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Despite intensive efforts motivated by the European Water Framework Directive, many water bodies still suffer from poor water quality in Germany. Intensively drained agricultural areas are still a critical source for nitrate which is responsible for negative effects on aquatic ecosystems. Basic measures such as the fertiliser ordinance are expected to be not sufficient to completely eliminate nitrate exports via drainage tiles and ditches. Consequently, there is the demand to manage the reduction of nitrate concentrations with new additional end of pipe solutions. For this, the presented study focuses on a simply to implement reactive ditch for denitrification, which is installed into drainage ditches to reduce nitrate concentrations. An existing drainage ditch that is fed by tile drainage water was filled with wood chips to keep installation efforts as simple and cheap as possible. In situ parameters and nutrient concentrations were determined at the inflow and the outflow of the reactive ditch for 2 years.The results reveal a promising potential for wood chipbased filters to reduce nitrate concentrations by 28 % on average over all seasons. Within the filter, favourable conditions for denitrification were predominantly found without flooding of surrounding areas. Investigations revealed decreasing nitrate concentrations especially in cases of low flow rates and high temperatures. These encouraging results demonstrate the successful application of reactive ditches under German lowland conditions in general. Since tile drainages are installed for many agricultural areas in this region, there seems to be high potential for the application of this easily implementable type of bioreactor.

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Large-scale re-establishment of wetland buffer zones (WBZ) along rivers is regarded as an effective measure in order to reduce non-point source nitrogen (N) and phosphorus (P) pollution in agricultural catchments. We estimated efficiency and costs of a hypothetical establishment of WBZs along all watercourses in an agricultural landscape of the lower Narew River catchment (north-eastern Poland, 16,444 km2, amounting to 5% of Poland) by upscaling results obtained in five sub-catchments (1087 km2). Two scenarios were analysed, with either rewetting selected wetland polygons that collect water from larger areas (polygonal WBZs) or reshaping and rewetting banks of rivers (linear WBZs), both considered in all ecologically suitable locations along rivers. Cost calculation included engineering works necessary in order to establish WBZs, costs of land purchase where relevant, and compensation costs of income forgone to farmers (needed only for polygonal WBZs). Polygonal WBZs were estimated in order to remove 11%–30% N and 14%–42% P load from the catchment, whereas linear WBZs were even higher with 33%–82% N and 41%–87% P. Upscaled costs of WBZ establishment for the study area were found to be 8.9 M EUR plus 26.4 M EUR per year (polygonal WBZ scenario) or 170.8 M EUR (linear WBZ scenario). The latter value compares to costs of building about 20 km of an express road. Implementation of buffer zones on a larger scale is thus a question of setting policy priorities rather than financial impossibility.

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