Gypsum precipitation/dissolution as an explanation of the decrease of sulphate concentration during seawater intrusion V. Gomis-Yagu ¨es * , N. Boluda-Botella, F. Ruiz-Bevia ´ Departamento de Ingenierı ´a Quı ´mica, Universidad de Alicante, Apartado 99, E-03080 Alicante, Spain Received 2 June 1999; received in revised form 16 September 1999; accepted 19 November 1999 Abstract The precipitation of gypsum during the intrusion of seawater in a coastal aquifer is shown to be a possible cause of the decrease of the sulphate concentration with respect to the conservative freshwater/seawater mixing observed in many field studies. Results of previously published laboratory column experiments and the application of a multicomponent reactive transport model show that gypsum precipitates during the first stages of seawater intrusion, causing a decrease in sulphate concentration. As the resultant front advances, the water becomes undersaturated with respect to gypsum when the content of seawater 50%. At that point, water samples exhibit a deficit of sulphate of several mmol/l, in the range of the field values reported in papers on intrusion in coastal aquifers. Thus, in addition to bacterial sulphate reduction by organic matter, gypsum precipitation is a possible explanation for the non-conservative behaviour of sulphate during seawater intrusion.  2000 Elsevier Science B.V. All rights reserved. Keywords: Hydrogeochemistry; Seawater intrusion; Gypsum; Sulphate 1. Introduction It is well known that the displacement of fresh water by salt water during seawater intrusion is accompanied by cation exchange processes (Appelo and Geirnaert, 1983). These processes can explain deviations of the concentrations of cations from conservative mixing of both waters. For instance, increases in the concentration of calcium and decreases in the concentrations of sodium, mag- nesium and potassium are commonly found. A decrease in concentration of sulphate, in excess to what would be expected for conservative mixing of salt water and fresh water, is another characteristic observation during salt water intrusion. Such a decrease leads to a chloride/sulphate molar ratio higher than the value for seawater which is 19. Most of the papers reporting data of analyses of well waters in aquifers salinised by intrusion show this character- istic sulphate depletion. Works published in the last few years with specific reference to seawater intru- sion, such as those of Hahn (1991), Stuyfzand (1992), Bencini and Pranzini (1992), Bocanegra et al. (1992), Manzano et al. (1992), Bosch and Custodio (1992), Custodio (1992), Fidelibus and Tulipano (1996), are some examples containing field data where the depletion in concentration of sulphate is obvious. These authors either provide no explanation for the decrease of sulphate, or assign it to degradation of organic matter by sulphate reducing bacteria. In these studies it was further found that the water Journal of Hydrology 228 (2000) 48–55 www.elsevier.com/locate/jhydrol 0022-1694/00/$ - see front matter  2000 Elsevier Science B.V. All rights reserved. PII: S0022-1694(99)00207-3 * Corresponding author. Tel.: +34-96-590-3867; fax: +34-96- 590-3825. E-mail address: vgomis@ua.es (V. Gomis-Yagu ¨es).