What is TDS in water and why does it matter?

When comparing and discussing bottled water, TDS is often mentioned as a standard quality metric. TDS stands for "total dissolved solids" and measures the minerals, salts and organic compounds that naturally dissolve in water via contact through rock and other surfaces. It is measured in milligrams per litre.

As a trending topic where brands, scientists, and consumers all have their own perceptions and opinions about mineral water, this is a basic roundup of how to understand TDS levels in drinking water.

Table of contents

* TDS: A measure of minerals in water

* Why is the TDS level of a water important?

* How is TDS measured?

* What types of minerals can be found in water?

* How do minerals get into the water?

* Examples of high and low TDS sources

* The difference between high-TDS water and hard water

* Is there a dangerous TDS level in drinking water?

* Is water with high mineral content healthier?

* Conclusion

TDS: A measure of minerals in water

Total Dissolved Solids (TDS) measures the concentration of all inorganic and organic elements dissolved in water. The TDS level will tell you how mineralised your water is, but it does not reveal which specific minerals it contains. The most commonly used unit for TDS is milligrams per litre (mg/l) and is an expression of the actual mass of minerals dissolved in a litre of water. The minerals are what create the specific flavour and mouthfeel characteristics of the water.

Water is not just water. With varying mineral content, they can taste and feel very different. There are huge differences in brands. For example, Hildon has a TDS of 312 mg/l, Vidago 2,900 mg/l and Svalbardi only 21 mg/l.

Dissolved gases like nitrogen, oxygen and carbon dioxide are commonly present in groundwater and the atmosphere. All of which can contribute significantly to the character of bottled water. However, TDS doesn’t measure dissolved gases, only dissolved solids.

Using a water filter will generally not reduce the TDS level. The exception would be a reverse osmosis (RO) water filtration system. Such a system passes the water supply through an extremely fine membrane tight enough even to desalinate sea water.

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Why is the TDS level of water important?

TDS affects the taste of the water, or things it is mixed with, such as a cocktail. It can also affect your health and the piping system in your home.

• Taste & mouthfeel: Depending on the type of dissolved solids, high TDS levels often cause a bitter, salty, or even a sulphuric taste or odor. Cooking food or brewing tea/coffee with elevated mineral water normally affects the end result. Mouthfeel refers to the physical sensations you get in your mouth when drinking. For example, silica highly impacts water’s texture, giving it a ‘slippery’ feel. To know which specific minerals each water contains, you will have to read the bottle label or use a chemical test kit.
  
  
• Gastronomical experience: When pairing water with food or other drinks, knowing the TDS level will help you find the perfect water to enhance your tasting experience. For instance, the ideal pairing for whiskey is a super-low TDS water that avoids polluting the nuances of the liquor. Low TDS waters also pair well with light foods as they allow the flavours in the dish to shine. When eating red meats, a carbonated water with salty tones (typically high TDS) will pair better, as high mineral water has weight to it and will complement the dominant flavours of the dish.

Pairing water with food can deliver new gastronomical experiences.

• Nutrition & health: While some substances, such as lead or copper, are health hazards, others are necessary in moderate amounts for proper body functions. Minerals, such as magnesium and calcium, may benefit those unable to achieve a balanced diet. A low TDS concentration is not harmful, it simply does not add minerals to one’s intake. It still has all the same hydration benefits of any water.
  
  
• Pipes & home: Elevated levels specifically of calcium and magnesium can cause hard water which may collect in pipes and build up in sinks, tubs, pools, and faucets. Hard water can also stain the tap, shower, and toilets. For example, if there are very high levels of calcium in your tap water, it can cause clogging or rust in the pipes, or break heating equipment. Iron at levels over 0.3 mg/l stains laundry and plumbing fixtures.

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How is TDS measured?

To measure total dissolved solids at home, you can use a TDS meter which measures the conductivity of water. Conductivity measures how well water carries an electric charge. Distilled water without any minerals does not conduct electricity and will show a TDS of 0. The more minerals present in the water, the more electricity it conducts. When measured at a standard 25°C temperature, the conductivity measurement can then be converted into a milligrams of minerals per litre equivalent. Most handheld meters will give you results in either conductivity or TDS terms.

In the laboratory, the standard method for measuring TDS is to weigh the minerals left after removing the water by evaporation. The liquid is evaporated at 104°C/219°F and then the remaining dry matter is placed in an oven at 180°C/356°F to further remove moisture from the mineral residue. The milligrams of material which remain per litre are weighed as is the TDS number. Despite the potential for loss of solids, 180°C is a good compromise between complete removal of water and minimisation of dry material loss.

A typical handheld TDS meter

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What types of minerals can be found in water?

The minerals that make up a particular water’s TDS have varying effects on flavour and mouthfeel, as well as potential health impacts.

Major ions are the most frequent minerals dissolved in water and comprise at least 90% of TDS. The most common minerals are:

Mineral	Effect	WHO Recommended Amount *
Calcium (Ca)	Helps maintain strong bones and dental health. Serves as a signal for vital physiological processes, including vascular contraction, blood clotting, muscle contraction, and nerve transmission. Tends to make water chalky or milky.	No guideline value**
Magnesium (Mg)	Helps convert blood sugar into energy. Preliminary scientific research suggests it may help reduce stress. It is also involved in protein and nucleic acid synthesis and needed for normal vascular tone and insulin sensitivity. Bitter and metallic taste.	No guideline value
Sodium (Na)	Promotes proper functioning of nerves and muscles. Creates that recognizable salty flavour.	< 200 mg/l
Potassium (K)	May help reduce blood pressure by counteracting the impact of sodium. May reduce the risk of osteoporosis and kidney stones. Essential for regulating muscle contractions and maintaining proper nerve function. May leave a metallic bitterness.	3000 mg/l (seldom found in drinking water at levels that could be a health concern)
Bicarbonate (HCO3)	Vital for the pH buffering system in the body. Also gives a salty taste.	< 500 mg/l
Chloride (Cl)	Helps maintain pH balance and transmission of nerve impulses. Small amounts of chlorides are required for normal cell functions. Salty taste.	< 250 mg/l
Sulphate (SO4)	Can have a laxative effect over 1000mg/l. Salty and bitter tones.	< 250 mg/l
Nitrates (NO3)	May interfere with the ability of red blood cells to carry oxygen. Tasteless in water.	< 50 mg/l
Silica (SiO4)	Helps promote healthy skin, hair and nails as it is a key ingredient in collagen creation. Gives a “slippery” mouthfeel.	No data
Iron (Fe)	Essential for liver functionality. Can tint water a red/brown colour. Disagreeable metallic taste.	No guideline value
Zinc (Zn)	Important for the immune system. Tasteless if you have a zinc deficiency, but metallic if you have a sufficient amount in your diet.	3 mg/l
Total dissolved solids (TDS)	All the minerals dissolved in water.	1000 mg/l

The following trace elements are examples of minerals with fixed maximum limits often set by national authorities which, if exceeded, may pose a risk to health:

Mineral	Effect	WHO Recommended Amount*
Copper (Cu)	Causes severe stomach aches.	< 1.0 mg/l
Bromide (Br)	Ability to oxidize into bromate, which is toxic.	No data
Mercury (Hg)	Found to cause kidney damage.	< 0.001 mg/l
Lead (Pb)	Risk of lead poisoning. Often a result of old rusty water pipes.	< 0.01 mg/l
Fluorides (F)	Excessively high intake may cause crippling skeletal fluorosis and possibly increases bone fracture risk. In very small amounts of 0.5 - 1.0 mg/l it may have a positive impact on dental health.	< 5.0 mg/l
Arsenic (As)	High levels increase the risk of cancer.	< 0.01 mg/l
Nitrites (NO2)	Can cause nitrite poisoning. Oxidizes the iron component of red blood cells (hemoglobin), rendering them unable to carry oxygen.	< 3mg/l

Other trace elements may come from air pollution, industrial leaks, or rusty piping.

* Table uses the acceptability aspect (not a health related limit) of the World Health Organization’s (WHO) Drinking Water Standards. Note that recommended daily intake of each element is set at national and international levels, and may vary from country to country.

** No guideline value: When absorbed in excess of need, the excess is excreted by the kidney in most healthy people.

How do minerals get into the water?

Most waters such as artesian, lake, spring, or glacier meltwater, have passed through or run over some ground strata. As rainwater or meltwater flows through the ground, it gradually acquires a chemical signature from the layers of rocks and clay. Water is an effective solvent ("the universal solvent"), and given enough time, dissolves most naturally occurring minerals.

A simplified illustration of how substances get into water. Graphic from BE WTR.

Depending on local geology and layers of soil, water will pick up different mixtures of minerals from areas it flows through. Which minerals dissolve as it travels depends on many factors, including the specific geology of the layers of earth it runs through, pressure, and rock porosity.

The longer water filters through soil and rocks, the more minerals it will absorb. Groundwater may continue to evolve chemically after many thousands of years underground.

While most minerals absorbed are naturally occurring, some may come from human activity. This is especially true of heavy metals, as well as other harmful substances and chemicals not measured in TDS. This can include agricultural runoff or urban runoff from industrial activity at a high enough concentration that it can also harm aquatic life.

Examples of high and low TDS water sources

Waters that haven't naturally drained through the ground - such as from icebergs or rain - typically have lower TDS levels than those sourced from springs or artesian wells.

This is because of three main factors:

1. How abundant the minerals are in the geological strata,
2. How easily these minerals can dissolve into water.
3. How long the water is in contact with the geologic mineral source.

The higher up towards the surface the water is flowing, the lower the TDS tends to be as minerals in surface soils have often been flushed out by repeated precipitation and runoff, though this is far from uniform. For example, shallow sources that pass through limestone may be high TDS. And deeper sources that pass through dense rocks such as granite that leach very few minerals may be low TDS.

The Fine Water Society has categorised water into the following mineral groupings according to TDS level:

• Super low: 0-50 mg/l
• Low: 50-250 mg/l
• Medium: 250-800 mg/l
• High: 800-1,500 mg/l
• Very high: over 1,500 mg/l

Legal and regulatory definitions do vary in different countries around the world though. Freshwater is normally below 1,000 mg/l, while seawater starts at 10,000 and goes up to 35,000 mg/l.

Water brands with very high TDS levels

Water Brand	Country	Source	TDS
Three Bays	Australia	Spring	1300 mg/l (still)
Gerolsteiner	Germany	Volcanic spring	2527 mg/l (sparkling)
Vichy Catalan	Spain	Hot spring	3052 mg/l (sparkling)
ROI	Slovenia	Spring	7400 mg/l (sparkling)

As in the wine world, the term terroir very much applies to water. In both cases it describes the environmental factors which impact the taste and other unique characteristics of the product. Some of the richest mineral sources are hot springs and springs near mineral-rich volcanic strata. For example, Vichy Catalan is sourced from a 60°C/140°F spring in the Catalan region of Spain where it acquires not just extremely high mineral levels, but natural carbonation as well. ROI from Slovenia comes from an extremely magnesium-rich spring giving it a TDS of 7,400 and a strongly metallic, complex and salty taste

There are plenty of exciting high TDS brands to choose from. Photo from Punchdrink.

High TDS waters are almost always sparkling precisely because they are often sourced from regions with past or current volcanic activity. This creates springs not just rich in bicarbonate, calcium and magnesium, but which also impart natural carbonation due to cooled magma releasing carbonic acid into the water. If the amount of carbonic acid is more than the water can absorb given the temperature and pressure of the specific spring, carbon dioxide gas (CO2) will dissolve. Regions without these characteristics generally do not have soils rich enough in minerals to cause very high TDS levels.

One rare high TDS still water is Three Bays from Australia’s Mornington Peninsula. It spends 2000 years passing through red soil rich in 23 different healthy minerals. Even this only imparts a TDS level of 1300, literally a fraction of many sparkling waters. Although the variety of minerals in Three Bays does give it an amazingly rich, creamy taste and mouthfeel.

Water brands with very low TDS levels

Water Brand	Country	Source	TDS
Isbre	Norway	Granite strata spring	4 mg/l (still)
Svalbarði	Svalbard	Iceberg	21 mg/l (still)
Cloud Juice	Australia	Rainwater	45 mg/l (still)

As mentioned, brands with the lowest TDS levels are typically from sources that have never touched ground or only passed through ground that doesn’t leach minerals such as granite.

Iceberg water is made from pieces of ice that calve off land-based glaciers (though the glacier face often hangs over the sea) and float in the sea until they melt and are lost forever. Once gathered, they are generally kept frozen until shortly before bottling when they are quickly rinsed off, melted and placed in a sealed holding tank. This ensures no contact with any geologic strata where the water could pick up minerals or terrestrial pollutants. As long as the icebergs are collected come from the inner part of the glacier which never scraped along ground, the water maintains the purity it had when it first fell as snow thousands of years ago.

Svalbarði is an example of a water with an ultra-low TDS level.

As iceberg water has almost no minerals, the taste difference becomes clear when compared side-by-side with any mineral-rich water. The low TDS and presence of naturally dissolved atmospheric gases is exactly what creates the unique, light-as-air taste of iceberg water.

Rainwater is another low TDS source as the rain falls directly from the sky into a holding tank, without passing through layers of mineral rich soil before it is bottled. However, rainwater from coastal areas may have slightly higher sodium levels because of local seawater evaporation even if it won't be tastable. Rainwater is soft and usually slightly acidic due to absorption of natural atmospheric carbon dioxide. Marble chips (calcium carbonate) are sometimes added to rainwater storage tanks which contributes to calcium intake and corrosion prevention.

King Island Cloud Juice from Australia - made from rainwater.

Surface waters sourced from rivers or lakes fed by precipitation or direct glacial runoff (as opposed to springs) often have a very low TDS since they have not spent much time in contact with the ground. Although their potential exposure to bacteria and terrestrial pollutants creates an entirely different set of issues.

Some have pointed out that distilled water is also ultra-low TDS (zero actually) and hence assume it would taste the same as iceberg or rainwater. This is not the case, because unlike distilled water, these natural sources come from precipitation which absorbs air that includes nitrogen and oxygen. This produces a fresh taste very distinct from the flat, laboratory-like taste of distilled water. Distilled water is factory processed and produced by boiling it to evaporation, and then condensing it back to liquid form stripped of all minerals and dissolved gases.

Some bottlers also add minerals manually, but these can’t be labeled as natural mineral water (NMW) in most countries due to strict regulations ensuring NMW is bottled in its unaltered state.

Difference between TDS and hardness

Hardness and TDS are not the same thing, as water can have high TDS levels without being categorized as hard. Hardness is specifically identified as the amount of calcium and magnesium salts in water, generally in the form of bicarbonates, chlorides, and sulphates. TDS, on the other hand, measures the total of all mineral elements - not only calcium and magnesium.

Hardness is sometimes measured by the capacity of the water to react with soap. Hard water requires considerably more soap to produce a lather. The most common way to calculate hardness is measuring the amount of calcium and magnesium.

Example of hard water stains. Photo from Reddit Plumbing.

Hard water may build limescale in equipment that heats water, such as kettles and boilers. Over time, this will build up and eventually break equipment. Using a water softener is a good idea if you have hard water and want to avoid calcifying the pipes and ruining kitchen equipment.

Is there a dangerous TDS level in drinking water?

TDS does not measure health benefits, it is merely a measurement of total mineral content. The specific minerals contained in a water determine if it is safe or harmful. There are minerals that help maintain a healthy body, and there are others that - depending on concentration - can cause harm or even poison a person.

There is ongoing discussion regarding the health benefits of extreme TDS levels. However, there is little reliable research on the topic.

Environmental engineer Andrew Whelton of Purdue University advises against drinking distilled water, which has a TDS level of 0 mg/l, saying the minerals in teeth can leach into the mineral starved water. This statement does not have any known research to substantiate it. Nonetheless, drinking distilled water is usually not preferred as it has a flat "laboratory" taste.

Is water with high mineral content healthier?

The human body needs certain minerals to build strong bones and teeth and turn food into energy. Intake of minerals through drinking water can benefit those unable to maintain a balanced diet. But, for most people, sustaining a varied diet is sufficient to secure the minerals needed.

Waters with low TDS can be enjoyed without worrying about excessive sodium intake.

Although it is generally agreed that sodium is essential to human life, it is estimated that a total daily intake of 500mg is sufficient. To avoid the risk of cardiovascular disease, the intake of natural mineral water with a high content of salts should not be excessive. Some brands have more sodium than a slice of pizza, and the average American already consumes about 3500 mg sodium. So very much like food, a moderate intake of high TDS waters is recommended.

Conclusion

Water is much more complex and intriguing than ‘just water’ once you are aware of all the invisible minerals that hide within this essential, life-giving liquid. The best way to learn about water is simply to drink a variety of waters with different TDS levels and compare them. Begin with the highest TDS level and work your way down, finishing with the lowest. Memorizing the most common minerals in drinking water, and how they impact the flavour and mouthfeel, will help you recognize the mineral content without even reading the label.

Experimenting with matching and contrasting water with food or other beverages, such as wine or coffee, is an ideal exercise to improve your fine water and gastronomy skills. And for a water enthusiast’s experience unlike any other, tasting iceberg water with ultra-low minerality is as easy as trying Svalbarði Polar Iceberg Water.