Time lags of nitrate, chloride, and tritium in streams assessed by dynamic groundwater flow tracking in a lowland landscape

Vince P Kaandorp,Ype Van Der Velde,Joachim Rozemeijer,Perry G B De Louw,Hans Peter Broers

doi:10.5194/hess-25-3691-2021

Vince P Kaandorp, Ype Van Der Velde + Show 3 more

Open Access

https://doi.org/10.5194/hess-25-3691-2021

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Abstract

Abstract. Surface waters are under pressure from diffuse pollution from agricultural activities, and groundwater is known to be a connection between the agricultural fields and streams. This paper is one of the first to calculate long-term in-stream concentrations of tritium, chloride, and nitrate using dynamic groundwater travel time distributions (TTDs) derived from a distributed, transient, 3D groundwater flow model using forward particle tracking. We tested our approach in the Springendalse Beek catchment, a lowland stream in the east of the Netherlands, for which we collected a long time series of chloride and nitrate concentrations (1969–2018). The Netherlands experienced a sharp decrease in concentrations of solutes leaching to groundwater in the 1980s due to legislations on the application of nitrogen to agricultural fields. Stream measurements of chloride and nitrate showed that the corresponding trend reversal in the groundwater-fed stream occurred after a time lag of 5–10 years. By combining calculated TTDs with the known history of nitrogen and chloride inputs, we found that the variable contribution of different groundwater flow paths to stream water quality reasonably explained the majority of long-term and seasonal variation in the measured stream nitrate concentrations. However, combining only TTDs and inputs underestimated the time lag between the peak in nitrogen input and the following trend reversal of nitrate in the stream. This feature was further investigated through an exploration of the model behaviour under different scenarios. A time lag of several years, and up to decades, can occur due to (1) a thick unsaturated zone adding a certain travel time, (2) persistent organic matter with a slow release of N in the unsaturated zone, (3) a long mean travel time (MTT) compared to the rate of the reduction in nitrogen application, (4) areas with a high application of nitrogen (agricultural fields) being located further away from the stream or drainage network, or (5) a higher presence of nitrate attenuating processes close to the stream or drainage network compared to the rest of the catchment. By making the connection between dynamic groundwater travel time distributions and in-stream concentration measurements, we provide a method for validating the travel time approach and make the step towards application in water quality modelling and management.

Highlights

Diffuse pollution with nutrients is one of the main pressures on Europe’s surface- and groundwaters (EEA, 2018)
This study is among the first to couple decades-long time series of water quality to dynamic travel time distributions based on a high-resolution spatially distributed groundwater flow model
We calculated solute concentrations in the Springendalse Beek between 1969 and 2018. By making this connection between dynamic groundwater travel time distributions and long-term in-stream concentration measurements, we were able to show that the seasonal and long-term fluctuations of in-stream solute concentrations were mainly caused by the dynamic contribution of different groundwater flow paths

Summary

Introduction

Diffuse pollution with nutrients is one of the main pressures on Europe’s surface- and groundwaters (EEA, 2018). Intensive agricultural land use and the accompanying use of manure and fertilizers have significantly increased the amount of nitrate in the hydrological system in the period after 1950 (Aquilina et al, 2012; Broers and van der Grift, 2004; Hansen et al, 2011; Worrall et al, 2015). From 1985, nitrate concentrations leaching towards the groundwater decreased as a result of national and European legislations (Dutch Manure Law, 1986; EU Nitrates Directive, 1991) that contain rules for the reduction of N applications in farming. Kaandorp et al.: Time lags of nitrate, chloride, and tritium in lowland streams face waters has been reported in few countries, including the Netherlands (e.g. van den Brink et al, 2008; Visser et al, 2007; Zhang et al, 2013; Rozemeijer et al, 2014) and Denmark (Hansen et al, 2011)

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