Modelling the nitrate retention capacity of a restored riparian wetland

Abstract

Riparian wetlands provide beneficial functions to the wider environment. They intercept surface as well as sub-surface flows and through water storage and retention serve as nutrient buffer zones for the river system. Eutrophication of surface waters is recognised as one of the most important challenges to achieving ‘good ecological status’ in Denmark. Implementing the Water Framework Directive (WFD) entails a significant reduction of nitrate loading in order to preserve and enhance the ecological status of water bodies. The restoration of streams and floodplains forms part of important strategy to reduce nutrient loadings. This study focuses on developing a model capable of predicting the nitrate reduction capacity of reconstructed wetlands using a dynamic ecological modelling tool ECO Lab, coupled with a detailed dynamic physically based numerical flow and transport model MIKE SHE. The restored riparian wetland Brynemade located next to Odense River on Funen Island in Denmark was modelled over seven years and the model outcomes were compared with field measurements. To quantify the impact of the restoration on nitrate reduction, a new model of the wetland under pre-restoration conditions was developed based on the calibrated model. By quantifying the nitrogen load reduction, the effectiveness of restoring riparian wetlands to achieve water quality objectives is assessed. The results show that in the current wetland, denitrification patterns in the overland flow compartment follow river flood patterns, which indicate that most nitrate input originates from the river. The pre-restoration wetland receives fewer floods and has groundwater drainage, which results in a larger unsaturated zone and thus more denitrification compared to the restored situation, as well as less denitrification in the overland flow compartment. Denitrification in the saturated zone occurs mainly at the eastern boundary of the wetland, where the agricultural nitrate inflow occurs, as well as along the river channel, where nitrate seeps through the river bed to the saturated zone. The peat layer is the major contributor to total denitrification in the saturated zone. Most of the nitrate inflow from the adjacent agricultural fields is removed before it wells up to the surface. Restoring a wetland to its natural state by restoring a river flood regime and removing drainage increases the nitrate retention capacity. Most of the extra denitrification capacity comes from the increase in floods and the subsequent denitrification in the overland flow compartment. In the pre-restoration wetland 391 kg N/year is removed, compared to 1287 kg N/year after restoration, which is close to a 230% increase. In addition, the nitrogen removal in the river doubled, from 0.23% to 0.47%. The post-restoration removal rates of 129 kg N/ha/year are within the typical range for a wetland of this type. However, since wetland restoration is such a space consuming measure, to reach WFD goals a mix of nitrate load reduction measures has to be implemented.