The waters from Speyside, Islay, and the Highlands are meant to be representative of the waters used by distillers in those regions to make scotch; for dilution of drinks in the bourbon-and-branch style.
As I learned in the book Whisky on the Rocks, even distilleries next to each other may have different water sources, so it shouldn't be assumed that all the distilleries in an area use waters just like these in their whisky, but it seems like a good place to start.
The really cool thing about Uisge Source that it's not just water they sourced from these regions; they actually tell you about the chemistry of the water.
Islay’s Ardilistry Spring produces water with higher natural acidity which is created by filtration through peat.
St Colman’s Well in the Highland region produces a hard water, high in minerals due to filtration through porous and brittle red sandstone and limestone.
The Cairngorms Well in the Speyside region produces a soft water, low in minerals as a result of being filtered through hard rock such as granite.
And they give a chart of each water's properties. I love charts! (Click to make it bigger.)
As you can see from the chart, the Highland water is full of minerals including calcium and magnesium. Islay water is high in potassium, chloride, and sodium, and has a lower (more acidic) pH. Speyside water is low in nearly all minerals and has a slightly higher (more basic) pH than the other waters.
So: How do they taste? Happily, they sent me some to experiment with.
Uisge Source Taste Test
Speyside: Tastes quite dry. I notice this in distilled waters without mineral content, though at 125 ppm dissolved solids this still has a lot more minerals in it than my tap water. There is a granite taste to the water as well - not a creamy soft minerality but a hard one.
Highland: I measured the total dissolved solids (TDS) in this one at 225 ppm. It tastes softer in body and sweeter than Speyside. It's also more earthy.
Islay: At 183 ppm TDS it is halfway in mineral content between the other two, but this water has the most flavor- it's got a pronounced dirt/earthiness to it but I also taste grainy minerality.
Then the natural test would be to try different whiskies with the different waters, so that's what I did. The results were surprising!
Tasting Uisge Source with Scotch
I tried a 25-year-old Highland single-malt with each water, and a 10 year-old cask-strength Islay with each. I was surprised to find that each whisky tasted best with its regional water! Maybe I just got lucky - I didn't measure quantities down to drops and such, but I really didn't expect these to align.
The Speyside water made both the Islay and Highland whisky taste sweet. The Highland water brought out honey notes from whiskies, but it was totally in synch with the flavor profile of the Highland scotch where it wasn't a perfect fit for the Islay. The Islay water brought out the creme brulee and smoke of the Highland scotch which was good but not the typical flavor profile I associate with it, while it did the same for the Islay scotch to great effect.
This could all be in my head (and down my throat at this point) but I was surprised at how much these waters with subtle differences brought out pronounced differences in the whisky. Awesome.
Looking to buy Uisge Source? Unfortunately it's not in the US yet, though they tell me they're in talks with an importer and I'll share the news when it's available. They have a list of retailers on the site for UK customers and you may find it in duty-free shops in some airports.
The Water Project on Alcademics is research into water in spirits and in cocktails, from the streams that feed distilleries to the soda water that dilutes your highball. For all posts in the project, visit the project index page.