Chemistry parameter
Salinity
The dissolved salt content of water. Average seawater is about 35 grams of salt per kilogram, and the reason the sea is salty is that rivers deliver dissolved minerals continuously while evaporation removes only the water.
Key facts
- Unit
- PSU
- Metric
- Salinity
What it measures
Total dissolved salt, in practice determined from conductivity, temperature and pressure on the Practical Salinity Scale.
How it is measured
By CTD instrument in oceanography, by conductivity meter elsewhere.
How to read a value
| Value (PSU) | What it indicates |
|---|---|
| Below 0.5 | Fresh water. |
| 0.5 – 30 | Brackish. Estuaries and some coastal aquifers. |
| 30 – 40 | Marine. Open ocean averages about 35. |
| Above 40 | Hypersaline. The Red Sea approaches 40; the Dead Sea exceeds 300. |
Bands are conventional interpretation aids, not regulatory limits. Where a legal standard exists it is stated on the relevant standards page.
What causes it
- Chemical weathering of rock delivering dissolved ions to the sea over geological time
- Hydrothermal exchange at mid-ocean ridges
- Evaporation concentrating salt in enclosed basins
- Sea ice formation, which excludes salt and increases the salinity of the water beneath
Why it matters
- Salinity and temperature together drive the density differences that power the global ocean circulation
- Over-abstraction from coastal aquifers draws seawater inland, permanently degrading the aquifer in a process that is far easier to cause than to reverse
- Desalination energy demand rises steeply with feedwater salinity, which is why brackish desalination costs a fraction of seawater desalination
Related intelligence
Sources
- World Health Organization — Guidelines for Drinking-water Quality. Public but restricted · CC BY-NC-SA 3.0 IGO
- National Institute of Standards and Technology — NIST Chemistry WebBook — thermophysical properties of water. Open · US Government work — public domain