Elsevier

Applied Geochemistry

Volume 23, Issue 12, December 2008, Pages 3290-3304
Applied Geochemistry

Rivers of North-Rhine Westphalia revisited: Tracing changes in river chemistry

https://doi.org/10.1016/j.apgeochem.2008.06.030Get rights and content

Abstract

Three rivers in North-Rhine Westphalia, Germany, were investigated for their hydrochemical properties including their stable isotopic composition of water (δ2H, δ18O) and dissolved river compounds (δ13CDIC, δ34SSO4 and δ18OSO4, and δ15NNO3 and δ18ONO3). The study focused on two objectives: an assessment of potential sources for river solutes (anthropogenic vs. natural sources), and the quantification of changes in river chemistry over the past 15 a (for the rivers Lippe and Ruhr). Decreasing concentrations were found for most of those river constituents that are commonly linked to anthropogenic activities, such as [NO3-], [Cl], [K+], and [Na+]. An observed increase in [SO42-] for the river Lippe reflects most likely varying discharges from mining activities. Variations in the isotopic composition of water display the influence of ocean water (river Ems) or of evaporation that occurred either in channels (river Ems), in reservoirs (river Ruhr) or due to the use of river water for cooling purposes (river Lippe). δ13CDIC values around −11‰ point to carbonate dissolution by carbonic acid as the major source for dissolved inorganic C. Modifications of this average δ13CDIC resulted from enhanced agricultural use, sewage inputs, and gas exchange with the atmosphere in reservoirs and channels. The isotopic composition of dissolved SO42- reveals atmospheric deposition and sulphide oxidation as its major sources. Sulphate from sulphide oxidation in parts reflects the local geology (river Ruhr); in the Kreidebecken leaching of sulphide seems to be linked to agriculture and drainage (rivers Lippe and Ems). However, SO42- introduced from mining activities into the Lippe and the Ems does not alter the isotopic composition of riverine SO42-, despite rather high discharges. Nitrogen and O isotopes reveal that manure and sewage are major sources of NO3 in most parts of the river Ruhr. Only a single value from the headwaters displays the signature of soil NO3. Downstream increasing δ15NNO3 and δ18ONO3 values (both by 2‰ on average) point to denitrification and to additional inputs from atmospheric deposition.

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 SO42- 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 SO42- 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, HCO3-, NO3-, SO42-) 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 NO3- 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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