Salinity is the total amount of dissolved salts in water. Ocean water has a salinity of about 35,000 parts per million (ppm), meaning roughly 35 grams of salt are dissolved in every liter. Fresh drinking water, by comparison, contains less than 1,000 ppm. This single measurement affects everything from whether water is safe to drink to whether fish can survive in it to whether it can irrigate crops.
How Salinity Is Classified
The U.S. Geological Survey breaks water into categories based on dissolved salt concentration:
- Fresh water: less than 1,000 ppm
- Slightly saline: 1,000 to 3,000 ppm
- Moderately saline: 3,000 to 10,000 ppm
- Highly saline: 10,000 to 35,000 ppm
Ocean water sits at the top of that range, around 35,000 ppm. Bodies of water that mix freshwater and seawater, like estuaries and coastal lagoons, fall somewhere in between and are called brackish. Water saltier than the ocean (found in salt lakes, brine pools, and some underground formations) can exceed 50,000 ppm or more.
What “Salt” Actually Means Here
When scientists talk about salinity, they don’t just mean table salt. Salinity includes every dissolved mineral ion in the water. That said, two ions dominate: chloride and sodium. Together, these two make up over 90% of all dissolved ions in seawater. The remaining fraction includes smaller amounts of sulfate, magnesium, calcium, and potassium.
Rivers and rain constantly dissolve minerals from rocks and soil, carrying them into lakes and eventually the ocean. The ocean has accumulated these salts over billions of years. Because water evaporates but salts stay behind, the ocean’s salinity remains relatively stable. Enclosed bodies of water with no outlet, like the Dead Sea or Utah’s Great Salt Lake, become even saltier over time for the same reason.
How Salinity Is Measured
There are several practical ways to test salinity, each suited to different situations.
A conductivity meter is the most widely used method. It works by passing an electrical current between two metal plates submerged in the water. Since dissolved salts carry electrical charge, saltier water conducts more electricity. The reading comes in milliSiemens per centimeter (mS/cm), which can be converted to parts per thousand or ppm. This is the standard tool in oceanography and water treatment.
A refractometer measures how much a water sample bends light. More dissolved salt means light refracts at a different angle. You place a drop of water on a glass plate, look through the eyepiece, and read the salinity off an internal scale. Refractometers are popular in aquariums and aquaculture because they’re cheap, portable, and require no batteries.
A hydrometer floats in the water and measures its density, since saltier water is heavier. These work well for aquariums but need temperature correction to be accurate. Most are calibrated to either 60°F or 77°F, and readings taken at other temperatures need adjustment.
In modern oceanography, salinity is formally defined by the Practical Salinity Scale of 1978, which calculates salinity from precise conductivity measurements. The resulting unit, the practical salinity unit (PSU), replaced the older parts-per-thousand system and gives more consistent results across different conditions.
Why Salinity Matters for Aquatic Life
Every aquatic organism has to manage the balance between salt and water inside its cells versus the water around it. This process, called osmoregulation, takes real energy, and salinity determines how hard an organism has to work to stay alive.
Freshwater fish face a constant influx of water through their skin and gills because their body fluids are saltier than the surrounding water. To compensate, they actively absorb salt from the environment and excrete large volumes of dilute urine. Marine fish have the opposite problem: the ocean is saltier than their bodies, so they risk dehydration. They conserve water and push excess salt out through specialized cells in their gills and kidneys.
Some species, called euryhaline organisms, can tolerate wide swings in salinity. Salmon are a classic example, moving between freshwater rivers and the open ocean. These animals use proteins that act as molecular water channels and ion pumps to rapidly adjust their internal chemistry. Crustaceans like crabs take a different approach, adjusting cell volume directly: shrinking cells in low-salinity water to prevent swelling and expanding them in high-salinity water.
Sharks and rays use yet another strategy. They maintain high concentrations of an organic compound in their blood that keeps their internal saltiness close to that of seawater, reducing the need for active pumping. Hagfish take the simplest approach of all: they let their body fluid match the ocean’s salt concentration, essentially going with the flow rather than fighting it.
When salinity changes abruptly, as it can during heavy rains or droughts, organisms that can’t adjust fast enough face cellular damage or death. This is why salinity is one of the first things ecologists measure when assessing the health of an estuary or coastal habitat.
Salinity in Drinking Water
The EPA sets a secondary standard of 500 mg/L (500 ppm) for total dissolved solids in drinking water. This isn’t a legally enforceable limit but a guideline based on taste and aesthetics. Water above this threshold starts to taste salty, bitter, or metallic, and it can leave scale buildup on pipes and appliances. At much higher concentrations, salty water causes dehydration rather than relieving it, because your kidneys need more water to flush out the excess salt than the water itself provides.
Effects on Agriculture and Soil
Salinity in irrigation water is one of the biggest threats to crop production worldwide. Different crops have very different tolerances. Peppers are highly sensitive: irrigation water at around 2,900 mg/L reduced pepper yields by over 50% when applied by sprinkler, though the damage dropped to 16% when the same water was applied directly to the soil surface. The difference matters because sprinklers deposit salt on leaves, where it causes direct tissue damage on top of the root-zone stress.
Some crops handle salt better at certain growth stages. Sweet corn seedlings grow more slowly in salty conditions, but once plants reach the tasseling and grain-filling stages, irrigation water can be as salty as 5,800 mg/L without reducing yield. The type of salt also matters. In grain sorghum trials, sodium sulfate cut shoot growth to 43% of normal, while sodium chloride at the same concentration only reduced it to 70%. Farmers dealing with saline water need to know not just how much salt is present but which salts dominate.
Over time, repeated irrigation with even moderately salty water causes salt to accumulate in soil, eventually making land unproductive. This process, called salinization, has degraded farmland across arid regions from California’s Central Valley to the Punjab in South Asia.
Climate Change and Rising Salinity
Salt intrusion into coastal freshwater supplies is getting worse. A 2025 study published in Nature Communications analyzed 18 estuaries worldwide under a high-emission climate scenario and found that 89% are projected to see increased salt intrusion by the end of this century. The median increase was 9.1%, with some estuaries facing up to 18.2% more saltwater pushing inland.
Two forces drive this trend. Sea-level rise pushes saltwater farther upstream, and reduced river flows during more frequent droughts mean less freshwater to push it back. Of the two, sea-level rise contributes roughly twice as much as declining river discharge. The study also found that extreme salt intrusion events, the kind that currently happen once in a century, are projected to become 10% more intense on average across most estuaries studied. For coastal communities that depend on estuaries for drinking water or irrigation, this shift means saltwater will increasingly threaten supplies that have historically been fresh.