Water is the only ingredient that a baker needs which is given for free.
Of course, water isn't free - it's just that we don't usually pay for it directly. In the cities and towns, it's gathered, treated, and piped to our home or workplace, and we barely think about it except if we need to pay a water levy at some point, or if it rains and our routine is interrupted.That's why bakers love high hydration dough - less to pay for in a loaf of bread.
In the country, we think about water a lot. If you have lived in the country and rely on tanks to capture water, you will be very aware of drought events, because you will be running out of water. Some, like us, can ring up a water tanker and pay to have some delivered, if there has been a long time between rain falls. Others. particularly here in Australia, live so far away from water that ringing up a tanker is far too expensive - so they rely on effective water storage, or they perish.
It's one of those things which divides the city and the country. City folk think nothing of showering twice a day, or leaving the tap running longer than necessary, or watering the lawn. Country people cringe at the thought of any of these wasteful extravagances. And I guess the point here is that water is a resource that is unevenly distributed, and not managed to the same benefit for everyone.
But that's the supply side of the discussion. What about quality?
In breadmaking, water is an ingredient which can dramatically effect the quality of the bread we make. If water is too chlorinated, our fermentation process can be slowed down. If it contains too many minerals, or not enough, our breads will be very different. If it is dirty, the flavour will change. All of these things come into the mix - though bakers, being the ingenious creatures they are, will learn to find ways to work with whatever they have.
'Hard' water, or water that has a lot of mineral ions, can effect gluten development, and will cause 'tough' dough - or dough that is lacking in elasticity, and so the rise will be smaller. However, calcium and magnesium ions, which are the minerals present in larger amounts in hard water, are in themselves a yeast food.
The problem is also compounded by the fact that very hard water is slightly alkaline, thus neutralising the acids needed in fermentation.
The easy fix here is to increase the yeast content, or the amount of sourdough starter used. A better solution, though, is to keep the level of leaven (yeast or starter) the same or less, and use prefermentation instead. A long preferment will increase the amount of enzyme activity, thereby adjusting acidity automatically. Have a look at the article in this section about prefermentation.
The problem with soft water is almost the opposite of hard water. Soft water is generally slightly acidic - too acidic for good gluten formation, so the gluten breaks, or breaks down, during the proofing processes, thereby causing bread that 'goes sideways' rather than up, or which becomes 'porous', meaning that the crust becomes full of holes.
In this case, the minerals that yeasts need in trace amounts are lacking, so the gluten has no strength. The easy, chemical fix is to increase the salt level ever so slightly. It would also help to use a natural salt, rather than a refined one - such as himalayan or celtic sea salt, which are both rich in mineral salts like magnesium and calcium. If using these salts, it is quite likely that an adjustment to your recipe will be necessary anyway, as they are very mild in flavour. You need to be mindful of whether you already have hard water though, because these salts do increase calcium and magnesium in your dough.
The other way to deal with soft water is to reduce your leavening times - particularly prefermentation and autolyse - as with a slightly acid water, fermentation occurs more quickly. Again, your preferment is very useful here, even a short preferment, as it behaves like a shock absorber in the dough, lessening acidity by the amount of time the prefermentation process is given.
Chlorine and Chemicals
If you are on the water grid, both of these things will be in the water. The former, chlorine, is not such a problem - it exits the water naturally, so you can just let the water stand overnight if chlorine bothers you, and in the morning it will be gone. Having said that, I'm not sure that a well established sourdough starter is affected by chlorine. Perhaps a young starter will be though.
The latter, flouride, is harder to remove. It bonds with the water molecules, and even filtration won't remove it, unless you do some kind of osmotic filtration, which in itself wrecks the water anyway. Having said that, I'm not convinced that flouride has a noticeable effect on dough as such - just more chemicals for our body to deal with down the track.
The easy fix here is to use rain water - if you are in an area with lots of trees to clean the air, the rain water will generally be pretty good. If you live in an urban area, or in a house with a tiled roof which is used to collect the water, then you might need to filter the water before you use it. A simple charcoal filter will do the trick most of the time.
The subject of water is huge, and I can barely touch on it here. I would very much love to discuss the water cycle from farm to crust, and how as an ingredient, water is constantly transforming itself - but there is simply not enough time in the day. I do hope this little snippet has been useful.
Happy Sourdough Baking!