HYDROCHEMICAL SITUATION OF SHARI PLAYA LAKE BRINES AND THE CONTRIBUTION OF THE FEEDING WATER TO THE FORMATION OF EVAPORITE MINERALS, CENTRAL IRAQ

Abstract

Shari Playa is a closed elongated basin about 20 Km long and (3 – 5) Km wide; located about 150 Km north of Baghdad. It is characterized by the presence of a lake in winter which dries out to form salt playa in summer. The lake water concentrates by evaporation to form salt crust; composed of sodium chloride and sodium sulphate. Since the playa is a structural depression, many springs are developed inside it as a result of faults intersections. The water and brines in the Shari Playa basin indicate three major groups of water: Na+ – Cl– – SO4=; Ca2+ – SO4= and Na+ – Ca2+ – SO4=. Where the second cations and anions concentrations vary in different water sources in Shari Playa basin, other subdivisions may be recognized giving different water types. The supplied water to the depression strongly contributes to the formation of glauberite, gypsum and thenardite in the lake sediments, especially in the central part which is characterized by the presence of organic matter in the sediments. The water and brines are of different hydrochemical characteristics and contributes to the formation of different evaporite minerals or to dissolve them depending on their concentrations. The lake water precipitates gypsum at first, then glauberite forms in the deepest part of the playa lake when the water volume decreases to occupy the central part only. The concentration of the remaining brine increases due to evaporation, where Na+ is high enough to alter gypsum to glauberite [Na2Ca(SO4)2]. Thenardite (Na2SO4) forms in the salt crust when all Ca2+ is consumed from the lake brine before the NaCl precipitation. It also forms within the black mud slurry after ending of glauberite formation, which consumes the Ca2+ in the brine.