2 Floodplains under pressure - 2.5 Pollution pressures

Although the Water Framework Directive and the Nitrates Directive in combination require reduction of nutrients and hazardous substances, those substances are still used. Floodplains commonly act as long term storage for water and sediments including those less desirable and hazardous substances. Nutrients and hazardous substances reaches floodplains from either the landside, from the river during floods, or from the atmosphere. Agricultural plant production uses nutrients and pesticides to promote plant growth. Often more nutrients are applied than taken up by plants, and unused nutrients are moved into streams either via groundwater (nitrates), or as attached to soil particles and moved with surface run-off (phosphorus). Those nutrients may cause eutrophication impacts on the floodplain but may also buffer against eutrophication impacts in the river. Active floodplains where vegetation is more prominent cycle nutrients into plants, and if soils are water logged, enable denitrification to take place. In the absence of an active floodplain, nutrients enters the river with less transformations and may cause eutrophication related impact on ecological status of rivers, lakes, transitional, and coastal waters.

During floods, sediments which may carry both nutrient and hazardous substance pollution are deposited on the floodplains, removing the polluting substances from the river, but in return polluting the floodplain. Especially in areas where mining and heavy industry were or still are important, heavy metal pollution of floodplains can be prominent, and may continue for decades after mining has been stopped (Ciszewski and Grygar, 2016). Examples of this can be found from most countries in Europe. Contaminated sediments are also of concern when performing river restoration as removing structural flood protection, weirs or dams. It has been found that contaminated sediments are often stored behind these structures, and may be released if structures are changed (Hahn et al., 2018). Atmospheric deposition of nutrients and mercury is a ubiquitous pressure, hence also occurs in floodplains (EEA, 2018b). Climate change induced increased temperatures is expected to alter the mobilization of chemicals in floodplains, but unfortunately, not much is known about environmental effects.