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Saltwater, or salt water, is a geological term that refers to naturally occurring solutions containing large concentrations of dissolved, inorganic ions. In addition, this term is often used as an adjective in biology, usually to refer to marine organisms, as in saltwater fish.

Saltwater most commonly refers to oceanic waters, in which the total concentration of ionic solutes is typically about 35 grams per liter (also expressed as 3.5%, or 35 parts per thousand). As a result of these large concentrations of dissolved ions, the density of saltwater (1.028 g/L at 4o C) is slightly greater than that of freshwater (1.00 g/L). Therefore, freshwater floats above saltwater in poorly mixed situations where the two types meet, as in estuaries and some underground reservoirs.

The ions with the largest concentrations in marine waters are sodium, chloride, sulfate, magnesium, calcium, potassium, and carbonate. In oceanic waters, sodium and chloride are the most important ions, having concentrations of 10.8 g/L and 19.4 g/L, respectively. Other important ions are sulfate (2.7 g/L), magnesium (1.3 g/L), and calcium and potassium (both 0.4 g/L). However, in inland saline waters, the concentrations and relative proportions of these and other ions can vary widely.

Other natural waters can also be salty, sometimes containing much larger concentrations of salt than the oceans. Some lakes and ponds, known as salt or brine surface waters, can have very large concentrations of dissolved, ionic solutes. These water bodies typically occur in a closed basin, with inflows of water but no outflow except by evaporation, which leaves salts behind. Consequently, the salt concentration of their contained water increases progressively over time. For example, the Great Salt Lake of Utah and the Dead Sea in Israel have salt concentrations exceeding 20%, as do smaller, saline ponds in Westphalia, Germany, and elsewhere in the world.

Underground waters can also be extremely salty. Underground saltwaters are commonly encountered in petroleum and gas well-fields, especially after the hydrocarbon resource has been exhausted by mining.

Both surface and underground saltwaters are sometimes "mined" for their contents of economically useful minerals.

Saltwater intrusions can be an important environmental problem, which can degrade water supplies required for drinking or irrigation. Saltwater intrusions are caused in places near the ocean where there are excessive withdrawals of underground supplies of fresh waters. This allows underground saltwaters to migrate inland, and spoil the quality of the aquifer for most uses. Saltwater intrusions are usually caused by excessive usage of ground water for irrigation in agriculture, or to supply drinking water to large cities.

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