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 (δ¹³C_DIC, δ³⁴S_SO4 and δ¹⁸O_SO4, and δ¹⁵N_NO3 and δ¹⁸O_NO3). 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 [SO₄^{2 -}] 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). δ¹³C_DIC values around -11‰ point to carbonate dissolution by carbonic acid as the major source for dissolved inorganic C. Modifications of this average δ¹³C_DIC resulted from enhanced agricultural use, sewage inputs, and gas exchange with the atmosphere in reservoirs and channels. The isotopic composition of dissolved SO₄^{2 -} 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, SO₄^{2 -} introduced from mining activities into the Lippe and the Ems does not alter the isotopic composition of riverine SO₄^{2 -}, 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 δ¹⁵N_NO3 and δ¹⁸O_NO3 values (both by 2‰ on average) point to denitrification and to additional inputs from atmospheric deposition.