The project Aquisafe assesses the potential of selected near-natural mitigation systems, such as constructed wetlands or infiltration zones, to reduce diffuse pollution from agricultural sources and consequently protect surface water resources. A particular aim is the attenuation of nutrients and pesticides. Based on the review of available information and preliminary tests within Aquisafe 1 (2007-2009), the second project phase Aquisafe 2 (2009-2012) is structured along the following main components: (i) Development and evaluation of GIS-based methods for the identification of diffuse pollution hotspots, as well as model-based tools for the simulation of nutrient reduction from mitigation zones (ii) Assessment of nutrient retention capacity of different types of mitigation zones in international case studies in the Ic watershed in France and the Upper White River watershed in the USA under natural conditions, such as variable flow. (iii) Identification of efficient mitigation zone designs for the retention of relevant pesticides in laboratory and technical scale experiments at UBA in Berlin. The present report provides a review of the properties and existing mitigation experience of the two herbicides Atrazine and Bentazone, which will be examined exemplarily in (iii). Whereas Atrazine is clearly the pesticide of greatest concern in the USA, Bentazone is mainly an issue in Europe with an increasing tendency. The sorption of Atrazine and Bentazone on soils is moderate. Moderate sorption in combination with medium to high persistency makes these compounds relatively mobile; therefore they can usually be observed in surface waters in general and in ground waters near places of their application. First experiences show that mitigation systems can be effective measures to decrease their concentrations by supporting biotic and abiotic dissipation processes, mainly at high residence times. Adding organic matter can improve adsorption of Atrazine and Bentazone, an important dissipation process in these systems. Degradation rates for Atrazine and for Bentazone can be increased by implementing highly microbiologically active conditions which can usually be accomplished in the presence of external carbon sources. While mineralization of both herbicides is favoured in aerobic -environments significant degradation of Atrazine was also observed under anaerobic conditions. A great number of open questions remain on how to design a mitigation system which is adequate to reduce herbicides in drainage water. For instance, there is no specific information on the degradation of diluted and adsorbed forms of the herbicides, very little information about necessary residence times, adsorption constants, half lives and leaching behaviour in specific substrates or comparable designs. Moreover, the influence of nitrogen, which is present in drainage water at high concentrations, on degradation of Atrazine and Bentazone remains uncertain. Finally, the behaviour of Atrazine and Bentazone (contained in agricultural drainage water) in mitigation systems in general and in bioretention swales in particular is poorly studied. Realistically, mitigation systems would only be implemented if they also allow significant reduction of nitrates. Given the existing knowledge, systems with both aerobic and anoxic zones are likely to bring most successful results regarding both herbicides and nitrates; though they may be difficult to implement. Both for nitrates and pesticides, the presence of external organic carbon sources (with a combination of fast accessible and sustainable substrate partitions) seems to be a good basis for dissipation processes and effective reduction.

The AQUISAFE research project aims at mitigation of diffuse pollution from agricultural sources to protect surface water resources. The project has several objectives including optimizing system-analytical tools for the planning and implementation of mitigation zones, demonstrating the effectiveness of mitigation zones in international case studies in the US Midwest and Brittany, France and developing recommendations for the implementation of near-natural mitigation zones, which are efficient in attenuating nutrients and selected pesticides. A series of different types of mitigation systems, including constructed wetlands and reactive trenches are being constructed in 2010 at identified agricultural sites in France and the USA. A preliminary monitoring of a drainage-fed surface flow wetland showed good nitrate retention when water infiltrated or had significant residence times, but no discernable effect during major storm events. As a result, future designs aim at higher reaction times by adapting size of end-of-drainage solutions to expected flows and by developing new mitigation systems for existing drainage ditches. Moreover, reaction rates are improved by forming favourable conditions for underground passage and by addition of organic carbon sources, such as straw or wood chips. Whereas nutrients are the focus for the field sites in France, both nutrients and atrazine are the focus in the US. Reactive trenches are being tested for pesticide retention at laboratory and technical scale at the experimental field of the German Federal Environment Agency. In the latter experiments, Bentazon and Atrazine are used as test substances, given their relevance for European and US surface waters, respectivelyseveral objectives including optimizing system-analytical tools for the planning and implementation of mitigation zones, demonstrating the effectiveness of mitigation zones in international case studies in the US Midwest and Brittany, France, and developing recommendations for the implementation of near-natural mitigation zones, which are efficient in attenuating nutrients and selected pesticides. A series of different types of mitigation systems, including constructed wetlands and reactive trenches are being constructed in 2010 at identified agricultural sites in France and the USA. A preliminary monitoring of a drainage-fed surface flow wetland showed good nitrate retention when water infiltrated or had significant residence times, but no discernable effect during major storm events. As a result, future designs aim at higher reaction times by adapting size of end-of-drainage solutions to expected flows and by developing new mitigation systems for existing drainage ditches. Moreover, reaction rates are improved by forming favourable conditions for underground passage and by addition of organic carbon sources, such as straw or wood chips. Whereas nutrients are the focus for the field sites in France, both nutrients and atrazine are the focus in the US. Reactive trenches are being tested for pesticide retention at laboratory and technical scale at the experimental field of the German Federal Environment Agency. In the latter experiments, Bentazon and Atrazine are used as test substances, given their relevance for European and US surface waters, respectively.

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 project Aquisafe assesses the potential of selected near-natural mitigation systems, such as constructed wetlands or infiltration zones, to reduce diffuse pollution from agricultural sources and consequently protect surface water resources. A particular aim is the attenuation of nutrients and pesticides. Based on the review of available information and preliminary tests within Aquisafe 1 (2007-2009), the second project phase Aquisafe 2 (2009-2012) is structured along the following main components: (i) Development and evaluation of GIS-based methods for the identification of diffuse pollution hotspots, as well as model-based tools for the simulation of nutrient reduction from mitigation zones. (ii) Assessment of nutrient retention capacity of different types of mitigation zones in international case studies in the Ic watershed in France and the Upper White River watershed in the USA under natural conditions, such as variable flow. (iii) Identification of efficient mitigation zone designs for the retention of relevant pesticides in laboratory and technical scale experiments at UBA in Berlin. The following report focuses on (ii), providing an overview of existing mitigation systems that may reduce transport of agricultural pollutants to surface waters, with a particular focus on nitrate. The report is based on an extensive review of scientific literature as well as practical guidelines. The review emphasizes on systems, which can treat pollutant loads from agricultural fields with surface or tile drainage. Such mitigation systems could play an important role in intensely used agricultural areas, where existing efforts in farm or crop management are not sufficient to reach water quality goals in receiving rivers. This is typically the case for agricultural catchments with high ratio of artificial drainage, which allows an almost complete transfer of water and contaminants, particularly during high flow events. For each identified mitigation system, its general approach, performance against nitrates and other contaminants, boundary conditions as well as expected cost are given. The systems are structured according to their place on the pathway between field and surface water into 1. systems which attempt to reduce contaminant loads in the drainage pipes and ditches (section 2), 2. systems, which can be placed between drainage system and surface water (section 3), 3. systems, which can be placed in the receiving surface water (section 4). The review shows that there are a number of feasible options with the potential to mitigate NO3 - pollution from drained agricultural land. The most promising approaches with high removal potential were found to be: - controlled drainage (section 2.2), - bioreactors at the tile level (section 2.3.2), - reactive swales (section 2.4.2), - constructed wetlands (section 3.2) and - river-diversion wetlands (section 4.2.2). Most practical experience exists for constructed wetlands with surface flow (globally) and for controlled drainage (mainly in the USA), whereas the other systems are currently at an experimental state. v For a model agricultural area, the above systems resulted in expected nitrate reduction between 14 and 82 % and cost efficiencies between 23 and 246 € kg-N-1. In terms of absolute nitrate removal, (i) wood chip walls parallel to tile drains and (ii) constructed wetlands with straw as carbon source were found to be most effective. However, for both systems there are relatively few experiences so further testing will be necessary. Regarding cost efficiency, (iii) constructed surface flow wetland with low construction cost (dam) and (iv) controlled drainage are most efficient. Whereas constructed surface flow wetlands can be implemented independently, drainage control structures need to be managed by farmers, which requires their active cooperation and proper training.

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 project Aquisafe assesses the potential of selected near-natural mitigation systems, such as constructed wetlands or infiltration,zones, to reduce diffuse pollution from agricultural sources and consequently protect surface water resources. A particular aim is the attenuation of nutrients and pesticides. Based on the review of available information and preliminary tests within Aquisafe 1 (2007-2009), the second project phase Aquisafe 2 (2009-2012) is structured along the following main components: (i) Development and evaluation of GIS-based approaches for the identification of diffuse pollution hotspots, as well as model-based tools for the simulation of nutrient reduction from mitigation zones (ii) Assessment of nutrient retention capacity of different types of mitigation zones in international case studies in the Ic watershed in France and the Upper White River watershed in the USA under natural conditions, such as variable flow. (iii) Identification of efficient mitigation zone designs for the retention of relevant pesticides in laboratory and technical scale experiments at UBA in Berlin.The present study focused on (i) and aimed at testing GIS approaches for the localization of critical source areas (CSAs) of diffuse NO3- pollution in rural catchments with low data availability as a basis for the planning of mitigation measures. We tested a universal GIS-based approach, which is a combination of published methods. The five parameters land use, soil, slope, riparian buffer strips and distance to surface waters were identified as most relevant for diffuse agricultural NO3 - pollution. Each parameter was classified into three risk classes, based on a literature review. The risk classes of the five parameters were then averaged in a GIS overlay in order to find areas with highest risk. The Ic catchment in Brittany, France, served as a study site to test the applicability of the chosen approach. The result of the overlay was compared (a) with measured NO3 - loads in seven subcatchments of the Ic catchment and (b) with the results of a previous analysis by the numerical model Soil and Water Assessment Tool (SWAT). Regarding (a) it was found that higher mean risk classes in a subcatchment correspond with higher measured NO3- loads. However, due to the small number of data points a reliable statistical analysis was not possible. Regarding (b), the plotting of the loads predicted by SWAT against the mean risk class for the 32 SWAT subcatchments show a similar, but poorer relationship. The GIS approach was further analyzed regarding its sensitivity to each of the parameters. The analysis showed that the method is not very sensitive to most of the parameters, i.e. risk class distribution (or the choice of CSA) does not change greatly if one parameter is omitted. Nevertheless, if data quality for some parameters is known to be low, sensitivity of the result to the parameter should be considered in addition.In summary, it can be stated that the applied GIS overlay is a promising, easy to handle approach. First experiences on the Ic catchment indicate that GIS-based approaches can be robust, even for lower data availability. As a result, further work is suggested towards developing a universally applicable GIS method for nitrate CSA identification. Main points to be assessed are the number of classes, the necessary weighting of parameters and the best inclusion of different nitrogen pathways between field and surface water.