Reactive ditches: A simple approach to implement denitrifying wood chip bioreactors to reduce nitrate exports into aquatic ecosystems?

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 chip-based 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.