Predicting contamination of farmland after flooding
Floods and the resulting damage to agricultural and urban environments are likely to increase as a result of climate change. Major floods in Europe, such as those affecting the Elbe River in Germany in August 2002, can result in contamination by heavy metals of flooded farmland. Taking zinc as an example, a new modelling approach showed that there was more than a one in three chance that contamination could exceed 500 mg zinc per kilo of sediment following future extreme events.
The approach - called a quasi-2D flood model, as it predicts movements in time as well as distance - was designed to predict sediment flows at low, medium, high and extreme river flows. It also predicts the rate of transport of contaminating substances in polder systems. Polders are drained areas of farmland able to contain or divert floodwater peaks in the event of excessive river flow. This means that they are likely to retain substances deposited during flooding. The potential for contamination during extreme flooding needs to be quantified as part of the environment flood risk assessment required under the new EU Floods Directive1. Data on zinc was used as an example of a toxic metal that can have negative effects on soil and crops.
The approach improves on traditional one-dimensional hydrodynamic and water quality models by capturing the spatial differentiation of flows and transport of substances taking place within a much-modified river system such as a polder. It was tested using data from the middle reaches of the Elbe River where there is a high risk of heavy metal contamination and where a new polder system is in the planning stages. The model has several achievements:
- It enables sampling strategies to be developed in the event of future flooding
- It determines the likely severity of contamination on polder surfaces from sediments deposited during floods
- It can be used to improve water quality elements of polder system design
The level of 500 mg zinc per kg of sediment constitutes a trigger level for soil-plant pathways under the German Soil Protection and Contaminated Sites Ordinances. Concentrations exceeding the trigger level can lead to impairment of cultivated plant growth and reduced crop yields. The trigger level means that precautions may have to be taken to avoid or reduce further pollutant inputs. The authors believe that the environmental risk due to zinc deposition remains small, but risks could increase if there continues to be more intense rainfall and further extreme flood events in the region. They recommend modifications to the proposed design of the polders in this region so that potentially contaminated sediment would be contained in a smaller area.
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