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Sediment Pollution Should Be Included In Water Quality Assessment

October 5, 2010

Under the Water Framework Directive (WFD) (Directive 2000/60/CE), member states are required to achieve Good Water Status for water (continental, estuarine, subterranean and coastal water bodies) in Europe by 2015. Surface water quality is assessed taking into account the ecological and chemical status.

The quality of aquatic systems is more accurately assessed using the status of both the water column and the underlying sediment. A recent study by researchers of AZTI-Tecnalia concluded that water bodies risk being misclassified if, on evaluating their chemical status, only the water column is considered and sediment assessment is not included. Hence, this can lead to unnecessary recovery costs in the zone under study and detracting from any ecological gain that might accrue. The study was published in the journal Marine Pollution Bulletin.

The ecological status of a body of water involves the study of a number of indicators: biological (the composition and abundance of phytoplankton, macroalgae, angiosperms, macrobenthos and fish); hydromorphological (depth, tides, etc.); as well as the chemical and physical conditions that affect the water’s biological quality (oxygenation, salinity, nutrients, priority substances, and so on). The chemical status is also determined according to environmental quality standards (EQS) of important pollutants, these including metals found in the water and organic compounds, as listed under the 2008/105/EC Directive on priority and dangerous substances.

Spillages and hydromorphological pressure

Researchers of the Marine Research Division of AZTI-Tecnalia investigated the quality of coastal and estuarine waters in the Basque Country, between 1995 and 2007. According to their report, the river catchments, estuaries and coastal waters of the area have been polluted by urban and industrial discharges. All this has resulted in an increase in organic material, oxygen consumption, metals and organic compounds. Further hydromorphological pressure comes from the construction of ports, dredging, sediment disposal and land reclamation.

The study focused on the trends of water and sediment contamination by metal pollutants (arsenic, cadmium, copper, chromium, mercury, nickel, lead and zinc) and the response of these areas to water treatment programmes in recent years.

Two methodologies

The chemical status of these water bodies was assessed using two approaches: (i) following the principle of one out, all out as established by the WFD “”any metal in waters over the EQS will result in the whole body of water failing to achieve the chemical status””, and (ii) combining an analysis of the chemical quality of both the surface waters and the underlying sediment, using a weighted methodology proposed by these researchers previously.

Using the first approach, few of the water bodies achieved good chemical status, and the percentage of systems meeting this status falls over time, having reached values close to zero in recent years. Using the second approach, more than 50 per cent of the water bodies achieved Good Status, with the percentage of systems meeting this status remaining steady over time.

The researchers argue that the second approach is more accurate in assessing chemical status as it is better at discriminating between less polluted water and that which is highly polluted, and more in line with the responses of the biological indicators. Moreover, when all the information was incorporated “”at the level of water bodies”” from various biological, hydromorphologic, physico-chemical and chemical indicators, the ecological quality of water bodies in the Basque Country showed progressive recovery, especially since 2000, when most of the water treatment plans were completed.

By considering both water and sediment analysis in determining the status of water quality, resources could better be targeted at those water bodies where levels of pollution have a greater negative effect on the biological elements. However, the researchers say further research is needed on EQS measurements in water and the interpretation of chemical concentrations of contaminants in sediments.

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