Rivers of North-Rhine Westphalia revisited: Tracing changes in river chemistry
Introduction
Dissolved river constituents provide information on natural processes and the impact of anthropogenic activities. Distinguishing these different sources is a prerequisite for assessing how mankind is modulating nature. In addition, quantification of river constituents has been utilized in climate studies, linking natural (silicate) weathering, drawdown of atmospheric CO2 and global climate (e.g., Gaillardet et al., 1999). For example, Telmer and Veizer (1999) focussed on the origin and fate of DIC and showed that rivers on a global scale act as source of CO2 rather than as a sink. Dissolved provides information on the importance of oxidative weathering of pyrite, a process that also affects the C cycle and that has been shown to be the major source, e.g. in the Mackenzie River basin (Calmels et al., 2007).
Three rivers in northwestern Germany were investigated for their hydrological and hydrochemical properties: the Ruhr, Lippe and Ems. For selected dissolved constituents, natural sources and human impacts on these river systems were distinguished using naturally occurring stable isotopes (δ2HH2O, δ18OH2O, δ13CDIC, δ34S and δ18OSO4, and δ15N and δ18ONO3) in combination with physico-chemical parameters (T, pH, O2-conc., conductivity) and concentrations of major anions (Cl−, , , ) and cations (Na+, Ca2+, K+, Mg2+). In particular, stable isotopes provide valuable information for identifying sources of river constituents, for characterizing in-river processes and for catchment hydrology, as for the river Rhine (Buhl et al., 1991), the upper Danube river (Pawellek et al., 2002), or the St. Lawrence riverine system (Yang et al., 1996). Changes in river chemistry were quantified for the Lippe and Ruhr in comparison to an earlier study (Flintrop et al., 1996).
Section snippets
Description of the catchment areas
The geographical setting of the rivers is shown on the geological map in Fig. 1. The rivers Ruhr and Lippe have a length of about 220 km and their catchment areas cover 4485 km2 and 4882 km2, respectively. The Ruhr is located in the Rheinisches Schiefergebirge. It drains mainly siliciclastic rocks of Devonian and lower Carboniferous age, but enters coal bearing upper Carboniferous siliciclastics near the city of Hagen/Ruhr area (∼120 km from source). In addition, some Devonian limestones and
Methods
River samples were collected in autumn, where discharge is lowest in the year (LANUV NRW, 2007). The Ems and tributaries were sampled in September 2005, the rivers Lippe and Ruhr and their tributaries in September 2006. Samples were taken from the middle part of the rivers, upstream from the inflow of a tributary and again some kilometres downstream of it. Immediately after recovery pH, electrical conductivity, temperature and O2 content were measured (multi 340i, WTW). Sub-samples for
Results
All data are listed in Appendix. In addition, the reader is referred to Flintrop et al. (1996), for the 1991 results obtained for the rivers Ruhr and Lippe.
Changes in rivers Ruhr and Lippe since 1991
The results for rivers Ruhr and Lippe mostly confirm earlier observations of Flintrop et al. (1996). The isotopic composition of water (δ2H, δ18O) in general mirrors the different geographic settings (Fig. 3). Variations in the downstream evolution of both isotopes are caused by mixing with tributaries and evaporation effects, which occur either in the reservoirs (river Ruhr) or due to the use of river water for cooling purposes (river Lippe) (Flintrop et al., 1996). Compared to 1991, the Lippe
Conclusions
This study aimed at distinguishing natural and anthropogenic sources of river constituents by combining traditional parameters in hydrochemistry and light stable isotopes. These were applied to the rivers Lippe and Ruhr, following an earlier investigation by Flintrop et al. (1996), as well as the river Ems.
Compared to previous data, salt concentrations that are commonly attributed to anthropogenic activity, in particular and K, were found to be lower. This improvement in water quality is
Acknowledgements
This study was initiated during the field- and laboratory-based student course “Introduction to Stable Isotopes”, and we are grateful to students from this class for their help and insightful discussions. ICP-OES analyses, ion chromatography, and alkalinity measurements were performed in the Umweltgeochemisches Labor of the Geologisch-Paläontologisches Institut, and we thank A. Reschka and members of the section for Applied Geology for their help. A. Fugmann is gratefully acknowledged for his
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