Diffuse nitrate (NO3-) contamination from intense agriculture adversely impacts freshwater ecosystems, and can also pose a risk to human health if receiving surface waters are used for drinking water production. Implementation of near-natural mitigation zones such as reactive swales or wetlands have been proven to be promising measures to reduce nitrate loads in agricultural drainage waters. However, the behaviour of these systems at low temperatures and its dependence on system design is not well known until now. In this part of the Aquisafe project, the behaviour of a full scale (length: 45 m) infiltration ditch and two parallel wetlands (surface flow wetland and infiltration wetland) treating drainage water of two agricultural watersheds in Brittany (France) with high nitrate concentrations in the receiving river, were constructed and monitored for 3 flow seasons in 2011, 2012 and 2013 to evaluate field scale performance of these systems. As the flow in both sites is usually restricted to winter and spring months (December – May), systems usually operate at low water temperatures of 5°C - 10°C. Tracer tests revealed shorter than designed retention times (average values for whole flow season 2013: 1.1 h for infiltration ditch, 4.3 h for infiltration wetland and 8.4 h for surface wetland) due to high inflows and preferential flow. This likely is the main reason for observed low average retention of nitrate loads of 1.5-3% during the whole flow season. However, increase of relative nitrate retention to up to 80% during low flow conditions at the end of flow season in May with higher HRT and increasing temperatures show that investigated systems generally work. Results show a stronger correlation between residence time and nitrate reduction for all three systems compared to correlation with temperature. Retention times necessary in existing systems to achieve nitrate retention >30% were 1 day for infiltration ditch and 3 days for wetlands. Performance was compared to results of two technical scale reactive swales (length: 8 m) operated for 1.5 years at two different residence times (0.4 and 2.5 days), situated at a test site of the German Federal Environmental Agency (UBA) in Berlin (Germany). Similar nitrate reduction was observed for comparable temperature and HRT values (during low flow conditions at end of flow season 2013), showing that up-scaling is a suitable approach to transfer knowledge gathered from technical scale experiments to field conditions. For the design of new mitigation systems, expected inflow volumes have to be investigated carefully in advance to ensure a sufficient residence time for effective nitrate reduction at low temperatures.

Der Einsatz von Filtern zur Reduzierung von Stickstoff- und Phosphoreinträgen aus der Landwirtschaft in die Oberflächengewässer wurde in Deutschland bisher kaum untersucht. In einem Workshop wurde der Stand der Untersuchungen von Projekten in Polen, Dänemark, Deutschland und Frankreich vorgestellt. Um das Potential dieser Maßnahmen auszuschöpfen, sind die Entwicklung von Entscheidungsunterstützungssystemen für geeignete Einsatzorte und weitere Demonstrationsprojekte unter Feldbedingungen notwendig.

In agricultural watersheds affected by diffuse pollution, limitation of fertilizer and pesticide application may not be sufficient to achieve good river water quality. After waterworks had to be closed in Brittany due to elevated nitrate concentrations in the river Ic (> 50 mg-NO3 L-1), the project Aquisafe has been initiated. The objective of Aquisafe is to reduce pollutant loads (nitrate and pesticides) from agricultural fields by implementation of near-natural mitigation zones at diffuse pollution hotspots at the head of watersheds. Simple and small solutions have to be designed in order to more efficiently reduce nitrate and pesticide concentrations in receiving rivers. In addition, a planning tool has to be developed to determine optimal locations to construct these systems. Finally, a tool to assess the effectiveness of these reactive zones on watershed water quality will be implemented. In order to reach the first objective, design features are tested on three scales: 1) laboratory scale, 2) technical scale and 3) field scale. 1) In the laboratory, column experiments were conducted with different organic substrates at short hydraulic residence times (HRT). The efficiency for parallel reduction of nitrate and two common herbicides in Europe, Bentazon and Isoproturon, was explored (Krause Camilo, 2012). 2) In technical scale, two parallel swales were filled with the most suitable material determined in (1) for a one year test. The influence of HRT and temperature was investigated. For nitrate, high reduction could be achieved at short HRT; results for herbicides still have to be confirmed. 3) One infiltration ditch and two simple wetlands were constructed in Brittany (France), taking into account experiences from other scales. These systems are now monitored to investigate the effects of upscaling. Site locations were chosen based on a validated and repeatable GIS-based overlay method that prioritises zones of potential contribution to nitrate pollution (Orlikowski et al, 2011). Additionally, a new wetland module is being developed for the Soil and Water Assessment Tool (SWAT). It allows to predict impacts of wetland constructions on nitrate concentrations in receiving rivers; the module is now implemented but still has to be calibrated with in situ monitoring results. The presentation will focus on results of the up-scaling approach, and will show how the tools of Aquisafe can be used for supporting the development of strategies at catchment scale.

The present study aimed at developing a universal method for the localization of critical source areas (CSAs) of diffuse nitrate (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- parameters were averaged in a GIS-overlay to localize areas with low, medium and high risk of NO3- pollution. The five parameters were 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, showed that identified CSAs were 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.

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.

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.