Isotopic Assessment of Sources and Processes Affecting Sulfate and Nitrate in Surface Water and Groundwater of Luxembourg

Abstract

Surface water and deep and shallow groundwater samples were taken from selected parts of the Grand-Duchy of Luxembourg to determine the isotopic composition of nitrate and sulfate, in order to identify sources and/or processes affecting these solutes. Deep groundwater had sulfate concentrations between 20 and 40mg/L, δ³⁴S_sulfate values between ?3.0 and ?20.0‰, and δ¹⁸O_sulfate values between +1.5 and +5.0‰ nitrate was characterized by concentrations varying between <0.5 and 10mg/L, δ¹⁵N_nitrate values of ～?0.5‰, and δ¹⁸O_nitrate values ～+3.0‰. In the shallow groundwater, sulfate concentrations ranged from 25 to 30mg/L, δ³⁴S_sulfate values from ?20.0 to +4.5‰, and δ¹⁸O_sulfate values from ～+0.5 to +4.5‰ nitrate concentrations varied between ～10 and 75mg/L, δ¹⁵N_nitrate values between +2.5 and +10.0‰, and δ¹⁸O_nitrate values between +1.0 and +3.0‰. In surface water, sulfate concentrations ranged from 10 to 210mg/L, δ³⁴S_sulfate values varied between ?9.3 and +10.9‰, and δ¹⁸O_sulfate values between +3.0 and +10.7‰ were observed. Nitrate concentrations ranged from 10 to 40mg/L, δ¹⁵N_nitrate values from +6.5 to +12.0‰, and δ¹⁸O_nitrate values from ?0.4 to +4.0‰. Based on these data, three sulfate sources were identified controlling the riverine sulfate load. These are soil sulfate, dissolution of evaporites, and oxidation of reduced S minerals in the bedrock. Both groundwater types were predominantly influenced by sulfate from the two latter lithogenic S sources. The deep groundwater and a couple shallow groundwater samples had nitrate derived mainly from soil nitrification. All other sampling sites were influenced by nitrate originating from sewage and/or manure. A decrease in nitrate concentration observed along one of the rivers was attributed to denitrification. It appears that sulfate within Luxembourg's aquatic ecosystem is mainly of lithogenic origin, whereas nitrate is often derived from anthropogenic activities.