Sediment Discharge Test
The Water Framework Directive (WFD) does not have compulsory standards to remediate contaminated sediments. There might be national legislation of course, but the only European obligation is that contaminated sediment may not hamper the WFD water quality standards and biological quality elements. However, it is not so easy to quantify the relationship between sediment and water quality or ecology in surface waters.
Several tools are developed in the Netherlands to solve this problem. We developed a Guidance document for sediment assessment (including an Excel tool SEDIAS) to determine whether existing sediments threat the WFD objectives. If so, remediation of the sediments will be part of the WFD measures.
However, sediments are often dredged of reused for other reasons, such as navigational depth. Licence agencies have to decide whether these activities do not hinder the WFD-objectives. The Sediment Discharge Test (zip, 1.7 MB) is a Guidance document accompanying a simple Excel-application for water managers and authorities to decide whether they can give a permit. It basically fits in the approach of discharge licenses; that is why it is called Sediment Discharge Test. Typical examples that can be evaluated by the SDT are: creating a (historically) contaminated top layer of sediment after maintenance dredging, erosion of historically contaminated river banks as a result of hydraulic changes in the river bed, deepening of a navigation channel, or expand the river area by adding a formerly industrial site.
The first step in the SDT is to check whether a trigger value in the sediment (defined as a total concentration) is exceeded and whether the sediment quality after the intervention will be worse than the actual sediment quality. If both criteria are answered with ‘yes’, the SDT calculates the contribution of the sediment to the water quality based on the flow rate, the thickness of the sediment layer that interacts with the surface water, and a number of assumptions, like chemical equilibrium in surface water.
The default calculation with the tool is a worst case. If the contribution of the sediment causes water quality problems, the user of the tool may adjust a number of parameters to a site specific value. This value should be founded by the user. If the criteria are still exceeded, it is possible to take measures to reduce the effects of the intervention.