Salinity

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

Salinity (i/) 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 chemistryof 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.

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 ‰
Other Languages
العربية: ملوحة
भोजपुरी: नुनछाहपन
български: Соленост
bosanski: Salinitet
català: Salinitat
čeština: Salinita
dansk: Salinitet
Deutsch: Salinität
eesti: Soolsus
español: Salinidad
Esperanto: Saleco
euskara: Gazitasun
فارسی: شوری (آب)
français: Salinité
Gaeilge: Salandacht
galego: Salinidade
한국어: 염분
हिन्दी: लवणता
hrvatski: Salinitet
Bahasa Indonesia: Keasinan
italiano: Salinità
Latina: Salsitudo
latviešu: Sāļums
lietuvių: Druskingumas
македонски: Соленост
मराठी: खारे पाणी
Bahasa Melayu: Kemasinan
Nederlands: Saliniteit
日本語: 塩分濃度
norsk: Salinitet
norsk nynorsk: Salinitet
occitan: Salinitat
Oromoo: Salinity
polski: Zasolenie
português: Salinidade
română: Salinitate
русский: Солёность
саха тыла: Туустаныы
Scots: Salinity
Simple English: Salinity
slovenčina: Salinita
slovenščina: Slanost
српски / srpski: Salinitet
srpskohrvatski / српскохрватски: Salinitet
Sunda: Salinitas
suomi: Suolaisuus
svenska: Salinitet
Türkçe: Tuzluluk
українська: Солоність
اردو: نمکینیت
Tiếng Việt: Độ mặn
吴语: 盐度
中文: 鹽度