Difference Between Salt, Brackish and Fresh Water

Difference Between Salt, Brackish and Fresh Water

Does It Matter Which Anode Metal You Use?

Yes. Different water types have different salinity levels, so the metal type determines which anode type will work correctly to protect your underwater parts.

Freshwater contains less than 0.05% salt, or less than 1% salt by some definitions. Brackish water contains less than 3% salt. And saltwater contains more than 3% salt.

Water is neutral, however it follows that the more salt you dissolve in water, the more basic it becomes (i.e pH greater than 7). Saltwater is therefore the most basic, followed by brackish water, and freshwater is the closest to neutral pH 7.

Seawater is very similar from place to place, where as freshwater varies a lot depending on the type of soil and rock the water is in contact with. Seawater can contain hundreds of trace elements including metals like lead and mercury. Seawater also contains large quantities of chlorine (twice as much even than salt), with somewhat smaller quantities of magnesium, sulphur, calcium, and potassium. Freshwater contains similar elements, but less of them making fresh water purer. Brackish is obviously where salt and fresh water meet and mix such as in estuaries and lakes.

The dissolved oxygen in water type also varies. Oxygen dissolves in freshwater more easily than brackish or salt water. Freshwater also gets the opportunity to dissolve oxygen more often, because of the nature of where you find it such as in small rivers where it flows faster. However temperature is probably the biggest factor when it comes to dissolved oxygen, not the type of water.

The salts that are found in salt water, as well as brackish water, along with other elements break down and become electrically charged ions. These particles are better electricity conductors. This means that electricity flows through salt water more readily and efficiently than it does through fresh water which is why choosing the right metal type for your water type is critical as they react differently depending on the salinity level.

We often get asked ‘what water type is the Thames river?’ It depends on where you are in relation to the tidal, salty water coming in from the mouth of the river, and your position upstream. As a rough guide, water between Teddington and the ocean is classed as brackish, anything north of Teddington is fresh. Obviously this may change with rainfall etc. In some marina’s it may well be fresh water – you’d need to check with other people in the marina to be sure.


  • Fresh Water – Magnesium
  • Brackish Water – Aluminium
  • Salt Water – Zinc or Aluminium


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