Salinity

Salinity (/səˈlɪnəti/) is the saltiness or amount of salt dissolved in a body of water, called saline water (see also soil salinity). This is usually measured in (note that this is technically dimensionless). Salinity is an important factor in determining many aspects of the chemistry of natural waters and of biological processes within it, and is a thermodynamic state variable that, along with temperature and pressure, governs physical characteristics like the density and heat capacity of the water.

A contour line of constant salinity is called an isohaline, or sometimes isohale.

WOA09 sea-surf SAL AYool
Annual mean sea surface salinity for the World Ocean. Data from the World Ocean Atlas 2009.[1]
IAPSO Standard Seawater
International Association for the Physical Sciences of the Oceans (IAPSO) standard seawater.

Definitions

Salinity in rivers, lakes, and the ocean is conceptually simple, but technically challenging to define and measure precisely. Conceptually the salinity is the quantity of dissolved salt content of the water. Salts are compounds like sodium chloride, magnesium sulfate, potassium nitrate, and sodium bicarbonate which dissolve into ions. The concentration of dissolved chloride ions is sometimes referred to as chlorinity. Operationally, dissolved matter is defined as that which can pass through a very fine filter (historically a filter with a pore size of 0.45 μm, but nowadays usually 0.2 μm).[2] Salinity can be expressed in the form of a mass fraction, i.e. the mass of the dissolved material in a unit mass of solution.

Seawater typically has a mass salinity of around 35 g/kg, although lower values are typical near coasts where rivers enter the ocean. Rivers and lakes can have a wide range of salinities, from less than 0.01 g/kg[3] to a few g/kg, although there are many places where higher salinities are found. The Dead Sea has a salinity of more than 200 g/kg.[4] Rainwater before touching the ground typically has a TDS of 20 mg/L or less.[5]

Whatever pore size is used in the definition, the resulting salinity value of a given sample of natural water will not vary by more than a few percent (%). Physical oceanographers working in the abyssal ocean, however, are often concerned with precision and intercomparability of measurements by different researchers, at different times, to almost five significant digits.[6] A bottled seawater product known as IAPSO Standard Seawater is used by oceanographers to standardize their measurements with enough precision to meet this requirement.

Composition

Measurement and definition difficulties arise because natural waters contain a complex mixture of many different elements from different sources (not all from dissolved salts) in different molecular forms. The chemical properties of some of these forms depend on temperature and pressure. Many of these forms are difficult to measure with high accuracy, and in any case complete chemical analysis is not practical when analyzing multiple samples. Different practical definitions of salinity result from different attempts to account for these problems, to different levels of precision, while still remaining reasonably easy to use.

For practical reasons salinity is usually related to the sum of masses of a subset of these dissolved chemical constituents (so-called solution salinity), rather than to the unknown mass of salts that gave rise to this composition (an exception is when artificial seawater is created). For many purposes this sum can be limited to a set of eight major ions in natural waters,[7][8] although for seawater at highest precision an additional seven minor ions are also included.[6] The major ions dominate the inorganic composition of most (but by no means all) natural waters. Exceptions include some pit lakes and waters from some hydrothermal springs.

The concentrations of dissolved gases like oxygen and nitrogen are not usually included in descriptions of salinity.[2] However, carbon dioxide gas, which when dissolved is partially converted into carbonates and bicarbonates, is often included. Silicon in the form of silicic acid, which usually appears as a neutral molecule in the pH range of most natural waters, may also be included for some purposes (e.g., when salinity/density relationships are being investigated).

Seawater

Full 3 minute NASA video Feb 27,2013 The NASA Aquarius instrument aboard Argentina's SAC-D satellite is designed to measure global sea surface salinity. This movie shows salinity patterns as measured by Aquarius from December 2011 through December 2012. Red colors represent areas of high salinity, while blue shades represent areas of low salinity.

The term 'salinity' is, for oceanographers, usually associated with one of a set of specific measurement techniques. As the dominant techniques evolve, so do different descriptions of salinity. Salinities were largely measured using titration-based techniques before the 1980s. Titration with silver nitrate could be used to determine the concentration of halide ions (mainly chlorine and bromine) to give a chlorinity. The chlorinity was then multiplied by a factor to account for all other constituents. The resulting 'Knudsen salinities' are expressed in units of parts per thousand (ppt or ).

The use of electrical conductivity measurements to estimate the ionic content of seawater led to the development of the scale called the practical salinity scale 1978 (PSS-78).[9][10] Salinities measured using PSS-78 do not have units. The suffix psu or PSU (denoting practical salinity unit) is sometimes added to PSS-78 measurement values.[11]

In 2010 a new standard for the properties of seawater called the thermodynamic equation of seawater 2010 (TEOS-10) was introduced, advocating absolute salinity as a replacement for practical salinity, and conservative temperature as a replacement for potential temperature.[6] This standard includes a new scale called the reference composition salinity scale. Absolute salinities on this scale are expressed as a mass fraction, in grams per kilogram of solution. Salinities on this scale are determined by combining electrical conductivity measurements with other information that can account for regional changes in the composition of seawater. They can also be determined by making direct density measurements.

A sample of seawater from most locations with a chlorinity of 19.37 ppt will have a Knudsen salinity of 35.00 ppt, a PSS-78 practical salinity of about 35.0, and a TEOS-10 absolute salinity of about 35.2 g/kg. The electrical conductivity of this water at a temperature of 15 °C is 42.9 mS/cm.[6][12]

Lakes and rivers

Limnologists and chemists often define salinity in terms of mass of salt per unit volume, expressed in units of mg per litre or g per litre.[7] It is implied, although often not stated, that this value applies accurately only at some reference temperature. Values presented in this way are typically accurate to the order of 1%. Limnologists also use electrical conductivity, or "reference conductivity", as a proxy for salinity. This measurement may be corrected for temperature effects, and is usually expressed in units of μS/cm.

A river or lake water with a salinity of around 70 mg/L will typically have a specific conductivity at 25 °C of between 80 and 130 μS/cm. The actual ratio depends on the ions present.[13] The actual conductivity usually changes by about 2% per degree Celsius, so the measured conductivity at 5 °C might only be in the range of 50–80 μS/cm.

Direct density measurements are also used to estimate salinities, particularly in highly saline lakes.[4] Sometimes density at a specific temperature is used as a proxy for salinity. At other times an empirical salinity/density relationship developed for a particular body of water is used to estimate the salinity of samples from a measured density.

Water salinity
Fresh water Brackish water Saline water Brine
< 0.05% 0.05 – 3% 3 – 5% > 5%
< 0.5 ‰ 0.5 – 30 ‰ 30 – 50 ‰ > 50 ‰

Classification of water bodies based upon salinity

Thalassic series
>300
hyperhaline
60–80
metahaline
40
mixoeuhaline
30
polyhaline
18
mesohaline
5
oligohaline
0.5

Marine waters are those of the ocean, another term for which is euhaline seas. The salinity of euhaline seas is 30 to 35. Brackish seas or waters have salinity in the range of 0.5 to 29 and metahaline seas from 36 to 40. These waters are all regarded as thalassic because their salinity is derived from the ocean and defined as homoiohaline if salinity does not vary much over time (essentially constant). The table on the right, modified from Por (1972),[14] follows the "Venice system" (1959).[15]

In contrast to homoiohaline environments are certain poikilohaline environments (which may also be thalassic) in which the salinity variation is biologically significant.[16] Poikilohaline water salinities may range anywhere from 0.5 to greater than 300. The important characteristic is that these waters tend to vary in salinity over some biologically meaningful range seasonally or on some other roughly comparable time scale. Put simply, these are bodies of water with quite variable salinity.

Highly saline water, from which salts crystallize (or are about to), is referred to as brine.

Environmental considerations

Salinity is an ecological factor of considerable importance, influencing the types of organisms that live in a body of water. As well, salinity influences the kinds of plants that will grow either in a water body, or on land fed by a water (or by a groundwater).[17] A plant adapted to saline conditions is called a halophyte. A halophyte which is tolerant to residual sodium carbonate salinity are called glasswort or saltwort or barilla plants. Organisms (mostly bacteria) that can live in very salty conditions are classified as extremophiles, or halophiles specifically. An organism that can withstand a wide range of salinities is euryhaline.

Salt is expensive to remove from water, and salt content is an important factor in water use (such as potability). Increases in salinity have been observed in lakes and rivers in the United States, due to common road salt and other salt de-icers in runoff.[18]

The degree of salinity in oceans is a driver of the world's ocean circulation, where density changes due to both salinity changes and temperature changes at the surface of the ocean produce changes in buoyancy, which cause the sinking and rising of water masses. Changes in the salinity of the oceans are thought to contribute to global changes in carbon dioxide as more saline waters are less soluble to carbon dioxide. In addition, during glacial periods, the hydrography is such that a possible cause of reduced circulation is the production of stratified oceans. In such cases, it is more difficult to subduct water through the thermohaline circulation.

See also

References

  1. ^ World Ocean Atlas 2009. nodc.noaa.gov
  2. ^ a b Pawlowicz, R. (2013). "Key Physical Variables in the Ocean: Temperature, Salinity, and Density". Nature Education Knowledge. 4 (4): 13.
  3. ^ Eilers, J. M.; Sullivan, T. J.; Hurley, K. C. (1990). "The most dilute lake in the world?". Hydrobiologia. 199: 1–6. doi:10.1007/BF00007827.
  4. ^ a b Anati, D. A. (1999). "The salinity of hypersaline brines: concepts and misconceptions". Int. J. Salt Lake. Res. 8: 55–70. doi:10.1007/bf02442137.
  5. ^ "Learn about salinity and water quality". Retrieved 21 July 2018.
  6. ^ a b c d IOC, SCOR, and IAPSO (2010). The international thermodynamic equation of seawater – 2010: Calculation and use of thermodynamic properties. Intergovernmental Oceanographic Commission, UNESCO (English). pp. 196pp.CS1 maint: Multiple names: authors list (link)
  7. ^ a b Wetzel, R. G. (2001). Limnology: Lake and River Ecosystems, 3rd ed. Academic Press. ISBN 978-0-12-744760-5.
  8. ^ Pawlowicz, R.; Feistel, R. (2012). "Limnological applications of the Thermodynamic Equation of Seawater 2010 (TEOS-10)". Limnology and Oceanography: Methods. 10 (11): 853–867. doi:10.4319/lom.2012.10.853.
  9. ^ Unesco (1981). The Practical Salinity Scale 1978 and the International Equation of State of Seawater 1980. Tech. Pap. Mar. Sci., 36
  10. ^ Unesco (1981). Background papers and supporting data on the Practical Salinity Scale 1978. Tech. Pap. Mar. Sci., 37
  11. ^ Millero, F. J. (1993). "What is PSU?". Oceanography. 6 (3): 67.
  12. ^ Culkin, F.; Smith, N. D. (1980). "Determination of the Concentration of Potassium Chloride Solution Having the Same Electrical Conductivity, at 15C and Infinite Frequency, as Standard Seawater of Salinity 35.0000‰ (Chlorinity 19.37394‰)". IEEE J. Oceanic Eng. OE-5 (1): 22–23. doi:10.1109/JOE.1980.1145443.
  13. ^ van Niekerk, Harold; Silberbauer, Michael; Maluleke, Mmaphefo (2014). "Geographical differences in the relationship between total dissolved solids and electrical conductivity in South African rivers". Water SA. 40 (1): 133. doi:10.4314/wsa.v40i1.16.
  14. ^ Por, F. D. (1972). "Hydrobiological notes on the high-salinity waters of the Sinai Peninsula". Marine Biology. 14 (2): 111–119. doi:10.1007/BF00373210.
  15. ^ Venice system (1959). The final resolution of the symposium on the classification of brackish waters. Archo Oceanogr. Limnol., 11 (suppl): 243–248.
  16. ^ Dahl, E. (1956). "Ecological salinity boundaries in poikilohaline waters". Oikos. 7 (1): 1–21. doi:10.2307/3564981. JSTOR 3564981.
  17. ^ Kalcic, Maria, Turowski, Mark; Hall, Callie (2010-12-22). "Stennis Space Center Salinity Drifter Project. A Collaborative Project with Hancock High School, Kiln, MS". Stennis Space Center Salinity Drifter Project. NTRS. Retrieved 2011-06-16.
  18. ^ "Hopes To Hold The Salt, And Instead Break Out Beet Juice And Beer To Keep Roads Clear". www.wbur.org.

Further reading

Aquarius (SAC-D instrument)

Aquarius was a NASA instrument aboard the Argentine SAC-D spacecraft. Its mission was to measure global sea surface salinity to better predict future climate conditions.Aquarius was shipped to Argentina on June 1, 2009 to be mounted in the INVAP built SAC-D satellite. It came back to Vandenberg Air Force Base on March 31, 2011.For the joint mission, Argentina provided the SAC-D spacecraft and additional science instruments, while NASA provided the Aquarius salinity sensor and the rocket launch platform. The National Aeronautics and Space Administration (NASA)'s Jet Propulsion Laboratory in Pasadena, California, managed the Aquarius Mission development for NASA's Earth Science Enterprise based in Washington, D.C., and NASA's Goddard Spaceflight Center in Greenbelt, Maryland, is managing the mission after launch.The observatory was successfully launched from Vandenberg Air Force Base on June 10, 2011. After its launch aboard a Delta II from Vandenberg Air Force Base in California, SAC-D was carried into a 657 km (408 mi) sun-synchronous orbit to begin its 3-year mission.On June 7, 2015, the SAC-D satellite carrying Aquarius suffered a power supply failure, ending the mission.

Brackish marsh

Brackish marshes develop by salt marshes where a significant freshwater influx dilutes the seawater to brackish levels of salinity. This commonly happens upstream from salt marshes by estuaries of coastal rivers or near the mouths of coastal rivers with heavy freshwater discharges in the conditions of low tidal ranges.

Brackish water

Brackish water is water having more salinity than freshwater, but not as much as seawater. It may result from mixing seawater with fresh water together, as in estuaries, or it may occur in brackish fossil aquifers. The word comes from the Middle Dutch root "brak". Certain human activities can produce brackish water, in particular civil engineering projects such as dikes and the flooding of coastal marshland to produce brackish water pools for freshwater prawn farming. Brackish water is also the primary waste product of the salinity gradient power process. Because brackish water is hostile to the growth of most terrestrial plant species, without appropriate management it is damaging to the environment (see article on shrimp farms).

Technically, brackish water contains between 0.5 and 30 grams of salt per litre—more often expressed as 0.5 to 30 parts per thousand (‰), which is a specific gravity of between 1.005 and 1.010. Thus, brackish covers a range of salinity regimes and is not considered a precisely defined condition. It is characteristic of many brackish surface waters that their salinity can vary considerably over space or time.

Ecology of the San Francisco Estuary

The San Francisco Estuary together with the Sacramento–San Joaquin River Delta represents a highly altered ecosystem. The region has been heavily re-engineered to accommodate the needs of water delivery, shipping, agriculture, and most recently, suburban development. These needs have wrought direct changes in the movement of water and the nature of the landscape, and indirect changes from the introduction of non-native species. New species have altered the architecture of the food web as surely as levees have altered the landscape of islands and channels that form the complex system known as the Delta.This article deals particularly with the ecology of the low salinity zone (LSZ) of the estuary. Reconstructing a historic food web for the LSZ is difficult for a number of reasons. First, there is no clear record of the species that historically have occupied the estuary. Second, the San Francisco Estuary and Delta have been in geologic and hydrologic transition for most of their 10,000 year history, and so describing the "natural" condition of the estuary is much like "hitting a moving target". Climate change, hydrologic engineering, shifting water needs, and newly introduced species will continue to alter the food web configuration of the estuary. This model provides a snapshot of the current state, with notes about recent changes or species introductions that have altered the configuration of the food web. Understanding the dynamics of the current food web may prove useful for restoration efforts to improve the functioning and species diversity of the estuary.

Freshwater fish

Freshwater fish are those that spend some or all of their lives in fresh water, such as rivers and lakes, with a salinity of less than 0.05%. These environments differ from marine conditions in many ways, the most obvious being the difference in levels of salinity. To survive fresh water, the fish need a range of physiological adaptations.

41.24% of all known species of fish are found in fresh water. This is primarily due to the rapid speciation that the scattered habitats make possible. When dealing with ponds and lakes, one might use the same basic models of speciation as when studying island biogeography.

Halocline

In oceanography, a halocline (from Greek hals, halo- 'salt' and klinein 'to slope') is a subtype of chemocline caused by a strong, vertical salinity gradient within a body of water. Because salinity (in concert with temperature) affects the density of seawater, it can play a role in its vertical stratification. Increasing salinity by one kg/m3 results in an increase of seawater density of around 0.7 kg/m3.

In the midlatitudes, an excess of evaporation over precipitation leads to surface waters being saltier than deep waters. In such regions, the vertical stratification is due to surface waters being warmer than deep waters and the halocline is destabilizing. Such regions may be prone to salt fingering, a process which results in the preferential mixing of salinity.

In certain high latitude regions (such as the Arctic Ocean, Bering Sea, and the Southern Ocean) the surface waters are actually colder than the deep waters and the halocline is responsible for maintaining water column stability, isolating the surface waters from the deep waters. In these regions, the halocline is important in allowing for the formation of sea ice, and limiting the escape of carbon dioxide to the atmosphere.

Haloclines are also found in fjords, and poorly mixed estuaries where fresh water is deposited at the ocean surface.

Halophyte

A halophyte is a salt-tolerant plant that grows in waters of high salinity, coming into contact with saline water through its roots or by salt spray, such as in saline semi-deserts, mangrove swamps, marshes and sloughs and seashores. These plants do not prefer saline environments but because of their ability to cope with high salinity in various ways they face much less competition in these areas. The word derives from Ancient Greek ἅλας (halas) 'salt' and φυτόν (phyton) 'plant'. An example of a halophyte is the salt marsh grass Spartina alterniflora (smooth cordgrass). Relatively few plant species are halophytes—perhaps only 2% of all plant species.

The large majority of plant species are glycophytes, which are not salt-tolerant and are damaged fairly easily by high salinity.

Halotolerance

Halotolerance is the adaptation of living organisms to conditions of high salinity. Halotolerant species tend to live in areas such as hypersaline lakes, coastal dunes, saline deserts, salt marshes, and inland salt seas and springs. Halophiles are organisms that live in highly saline environments, and require the salinity to survive, while halotolerant organisms (belonging to different domains of life) can grow under saline conditions, but do not require elevated concentrations of salt for growth. Halophytes are salt-tolerant higher plants. Halotolerant microorganisms are of considerable biotechnological interest.

Hypersaline lake

A hypersaline lake is a landlocked body of water that contains significant concentrations of sodium chloride or other salts, with saline levels surpassing that of ocean water (3.5%, i.e. 35 grams per litre or 0.29 pounds per US gallon). Specific microbial and crustacean species thrive in these high-salinity environments that are inhospitable to most lifeforms. Some of these species enter a dormant state when desiccated, and some species are thought to survive for over 250 million years. The water of hypersaline lakes has great buoyancy due to its high salt content.

The most saline water body in the world is the Don Juan Pond, located in the McMurdo Dry Valleys in Antarctica. Its volume is some 3,000 cubic meters, but is constantly changing. The Don Juan Pond has a salinity level of over 44%, (i.e. 12 times as salty as ocean water). Its high salinity prevents the Don Juan from freezing even when temperatures are below −50 °C (−58 °F). There are larger hypersaline water bodies, lakes in the McMurdo Dry Valleys such as Lake Vanda with salinity of over 35% (i.e. 10 times as salty as ocean water). They are covered with ice in the winter.

The most saline lake outside of Antarctica is Lake Assal, in Djibouti, which has a salinity of 34.8% (i.e. 10 times as salty as ocean water). Probably the best-known hypersaline lakes are the Dead Sea (34.2% salinity in 2010) and the Great Salt Lake in the state of Utah, USA (5–27% variable salinity). The Dead Sea, dividing Israel and the Palestinian West Bank from Jordan, is the world's deepest hypersaline lake, and the Araruama Lagoon in Brazil is the world's largest. The Great Salt Lake, located in Utah, while having nearly three times the surface area of the Dead Sea, is shallower and experiences much greater fluctuations in salinity. At its lowest recorded levels, it approaches 7.7 times the salinity of ocean water, but when its levels are high, its salinity drops to only slightly higher than that of the ocean.Hypersaline lakes are found on every continent, especially in arid or semi-arid regions. The Devon Ice Cap contains two subglacial lakes that are hypersaline.

Mediterranean Sea

The Mediterranean Sea is a sea connected to the Atlantic Ocean, surrounded by the Mediterranean Basin and almost completely enclosed by land: on the north by Southern Europe and Anatolia, on the south by North Africa and on the east by the Levant. Although the sea is sometimes considered a part of the Atlantic Ocean, it is usually identified as a separate body of water. Geological evidence indicates that around 5.9 million years ago, the Mediterranean was cut off from the Atlantic and was partly or completely desiccated over a period of some 600,000 years, the Messinian salinity crisis, before being refilled by the Zanclean flood about 5.3 million years ago.

It covers an approximate area of 2.5 million km2 (965,000 sq mi), representing 0.7% of the global ocean surface, but its connection to the Atlantic via the Strait of Gibraltar — the narrow strait that connects the Atlantic Ocean to the Mediterranean Sea and separates Spain in Europe from Morocco in Africa — is only 14 km (8.7 mi) wide. In oceanography, it is sometimes called the Eurafrican Mediterranean Sea or the European Mediterranean Sea to distinguish it from mediterranean seas elsewhere.The Mediterranean Sea has an average depth of 1,500 m (4,900 ft) and the deepest recorded point is 5,267 m (17,280 ft) in the Calypso Deep in the Ionian Sea. The sea is bordered on the north by Europe, the east by Asia, and in the south by Africa. It is located between latitudes 30° and 46° N and longitudes 6° W and 36° E. Its west-east length, from the Strait of Gibraltar to the Gulf of Iskenderun, on the southwestern coast of Turkey, is approximately 4,000 km (2,500 miles). The sea's average north-south length, from Croatia's southern shore

to Libya, is approximately 800 km (500 miles).The sea was an important route for merchants and travellers of ancient times, facilitating trade and cultural exchange between peoples of the region. The history of the Mediterranean region is crucial to understanding the origins and development of many modern societies.

The countries surrounding the Mediterranean in clockwise order are Spain, France, Monaco, Italy, Slovenia, Croatia, Bosnia and Herzegovina, Montenegro, Albania, Greece, Turkey, Syria, Lebanon, Israel, Egypt, Libya, Tunisia, Algeria, and Morocco; Malta and Cyprus are island countries in the sea. In addition, the Gaza Strip and the British Overseas Territories of Gibraltar and Akrotiri and Dhekelia have coastlines on the sea.

Ocean

An ocean (from Ancient Greek Ὠκεανός, transc. Okeanós) is a body of water that composes much of a planet's hydrosphere. On Earth, an ocean is one of the major conventional divisions of the World Ocean. These are, in descending order by area, the Pacific, Atlantic, Indian, Southern (Antarctic), and Arctic Oceans. The word "ocean" is often used interchangeably with "sea" in American English. Strictly speaking, a sea is a body of water (generally a division of the world ocean) partly or fully enclosed by land, though "the sea" refers also to the oceans.

Saline water covers approximately 361,000,000 km2 (139,000,000 sq mi) and is customarily divided into several principal oceans and smaller seas, with the ocean covering approximately 71% of Earth's surface and 90% of the Earth's biosphere. The ocean contains 97% of Earth's water, and oceanographers have stated that less than 5% of the World Ocean has been explored. The total volume is approximately 1.35 billion cubic kilometers (320 million cu mi) with an average depth of nearly 3,700 meters (12,100 ft).As the world ocean is the principal component of Earth's hydrosphere, it is integral to life, forms part of the carbon cycle, and influences climate and weather patterns. The World Ocean is the habitat of 230,000 known species, but because much of it is unexplored, the number of species that exist in the ocean is much larger, possibly over two million. The origin of Earth's oceans is unknown; oceans are thought to have formed in the Hadean eon and may have been the impetus for the emergence of life.

Extraterrestrial oceans may be composed of water or other elements and compounds. The only confirmed large stable bodies of extraterrestrial surface liquids are the lakes of Titan, although there is evidence for the existence of oceans elsewhere in the Solar System. Early in their geologic histories, Mars and Venus are theorized to have had large water oceans. The Mars ocean hypothesis suggests that nearly a third of the surface of Mars was once covered by water, and a runaway greenhouse effect may have boiled away the global ocean of Venus. Compounds such as salts and ammonia dissolved in water lower its freezing point so that water might exist in large quantities in extraterrestrial environments as brine or convecting ice. Unconfirmed oceans are speculated beneath the surface of many dwarf planets and natural satellites; notably, the ocean of the moon Europa is estimated to have over twice the water volume of Earth. The Solar System's giant planets are also thought to have liquid atmospheric layers of yet to be confirmed compositions. Oceans may also exist on exoplanets and exomoons, including surface oceans of liquid water within a circumstellar habitable zone. Ocean planets are a hypothetical type of planet with a surface completely covered with liquid.

Osmotic power

Osmotic power, salinity gradient power or blue energy is the energy available from the difference in the salt concentration between seawater and river water. Two practical methods for this are reverse electrodialysis (RED) and

pressure retarded osmosis (PRO). Both processes rely on osmosis with membranes. The key waste product is brackish water. This byproduct is the result of natural forces that are being harnessed: the flow of fresh water into seas that are made up of salt water.

In 1954, Pattle suggested that there was an untapped source of power when a river mixes with the sea, in terms of the lost osmotic pressure, however it was not until the mid ‘70s where a practical method of exploiting it using selectively permeable membranes by Loeb was outlined.

The method of generating power by pressure retarded osmosis was invented by Prof. Sidney Loeb in 1973 at the Ben-Gurion University of the Negev, Beersheba, Israel. The idea came to Prof. Loeb, in part, as he observed the Jordan River flowing into the Dead Sea. He wanted to harvest the energy of mixing of the two aqueous solutions (the Jordan River being one and the Dead Sea being the other) that was going to waste in this natural mixing process. In 1977 Prof. Loeb invented a method of producing power by a reverse electrodialysis heat engine.The technologies have been confirmed in laboratory conditions. They are being developed into commercial use in the Netherlands (RED) and Norway (PRO). The cost of the membrane has been an obstacle. A new, lower cost membrane, based on an electrically modified polyethylene plastic, made it fit for potential commercial use. Other methods have been proposed and are currently under development. Among them, a method based on electric double-layer capacitor

technology. and a method based on vapor pressure difference.

Saline water

Saline water (more commonly known as salt water) is water that contains a high concentration of dissolved salts (mainly NaCl). The salt concentration is usually expressed in parts per thousand (permille, ‰) or parts per million (ppm). The United States Geological Survey classifies saline water in three salinity categories. Salt concentration in slightly saline water is around 1,000 to 3,000 ppm (0.1–0.3%), in moderately saline water 3,000 to 10,000 ppm (0.3–1%) and in highly saline water 10,000 to 35,000 ppm (1–3.5%). Seawater has a salinity of roughly 35,000 ppm, equivalent to 35 grams of salt per one liter (or kilogram) of water. The saturation level is dependent on the temperature of the water. At 20 °C one liter of water can dissolve about 357 grams of salt, a concentration of 26.3%. At boiling (100 °C) the amount that can be dissolved in one liter of water increases to about 391 grams, a concentration of 28.1%.Some industries make use of saline water, such as mining and thermo-electric power.

Salt lake

A salt lake or saline lake is a landlocked body of water that has a concentration of salts (typically sodium chloride) and other dissolved minerals significantly higher than most lakes (often defined as at least three grams of salt per litre). In some cases, salt lakes have a higher concentration of salt than sea water; such lakes can also be termed hypersaline lakes. An alkalic salt lake that has a high content of carbonate is sometimes termed a soda lake.

Saline lake classification:

subsaline 0.5–3 ‰

hyposaline 3–20 ‰

mesosaline 20–50 ‰

hypersaline greater than 50 ‰

Salton Sea

The Salton Sea is a shallow, saline, endorheic rift lake located directly on the San Andreas Fault, predominantly in the U.S. state of California's Imperial and Coachella valleys.

The lake occupies the lowest elevations of the Salton Sink in the Colorado Desert of Imperial and Riverside counties in Southern California. Its surface is 236.0 ft (71.9 m) below sea level as of January 2018. The deepest point of the sea is 5 ft (1.5 m) higher than the lowest point of Death Valley. The sea is fed by the New, Whitewater, and Alamo Rivers, as well as agricultural runoff, drainage systems, and creeks.

Over millions of years, the Colorado River has flowed into the Imperial Valley and deposited soil (creating fertile farmland), building up the terrain and constantly changing the course of the river. For thousands of years, the river has alternately flowed into and out of the valley, alternately creating a freshwater lake, an increasingly saline lake, and a dry desert basin, depending on river flows and the balance between inflow and evaporative loss. The cycle of filling has been about every 400–500 years and has repeated many times. The latest natural cycle occurred around 1600–1700 as remembered by Native Americans who talked with the first European settlers. Fish traps still exist at many locations, and the Native Americans evidently moved the traps depending upon the cycle.

The most recent inflow of water from the now heavily controlled Colorado River was accidentally created by the engineers of the California Development Company in 1905. In an effort to increase water flow into the area for farming, irrigation canals were dug from the Colorado River into the valley. The canals suffered silt buildup, so a cut was made in the bank of the Colorado River to further increase the water flow. The resulting outflow overwhelmed the engineered canal near Yuma, Arizona, and the river flowed into the Salton Basin for two years, filling the historic dry lake bed and creating the modern sea, before repairs were completed.While it varies in dimensions and area with fluctuations in agricultural runoff and rainfall, the Salton Sea is about 15 by 35 miles (24 by 56 km). With an estimated surface area of 343 square miles (890 km2) or 350 square miles (910 km2), the Salton Sea is the largest lake in California. The average annual inflow is less than 1.2 million acre⋅ft (1.5 km3), which is enough to maintain a maximum depth of 43 feet (13 m) and a total volume of about 6 million acre⋅ft (7.4 km3). However, due to changes in water apportionments agreed upon for the Colorado River under the Quantification Settlement Agreement of 2003, the surface area of the sea is expected to decrease by 60% between 2013 and 2021.The lake's salinity, about 56 grams per litre (7.5 oz/US gal), is greater than that of the Pacific Ocean (35 g/l (4.7 oz/US gal)), but less than that of the Great Salt Lake (which ranges from 50 to 270 g/l (6.7 to 36.1 oz/US gal)). Recently, the concentration has been increasing at a rate of about 3% per year. About 4 million short tons (3.6 million t) of salt are deposited in the valley each year.

Seawater

Seawater, or salt water, is water from a sea or ocean. On average, seawater in the world's oceans has a salinity of about 3.5% (35 g/L, 599 mM). This means that every kilogram (roughly one litre by volume) of seawater has approximately 35 grams (1.2 oz) of dissolved salts (predominantly sodium (Na+) and chloride (Cl−) ions). Average density at the surface is 1.025 kg/L. Seawater is denser than both fresh water and pure water (density 1.0 kg/L at 4 °C (39 °F)) because the dissolved salts increase the mass by a larger proportion than the volume. The freezing point of seawater decreases as salt concentration increases. At typical salinity, it freezes at about −2 °C (28 °F). The coldest seawater ever recorded (in a liquid state) was in 2010, in a stream under an Antarctic glacier, and measured −2.6 °C (27.3 °F). Seawater pH is typically limited to a range between 7.5 and 8.4. However, there is no universally accepted reference pH-scale for seawater and the difference between measurements based on different reference scales may be up to 0.14 units.

Soil Moisture and Ocean Salinity

Soil Moisture and Ocean Salinity, or SMOS, is a satellite which forms part of ESA's Living Planet Programme. It is intended to provide new insights into Earth's water cycle and climate. In addition, it is intended to provide improved weather forecasting and monitoring of snow and ice accumulation.

Soil salinity

Soil salinity is the salt content in the soil; the process of increasing the salt content is known as salinization. Salts occur naturally within soils and water. Salination can be caused by natural processes such as mineral weathering or by the gradual withdrawal of an ocean. It can also come about through artificial processes such as irrigation and road salt.

Soil salinity control

Soil salinity control relates to controlling the problem of soil salinity and reclaiming salinized agricultural land.

The aim of soil salinity control is to prevent soil degradation by salination and reclaim already salty (saline) soils. Soil reclamation is also called soil improvement, rehabilitation, remediation, recuperation, or amelioration.

The primary man-made cause of salinization is irrigation. River water or groundwater used in irrigation contains salts, which remain behind in the soil after the water has evaporated.

The primary method of controlling soil salinity is to permit 10-20% of the irrigation water to leach the soil,that will be drained and discharged through an appropriate drainage system. The salt concentration of the drainage water is normally 5 to 10 times higher than that of the irrigation water, thus salt export matches salt import and it will not accumulate.

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Quality indicators
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