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

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.