Evolution of the isotopic composition of dissolved sulphate in Calders stream (Llobregat basin, NE Spain)

The Calders stream is a tributary of the Llobregat River characterised by negative values of δ34S of dissolved sulphate, whereas in the Llobregat basin most of the reported values are positive. Stream waters were sampled monthly between 1997 and 1998 and quarterly in 1999. Data show an overall increase in δ34S from -10 per mille to 0 per mille, coupled with an increase in Na and Cl concentrations. This trend has a break on May 1998, with a maximum δ34S of +2 per mille. The oxygen isotopic composition of dissolved sulphate, δ18O, displays an opposite trend with a slightly decrease from +9 per mille to +6 per mille. The aim of the present study is to elucidate the origin of these negative values and to understand their evolution. Pristine stream water from a tributary has a δ34S = -18 per mille and δ18O = +2.5 per mille, in accordance with the reported values for the sulphate produced by pyrite oxidation in this area. Moreover, these values fall in the field of sulphate produced by sulphide oxidation in a δ18OSO4 vs 18OH2O plot. Therefore, negative δ34S values of Calders stream are interpreted as natural values obtained by leaching of bedrock, pyrite-bearing marls and limestones. In order to determine which processes caused the isotopic evolution described above, a detailed sampling up stream was performed on November 2000. It is interesting to notice the significant variation in major ions, with an increase of Cl and Na content in sample 6, which is progressively diluted downwaters. This change is recorded by a drastic δ34S enrichment from +0.3 per mille in site 7, to +21 per mille in site 6, and back to lower values downstream. Water chemistry and isotopic values of sample 3 can be explained by a mix between natural sources and fertilisers. Sample 7 has the same inputs that sample 3, with the contribution of pig manure, which is increasingly spread onto the field as a fertiliser (one sample of pig manure analysed has a δ34S = 0 per mille and a δ18O = +5.6 per mille). Sewage effluents in the Calders Basin flow directly into the stream and only a small volume is plant-treated since April 1999, and hence their contribution cannot be discarded. Therefore, the δ34S of sample 7 could be interpreted as a three end-member mix, natural, agricultural and sewage sources. Wastewater collected from a dye industry located near sample 6 is sodium-chlorine type with a δ34S = -0.8 per mille and four times more sulphate than sample 7. The increase in Na and Cl detected in sample 6 (with values up to 110 and 175 ppm, respectively) is interpreted as a contribution of 10% in volume of water coming from this industry. However, this balance cannot account for the 34S-rich value found and its low sulphate concentration, even lower than in sample 7. It is important to point that occasionally oil spillage from this dye industry into the stream has been reported. The last one took place in spring 1998, and during the November 2000 sampling, part of the oil was still visible. The δ34S of the stream water at this site has a value of +21 per mille, sulphate reduction mechanism accounts for such increase of the isotopic signature coupled with a lowering of sulphate concentration. Downwaters of site 6, stream waters show a decrease in the δ34S, [Na] and [Cl], with values down to -4.5 per mille, 30 and 50ppm, respectively at the last sampling site. This evolution is interpreted as a dilution with waters similar to sample 3, a mix between pristine waters and fertilisers. This detailed sampling indicates that the negative isotopic composition of dissolved sulphate in the area has a natural origin. Regardless to most of the surficial waters of the Llobregat basin, with a δ34SSO4 mainly controlled by evaporites, in the Calders stream sulphate is derived from pyrite oxidation. Moreover δ34S-rich sulphate effluents from anthropogenic sources, mainly, agricultural sources, dye industry effluent, sewage and oil spills, controls the δ34S of the stream waters. Sudden increases of δ34S of dissolved sulphate in stream waters are believed to be caused by oil spillage. The long-term enrichment (from Jan-97 to Aug-99) in the area is interpreted as a progressive increase in the agricultural and industrial activities