Nitrate removal in a restored riparian groundwater system: functioning and importance of individual riparian zones

摘要

For the design and the assessment of river restoration projects, it isimportant to know to what extent the elimination of reactive nitrogen (N) canbe improved in the riparian groundwater. We investigated the effectiveness ofdifferent riparian zones, characterized by a riparian vegetation succession,for nitrate (NO₃−) removal from infiltrating river water in arestored and a still channelized section of the river Thur, Switzerland.Functional genes of denitrification (nirS and nosZ) wererelatively abundant in groundwater from willow bush and mixed forestdominated zones, where oxygen concentrations remained low compared to themain channel and other riparian zones. After flood events, a substantialdecline in NO₃− concentration (> 50%) was observed in thewillow bush zone but not in the other riparian zones closer to the river. Inaddition, the characteristic enrichment of 15N and 18O in theresidual NO₃− pool (by up to 22‰ for δ15N and upto 12‰ for δ18O) provides qualitative evidence that thewillow bush and forest zones were sites of active denitrification and, to alesser extent, NO₃− removal by plant uptake. Particularly in thewillow bush zone during a period of water table elevation after a floodingevent, substantial input of organic carbon into the groundwater occurred,thereby fostering post-flood denitrification activity that reducedNO₃− concentration with a rate of ~21 μmol N l?1 d?1. Nitrogen removal in the forestzone was not sensitive to flood pulses, and overall NO₃− removalrates were lower (~6 μmol l?1 d−1). Hence,discharge-modulated vegetation–soil–groundwater coupling was found to be akey driver for riparian NO₃− removal. We estimated that, despitehigher rates in the fairly constrained willow bush hot spot, totalNO₃− removal from the groundwater is lower than in the extendedforest area. Overall, the aquifer in the restored section was more effectiveand removed ~20% more NO₃− than the channelized section.