Brackish water is water that is part saline and part freshwater. It is situated where rivers or lakes water flows into seawater and can also be found in estuaries, mangrove swamps, deltas and some inland seas such as the Caspian Sea. There is a technical standard for brackish water of between 3 and 30 ppt salt or 1000 to 4000 ppm salt.
The increasing need for fresh water supplies for urbanized areas is creating conditions where freshwater is being diverted away from its natural flow to the sea through brackish areas, or where desalinization of brackish water is threatening brackish biomes and environments.
There are brackish lifeforms that have either adapted from freshwater species or saline species in order to thrive in these specific environments. Changes to the salinity are, of course, threatening these species. Included are finned fish, crustaceans, and aquatic plants.
In Sri Lanka, the conflict between industrial development and the existing brackish water shrimp industries as well as the protections that need to be in place for the overall wetlands ecosystems, is indicative of the worldwide implications of the increasing need for both freshwater and for the maintenance of brackish environments that provide food and habitat.
In India, changes to land management, where changing levels of land farming or shrimp cultivation provide conflicts between the diversion of freshwater from aquaculture that goes on in brackish areas, or in the reduction of water that is available for land farming. Also industrial, agricultural and urban effluent release into brackish areas is of major concern, as the chemical balances or bacterial balances can get out of control.
The Middle Eastern and North African countries are said to account for about 60 percent of the entire world's desalination capacity, with Saudi Arabia accounting for about half of that capacity. Some of this is desalinated brackish water or brackish water that is blended with freshwater. Complete desalination of sea water is more viable there because of the readily available energy in the oil producing countries.
The United States has huge areas of deep saline aquiferswhich will require some work to use, which may help to reduce the need to divert water from brackish or partially brackish regions, such as the California Sacramento-San Joaquin Delta. There are increasingly volatile water issues between the far more water rich Nothern and Eastern part of the state and the arid southern part of the state due to a multi year drought. Currently, the San Joaquin Delta is experiencing environmental degradation and is vulnerable to incursion of far more saline sea water from the San Francisco Bay that will upset the delicate ecological balance of the brackish and wetlands region.
The bottom line is that, throughout the world, increasing human urban and rural populations, increasing industrialization and urbanization and increasing needs for fresh water combine with increasing threats from pollution to create complicated sets of competing needs for the preservation of brackish environments and the production of more safe fresh water.