The total amount of water on Earth exists in a continuous cycle, but its dissolved mineral content varies dramatically. This variation determines whether a body of water is considered fresh or saline, with the primary dissolved mineral being sodium chloride (table salt). The distinction between these water types is based on the concentration of dissolved solids, a measurement that has profound effects on both the environment and human use.
Defining Salinity: The Three Main Water Categories
Salinity is the measure of the total concentration of dissolved salts in water, typically expressed in parts per thousand (ppt) or Practical Salinity Units (PSU). This measurement represents the mass of dissolved solids found in one kilogram of water. The standard classification system uses specific concentration ranges to categorize natural waters.
Water Categories
Freshwater is defined as having a salinity of less than 0.5 ppt, including most rivers, lakes, and glaciers. Brackish water contains a transitional concentration, ranging from 0.5 ppt up to about 30 ppt. Saline water, or saltwater, generally begins at 30 ppt and can extend higher, encompassing the vast majority of the world’s oceans.
The Primary Source of Saline Water
The oceans constitute the largest reservoir of saline water, holding an average salinity of approximately 35 ppt. This concentration results from geological processes that have accumulated dissolved minerals over billions of years.
Sources of Ocean Salt
One main source is the chemical weathering of continental rocks, where slightly acidic rainwater erodes minerals like sodium and chloride. These dissolved ions are transported by rivers into the ocean basins. Another significant source is hydrothermal activity at seafloor fissures, such as mid-ocean ridges. Here, seawater seeps into the crust, dissolves minerals from the surrounding rock, and is vented back into the ocean.
The concentration remains high because evaporation removes pure water molecules from the surface but leaves the salt compounds behind. While new salts are continuously introduced, the ocean’s overall salinity remains stable because other processes, like the formation of sea spray and the reaction of dissolved salts with ocean-floor rock, remove them. About 85% of all dissolved solids in the ocean are sodium and chloride ions.
Freshwater Sources and Why They Lack Salt
Freshwater sources like ice caps, glaciers, lakes, and rivers maintain a very low salinity, typically under 0.5 ppt. The mechanism that keeps these waters fresh is the hydrologic cycle, which acts as a continuous purification system.
When water evaporates from the Earth’s surface, it leaves behind all dissolved salts and minerals. The pure water vapor rises, condenses to form clouds, and returns to the surface as precipitation, such as rain or snow. While rivers pick up some dissolved minerals as they flow across the land, the continuous transport of this water to the ocean prevents the salt from concentrating. The constant movement of river water ensures that dissolved salts are quickly flushed out to the sea. Freshwater lakes are similarly maintained by a balance of inflow and outflow through rivers, which prevents significant salt accumulation.
Special Cases of Saline Water
Saline water is not exclusive to the open ocean; it also occurs in special environments where geological or hydrological conditions prevent the natural flushing of salt.
Estuaries and Salt Lakes
One common transitional zone is the estuary, a brackish environment where river freshwater meets and mixes with ocean saltwater. Salinity here fluctuates widely, ranging from 0.5 ppt near the river mouth to over 30 ppt near the open sea.
Another example involves endorheic, or closed-basin, salt lakes, such as the Dead Sea or the Great Salt Lake. These lakes lack an outflowing river, meaning water leaves the system only through evaporation. This process progressively concentrates dissolved minerals, leading to extremely high salinity levels, sometimes exceeding 300 ppt.
Saline Groundwater
Saline water can also be found underground as saline groundwater, which occurs through two main mechanisms. In coastal regions, denser seawater can naturally intrude into freshwater aquifers, a process known as saltwater intrusion. Deep groundwater can also become saline by dissolving ancient mineral deposits or by mixing with residual seawater trapped in rock formations from prehistoric periods.