The USGS Water Science School
Care to guess what percentage of Earth's water is saline?
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:
Mono Lake in California, showing salt deposits left onshore as the water level declines to serve the water needs of Los Angeles.
Credit: C.D. Miller, USGS.
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.
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.
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're one of the lucky ones, as these students from Idaho were, to have experienced floating in the Dead Sea in the Middle East, you've experienced the unique sensation of floating in the exteremly 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.
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.
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 15 percent of all water used in the United States in 2005 was saline. The second chart shows that almost all saline withdrawals, over 95 percent, was used by the thermoelectric-power industry to cool electricity-generating equipment. About five percent of the Nation's saline water was used for mining and industrial purposes.
Note: For 2000, mining data was compiled only for the 22 States that reported significant withdrawals in 1995.