Salinity is the total concentration of dissolved salts in water, most commonly sodium chloride. It is a key parameter for classifying water bodies and determining suitability for drinking, irrigation, and aquatic life.
Salinity represents the total mass of dissolved salts per unit mass or volume of water, typically expressed in parts per thousand (ppt), practical salinity units (PSU), or milligrams per liter. Freshwater is generally defined as having salinity below 0.5 ppt, brackish water ranges from 0.5 to 30 ppt, and seawater averages about 35 ppt. Salinity in surface and groundwater is influenced by evaporation, precipitation, rock dissolution, tidal mixing, and anthropogenic inputs such as road salt, irrigation return flows, and brine disposal from oil and gas operations. Elevated salinity affects water's suitability for irrigation by potentially causing soil salinization, which reduces crop yields and degrades soil structure. Aquatic organisms have species-specific salinity tolerances, and increases in salinity can alter community composition and ecosystem function in freshwater habitats. Salinity is readily estimated in the field from conductivity measurements, as dissolved salts are the primary contributors to electrical conductivity. Climate change and sea level rise are expected to increase saltwater intrusion into coastal aquifers and estuaries, threatening freshwater supplies for millions of people.