Nitrogen-loads to streams across two different riparian zones (crop
field and wetland): Importance of bypass flow and subsurface removal
processes
Abstract
N-loads from subsurface, drains, and groundwater-fed surface (bypass)
flows via two riparian zones (crop field and wetland) to a second order
stream were investigated by sampling of shallow and deep groundwater on
both sides and monthly measurements of flows from springs, drains, and
stream including water quality (nitrate). A push-pull test in the crop
field gave estimates of first-order denitrification rate (0.23
day–1). Reactive transport modelling evaluated
observations of water chemistry and denitrification processes in the
groundwater below the crop field showing that nitrate was completely
removed by denitrification with pyrite in the aquifer (model rates of
0.6–2.5 mmol NO3 L−1
yr−1). A drain in the crop field routed approximately
10% (bypass) of the regional groundwater inflow to the stream. Buffer
efficiency was high at 90%. The wetland on the other side of the stream
hosts several locations of focused nitrate-rich groundwater-fed spring
discharge, predominantly through a non-maintained drainage system of
drainpipes and ditches with bypass accounting for 59% of the regional
flow input. Nitrate was completely removed in groundwater by
denitrification with dissolved organic matter in shallow groundwater.
The regional inflow and N load to the wetland is amongst the highest
recorded and data shows that the N load to the stream is very high. The
buffer efficiency ranged from 45–83% depending on if all springs
contributed to the stream or only the two with visible outflow. A
conceptual model for nitrate removal efficiency as a function of
Damköhler number and percent bypass flow is proposed.