biological effects occur occasionally. Concentrations at, or above ERM values represent a probable-effects range within which adverse biological effects frequently occur. The ANZECC sediment quality guidelines (SQGs) use the Interim Sediment Quality Guidelines-Low (ISQG-L) value (equivalent to ERL) as a threshold level that triggers the requirement for additional investigative work.

To estimate the adverse effects of mixtures of chemicals, NOAA uses mean ERM quotients which are determined by normalising the concentration of each substance to its ERM value, summing the quotient for each substance, and dividing the resultant sum by the total number of contaminants for which guidelines are available. In the Sydney University regional assessment of the contaminant status of NSW estuaries, the mean ERM quotient (MERMQ). is determined for three metals (Cu, Pb and Zn) only (MERMQ3m). The advantages of this approach are that analysis of these metals is cheap and accurate, these metals are ubiquitous and concentrations are available for all NSW estuaries. Also, for urban estuaries at least, concentrations of these metals follow trends for other metals and organic contaminants [4].

Severity of Impact

The extent to which chemicals exceed ?natural’, ?pristine’, or ?pre-anthropogenic’ concentrations is a measure of the degree of impact in estuarine sediments. To measure the magnitude of this exceedence requires an accurate determination of pre-anthropogenic, or ?background’ concentrations of contaminants. Enrichment is the extent to which contaminant concentrations exceed geochemical background levels. This can be expressed as an enrichment factor (EF) = Cn/Bg, where Cn is the concentration of the contaminant and Bg is the background concentration. When evaluating EFs, it is important to consider uncertainties in acquiring the data, e. g. spatio-temporal variation and field error. Enrichment factors above a certain threshold should be considered indicative of contamination. This threshold varies between studies, but is usually between 1.5 and 3. Other methods of estimating the degree of contamination, e. g. the Index of Geoaccumulation (Igeo) [5] have been attempted, but EFs have been adopted in the University of Sydney study for simplicity and clarity. Classes of enrichment are: EF<1 indicates no enrichment, <3 is minor; 3-5 is moderate; 5-10 is moderately severe; 10-25 is severe; 25-50 is very severe; and >50 is extremely severe.

Identification of Source and Dispersion Pathways

The other information required by environmental managers and planners is the source of contamination and to what extent toxicants have been dispersed through the system. To acquire this type of information necessitates the use of normalised contaminant data to minimise the confounding introduced by variable grain size. Normalisation can be undertaken by physical separation and analysis of a certain size fraction of the sediment (size normalisation), by elemental normalisation and by a post-extraction normalisation (PEN) procedure [6]. The latter two techniques use total chemical data, which is faster, more economical and employs only one chemical analysis, but if accurate and detailed results are required, size normalisation should be undertaken. All three techniques are used in the Sydney University study for most of the estuaries investigated.

1. ANZECCARMCARZ, 2000. Australian and New Zealand Guidelines for Fresh and Marine Water Quality. Australian and New Zealand Environment and Conservation Council/Agricultural and Resource Management Council of Australia and New Zealand, Canberra.
2. Long. E. R., MacDonald, D. D., Smith, L. and Calder, F. D., 1995. ?Incidence of Adverse Biological Effects Within Ranges of Chemical Concentrations in Marine and Estuarine Sediments’, Environmental Management, 19, 81-97.
3. Long, E. R. and MacDonald, D. D., 1998. ?Recommended uses of empirically derived sediment quality guidelines for marine and estuarine ecosystems’, Human and Ecological Risk Assessment, 4/5, 1019-1039.
4. Birch, G. F. and Taylor, S. E., 2002. The use of sediment quality guidelines in the environmental assessment of Port Jackson estuary, Sydney, Australia. Hydrobiologia, 472, 19-27.
5. Fostner and Muller, 1981
6. Birch, G. F., in press. A test of normalisation methods for marine sediments, including a new post-extraction normalisation (PEN) technique. Hydobiologia.

Author

Gavin Birch, Environmental Geology Group, School of Geosciences, The University of Sydney, NSW, 2006.