Hydrochemical characteristics of sabkha brines, evaporite crystallization and microbial activity in Al-Kharrar sabkha and their implication on future infrastructures in Rabigh area, Red Sea coastal plain of Saudi Arabia

摘要

Al-Kharrar sabkha area is characterized by a distinctive distribution of varied evaporite minerals and microbial activities. The lowest topographic depressions are dominated with primary halite crystals in a high salinity brine (<220 parts per thousand) or microbial mats at a relatively low salinity value <80 parts per thousand. As a consequence of desiccation of these pans and in saline mudflat/sandflat areas, gypsum, anhydrite and halite are displacively, inclusively and replacively grown below the sediment surface of the sabkha. Crystallization pressures exerted by these evaporite minerals are very effective in sediment destruction of the sabkha and consequently possible deterioration of the infrastructures of future buildings. In addition, bacterial sulfate reduction corrodes gypsum and results in the formation of carbonate minerals and occasional pyrite. The salinity of sabkha water and brine samples ranges from 21.3 to 141 g/l which is conformable to the concentration values of Cl-. The concentration order of the sabkha water and brine samples is: Na+ > Mg2+ > Ca2+ >K+ or Mg2+ > Na+ > Ca2+ > K + for cations and Cl - > SO42- > HCO3- for anions. No distinct correlation is observed between the various cations, anions, alkalinity and TDS, with the exception of the positive relation between Cl -and TDS. The dominant brine type for most samples is NaCl or MgNaCl. Mg/Ca ratio in some water samples is high (12.14-23.66) which allows dolomitization of the early deposited carbonate minerals. Most water and brine samples are located in the field of recent or old marine water and of MgCl2 or CaCl2 composition, respectively. The resultant crystallization pressure by evaporite minerals, microbial activity and varied brine compositions are factors that increase the future corrosion for foundations in Al-Kharrar sabkha area.