Saline Water and Salinity

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In your everyday life you are not involved much with saline water. You are concerned with freshwater to serve your life's every need. But, most of Earth's water, and almost all of the water that people can access, is saline, or salty water. Just look at the oceans and remember that oceans comprise about 97% of all water on, in, and above the Earth.

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View of Montevideo, Uruguay area of South America

Why is the ocean salty? Rivers discharge mineral-rich water to the oceans is from outflow from rivers, which drain the landscape, thus causing the oceans to be salty. (Credit: NASA)

What is saline water?

First, what do we mean by "saline water?" Water that is saline contains significant amounts (referred to as "concentrations") of dissolved salts, the most common being the salt we all know so well—sodium chloride (NaCl). In this case, the concentration is the amount (by weight) of salt in water, as expressed in "parts per million" (ppm). If water has a concentration of 10,000 ppm of dissolved salts, then one percent (10,000 divided by 1,000,000) of the weight of the water comes from dissolved salts.

Here are our parameters for saline water:

  • Fresh water - Less than 1,000 ppm
  • Slightly saline water - From 1,000 ppm to 3,000 ppm
  • Moderately saline water - From 3,000 ppm to 10,000 ppm
  • Highly saline water - From 10,000 ppm to 35,000 ppm
  • By the way, ocean water contains about 35,000 ppm of salt.

 

Saline water is not just in the oceans

Naturally, when you think of saline water you think of the oceans. But, hundreds of miles from the Pacific Ocean, the residents of states such as Colorado and Arizona can "enjoy a day at the beach" by just walking outside their house, for they may be right next to saline water. There is an extensive amount of very salty water in the ground in the western United States. In New Mexico, approximately 75 percent of groundwater is too saline for most uses without treatment (Reynolds, 1962). Water in this area may have been leftover from ancient times when saline seas occupied the western U.S., and, also, as rainfall infiltrates downward into the ground, it can encounter rocks that contain highly soluble minerals, which turn the water saline. Groundwater can exist and move for thousands of years and can thus become as saline as ocean water.

Aerial view from above Mono Lake towards the west-southwest.

The declining water level of the lake is clearly seen by the parallel lines and white-colored lake deposits ringing the shore. The diversion of fresh-water inflow to the city of Los Angeles and evaporation has led to the decline in water level at a rate of about 1 m per year. The snow-covered mountains in the background are the Sierra Nevada. (Credit: C.D. Miller, USGS)

Mono Lake in California is the saline remnant of a much larger lake (Lake Russel) that filled the Mono basin millions of years ago. The ancient fresh-water lake was once about 130 meters higher than the current water level. Mono Lake is now a highly-saline remnant of Lake Russel, having much of its fresh water drained off to serve the water needs of the city of Los Angeles. Water levels are currently falling about 1 meter per year. This has resulted in salty deposits left onshore as the water recedes.

 

Can saline water be used for anything?

So, with all of the water available on Earth and all that saline water sitting offshore of our coasts, how come we are worried about water shortages? You can think of it as a water-quality situation rather than water-quantity situation. In its raw state, saline water just cannot be used for many of the purposes we need water for, such as drinking, irrigation, and many industrial uses. Slightly saline water is sometimes used for similar purposes as freshwater. For example, in Colorado, water having up to 2,500 ppm of salt is used for irrigating crops. Normally, though, moderate to high saline water has limited uses. After all, you don't drink salt water at home; you don't use it to water your tomatoes or brush your teeth; farmers don't usually irrigate with it; some industries can't use it without damaging their equipment; and, farmer Joe's cows won't drink it.

If nothing else, saline water can be just plain fun. If you happen to be one who has been to the Dead Sea in the Middle East, you could have experienced the unique sensation of floating in the extremely dense (and salty) water that apparently holds you up like a mattress. The water is so dense that you truly do not sink, as you do in normal, even ocean, water. Closer to home, many homeowners who have backyard pools fill them with saline water, rather than have to use freshwater and added chlorine. 

So, what else can saline water be used for, and can it be made more usable?

There are two answers—both "yes." Saline water is useful for some water-use purposes, and saline water can be turned into freshwater, for which we have many uses.

Fresh and saline water withdrawals and saline water withdrawals by category of use, 2015

Saline water withdrawals in the United States, by category of use, for 2015.

Saline water use in the United States in 2015

In today's world we are all more aware of the need to conserve freshwater. With the ever-growing demand for water by growing populations worldwide, it makes sense to try to find more uses for the abundant saline water supplies that exist, mainly in the oceans. As these pie charts of the Nation's water use show, about 16 percent of all water used in the United States in 2015 was saline. The second chart shows that almost all saline withdrawals, over 97 percent, was used by the thermoelectric-power industry to cool electricity-generating equipment. About three percent of the Nation's saline water was used for mining and industrial purposes.