USING DUAL-ISOTOPE DATA TO TRACE THE ORIGIN AND PROCESSES OF DISSOLVED SULPHATE: A CASE STUDY IN CALDERS STREAM (LLOBREGAT BASIN, SPAIN)

Abstract

Whereas most of the reported δ34S values of dissolved sulphate are positive in the Llobregat basin, Calders stream, which is a tributary of the Llobregat River, is characterised by negative values. Stream waters, sampled monthly between 1997 and 1998, and quarterly in 1999, show an overall increase in δ34S from −10‰ to 0‰, coupled with an increase in Na and Cl concentration. On the other hand, the oxygen isotopic composition of dissolved sulphate, δ18O, displayed an opposite trend with a slight decrease, from +9‰ to +6‰. Detailed sampling up stream in November 2000 indicated that, contrary to most of the surficial waters of the Llobregat basin with a δ34SSO4 mainly controlled by evaporites, in Calders stream, sulphate is derived from pyrite oxidation. The dual-isotope approach, coupled with chemical data, allowed us to identify the contribution of 34S-rich sulphate effluents from anthropogenic sources, while mixing models, calculated between natural and anthropogenic sources, enabled us to estimate their contribution. Sudden increases of δ34S and δ18O of dissolved sulphate in stream waters are believed to be caused by a sulphate reduction process related to oil spillage. The long-term enrichment in δ34S, coupled with a decrease in δ18OSO4, from Jan-97 to Aug-99, is interpreted as a progressive increase in the contribution of pig manure.