Composition and origin of the sabkha brines, and their environmental impact on infrastructure in Jizan area, Red Sea Coast, Saudi Arabia

摘要

The sulfate evaporite minerals (gypsum and anhydrite) and brines of Jizan sabkha cause corrosion of the steel reinforcement and deterioration of the concrete, and consequently hinder the development activity for building new urban communities and industrial zones in Jizan area, Red Sea coastal plain of Saudi Arabia. The sabkha evaporite minerals below the sediment surface are represented by displacive and inclusive growth of lenticular and rosette gypsum, and nodular anhydrite. In small saline pans, halite precipitates form rafts, chevrons and cornets. The salinity (TDS) of the groundwater in the sabkha area is highly variable, and ranges from 12,900 to 495,000 mg/l, compared to the average value of the Red Sea water of 40,366 mg/l. The low salinity values of the sabkha brines are most probably caused by localized influx of groundwater of meteoric origin from direct rain fall and/or temporary floods, in addition to seepage of sewage water from septic tanks. The electric conductivity (EC) values range from 20,000 to 199,100 lS/ cm which are conformable to the salinity values of the brine. The dominant cation concentration order in seawater and brines of the sabkha is Na+ > Mg2+ > Ca2+ > K+, or Na+ > Mg2(+) > K+ > Ca2(+) >. The dominant anion concentration order is Cl- > SO42- > HCO3-. The dominant brine type for most samples is sodium chloride, with variable proportions of the major cations Ca2(+) and Mg2+ and the major anion SO42-. Most brine samples indicate their source is of modified marine water having an elevated CaCl2 content, which may be derived from dissolution of mixed salt from the Miocene salt dome in Jizan area. The chemical composition and origin of the brines, and mineralogy and textures of the evaporite minerals in Jizan sabkha help in understanding the nature of the corrosive factors to the foundations in Jizan area.