Implementation of small organically enriched constructed wetlands to mitigate agricultural nitrate hotspots in Brittany, France

Rural watersheds often face diffuse pollution problems due to agricultural activities. In the Ic watershed in Brittany (France), nitrate concentrations in rivers frequently exceed the EUthreshold of 50 mg-NO3 L-1, despite various actions to reduce the impact from agriculture. As a result, other solutions are considered, such as mitigation systems that can prevent transfer of agricultural pollutants from cropland to the streams. Constructed wetlands have been shown to fit this aim, because they can reach significant N removal for water residence times above ~12 hours, can be implemented decentrally within rural watersheds, while meeting cost and policy requirements. However, constructed wetlands require space, which is particularly scarce and costly in intensively used agricultural watersheds. As a consequence, it was decided to test a more area-effective solution in three pilot systems. On the one hand land-use itself was optimized (i) at site 1 by placing two wetlands with same inflow and dimension on an area of minor agricultural value adjacent to a stream (one surface and one subsurface-flow, both 20 x 10 meters) and (ii) at site 2 by building an elongated infiltration wetland (45 x 2 meters) directly in an existing drainage ditch, thus preventing any use of agricultural surface. In both cases farmers agreed to the placement of the wetlands free of charge. On the other hand it was attempted to raise the areal removal efficiency, with a focus on denitrification, since nitrate is of most concern with inflow concentrations to the sites ranging between 30 and 66 mg-NO3 L-1. This increase in denitrification is attempted (a) by increasing the range of anoxic zones within the wetlands and (b) by adding carbon sources. For (a) one wetland at each site is filled with gravel with bottom outlets to enforce underground passage. Moreover saturation level within the infiltration wetlands and thus hydraulic retention time, can be controlled at drain outlets. For (b) organically rich soil is added to both wetlands at site 1 and carbon sources are mixed with the gravel at site 2. The three wetlands have been constructed in 2010 and are currently monitored for flow and water quality at inlets, as well as at surface and subsurface outlets. The monitoring will allow the calculation of substance mass balances for the entire rain season, expected from December 2010 to May 2011.

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