[en] In the middle section of the Llobregat Basin (NE Spain) several potash mines exist. This activity produces large salt tailings, with an estimated mass of 61Mt, which are stored around the mining sites, with no waterproofing. Some aquifers near the mining zones are highly salinized (chlorine up to 14%), and the origin of the salinity is controversial, as it can be related either to contamination from mine tailing effluents or to natural water interaction with saline formations. Moreover, with less intensity fertilisers could also contribute to groundwater salinization, as agriculture is an important economic activity in the Llobregat Basin. These three sources, natural, mine effluents and fertilisers, where chemical and isotopically (δ³⁴S) characterised in a previous paper, in addition, samples of uncertain origin were analysed and the sources of salinization were determined coupling chemical with δ³⁴S data. On the basis of the previous results, the analysis of the δ¹⁸O_SO4 and δ¹⁸O_H2O of the same samples is presented in order to: (a) asses the extent of redox processes, and (b) quantify the contribution of each source using a δ¹⁸O_SO4 - δ³⁴S_SO4 diagram. The δ¹⁸O_SO4 was determined by TC/EA-IRMS, and the δ¹⁸O_H2O was measured by CO₂ equilibrium and IRMS. Notation is expressed relative to V-SMOW. All samples plot away from the field of sulphate produced from sulphide oxidation in a δ¹⁸O_SO4 vs δ¹⁸O_H2O diagram therefore oxidation processes are discarded. Results of δ¹⁸O_SO4 vs δ³⁴S_SO4 are shown. Values for natural samples and mine effluents are in accordance with the δ¹⁸O_SO4 of bedrock evaporites and Potash Unit respectively, ranging from +12.2 per mille to +14.6 per mille the natural samples; and from +7.6 per mille to +9.8 per mille the mine effluents. Fertiliser oxygen isotopic composition is +9.5 per mille for ammonium sulphate, and +14.6 per mille for NPK. The δ¹⁸O_SO4 allows to discriminate natural and mine samples, but NPK plot within the area of natural values and ammonium sulphate coincides with mine effluents. Despite the overlapping of the δ¹⁸O_SO4 values, the δ³⁴S_SO4 is different, and quantification of the contribution of each source can be done. The mixing lines between the end members considered is shown. These models allow to quantify different source contribution for samples of uncertain origin. Samples from site 9 are in accordance with sulphates from the bedrock. Samples from site 7 are a mix between mine effluents and natural values, the contribution of mine effluents varies from 60 to 30% and fertilisers are discarded. For samples from sites 8 and 10, a better characterisation is possible, with a distinction of different fertilisers. At site 8 samples are a mix of NPK fertiliser with natural values, and at site 10, samples are mainly controlled by ammonium sulphate, in both cases fertilisers contribution is up to 90 %. This study confirms the usefulness of isotopic geochemistry to trace the origin of water salinization in the middle section of the Llobregat River. As dilution processes do not affect isotopic composition, coupling the analysis of δ³⁴S_SO4 and δ¹⁸O_SO4 in waters provides an excellent tool for quantifying the anthropogenic inputs existing in the area, potash mine effluents and fertilisers