Brine geochemical changes and salt crust evolution of Lake Urmia in Iran

Abstract

Gradual desiccation of Lake Urmia (LU) in the northwest of Iran has led to catastrophic water loss by 2022 promoting the salt crust (SC) deposition on the lake bed. Therefore, this study aimed to investigate 1) the effect of water level on the brine composition, 2) the effect of gradual lake desiccation on SC formation, mineralogy and stratification and 3) the SC formation and mineral precipitation in a lab-scale experiment. In 2019, brine and SC samples were collected in July and October, and analyzed for the prevailing ions and minerals. For the simulation experiment, brine samples were added in two vessels and the change in pH, ionic composition and SC evolution were investigated. The wet season brine type changed from Mg-Na-Cl to Na-Mg-Cl and remained unchanged throughout the dry season. In vessels, Mg2+ and Na+ became dominant ions in the surface and subsurface layers respectively, while Cl- remained roughly unchanged; and minerals gradually precipitated from the margins to the center of the vessels based on their solubility. Similar to the lake SCs, in the simulation vessels, halite dominated the SCs and bischofite (MgCl2·6H2O) was detected in most of the vessels’ layers. The formation of oldhamite (CaS) in both in the vessels and the lake SC indicated the prevalence of low oxygen conditions. In the simulation vessel, Calciolangbeinite (K2CaSO4(H2O)4) was one of the first minerals to form along with halite, while Starkeyite (MgSO4·(H2O)4) and Kieserite (MgSO4·(H2O)) were the latest minerals to precipitate. The absence of magnesium sulfate minerals under natural conditions in both newly formed SCs in 2019 and those dated back to 2005, suggested that if the lake level continues to drop, magnesium sulfate minerals will be the final product of the brine. The existence of a dark silicate layer within SC stratification helped to investigate the evaporate deposition at the early stages of SC formation and after the catastrophic shrink. Based in the precipitation of a new layer on top of the submerged SC samples during 2019, a sedimentation rate between 10 and 30 mm/y is calculated with NaCl and CaS as major precipitants.