资源环境科技发展态势分析平台

A restored riparian zone was characterized to understand the effects of flooding on subsurface hydrological flow paths and nitrate removal in groundwater. Field and laboratory investigations were combined with numerical modeling of dynamic flow and reactive nitrate transport. Flooding enhances nitrate removal in groundwater primarily by two mechanisms. First, by creating a stagnant flow zone beneath the flooded area thereby increasing the residence time and leaving more time for nitrate removal. Secondly, nitrate removal is increased by enhancing upward flow into the highly reactive organic-rich top layers. Flooding therefore contributes to nitrate removal in hot spots, where nitrate is transported to the peat and during hot moments, when flow is stagnant. The permeability of the capping peat layer relative to the aquifer is important as it controls both mechanisms. The model shows that the deep-seated nitrate removal is greater than projected from the laboratory nitrate reduction experiments.

Plain Language Summary Managed riparian zones are often used as buffer zones to enhance removal of nitrate. Restoration of riparian zones has included re-meandering of the river with the purpose of creating frequent flooding events so that river-associated nitrate could be removed due to denitrification and/or plant uptake on the floodplain. Little focus, however, has been on the effects of flooding on nitrate removal in groundwater. This study demonstrates that flooding also has a beneficial effect on nitrate removal, because of two physical flow mechanisms. When a riparian zone is flooded, groundwater beneath the flooded area will be more stagnant and any nitrate trapped inside this zone will be more quickly removed. Secondly, groundwater will also flow to the surface of the flooded part and bring nitrate into contact with organic-rich peat layers at the surface, also enhancing nitrate removal.