Nitrate attenuation in a restored river floodplain system: River Cole (Oxfordshire - UK)

Abstract

Restoring river-floodplain connectivity has been proposed as an alternative management measure for natural flood defence through the temporary storage of floodwaters and the attenuation of flood peaks downstream. Whilst several studies have documented the associated ecological and landscape amenity values of such hydrological measures, the water quality benefits to the adjacent water bodies have been inadequately studied. To date, the focus of scientific research and natural resource management has been on the role of riparian buffer zones for the alleviation of agricultural diffuse nitrate pollution. This research investigated the potential for nitrate attenuation in a restored riverfloodplain system, the River Cole (Coleshill, England), with the aim of informing future restoration schemes of the best management practices for enhanced nitrate removal. Following restoration, the increased river-floodplain connectivity has encouraged overbank flooding of the different land use zones throughout the year. The flood pulse supplies the floodplain soil with river water nitrate and creates the necessary anaerobic conditions for the effective removal of nitrate via heterotrophic denitrification, while organic carbon is supplied mainly through the traditional land use management practices of grazing and mowing. The conservation of nitrogen via DNRA is of minimal importance in this lowland agricultural catchment setting, mainly due to the nonlimiting nitrate supply from the surrounding agricultural land but also the intermittent saturation regime that restricts the low redox conditions to the low elevation riparian areas. This presents the added benefit of restricting methane emission to the more frequently waterlogged riparian soils, while denitrification is effective across the whole floodplain area. Additionally, more than 90% of nitrate removal occurs in the top 30 cm of the soil during the flood, while the role of subsurface denitrification is restricted by the limited availability of organic carbon and nitrate. Based on these findings, this study demonstrates that, for similar catchments, the nitrate removal capacity of a floodplain can be assessed by the denitrification capacity of the surface soil. The assessment of the denitrification capacity can be undertaken inexpensively using a simple empirical model that requires a single microbial denitrification potential measurement, and a seasonal or monthly record of soil nitrate content, soil moisture, and temperature. Assessments can be undertaken as part of the design process to optimise nitrate removal or post restoration to appraise the functioning of the scheme.