Salt evaporation pond

A salt evaporation pond is a shallow artificial salt pan designed to extract salts from sea water or other brines. The salt pans are shallow and expansive, allowing sunlight to penetrate and reach the seawater. Natural salt pans are formed through geologic processes, where evaporating water leaves behind salt deposits. Some salt evaporation ponds are only slightly modified from their natural version, such as the ponds on Great Inagua in the Bahamas, or the ponds in Jasiira, a few kilometres south of Mogadishu, where seawater is trapped and left to evaporate in the sun.

During the process of salt winning, seawater or brine is fed into artificially created ponds from which water is drawn out by evaporation, allowing the salt to be subsequently harvested.^[1]^: 517^[2]

The ponds also provide a productive resting and feeding ground for many species of waterbirds, which may include endangered species.^[3] However, Ghanaian fisheries scientist RoseEmma Mamaa Entsua-Mensah also noted that salt winning can destroy mangrove forests and mudflats, altering the environment and making it unproductive for other development or fish growth.^[4] The ponds are commonly separated by levees. Salt evaporation ponds may also be called salterns, salt works or salt pans.

Metrics and energetics

There is an associated loss of available energy when evaporating into dry air. This Gibbs Free Energy becomes positive when the salinity is high enough (or air humid enough) for the salt solution to cause water to condense into it. That is how liquid desiccants work.^[5]

Evaporation systems are also often evaluated by the water evaporation rate per unit area. When the energy is largely provided by sunlight, these are often evaluated with a solar efficiency, ( $\eta _{\rm {sol}}$ ), which is a thermal efficiency that compares incoming light energy to the enthalpy of vaporization. This is the same as the gained output ratio (GOR) in desalination.^[5]

Algae and color

Due to variable algal concentrations, vivid colors (from pale green to bright red) are created in the evaporation ponds. The color indicates the salinity of the ponds. Microorganisms change their hues as the salinity of the pond increases. In low- to mid-salinity ponds, green algae such as Dunaliella salina are predominant, although these algae can also take on an orange hue. Halobacteria, a type of halophilic Archaea (also known as Haloarchaea), are responsible for changing the color of middle to high-salinity ponds to shades of pink, red, and orange. Other bacteria such as Stichococcus also contribute tints.^{[citation needed]}

Examples

Notable salt ponds include:

The Salterns of Guérande, in Loire-Atlantique, France. The salt produced in the salterns are a protected geographical indication in Europe.
The Cáhuil salt ponds, in the O'Higgins Region, Chile.
The Manaure salt ponds, in La Guajira Department, Colombia.
The Salineras de Maras, Peru, in the Cusco Region.
The saltworks of Alcácer do Sal, Comporta, and Castro Marim in Portugal
The El Caracol solar evaporator, on the outskirts of Mexico City, Mexico.
The Sečovlje and Strunjan salt ponds on the northern edge of the Adriatic Sea in Slovenia.
The San Francisco Bay salt ponds in the United States, formerly operated by Cargill,^[6]^[7]^[8]^[9] including Charleston Slough. Cargill has since ended salt production in the area, and most of the ponds are being restored to a more natural state.
South Bay Salt Works in San Diego, California.
The Dead Sea salt ponds in the West Bank, Israel and Jordan.
The salt ponds in Salina, Malta. The name of the village is the Maltese word for salt pan.
The Port Hedland, Dampier, Lake McLeod, Useless Loop and Onslow salt ponds in Western Australia.
Yellow Walls, Malahide, Ireland; active from 1770 to 1837.^[10]
Lake Grassmere in New Zealand
The ancient salt pans in Marsala and Trapani, Sicily. Salt has been farmed here since the Phoenician period, with archaeological evidence still present in nearby Motya.
The Nature reserve of Margherita di Savoia, in Apulia, Italy. Known since antiquity, it's one of the largest in Europe. Flamingos often nest in the area.
The salt works on the island of Great Inagua owned by Morton Salt.
The salt harvesting by the Tsonga women of Baleni on the Small Letaba River, Limpopo, South Africa.^[11]
The Sorae Salt Pond in South Korea which operated from 1935 to 1996

Until World War II, salt was extracted from sea water in a unique way in Egypt near Alexandria.^[12] Posts were set out on the salt pans and covered with several feet of sea water. In time the sea water evaporated, leaving the salt behind on the post, where it was easier to harvest.

Production

Salt pans are shallow and open, and metal pans are often used to evaporate brine. They are usually found close to the source of the salt. For example, pans used in the solar evaporation of salt from seawater are usually found on the coast, while those used to extract salt from solution-mined brine will be found near the brine shaft. In this case, extra heat is often provided by lighting fires underneath.

History

Archaeological evidence

Archaeological evidence for early salt production is fragmentary. Coastal installations have been lost to rising sea levels, furnaces were often dismantled after use, and the ceramic containers used to hold and heat brine were broken up, so few direct traces survive.^[13]

Europe

From the Neolithic to the Iron Age, salt making in Europe relied mainly on inland brine sources and on evaporation over a fire, using the ceramic vessels and supports known as briquetage. Methods varied with local conditions, and large coastal evaporation ponds were best suited to warmer and drier regions.^[14]

Marine salt making in western Europe may go back to the 5th millennium BC, when seasonal lagoons behind low sandy shores could have served as natural basins in which seawater was concentrated before being heated.^[13]

Organised production is attested archaeologically in west-central France from the 3rd millennium BC, where distinctive ceramic forms indicate installations for heating brine. This does not rule out earlier concentration by solar evaporation in coastal lagoons, such as those near Locmariaquer and Carnac in Morbihan, or at Varna in Bulgaria.^[13]

China

Excavation and chemical analysis at Zhongba, in the Three Gorges region, show substantial salt production by the first millennium BC, with signs of earlier activity; brine was boiled rather than evaporated in the open.^[15]

On the northern coast, at sites around southern Laizhou Bay in Shandong such as Shuangwangcheng and Nanheya, sea salt was produced from the late Neolithic through the Shang and Zhou periods. Work at these and other coastal sites relates the organisation of production to Holocene shoreline change and to recurrent flooding.^[16]^[17]

India

On the Saurashtra coast and the Gulf of Khambhat, the Harappan site of Padri (Kerala-no-dhoro) has been interpreted as a settlement specialising in salt production during the Mature Harappan period, about 2200–2000 BC.^[18] A later geoarchaeological study describes the site's economy as based on salt manufacture, in a stable estuarine setting close to the present sea level.^[19]

Mesopotamia

Cuneiform sources record salt under the Sumerian word mun.^[20] Salt was obtained mainly from the natural salt flats and brines of the alluvial plain and the southern marshes. Purpose-built evaporation complexes are difficult to identify archaeologically.^[21]

Mesoamerica

At El Salado in central Veracruz, stratified deposits and residues associated with the evaporation of brine indicate two main phases of salt production: an Early Formative phase of about 1400–1000 BC and a Late Classic phase of about AD 650–1000.^[22]

Marine salterns in antiquity

In the classical world, from the Mediterranean and the Near East to India and Han China, the techniques of salt making changed little. What changed was their administration: written records and management by states, cities or officials allowed larger output, wider distribution and higher consumption.^[23] Sea salt became more important over the same period. It was increasingly carried by ship rather than overland, and by about AD 500 it accounted for most of the salt consumed, in large part because of its use in preserving fish.^[23]

Large marine salterns (salinae) operated around the Mediterranean by classical antiquity. Seawater was let in by gravity and passed through a series of connected shallow ponds, growing more concentrated at each stage until the salt crystallised and could be harvested.^[14] Where the coast allowed, these ponds were laid out in sheltered lagoons and on low-energy shores and were linked by channels and sluices.^[24]

Salt was essential to food preservation, to animal husbandry and to several crafts, but the salinae themselves have left comparatively few remains, which makes their layout and operation difficult to reconstruct.^[24] The lagoons of the Tyrrhenian coast, including those near the mouth of the Tiber, were used for salt making from an early date and remained in use for centuries.^[24] Roman salterns are documented at O Areal in Vigo in north-western Spain, at Kaunos in southern Turkey, and in the lagoons near Maccarese and Fiumicino outside Rome, where ponds, channels and working surfaces survive.^[24]

Ownership took several forms. From the late Republic to the second century AD, some salinae were state property leased to tax-farming companies, while others were held by cities or by private owners. The evidence does not support a general state monopoly.^[24]

In the ancient Near East, salt also had ritual and legal uses. It was prescribed for offerings at the Temple in Jerusalem, appears in treaty language such as the "covenant of salt", and was sown on conquered land as a curse. Rabbinic texts describe a storehouse for the salt used in Temple sacrifices, and early Christian writers used salt as a moral metaphor.^[25] Surveys by the Israel Antiquities Authority in Western Galilee and on the northern Carmel coast have recorded ancient installations, including sequences of shallow evaporation basins and channels.^[25]

In the Pacific, salterns of the seventh century AD have been excavated at the Sigatoka sand dunes on Viti Levu, Fiji, where seawater was evaporated in large flanged clay dishes. This is the earliest salt working identified in the region.^[26] Salt pans were also built in the Hawaiian Islands, including at Kawaihae on the west coast of Hawaiʻi.^[27]

Marine salterns in the Middle Ages

Purpose-built solar salterns took shape on the French Atlantic coast from the early Middle Ages: around Guérande and Batz, in the Bay of Bourgneuf and the Marais breton, and in Saintonge near Brouage.^[28]^[29]^[30]

At Guérande, pond systems built in the marshes are attested by the 9th century and became the dominant form of production.^[28]^[31] In Saintonge and around Brouage, sheltered lagoons were converted into graded ponds fed by tidal channels known as étiers.^[29]

Later medieval documents record how these salterns were held and worked. The accounts of the Blanchet family, covering 1463 to 1512, describe ownership, reorganisation and the return to production in the Bay of Bourgneuf after the disruption of the mid-14th century.^[32] A rent survey of 1478 for the prévôté of Hiers records a similar recovery in the marshes of Saintonge.^[33] Atlantic "bay salt" was traded widely from the Merovingian and Carolingian periods onwards, reaching northern France and the British Isles by river and coastal routes.^[34]

Salterns are recorded elsewhere in medieval Europe. The Domesday Book of 1086 lists more than 1,195 salinae in England, mostly on the southern and eastern coasts.^[35] In Portugal, a donation charter of 959 issued by Countess Mumadona Dias refers to land in Alavario et salinas, an early reference to salt pans on the Ria de Aveiro.^[36]^{[better source needed]}

Early modern period and industrialisation

After the Middle Ages, coastal solar salterns expanded while inland brine works were reorganised along more industrial lines. In the northern Adriatic, the pans of Piran and Sečovlje were laid out under Venetian rule as regular sequences of evaporation and crystallisation basins, and remained in production through the 15th century, when salt pans were being destroyed elsewhere in the region.^[37]^{[better source needed]}

Gallery

Natural salt evaporation ponds at Pedra de Lume, Sal island, Cape Verde
A salt pan worker in a salt evaporation pond in Tamil Nadu, India.
Salt evaporation ponds at a lithium mine in Argentina. The brine in this salar is rich in lithium, and the mine concentrates the brine in the ponds.
Contemporary solar evaporation salt pans on the island of Lanzarote at Salinas de Janubio
Solar evaporation ponds in the Atacama Desert
Solar evaporation ponds in the Salt Valley of Añana, Spain
Solar evaporation ponds in the Salt Valley of Añana, Spain
Salt evaporation ponds at Pag, Croatia
Aerial view of the Saline di Margherita di Savoia, Italy
Aerial view of the salt evaporation ponds in Ston, Croatia
Salt pile in Argentina