Insights in nutrient sources and transport from high-frequency monitoring at the outlet pumping station of an agricultural lowland polder catchment

Abstract:

In this study, we present new insights in nutrient sources and transport processes in an agricultural-dominated lowland water system based on high-frequency monitoring technology. Starting in October 2014, we have collected semi-continuous measurements of the TP and NO₃ concentrations, conductivity and water temperature at a large scale pumping station at the outlet of a 576 km² polder catchment. The semi-continuous measurements complement a water quality monitoring program at six locations within the drainage area based on conventional monthly or biweekly grab sampling. The NO₃ and TP concentrations at the pumping station varied between 0.5 and 10 mgN/L and 0.1 and 0.5 mgP/L. The seasonal trends and short scale concentration dynamics clearly indicated that most of the NO₃ loads at the pumping station originated from subsurface drain tubes that were active after intensive rainfall events during the winter months. A transfer function-noise model of hourly NO₃ concentrations reveals that a large part of the dynamics in NO₃ concentrations during the winter months can be predicted using rainfall data. In February however, NO₃ concentrations were higher than predicted due to direct losses after the first manure application. The TP concentration almost doubled during operation of the pumping station. This highlights resuspension of particulate P from channel bed sediments induced by the higher flow velocities during pumping. Rainfall events that caused peaks in NO₃ concentrations did not result in TP concentration peaks. Direct effects of run-off, with an association increase in the TP concentration and decrease of the NO₃concentration, was only observed during rainfall event at the end of a freeze-thaw cycle. The high-frequency monitoring at the outlet of an agricultural-dominated lowland water system in combination with low-frequency monitoring within the area provided insight in nutrient sources and transport processes that are highly relevant for water quality management.