Located in Java (between Banyuwangi Regency and Bondowoso Regency), Indonesia is a group of composite volcanoes called the Ijen volcano complex. Within one of these volcanoes is a lake which is recognised as the largest, most acidic crater lake in the world. This lake is found inside a large caldera Ijen.

A caldera is a large volcanic crater, it is formed by a major eruption leading to the collapse of the mouth of the volcano.........beautiful turquoise-coloured acidic crater lake. This turquoise-coloured acidic lake is the site of a Sulfur mining operation today, in which elemental Sulfur (S) is removed from the crater floor. This lake is recognised as the most highly acidic crater lake in the world. The pH of the water in the lake's edges was measured to be pH 0.5 and in the middle of the lake pH 0.13 due to high sulfuric acid (with molecular formula H2SO4) concentration. One of the wonders that exists for this acidic lake is the existence of a Blue Flame color at night in Ijen Crater. This Blue Flame Ijen Crater Volcano contains stunning electric flames that erupt from the volcano every night where a dazzling electric-blue fire can often be seen streaming down the volcano with a blue fire glow of lava from the volcano. The glow of this flame is actually the light from the combustion of sulfuric gases from elemental Sulfur (S). High concentrations of sulfate {sulfate ion is a polyatomic anion with the empirical formula (SO4)-2} , chloride (the anion, Cl−) and rock-forming major cations (a positively charged ion metals) are also found within the crater lake waters.

Craters that you can find at the Ijen PlateauThe walls of the Kawah Ijen are light ocher, but the water has a turquoise colour, with emerald reflexes. The temperature of the water is around 34°C. The smoke emitted by the volcano is filled with sulphur dioxide. Depending on the amount the volcano is sending into the air, it is full of a sulphur smell. This smell is pungent and irritating. Where the sulphur pours out of the ceramic pipes it reaches a temperature of 120°C. The colour then is bright red, which gradually turns yellow while the sulphur is solidifying.


Level of the lake might rise 4 meters in rainy season
Crater lakes cover active volcanic vents and the water within them is often acidic, saturated with volcanic gases and cloudy with a greenish colour. Ijen volcano has been hosting an acidic crater lake for at least 200 years. Ijen crater lake is by far the largest acidic lake on Earth, having a maximum depth of 212 m, and an average depth of 176 m. The Ijen crater lake has a regular oval shape of 600 times 1000 meters.
Acidic lakes result from the mix of rainfall water with gasses coming from the depths of the volcano. The outlet of the Ijen crater lake is located in western side that becomes the upper course of Banyupahit and Banyuputih rivers.
Detailed monitoring by Dutch volcanologists in the thirties of the nineteenth century revealed a clear relation between rainfall, lake water level and lake temperature. Yearly precipitation in the Ijen area is variable with maximum amounts op to 2.5 meters. There are significant fluctuations between a dry (May-October) and a wet season (November-April) when the lake level may rise by up to 4 meters. Surface lake temperatures are always higher than air temperature and generally decrease in the rainy season.

Acidity of the Ijen crater lake
The Ijen lake is an extreme example of acid sulphate-chloride type crater-lake waters carrying a high load of dissolved elements. A very low pH value is accompanied by high loads of TDS (~100 g/kg, tds = the total dissolved solids in water). The lake contains 600,000 tonnes of hydrogen chloride, 550,000 tonnes of sulphuric acid, 200,000 tonnes of aluminium sulphate and 170,000 tonnes of iron sulphate, with an acidity around pH 0.5. Any eruption through this lake would expel the water and create deadly mud-flow that can claim hundreds of lives and properties. The high acidity and high concentrations of sulphates, chloride and fluoride can be attributed to magmatic volatiles at the bottom of the lake.

Test acidity Ijen crater lake by boat - East Java     
The lake chemistry is determined by dissolution of magmatic volatiles, fluid-rock interaction, evaporation of the lake water, dilution by meteoric water and recycling of lake water through seepage into the subsurface hydrothermal system. The lake acts as chemical condenser for volatiles and as a calorimeter trapping heat supplied by a shallow magmatic reservoir. Magmatic volatiles can be supplied to the crater lake system by direct injection of magmatic vapours (SO2, H2S, HCl and HF) via sub aqueous fumaroles or via hot brines entering at the lake bottom.


Origin of the acidic river
As mentioned, the outlet of the crater lake is located in western side. The first stretch within the caldera is called Banyupahit, meaning bitter water. Shortly before breaking through the caldera wall near Blawan village, two major tributaries - Kalisat and Kalisengon - and discharges from hot springs change the river’s chemistry and appearance and give it a whitish colour caused by a milky-white precipitate. From there on, the river is called Banyuputih, meaning white water.
profile ijen crater lakeNear the summit area where the initial discharge rate is about 50 litres/second, the water composition and pH (<0.5) are similar to that of the lake. Water compositions and acidity show a downstream trend with strong jumps of about two pH units at the tributary inlets, largely due to dilution with near neutral water. At an irrigation dam in the coastal plain near Liwung village (Asembagus area), discharge rates have increased about thousand-fold and pH values in the dry season may range between 3 and 4.5.

Not only picturesque but also dangerous
While Ijen is a very picturesque crater lake, it can also be dangerous. Gas discharges near the lake contain a lot of sulphur, as a permanent solfatara on the lakeshore continuously produces native sulphur. This can make it impossible to breath. In 1976, 50 people were surprised inside the crater by an enormous bubble of sulphuric dioxide, which, after raising the surface of the lake, killed 11 of them by suffocation. The local people said it was the volcano who "asked" for a sacrifice as it offers all its richness.
A strongly active solfatara field is present in the south-eastern part of the crater near the lake shore, where sulphur-mining activities have existed for many decades. Gas temperatures fluctuate around 200°C. The fumarolic gases are led through pipes where native sulphur is deposited upon cooling. These gases can make breathing difficult, while miners are working with hardly any protection. The sulphur is collected by local workers and carried out of the crater, after passing a weighing station on the outer flanks.

Fishes in the Java sea die of Ijen's heavy metals
The acid nature of the water in the lake and river also generates environmental problems. After a first stretch within the caldera, it breaks through the caldera rim in the north and reaches a cultivated alluvial plain before reaching the Java Sea. In this area, virtually all of the acid river water is used for irrigation.
When entering the coastal plain near Asembagus, water of the Banyuputih river is used for irrigation of 3500 hectare of agricultural land where rice and sugar cane are the main crops. The area is inhabited by more than 100,000 people. In the dry season all of the water is diverted from the main riverbed. In general, pH-values varying between 3 and 4.5 have been measured during several campaigns in the late 1990s.
During increased water discharges and floods in the rainy season the river follows its original bed and reaches the sea. As local reports indicate, fish and other near-shore organisms perish at these occasions. According to 1996 data about 150 tons of sulphur, 2.8 tons of fluoride, 50 tons of chlorine, 10 tons of aluminum, 34 tons of quartz, 420 kg of manganese, 35 kg of titanium and 4 kg of copper are discharged daily into the irrigation network. Long-term activity of the lake and river system must have brought millions of tons of heavy metals to the coastal area. Because of high contents of fluoride in drinking water, tooth problems in local residents are widespread.