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Lake Grassmere, 40 km south of Blenheim and 9 km south of the mouth of the Awatere River, is a shallow lagoon protected from the open sea by a shingle barrier beach covered by sand dunes. It is on the north-easterly extension of the Ward depression.
Turn off SH1 at Kaparu Road and travel along this access road to get a closer look at the Salt Works on the lake shore.
The lake area varies between 3,500 and 4,400 acres. Because the watershed is small the maximum is only attained in rare floods. The climate, with a low average rainfall of 585 mm (24 in.) and prevailing strong and dry north-westerly winds, provides Lake Grassmere with the suitable conditions required for natural economic salt production.
The Maori name for the lake is Kaparatehau ("wind-blown lake"). Early whalers on the coast rendered this name “Cobblers' Hole”. According to legend, the lake occupies the site of early cultivations. Kupe, the navigator, is said to have poured salt water on these cultivations, thus creating the lake.
In pre-European times it was used as a ready source of food, as it attracts a wide range of waterfowl.
Until the 1940s the lake bed at Lake Grassmere, was a mud bath in winter and a dustbowl in summer, with the occasional natural deposit of salt. The lake having no natural inflow was a very high in salinity.
Today the large shallow lake has ponds along the fringe that develop a deepening pink colour during the summer months. At the same time, huge white mounds appear on the shore. This fascinating and rather alien landscape is the result of natural salt production. The pink to purple colour of the crystallisation ponds is caused by natural microscopic green algae that change to pink in the high salt concentration. The same phenomena gives the Red Sea its name. There are also small pink shrimps in the water that thrive in this salty environment.
Other salt works in the world are generally much closer to the equator, but Marlborough's abundance of warm north-westerly winds, long hours of sunshine and low summer rainfall provide the evaporation needed to extract salt from the sea at this latitude.
What is salt?
Salt, or sodium chloride (NaCl), is a common mineral used in the manufacture of many products and as a food seasoning. Salt is the main mineral in sea water, making up three-quarters of the 3.5% of dissolved minerals in sea water. Salt also occurs naturally in mineral deposits, where it is known as rock salt. Because New Zealand has no rock salt deposits, salt was imported from the time of European arrival in the early 1800s until the country developed a sea-salt industry in the 1950s
Almost 1% of human blood is salt. The salt-derived sodium in the blood helps regulate the body’s water balance, plays a central role in electrical impulses in the nerves, and assists cells to take up nutrients. Although some salt is vital to health, most people in the developed world eat too much of it.
Salt is used to flavour food, and to preserve foods such as butter, meat and fish. During the 19th century it was added to soils as a fertiliser. It was also sprinkled onto hay during stacking so that livestock would get the benefit of sodium when the hay was fed out. Since the 20th century salt has been also been used in textile dyeing, bleaching paper, curing leather and in the dairy, food manufacturing and pharmaceutical industries.
Salt making at Lake Grassmere
Sea water, containing 2.8% salt fresh from the Pacific Ocean, is pumped into Lake Grassmere. Warm north-west winds blow across the exposed lake, evaporating water and increasing the concentration of salt.
When the salt content in the lake has increased to 5% the brine is transferred to a series of large concentration ponds. Calcium sulphate, the first mineral to crystallise, coats the bottom of the concentration ponds.
When the brine has increased to 25% salt, it is called a saturated solution. At this point around 90% of the original sea water has evaporated. The solution is then pumped from the concentration ponds into deep holding ponds, where it is held over winter.
In October of each year the saturated solution is pumped from the deep holding ponds into smaller crystallisation ponds.
In March, when the salt is ready for harvest, the remaining brine containing other unwanted minerals (called ‘bitterns’ or ‘mother liquor’) is pumped from the crystallisation ponds into the sea. Unseasonable rain during harvesting, as occurred in 1986, can mean that no salt is harvested. Usually only 50 millimetres of rain falls during the autumn harvest period, and as rain water is less dense than brine, it forms a surface layer which can be drained away.
After the crystallisation ponds are drained, mechanical harvesters work around the clock for about five weeks scooping up the crystallised salt crust or ‘cake’, which varies in thickness from 25 to 100 millimetres. The salt is loaded onto trucks and taken to the washery. There, it is washed with saturated brine to remove mud and other impurities. The salt is then stacked in huge piles. After it has been crushed and sieved to produce similar-sized grains, it is sent to industrial users.
A large range of salts with slightly different chemical compositions, grain sizes and shapes are produced. All table salt produced in New Zealand is solar salt, and both iodised and non-iodised table salt is available. Iodised salt contains added iodine (to prevent goitre) and silicon dioxide (to make the salt run). Specialty salts, including flaky salt, are also produced at Lake Grassmere. They are as good as any in the world, despite the New Zealand fashion for sprinkling imported salt on fine cuisine. Animal health products such as salt licks for farm animals are also produced at Lake Grassmere.
At the end of summer Lake Grassmere’s gleaming white salt piles are easily seen from State Highway 1. This seasonal landmark forms a vivid contrast to the burnt brown Marlborough hills. And from overhead, air passengers can gaze down at the series of pink-coloured ponds where drying winds help produce half of the country’s salt.
Requirements to log this Earthcache
You must visit given coordinates during the day.
Questions to answer – Email these answers to owner.
1. What action maintains the natural barrier protecting the lake from the open sea?.
2. Describe what you see on the yellow sign near the given coordinates and what significance if any does it have to the production process now or in the past?.
Your log - You do not have to wait for permission to log your find.
1. Write a brief weather report for the time of your visit including date, time of day, temperature, sky condition and wind direction. Do you consider this to be a poor, average, good or excellent water evaporation day.
2. Either
(a) Take a photo any where on the access road of yourself or GPS checking the weather with either a salt mountain or pink pond in the background. Download with your log.
(b) Count the number of ponds visible from the access road which are showing signs of the pink/purple phenomenen. Document your finding indicating depth of colour weak, medium or strong.
Additional Hints
(Decrypt)
Zbqrea pnef znl unir na bhgqbbe grzcrengher thntr.