GEOCHEMISTRY OF SALINE SOILS IN 2 ARID ZONES OF THE MEDITERRANEAN BASIN .1. GEOCHEMISTRY OF THE CHOTT MELGHIR-MEHROUANE WATERSHED IN ALGERIA

An endoreic region in South Algeria was studied, which included the so uthern slopes of the Aures, the Biskra and Touggourt areas, and the Me lghir and Merouane chotts, which are large depressions located below s ea level. The chemistry of surface waters and groundwater and the mine ralogy of efflorescences were studied along a 100-km-long toposequence . The ionic strength of natural solutions varied from 22 x 10(-3) in t he Aures Mountains to over 5.0 in the chotts. Thermodynamic equilibriu m models indicated that solutions were controlled by the following pre cipitation sequence: calcite > gypsum > mirabilite = bloedite > halite . This sequence, confirmed by X-ray diffraction of efflorescences, ind icated that the landscape had behaved as a chromatographic separator. Chemical composition variations related to this separation were consis tent with generalized residual alkalinity, thereby indicating that a s ingle chemical family was present in the study area, which suggests th at a single hydrologic system was also present. Gradual concentration and precipitation of minerals in the downstream direction accounted fo r most of the chemical facies variations. This study provided a chance to test successfully a new extension to the Debye-Huckel law for chlo rurated sodic brines.