Tomography of anthropogenic nitrate contribution along a mesoscale river
Graphical abstract
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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