WATER QUALITY PROBLEMS OF A GYPSIFEROUS WATERSHED: UPPER KIZILIRMAK BASIN, S ˙ IVAS, TURKEY F ˙ IKRET KAÇARO ˘ GLU 1∗ , MUSTAFA DE ˘ G ˙ IRMENC ˙ I 2 and ORHAN CER ˙ IT 2 1 Department of Geological Engineering, Cumhuriyet University, 58140 Sivas, Turkey; 2 Department of Environmental Engineering, Cumhuriyet University, 58140 Sivas, Turkey (∗ author for correspondence, e-mail: fkacar@cumhuriyet.edu.tr) (Received 18 August 1999; accepted 25 May 2000) Abstract. The solubility of the gypsum is generally very high in comparison to many other minerals. The surface and groundwater contacting gypsum formations can easily attain high amounts of total dissolved solids (TDS), calcium, and sulphate. This composition of the waters prevent their usage for drinking, domestic, industrial and irrigation purposes. Gypsum formations, containing halite (rock salt) interlayers, crop out in a large area in Upper Kızılırmak basin, Sivas, Turkey. In this study, the effects of the lithological composition of the catchment area on the water chemistry and quality were studied on the Upper Kızılırmak basin. Surface waters draining gypsiferous area, and Göydün and Seyfe springs create high TDS concentration in Kızılırmak river (EC = 1100–5200 μS cm −1 ). Göydün and Seyfe springs which issue from gypsum are very rich in TDS (EC = 12 825–13 900 μS cm −1 ) and are of poor quality. Surface and groundwater resources in non-gypsiferous part of the basin (Yıldız river, Kaynarca and Gaziköy springs, Tavra valley) have lower TDS and higher quality. EC of these waters range between 495–630, 795–995, and 530–575 μS cm −1 , respectively. The Kızılırmak river (with the exception of the upstream part which drains non-gypsiferous area), Göydün and Seyfe springs, and the well in Kızılırmak river alluvium (SK-1) are not suitable for drinking, irrigation and industry. TDS, hardness, sulphate and chloride concentrations of these waters exceed maximum permissible limits for these parameters in Turkish Drinking Water Standards (TDWS). The above mentioned water resources have high to very high salinity and sodium (alkali) hazard from the point of irrigation water quality, and are not also suitable for allmost all industrial usage. Keywords: groundwater, gypsum aquifer, saturation index, solubility, water quality 1. Introduction Gypsum has relatively high solubility in comparison to many other minerals. The relationship between environmental conditions and the solubility is simple. There is positive correlation between temperature, pressure and solubility of the gypsum (Hem, 1985; Ford and Williams, 1989; Langmuir, 1997). Because of these prop- erties, the surface water and groundwater contacting gypsum become enriched in total dissolved solids (TDS) with calcium (Ca) and sulphate (SO 4 ) as the dominant ions (Drever, 1997). Gypsum formations which contain rock salt (halite) interlayers crop out in a large area in Upper Kızılırmak basin, Central Anatolia, Turkey. Some rock salt beds in the gypsum located to the southeast of Sivas city are used for salt production Water, Air, and Soil Pollution 128: 161–180, 2001. © 2001 Kluwer Academic Publishers. Printed in the Netherlands.