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The quality of surface water is best assessed using the status of both the water and underlying sediment. A recent study concluded that water bodies risk being misclassified if sediment assessment is not included, which can lead to unnecessary recovery costs.

Under the Water Framework Directive1 (WFD), Member States are required to achieve at least ‘good water status’ for surface water (inland, estuarine and coastal water bodies) in Europe by 2015.

Surface water quality is assessed on both its ecological status and chemical status. Ecological status includes the physical and chemical conditions that affect the water’s biological quality, such as nutrients and oxygen levels.

The chemical status is also determined according to levels (or environmental quality standards (EQS)) of important pollutants, including metals, found in the water, as listed under the EC’s Directive2 on priority dangerous substances.

In this study, Spanish researchers investigated the quality of Basque coastal and estuarine waters in northern Spain. The study focused on the long-term trend (from 1995-2007) 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 programs.

In addition, the chemical status of these water bodies was assessed using two approaches: (1) following the principle of ‘one out, all out’ under the WFD, whereby any metal in waters over the EQS will result in the whole station failing to achieve the chemical status (and for concentrations below the EQS, the chemical status is met), and (2) Combining the chemical quality of both the surface waters and the underlying sediment, using a methodology proposed by these researchers.

The river catchments, estuaries and coastal waters of the study area have been polluted by urban and industrial discharges, particularly from iron ore mining in the region. Additional pollution comes from the construction of ports, dredging, sediment disposal, and land reclamation. Emission control measures and water treatment programs have been implemented to help tackle these pressures.

Using the first approach, few of the water bodies achieved good status, and the percentage of systems meeting this status falls over time. 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, which has less impact on wildlife, and that which is highly polluted. In addition, this approach reflects the drop in pollution of river catchments in recent years, which has improved water quality in many bodies.

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

Source: Tueros, I., Borja, A., Larreta, J. et al. (2009). Integrating long-term water and sediment pollution data, in assessing chemical status within the European Water Framework Directive. Marine Pollution Bulletin. 58:1389-1400.

This article was originally posted by Environmental Expert on November 27, 2009.

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