1. Algae and Tiny Microbes in Water Make Geosmin and 2-MIB, the Muddy Smells
Two main substances, geosmin and 2-MIB, are produced when blue-green algae and actinomycetes (a group of tiny soil and water bacteria) grow. In water sources like dams, lakes, and rivers, these substances cause a musty or earthy smell [1]. Geosmin is described as having a musty, earthy odor, while 2-MIB is described as smelling like ink. These smells can also attach to fish living in the water. A guide on seafood odors notes that freshwater fish like rainbow trout and catfish, as well as freshwater shrimp, can sometimes taste muddy [3]. However, other substances like pyrrolidine are also thought to be involved, so geosmin alone does not explain everything. For now, we will focus on these two main substances.
2. Warm Water and Nutrients Help Algae and Plankton Grow in Lakes and Reservoirs
According to explanations from Chiba Prefecture, when water temperatures rise in lakes and reservoirs, phytoplankton can multiply, leading to musty or ink-like smells [2]. One trigger for this growth is nutrients entering the water. Nitrogen and phosphorus from household wastewater and fertilizer from fields can be sources of these nutrients[2]. However, it is not decided by nitrogen and phosphorus alone. Water temperature and the types of living things that grow also play a role, so we cannot point to just one single cause.
3. Geosmin and 2-MIB Are Not Poisonous, but Human Noses Detect Them at Tiny Amounts
The odor threshold for geosmin and 2-MIB is said to be about 5 to 10 nanograms per liter of water for humans. Japan’s water quality standards require both substances to be 10 nanograms or less at the tap [1]. Chiba Prefecture explains that this standard is based on the limit where people begin to notice the smell [2]. To understand 10 nanograms, imagine a 25-meter pool holding 250,000 liters of water (assuming a depth of 1 meter). That would contain about 2.5 milligrams of the substance. This is one-four-hundredth the weight of a 1-yen coin (which weighs 1 gram). Toxicity has not been recognized in these substances [1]. Yet, the musty smell is taken seriously. A report from the Oita City Waterworks Bureau suggests this might be because we instinctively feel a "threat of mold" when we smell it. However, this is the author’s speculation, not a proven fact.
4. Fish Take In the Smell Through Their Gills and Store It in Their Fat
Geosmin and 2-MIB enter fish bodies through their gills, skin, and digestive tract. In a study of European whitefish, the gills were found to be the main way these substances enter, and they accumulate in fatty tissues [4]. The fish do not make these smells themselves; they come from microbes in the water. Freshwater fish are said to easily take in surrounding water through their body surface and gills due to osmotic pressure. This makes it easier for them to absorb odors [3]. It is said that people can detect these smells in fish meat at levels below 1 nanogram per gram [4]. Even at very low concentrations in the water, the substances can build up in fat until they reach a level humans can notice.
5. Fish Farms Remove the Smell by Keeping Fish in Clean Water Before Selling
In freshwater fish farming, it is common to keep fish in clean water for a while before selling them to remove the smell. This process is called "purging" [3]. In a study of European whitefish (weighing about 600 grams) raised in a recirculating system, the fish meat initially contained 32 nanograms per gram of geosmin and 24 nanograms per gram of 2-MIB. After keeping them in clean water for 16 days, both substances fell below detection limits. Geosmin was nearly below the detection limit after 13 days. The incoming water was near 0°C in winter [4]. In a test at the Aichi Prefectural Fisheries Experimental Station with eels (at 28°C), the eels were judged to have almost no smell after 7 days in a test group with a high water exchange rate and no feeding. This test suggests that water exchange is involved in removing the smell [5]. Since this was a preliminary test, we do not use its detailed numbers. The 16-day figure applies only to those specific fish and conditions. If the fish type or water temperature changes, the time needed would likely change too, so we cannot generalize it.
6. Water Treatment Plants Use Activated Carbon and Ozone to Remove Musty Odors
Standard rapid filtration at water treatment plants (which includes coagulation, sand filtration, and chlorine disinfection) cannot remove these musty substances dissolved in water [1]. Instead, processes using powdered activated carbon, ozone, or granular activated carbon are added to absorb or break down the odors. Chiba Prefecture uses similar treatments [2]. Powdered activated carbon is good for sudden outbreaks, but it does not always remove a high percentage of the smell, and it can be expensive [1]. In October 2014, blue-green algae producing 2-MIB increased in the Serikawa Dam upstream of Oita City. The concentration of 2-MIB reached 1,000 nanograms per liter, which is 100 times the standard of 10 nanograms per liter. Musty smells from taps lasted for 51 days, from October 9 to November 28, affecting about 120,000 households and causing 450 complaints [1]. The city added up to 30 milligrams per liter of powdered activated carbon, but removal was difficult, and water quality standards were temporarily exceeded. The report states that powdered activated carbon at about one million times the concentration of the musty substance was needed. Note that this case involved 2-MIB, not geosmin. The city also advised that boiling water is more effective than letting it sit [1].
7. How to Learn More About Water Quality from Your Local Water Bureau
If you want to investigate further, you can look at the website of your local water bureau with your family. Search for "water quality" to see if test results are posted. It might be interesting to see if musty odor substances are listed. You might also find water treatment plants that allow visits. Check your local water bureau’s announcements to see if you can see equipment like activated carbon or ozone systems. For official information, Chiba Prefecture’s page "What is 'Musty Smell' in Tap Water?" briefly summarizes the causes and solutions [2].