Lake Macleod, West Australia

Lake Macleod, West Australia, is a halite-filled salina some 120 km long and 40 km wide, lying just to the north of Shark Bay. Climate is arid, with an average rainfall around 230-260 mm and annual potential pan evaporation above 2,600 mm (BWh Köppen Climate zone). It is the westernmost salt lake in Australia. The current saltflat surface of much of Lake Macleod still lies about a metre below sea level indicating evaporative drawdown and a groundwater feed from the nearby Indian Ocean.

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Region of seawater springs on the eastern side of Lake Macleod.

Some parts of the lake depression, especially near the marine seeps, are still covered with semipermanent spring fed water channels and  sheets. In its northern part, and across the current lake surface, the lake sediment fill is a Holocene gypsarenite unit; it is as much as 5 meters thick along the northern and western (seaward) sides of the lake. To the south and beneath the laminated gypsum unit is a widespread halite bed up to 7 m thick. It is made up of stacked halite crusts, dominated by growth-aligned chevrons and cornets. Brines from this halite unit in the south are pumped into saltwork ponds to produce halite for the chemical industries of southeast Asia. Gypsum was dredged in the central part of the lake and mostly used for wallboard manufacture. Both the gypsum and the halite units overlie a thin unit of Holocene lagoonal carbonate deposited before the depression’s transition from marine embayment to coastal seepage lake.

Lake Macleod never possessed a surface (hydrographic) connection to the ocean at any time in its salt-accumulating Holocene history. Seawater seeps onto the subsealevel lake floor via a series of marginward carbonate-precipitating spring pavements and beds, fed from fractures and caves in the underlying and surrounding calcareous coastal dune aquifer. As the escaping water exits the carbonate-dune aquifer, it degasses and concentrates to salinities where it precipitates seepage or strandzone carbonates, including aragonite pisolites and tepees, especially about seeps along the eastern and northern edges of the Macleod salina.

Today, the northern part of the lake is a perennial seepage pond, called Ibis Pond, where the perennial inflow water gathers in ponds that are over a metre deep. When the water first escapes into the pond, it has normal to near normal marine salinities and supports a restricted population of halotolerant marine gastropods and small fish. A similar marine biota can be found in marine seepage ponds on the seaward side of gypsum-filled Lake Macdonnell of coastal South Australia. Spring-fed normal marine waters are thought to have supplied the Lake Macleod brine lake for the last 5,000 years, ever since the Australasian Sea rose to a level that was within a metre or two of its present level. At that time a series of laterally accreting beach ridges cut off the lake’s southern surface connection to the Indian Ocean (5,100 years ago). Since then, evaporative drawdown has pulled seawater into the lake depression, and bedded salts have accumulated (see Warren 2016, Chapter 4 for more detail).

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 Lake Macleod, Western Australia. Stratigraphic cross sections showing distribution of halite and gypsum, as well as the effect of seepage decreasing the salinity, so that the gypsum thickness increases toward areas of carbonate seepage defined by Ibis Pond and the western platform (after Logan and Brown, 1986).

Salt Production

The saturated brine contained in Lake Macleod is approximately ten times saltier than normal seawater, eliminating the need for a series of concentration ponds usually required to evaporate water to reach "salting" point (sodium chloride saturation). A collection ditch was cut to the halite layer to recover brine. The brine is pumped at an average rate of 55 cubic metres per minute from the collection ditch into 8.5 kilometres of transport channel to a common collection point before being pumped into the crystallisers.

A total of 1,650 hectares of evaporator pans have been constructed, and there are thirty-three crystalliser pans, averaging 23 hectares, each used for salt production. The excess area is used to store residual brine containing other dissolved salts.

The crystallisers for salt production have a salt pavement to support the equipment used in the harvesting operation and to prevent contamination of the salt during harvesting. Salt precipitation is stopped by draining the remaining brine when about three-quarters of the sodium chloride has been deposited and before other dissolved salts come out of solution in significant quantities. The residual brine contains high concentrations of potassium, magnesium and other salts and is a potential source of these minerals.


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Salt harvesting

Harvesting of salt is carried out using a GPS controlled salt cutter with an average capacity of 1,000 tonnes per hour discharging directly into three 60 tonne trailers hauled by a prime mover. GPS equipment enables accurate control, leaving an even surface for the next year's growth of crystals. After harvesting, the crystallisers are cleaned and refilled with brine. To ensure high-quality salt with a minimum of impurities, the harvested salt is washed with saturated brine at the wash plant. This removes the soluble impurities (magnesium and potassium) and flushes the insoluble solids (gypsum and soil).

Washed salt is then drained on stainless steel mesh belts before being conveyed to the washed stockpile. The salt drains in this stockpile for approximately two months for the moisture content to decrease. Dry salt is hauled by road trains of up to 180 tonne capacity, 24 kilometres to the 400,000-tonne stockpiles at Cape Cuvier for shipment.

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Remote monitoring of inflow channel

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Stockpiling salt

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