Elsevier

Science of The Total Environment

Volume 615, 15 February 2018, Pages 773-783
Science of The Total Environment

Tomography of anthropogenic nitrate contribution along a mesoscale river

https://doi.org/10.1016/j.scitotenv.2017.09.297Get rights and content

Highlights

  • Spatially highly resolved water quality was evaluated along a meso-scale river.

  • Systematic changes in NO3 concentrations and isotope signature were observed along the river.

  • Agricultural land use and treated wastewater were the main sources of NO3.

  • Isotope signature and concentration of agricultural NO3 hardly varied over time.

  • Wastewater sources spatiotemporally changed isotope signature in the river.

Abstract

Elevated nitrate concentrations are a thread for water supply and ecological integrity in surface water. Nitrate fluxes obtained by standard monitoring protocols at the catchment outlet strongly integrate spatially and temporally variable processes such as mobilization and turnover. Consequently, inference of dominant nitrate sources is often problematic and challenging in terms of effective river management and prioritization of measures. Here, we combine a spatially highly resolved assessment of nitrate concentration and fluxes along a mesoscale catchment with four years of monitoring data at two representative sites. The catchment is characterized by a strong land use gradient from pristine headwaters to lowland sub-catchments with intense agricultural land use and wastewater sources. We use nitrate concentrations in combination with hydrograph separation and isotopic fingerprinting methods to characterize and quantify nitrate source contribution.

The hydrological analysis revealed a clear dominance of base flow during both campaigns. However, the absolute amounts of discharge differed considerably from one another (outlet: 1.42 m3 s 1 in 2014, 0.43 m3 s 1 in 2015). Nitrate concentrations are generally low in the pristine headwaters (< 3 mg L 1) and increase downstream (15 to 16 mg L 1) due to the contribution of agricultural and wastewater sources. While the agricultural contribution did not vary in terms of nitrate concentration and isotopic signature between the years, the wastewater contribution strongly increased with decreasing discharge. Wastewater-borne nitrate load in the entire catchment ranged between 19% (2014) and 39% (2015). Long-term monitoring of nitrate concentration and isotopic composition in two sub-catchment exhibits a good agreement with findings from spatially monitoring. In both datasets, isotopic composition indicates that denitrification plays only a minor role. The spatially highly resolved monitoring approach helped to pinpoint hot spots of nitrate inputs into the stream while the long-term information allowed to place results into the context of intra-annual variability.

Introduction

Nitrate concentrations in surface and groundwater ecosystems have increased in recent decades due to land use change and accompanying application of fertilizer in agriculture as well as from fossil fuel combustion and subsequent atmospheric deposition (Galloway et al., 2003, Pattinson et al., 1998, Zweimüller et al., 2008). Although since the 1980s, nitrogen oxide and ammonia emissions in Europe declined by 49% and 18%, respectively (Hettelingh et al., 2014), inputs especially from agricultural fertilizers are still high (Federal Ministry for the Environment and Federal Ministry of Food, 2012) resulting in water quality deterioration in groundwater and surface water (Altman and Parizek, 1995, Sebilo et al., 2003, Wassenaar, 1995) and are a major control of eutrophication, especially for coastal environments (Decrem et al., 2007, Prasuhn and Sieber, 2005). Moreover, nitrate increases primary production and has the ability to change food web structures of riverine and coastal ecosystems (Howarth et al., 1996, Turner and Rabalais, 1991). Similarly, elevated nitrate concentrations are the cause for the bad chemical status of 26% of all groundwater bodies in Germany (Völker et al., 2016). In 2016, the European Commission filed a law suit against the German Federal Government related to constantly elevated nitrate concentrations in groundwater (ZEIT-ONLINE, 2016). Existing and partly legally binding targets failed for river and lake protection, air quality control and natural conservation. In a report from the German Advisory Council on the Environment (SfU, 2015) 40 proposed measures coping with nitrate as an environmental pollutant were listed. To draft an amendment for a fertilization ordinance regulating the application of manure and fermentation waste products and to implement a pollution tax for nitrate surplus from agricultural practice are two of the highest priorities. To make effective use of these measures, it is important to characterize and quantify potential nitrate sources and in-stream nitrate processing and its controls in individual catchments. Different sources of nitrate are often characterized by individual isotopic signatures that can be used as fingerprints for source delineation or process mapping in hydrological systems (Rock and Mayer, 2004, Xue et al., 2009). For instance, atmospheric NO3, nitrified soil‑nitrogen can be distinguished from synthetic fertilizer by its distinct nitrate isotopic signatures (Aravena et al., 1993, Kendall and McDonnell, 1998, Wassenaar, 1995). However, a clear isotope-based distinction between different sources is not always possible. Sometimes N and O isotope signatures overlap as observed for NO3 from animal manure and wastewater effluents (Aravena et al., 1993). Therefore, a combination of stable isotope information with other environmental tracers (i.e. chloride, bromide, manganese, ammonium and iron) (Altman and Parizek, 1995, Mengis et al., 1999) as well as a land use analysis (Mueller et al., 2016, Nestler et al., 2011) can enhance the ability to describe the origin of nitrate. To characterize the mobilization of different nitrate pools, it is also important to investigate discharge and corresponding nutrient loads during different seasonal discharge scenarios. High seasonal and interannual variations in discharge and nutrient flows are associated with changing landuse patterns (Klose et al., 2012). Fairbairn et al. (2016) investigated micropollutants in a small watershed under different seasonal and hydrological conditions. They found out that agricultural herbicides showed the highest loadings during increased flows. In agriculturally-influenced prairie streams, Kemp and Dodds (2001) found out that nitrate concentrations are negatively correlated with discharge. Therefore, hydrological effects can have various impacts on the water quality of the gaining stream.

The objective of this study is to apply a fingerprint monitoring method to assess spatial and temporal variability of nitrogen sources within a mesoscale river catchment. The studied Holtemme catchment represents a blueprint example of pristine mountainous headwaters and agricultural as well as urban impacts in the downstream parts. The novelty of this study is the combination of spatially highly resolved assessments along the river with a longer-term monitoring in typical land use types: More specifically, two spatially highly resolved snapshot monitoring campaigns were conducted in October 2014 and 2015 during comparable hydrological base flow conditions. Sampling included 27 points within the river, 12 tributaries and two wastewater treatment plants. We measured nitrate isotopic compositions in concert with major ion concentrations and discharge to differentiate the impact of different nitrate sources and to quantify nitrate loads. This data is combined with a multi-annual, monthly monitoring at two stations representing undisturbed and agricultural land use sites. With this concept, we aim at identifying critical spatial areas as well as seasonal variations of nitrate related aspects of the water quality at catchment scale.

Section snippets

General information

The Holtemme River is a major tributary of the Bode River in the Harz Mountains, Germany (Fig. 1). The stream is part of the Terrestrial Environmental Observatories' (TERENO) network and therefore one of the best equipped regions for Meteorology and Hydrology in Central Germany (Wollschläger et al., 2016, Zacharias et al., 2011). The Holtemme basin has a total size of 282 km2 (Mueller et al., 2015) and a mean annual discharge (MQ) at the outlet of 1.55 m3 s 1 (1982–2013), monitored by the State

Discharge separation

Discharge may represent a mixture of base flow and quick flow depending on hydrological events such as precipitation or snow melt. The varying mixing ratio is likely to impact nitrate concentrations and isotopic compositions as different pathways for mobilization of nitrogen from natural and anthropogenic sources may be associated with the two flow components. Therefore, we conducted a separation of the total flow into the quick flow and base flow component at all locations where discharge was

Hydrological situation and nitrate loads

The amounts of discharge differed substantially between both sampling campaigns (Fig. 3A). In 2014, we determined higher discharge at all discharge monitoring stations along the main stream. The runoff at the outlet of the catchment was about 0.815 m3 s 1. In contrast, the discharge in 2015 was on average 66% lower compared to 2014, especially in the headwater region. The outlet of the Holtemme catchment had a runoff in 2014 of about 0.247 m3 s 1 (Fig. 3A). Nitrate concentrations were similar for

Conclusion

This study reveals how different sources shape the spatial and temporal variability of nitrate concentration and loads along a mesoscale river system that is characterized by a strong land use gradient from pristine headwater catchments to catchments with intense agricultural land use. The sampling water of headwater catchments offers valuable information on the dependency of nitrate dynamics on variable hydrological conditions. The headwater nitrate concentrations are generally low (between 1

Acknowledgment

We thank the State Office of Flood Protection and Water Management (LHW) particularly our contact person, Elke Stobrawe, for suppling discharge data from public measuring stations. We also thank Martina Neuber, Petra Blümel and Kathrin Singer for laboratory support. Additionally, we thank Ilona Bärlund and Markus Weitere for organizing this project and Liza-Marie Beckers, Thomas Grau and Cornelia Wilske for valuable discussions on this topic. We also would like to thank the European Commission

Funding

Precipitation data were provided by the German Weather Service (DWD), the Joint Research Center of the European Commission, the European Environmental Agency, the Federal Institute for Geosciences and Natural Resources (BGR), the European Water Archive, and the Global Runoff data Centre. The River Network is supported by ATKIS R DLM 1000 © Bundesamt für Kartographie und Geodäsie, 2003. Topographical information is based on Geobasisinformation on the Vermessungs- und Katasterverwaltung by

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