Effects of the adjacent evaporitic formations on groundwater quality of alluvial sand-gravel aquifers: a study in the Derincay basin (north-central Turkey)

Evaporite bearing formations extending in large areas in Turkey affect quality of water resources. The Derincay basin is an example to the impacts of evaporites on the adjacent alluvial aquifers. Miocene-Pliocene aged evaporites cover nearly one-third of the basin and mostly surround the low elevated alluvial sand-gravel aquifer. The higher quality of the groundwater in the upstream regions, where the evaporites do not crop out or partially crop out, turns into low to very low quality in the downstream areas due to the large extents of the evaporites. Waters from the evaporites increase TDS, Ca, SO4, Na, and Cl in the alluvium aquifer, particularly in the downstream region of the basin. EC values rise from 1000-1300 mu S/cm to over 5000 mu S/cm from upstream to downstream. "High salinity but low sodium hazard'' category for the upstream parts turns into "very high salinity'' and "medium-high sodium hazard'' for the downstream regions on the salinity diagram. Groundwater in the upstream parts falls in "good to permissible'' class; however, the class turns into "permissible to doubtful'' and "doubtful to unusable'' classes for the downstream on the Wilcox diagram. Ca-bicarbonate type in the upstream region turns into Ca-sulfate type in the downstream region due to the evaporite effect. This study indicates three main sources affecting the quality of groundwater of the alluvium aquifer in the Derincay basin: extent of the evaporite formations in the catchment area, recharge from the evaporite springs, and dilution with relatively high-quality stream waters.