Valérie Cappuyns and Rudy Swennen at the Catholic University of Leuven, Belgium, have assessed the risks associated with depositing dredged soil on floodplains. 

Riverbed sediments are known sinks for heavy metal contamination in river water, said Cappuyns. This sediment is deposited on the river's floodplain when the river bursts its banks. Dredging of riverbeds is used to keep rivers navigable, and the dredged sediment can end up being spread on nearby farmland within the floodplain.  

Cappuyn and Swennen argue that this deposited sediment acts as a secondary source of river contamination. In the event of a flood, contaminants in the deposited sediment can be re-released into the river and surrounding environment.

For example, after Hurricane Katrina in August 2005, 80 per cent of New Orleans was flooded for almost a month.  When the water finally drained away, it left behind heavy metal contamination, originating from dredged soil deposited on the cities floodplains.

Most environmental legislation requires that only the total amount of metals in the sediment must be checked prior to redistribution.  But Cappuyns says that considering other parameters, such as soil type and land-use, greatly improves the estimation of the risk of metal contamination.  The proportion of metal released back into the river water in the event of a flood is more important that the total amount of metal in the soil, she said.     

Cappuyns studied soil samples from Belgium floodplains, measuring parameters such as soil pH and organic matter content, to determine the solubility of the metals in the soil.  Using this information she categorized dredged sediment according to actual and potential risk to the environment.

Andrew Hursthouse, an environmental chemist from the University of Paisley, UK, said the method offers a practical approach to easily defining hazards within floodplains, and hopes it will have an impact on key environmental policies.  

Nina Athey-Pollard