1. Why is the underground water in Chiba special?
Deep underground in Chiba, at depths of several hundred meters to about 2000 meters, there is old water often called "fossil seawater." The gas field there, including the South Kanto gas field, is a water-soluble natural gas field [4]. In Japan, iodine is said to be produced commercially in Chiba, Niigata, and Miyazaki [3]. The methane gas is dissolved in the water underground. When the water is pumped up and pressure drops, the gas separates from the water, similar to how bubbles appear when you open a carbonated drink, though the gas here is methane, not carbon dioxide [4].
The special feature of this water is its high concentration of iodine. Professor Kyoichi Saito of Chiba University wrote about his experience tasting the brine from the South Kanto gas field, noting that the iodine is concentrated about 2000 times more than in normal seawater [9]. In ordinary seawater, iodine is present in very small amounts. Why is only the underground water so concentrated? There are several theories, such as it originating from seaweed or marine life, but the exact reason is not yet confirmed.
2. How does the blow-out method use air to push out iodine?
One method for extracting iodine from brine is called the "blow-out method." As the name suggests, it uses air [5]. The process generally has three steps. First, an oxidizer like chlorine is added to the pumped-up brine to change the dissolved iodine into its elemental form. Next, air is blown in from the bottom of a tower, pushing the iodine out as a gas. Finally, the air containing iodine is absorbed into another liquid to concentrate it, and it is oxidized again to extract the iodine [5].
You can imagine blowing air through a straw into a drink: bubbles rise as the air passes through the liquid. In the tower, the iodine moves into the air as it passes through the water. The types of chemicals and detailed steps differ from factory to factory, so this description covers only the general process [5]. The name "iodine" comes from the purple color of its gas, derived from the Greek word for "violet-colored" [5].
3. How did the source of iodine change from seaweed to underground water?
Iodine was first discovered in 1811 by Courtois in France, found in the ash of seaweed [5]. Seaweed can accumulate a lot of iodine; some kelp relatives are said to contain up to 0.45% iodine by dry weight [2]. The USGS (United States Geological Survey) states that before 1959, seaweed was the main source of iodine [2].
The turning point was extraction from underground water. Ise Chemical Industries, a company in Chiba, records that they started manufacturing iodine from seaweed in 1927. In the 1950s, they used a method involving activated carbon. In October 1961, they established the air-blowing method and began production combined with natural gas at their Shirako factory in Chiba. By 1970, they had switched all factories to this method and became the world’s largest iodine manufacturer, according to company records [8].
Today, global iodine supply is concentrated in three regions: nitrate mines in the Chilean desert, gas and oil fields in Japan, and brine wells in northwestern Oklahoma, USA [2]. USGS estimates for 2025 production are 23,000 tons for Chile and 9,000 tons for Japan [2]. Chile produces about 2.6 times more than Japan, but Japan is still the second-largest producer in the world.
4. What agreements were made to prevent ground subsidence?
When brine is pumped up, the amount of underground water decreases, which can cause the ground to sink slowly. This is called "ground subsidence." Documents from the Kujukuri Plain state that brine extraction is said to have caused ground subsidence [4].
To address this, Chiba Prefecture and companies extracting brine signed an agreement in 1973 to suppress ground subsidence. Records show that 342 gas wells were reduced over the three years from fiscal year 1973 to 1975 [4]. The extraction volume peaked in 1968 at about 206,000 cubic meters per day. It has continued to decrease, reaching about 134,000 cubic meters per day in 2024 [4]. This is about two-thirds of the peak volume (13.4 divided by 20.6 is approximately 0.65). Some of the brine is returned to the underground [4]. However, these figures are based on data from the Kujukuri Plain and may differ from the total extraction volume for all of Chiba Prefecture.
5. What is iodine used for, and why is it sent abroad?
The uses of iodine have expanded over time. It began with iodine tincture disinfectants in the 1870s. Since 1900, it has been used in contrast agents for X-ray examinations. Around 1950, it began being used in polarizing films for liquid crystal displays. Today, the largest use is still X-ray contrast agents [6]. Globally, the uses in order of volume are X-ray contrast agents, liquid crystal displays, pharmaceuticals, iodine-based disinfectants, and feed [6].
Iodine is also an essential element for the human body. In Japan, deficiency is rare because of the habit of eating seaweed, but in other countries, deficiency can be severe, causing thyroid swelling and developmental delays [6]. Many countries add iodine to table salt [6].
Chiba Prefecture, cooperating with the Japan Iodine Industry Association, donates potassium iodate to countries with iodine deficiencies. Between 1996 and 2000, 3,400 kg was sent to Mongolia. Between 2006 and 2008, 3,400 kg went to Cambodia. In 2014, 850 kg was sent to Sri Lanka. Since 2018, donations have continued to Madagascar [7]. The iodine taken from underground water in Chiba is also used to support countries short of iodine.
6. Can iodine production increase without pumping more water?
Japan’s iodine production is difficult to increase because pumping limits are set to prevent ground subsidence [6]. Therefore, efforts are underway to recover and reuse used iodine, and plans are advancing to cultivate seaweed in Chiba to extract iodine from it [6]. In 2018, the Chiba Iodine Resource Innovation Center was established at Chiba University [6].
There are also fields where demand is expected to grow. Chiba Prefecture’s public relations note that iodine is a key raw material for perovskite solar cells, which are currently attracting attention [1]. The first iodine was taken from seaweed. Now it is taken from ancient seawater, and plans are to take it from seaweed again in the future. The method of extracting iodine seems to be going in a full circle. However, how much production will actually come from seaweed cultivation remains to be seen.
7. How can you find iodine at home and calculate iodine numbers?
Find it at home: With your family, look at the labels of disinfectants and gargles in your medicine cabinet. Look for the word "iodine." Just look; do not drink it or get it in your eyes, and follow the cautions on the label.
Calculate it: The iodine in seawater is about 0.06 ppm [2]. If you multiply that by 2000, you get about 120 ppm. If you consider 1 ton of water as about 1000 kg, how many grams of iodine is 120 ppm per ton? 1 ppm is one-millionth. Try calculating it yourself.
Open the Chiba page: The "Iodine Support" page of Chiba Prefecture has a list of countries and amounts donated. Compare the differences between countries.