Saline waters

Saline waters often imply natural waters with salinity or total dissolved salts (TDS) above biological tolerance (1-5 g/L), while the term brines is used for waters with salinity greater than that of sea water (35 g/L) or 100 g/L. Brackish waters are detween freshwater (TDS <1 g/L) and sea water. In this article, saline waters are efined as natural waters with salinity greater than that of sea water. Saline waters are currently encountered in diverse geologic settings at the surface and in the upper crust of the Earth; saline lakes in the arid zones, waters in sedimentary basins and crystalline rocks, brines in anoxic marine basins, interstitial waters in marine sediments, hydrothermal waters from mid-ocean ridge and continental geothermal systems. Petrologic evidence, most notably from fluid inclusions, also clearly documents that hot, saline waters have been responsible for large-scale fluid transport, water-rock interactions, and ore depositions in various geologic processes in the past. Among these rather widespread occurrences of saline waters in various geologic settings, both present and past, our main interest here is in those in modern, w-temperature environments (saline lakes, brines in sedimentary basins, crystalline basements, and anoxic marine basins). The first and foremost question regarding the occurrence of saline waters is; what hydrologic regimes and geochemical processes have led to their formations? The sources of salinity can vary widely, including aeolian salts, surficial weathering products, seawater intrusion, subsurface water-rock interactions at elevated mperatures and pressures, and metamorphic-magmatic fluids. However, the fundamental requirement for the formation of saline waters is a closed or semiosed nature of hydrologic regimes or geochemical systems wherein dissolved solutes, regardless of their sources and geochemical mechanisms, can build up within a water body, either subaerial or subsurficial. Otherwise, hydrologic openness either dilutes or flushes dissolved salts out of the water body. Hydrologic and geochemical processes that can produce saline waters are also diverse. Under subaerial conditions, the evaporation of water into the atmosphere from hydrologically restricted water bodies (lakes, marginal seas, etc.) and soil water/groundwater is the primary processes for the build-up of salinity in the arid zone. The formation of ice by freezing in the polar regions can also produce brines subaerially and in soils. Subsurface processes leading to the formation of brines are perhaps even more complex, including boiling and phase separation of water, various water-rock interactions (dissolution of evaporite deposits and other minerals, diagenetic formation of hydrous minerals, etc.), and membrane filtration by shales. A number of geochemical processes at the surface and especially in the subsurface modify the chemical and isotopic compositions of saline waters over prolonged geologic time; compaction and dewatering of formations, redox reactions involving dissolved and gaseous species, maturation and decay of organic matter, and various water-rock interactions (dehydration, hydrolysis, cation exchange, alteration of detrital and igneous/metamorphic rocks, etc.).