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Tap water comes from surface water (eg. rivers, lakes, streams and reservoirs) or ground water taken up from wells or bore holes. Meanwhile, both mineral water and spring water come from natural underground sources.

As water passes by rocks it collects salts and minerals of calcium, magnesium, sodium and potassium. Bottled water has lots of these minerals in it but tap water can have similar levels, especially if it has come from groundwater sources.

2. Tap water undergoes more treatment than bottled water

Basic water treatment for tap water, according to lecturer in water chemistry at the Cranfield Water Institute Dr Emma Gosling, "removes the particles, the organics (eg. decaying vegetation) and microorganisms," while advanced water treatment "removes all those things, but it will also remove micro pollutants such as pesticides."

Chlorine has a crucial role to play in the treatment of tap water. It not only removes harmful bacteria at the treatment centre, but a residual amount of it will also do the same for the pipes that the water is carried through.

Meanwhile, bottled waters do get treated, but there is a difference between mineral and spring. Both need to come from a natural underground water source that is protected from pollution, and both types of bottled water need to be free from parasites and bacteria but mineral water must retain its "original purity" from source to bottle, whereas spring water can undergo any form of treatment.

In one set of tests, bottled water had less bacteria in it than tap water in three out of four samples. But it's worth pointing out that both tap and bottled water have to adhere to strict safety standards so both are safe to drink.

4. When it comes to health benefits, you are what you eat, not what you drink

A World Health Organisation conference in 2005 suggested that the presence of higher levels of some minerals – such as magnesium and calcium – in mineral water could give health benefits. However, different bottled waters have different types and amounts of minerals in them, and very hard tap water from groundwater sources might have as much mineral content as some bottled waters.

However, these differences between waters pale into insignificance in the face of a healthy diet. "If you're eating a diet with fruit and green leafy vegetables," says Dr Gosling, "you'll be getting those minerals anyway, usually at higher levels than in drinking water."

5. Bottled water is around 1,000 times more expensive than tap water

While health benefits might be a nuanced conversation, there's not much treading water where price comparison is concerned. With everyday branded bottled water ranging from 60p to £2 a litre, and tap water costing around 0.19p a litre according to Dr Gosling's own bill, the price multiple between the two ranges from more than 300 times to 1,000 times. "We are paying for the convenience of the package when we buy bottled water, and possibly the taste," says Dr Gosling.

The "taste of water" might seem like a paradoxical notion, but it does vary between bottled water brands and between hard and soft tap water. "The description of water is all about the natural minerals within the water," explains water sommelier Milin Patel. "Some natural waters from volcanic regions may have a high level of silica, which gives it a beautiful, silky velvet, smooth mouthfeel, which is very distinctive. Waters with high levels of calcium, magnesium can taste creamy."

Tasting some of these nuances would have you spending money like, well, water. Some examples of "unique experience waters" include: £5 a litre for water from a 90-million-year-old Czech glacier, £24 a litre for water from clouds above the Canary Islands and a gulp-inducing £50 a litre for water from an 8,000-year-old spring in Slovenia.

7. Water bottles are highly recyclable… if they are recycled

Then there's the question of environmental sustainability. Water bottles are made from the plastic polyethylene terephthalate, or PET, described as "highly recyclable and very circular material" by Adam Herriott from a climate change charity WRAP (the Waste and Resources Action Programme). Every council in the country has provision to collect this lightweight plastic for recycling into new products, and some manufacturers, like Buxton, say that their bottles are already made from 100% recycled material.

However, the "circularity" of PET is frustrated by a low recycling rate of somewhere between 22% and 28%, with the national recycling campaign Recycle Now estimating that only 20 million of the 36 million plastic bottles used every day are recycled. Like a lot of plastic packaging, if PET products are not disposed of properly they can break down into microplastics and get into various ecosystems.

Isla knew that tap water goes through a lot of cleaning processes so her hunch was that it would be cleaner than bottled water. Because bottled water – particularly mineral water – has to maintain its original purity and essentially be bottled at source, Greg wondered this too. "But," says Greg, "an expert told me that in tests, three quarters of bottled water samples had less bacteria than tap water."

"The bottom line here – because I don't want anyone to worry about this – is there are strict safety standards that both tap water and bottled water have to meet. So, they're both safe to drink."

9. Bottled water may have a small health benefit… but it's close

And finally, on the key question of whether bottled water is healthier than tap, if you look at the labels for most bottled waters you will see that they contain various minerals. These are minerals that have come from the rocks that the water has passed through. Different water passes through different types of rock, and some bottled water have more minerals than others, which affects the taste.

There is some thought, Greg explains, that some of these minerals – like calcium and magnesium – could be good for you. So bottled water may have a small health benefit. However, some tap water has just as many minerals as some bottled waters, and – as Dr Gosling pointed out – if you eat a good diet of fruit and vegetables, you're going to get a lot more minerals from that than the water that you drink.

Hear Greg's full summary, and find out how it will affect Isla’s water consumption in future, by listening here.

The information contained in this article was correct at the time of broadcast on 16 February, 2023.