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.

A mixed surface and sub-surface flow riparian zone in Brittany (France), which is mainly fed by water from drainage ditches, was monitored for nitrate retention over three years from 2005 to 2007. Results show high time-averaged nitrate retention of >90 % for subsurface and ~70 % for surface passage. However, no retention could be detected during major rain events, which reduced the overall (flow-averaged) retention to ~40 %. Based on the findings, higher nitrate retention can be reached by increasing (i) the water residence time in buffer systems, (ii) the fraction of subsurface passage or (iii) denitrification rates in the system. (i) is only feasible if (active) buffer volume is enlarged, which may be difficult in practice. In the case of Brittany an enlargement can also be reached by extending buffer systems into existing drainage ditches. (ii) is of particular importance in areas with low soil permeability. In such areas, addition of gravel or sand beds can be considered. Regarding (iii), denitrification turns maximal under anaerobic conditions if sufficient carbon sources are available. In straw- and bark-filled column experiments we found high nitrate retention rates of >99 % and ~40 %, respectively, during a comparably low residence time of ~5 hours. As a result, the addition of external carbon sources to buffer systems is suggested. Currently, several pilot sites are constructed in the Ic watershed in Brittany attempting to take into account points (i) to (iii). For the following four buffer types, monitoring will start in February 2010: (a) two short drainage ditches, filled with carbon sources, (b) one drainage ditch and (c) one riparian wetland, each filled with a gravel filter, and optional upstream addition of carbon sources.

The present study aims at developing a universal method for the localization of critical source areas (CSAs) of diffuse NO3- pollution in rural catchments with low data availability. Based on existing methods land use, soil, slope, riparian buffer strips and distance to surface waters were identified as the most relevant indicator parameters for diffuse agricultural NO3-pollution. The five parameters are averaged in a GIS-overlay to localize areas with low, medium and high risk of NO3- pollution. A first application of the GIS approach to the Ic catchment in France, shows that identified CSAs are in good agreement with results from river monitoring and numerical modelling. Additionally, the GIS approach showed low sensitivity to single parameters, which makes it robust to varying data availability. As a result, the tested GIS-approach provides a promising, easy-to-use CSA identification concept, applicable for a wide range of rural catchments.

The Aquisafe project is a cooperation of the Indiana University Purdue University Indianapolis (IUPUI, USA), the German Federal Environment Agency (UBA, Germany) and the Berlin Centre of Competence for Water (KWB, Germany). The aim of the project is the development of a scheme for natural mitigation zones to protect surface waters from diffuse pollution in rural and semi-rural environments. In particular, key contaminants, applicable management and modelling tools and potential substance removal by constructed wetlands or riparian zones are being studied. Within these frameworks, two case studies are carried out in Brittany, the number one agricultural region in France. A hydrological model is currently being applied on the Ic catchment (92 km2) to test its capability of (i) understanding hydrological, basin-scale regimes, (ii) predicting the effect of mitigation measures and (iii) distinguishing diffusion pathways for different types of contaminants. In the second case study, a constructed wetland in Iffendic on the River Meu is monitored as an example of a natural and inexpensive mitigation option. On the way through the wetland nitrate concentrations from drainage inflows to the river decreased more than tenfold. In the ongoing monitoring, knowledge on hydrological flowpaths is improved to be able to quantify the retention potential of constructed wetlands in Brittany for nitrate and other agriculturally-based pollutants, such as pesticides.