Lake Magadi, Kenya
Lake Magadi lies within Kenya, slightly to the north of Lake Natron and at the bottom of a steep-sided valley, which is the lowest point in the eastern or Gregory Rift Valley. The lake extends roughly 20 km N-S and is up to 6 km wide. Both lakes lie at an altitude of some 600 m above sea level and are surrounded by plateaus and active natrocarbonatite volcanoes, reaching to more than 3,000 m asl. With an area of only 90 km2, Lake Magadi is one of the most saline, but also is one of the smallest, alkaline lake sumps in the Rift Valley.
There is no perennial stream flow into Lake Magadi. The lake is recharged mainly by saline hot springs (temperatures up to 86 °C) feeding into alkaline "lagoons” or “moat facies” around the lake margins. Most of the hot springs crop out along the northwestern and southern shorelines of the lake. During the rainy season, a thin (less than 1 m) layer of brine covers much of the saline pan, but this evaporates rapidly leaving a vast expanse of white trona pavement that cracks in large polygons. A single vertebrate, a species of fish, a cichlid Alcolapia grahami, inhabits the hot, highly alkaline waters of this lake basin in areas where water temperatures can be as high as45 °C.
Trona accumulates across the central portions as flat pavements composed of stacked saline pan crusts. The Lake Magadi trona pan is some 74 km2 in area and is 7–50 m thick, known locally as the “Evaporite Series.” It is made up of cm-scale stacked trona-detrital couplets. Light-coloured trona layers are composed of rosettes and splays of upward-pointing, growth- aligned, trona crystals. Thin bands of finer trona, coloured by dark, mainly windblown dust separate the crust layers from each other. Even as it stacks and dissolves, each layer retains some porosity between interlocking growth-aligned crystal splays.
Lake Magadi, Kenya, looking North
Lake Magadi, extraction and processing areas.
Magadi contains more than 30 billion metric tons of trona in a bed between 7 and 40 metres thick and believed to have formed in the last 9,000 years. Thermal springs, driven by the area’s volcanic activity, are the primary source of sodium in the lake and are sourced from a deep actively circulating groundwater reservoir. Trona precipitates in the more central areas of Lake Magadi as a bottom-nucleated crystal pavement. The annual crop is estimated at 1.5 million tonnes/year, although the bulk of this crop (≈75%) redissolves each wet season. When trona-saturated surface brines in Magadi dry up, the brine level stays close to the trona pavement surface. Even during exceptionally dry years, the brine surface drops only 1 or 2 cm into the trona bed.
Trona saturation is achieved when the shrinking brine sheets are 2–10 cm deep. At that stage, the trona prisms grow upward from the pan surface as cm-scale aligned elongate blades and splays. The interlocking of the crystal splays gives the accreting trona sheet an inherent strength that allows it to support expansion polygons or pressure ridges, with saucers up to several tens of meters wide and ridges up to a metre high.
Ridging is the result of pavement overthrusting, driven by the sideways expansion of trona layers jostling for space in the various growing crystal pavements. The resultant cracking and thrusting of the layers cut the trona pavement up into a series of overthrust-edged saucers, with small bright pink brine pools in the centre of each polygon; the colour is the result of a haloarchaeal bloom (see Warren, 2016; Chapters 3 and 12 for detailed literature compilation).
Bright pink colours of Lake Magadi waters from a bloom of halophiles
Trona in Lake Magadi has been mined since 1914 and is currently quarried from the central pavement using dredging barges. Today, the facility is owned and operated by Tata Chemicals Magadi Ltd and produces around 350,000 tonnes/year. As a digger belt removes the solid material, liquor from the surrounding trona pavement drains into the resulting cavity, forming a pool or ‘paddock’ in which dredges float and move forward. Crystals of trona produced by the dredges are coarse crushed, mixed with lake liquor and pumped as a slurry back to the factory where the liquor is discarded, and the crystals washed and centrifuged. Water washing reduces the sodium chloride level associated with the liquor from 11.5 per cent to around 0.33 per cent. The damp crystals are then fed into calciner plant. The crushed refined soda is then conveyed to the calciner plant. There it is fed into two large rotary kilns and subjected to a continuous flow of hot gases across a reaction bed to chemically decompose the trona (sodium sesquicarbonate) into soda ash, carbon dioxide and water vapour. This calcined material is conveyed to the grinding and screening plant, where the physical grading of the product takes place, before the product is conveyed to the storage silos, followed by packaging and transport to market.
If for any reason a dredging barge breaks down for more than a day or two, trona precipitating in the paddock about the base of the barge can lock the barge in place. This trona has to be physically chipped away before dredging operations can continue. Replenishment of trona in the lake is so rapid that at present extraction rates the Magadi trona could be considered a renewable resource.
The Lake Magadi facility also produces NaCl in a series of purpose-constructed solar evaporation ponds used to concentrate natural lake brines pumped into the pans. Lake brines are typically saturated, with a total dissolved solids (TDS) content of approximately 32%. Of this, sodium chloride content varies from 30 - 34%, the rest being almost exclusively sodium carbonate compounds with some sodium fluoride in small quantities.
Processing plant on the edge of Lake Magadi