Brackish Water: Definition, Salinity & Density

Instructor: Yuanxin (Amy) Yang Alcocer

Amy has a master's degree in secondary education and has taught math at a public charter high school.

You might think only fresh or salt water is on our planet. Did you know there is a third kind called brackish water? In this lesson, we will learn its characteristics and discover what kind of wildlife inhabits it!

What is Brackish Water?

Go to a sea or ocean and what kind of water do you find? Salt water. Head to a lake or pond and what will it be? Fresh water. What do you think happens when you visit an estuary connecting a fresh water stream with seawater from the ocean? You now have the seawater mixing with your fresh water and the result is called brackish water! The specific definition of brackish water is, ''water that is saltier than fresh water, but not as salty as seawater.''

Brackish water can be found in estuaries and watersheds
brackish water

Salinity of Brackish Water

Do you know how to tell if water is brackish water instead of seawater? There is no universal scale stating how much salt brackish water has in it, but generally, there is an accepted salinity range; sort of like measuring how much sugar to put into water for hummingbird food. Salinity tells you how much salt is dissolved in a given liquid and is usually measured in parts per thousand (ppt) or parts per million (ppm). 1 ppt means there is one ounce of salt for every 1000 ounces of water. Sugar water, where you have 20 percent sugar and 80 percent water equates to 200 ppt of dissolved sugar. This range was mentioned by Daniel Hillel when he wrote for Salinity Management for Sustainable Irrigation: Integrating Science, Environment, and Economics. According to this scale, brackish water has between 0.5 and 2 parts per thousand (ppt) of total dissolved salts, fresh water has less than 0.5 ppt of dissolved salts and seawater has over 2 ppt of dissolved salts.

Water Type Salinity
Fresh water <0.5 ppt
Brackish water 0.5 to 2 ppt
Salt water >2 ppt

Density of Brackish Water

In addition to the salinity, another measurement used to differentiate between brackish, salt and fresh water is its density. Density tells you how thick your substance is by dividing its mass by its volume.

  • Density = Mass / Volume

When you take the density of something and divide by the density of water, then you have what is called the specific gravity of that liquid.

  • Specific gravity = Density / Density of water

An interesting point here, is that both your specific gravity and your density are affected by temperature. The warmer the temperature, the less dense your substance is with a corresponding lower specific gravity.

For water with a temperature of 77 degrees Fahrenheit, brackish water has a density between 997.453 kg/m3 and 998.584 kg/m3. Fresh water with a salinity of 0 at 77 degrees has a density of 997.075 kg/m3. Dividing the density of the brackish water by the density of fresh water, you get a specific gravity range for brackish water between 997.453 / 997.075 = 1.0004 and 998.584 / 997.075 = 1.0015.

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