The Islay Peat Sponge

Islay is a large rock with a huge peat sponge on top.

The peat sponge varies in thickness from 2 to 15 metres thick. Between the bed rock and the peat there is a 5 cm thick band of clay that created the environment for the peat to form in the first place. Islay also has glacial and loess deposits too.

 

As rain falls it percolates and wicks its way through the fibrous peat to the bedrock below, before hitting the impervious clay band and running off into the burns and lochs. If the peat layer dries out – the burns stop; if the burns stop, the river levels drop dramatically, the lochs ultimately empty as evaporation out performs replenishment.

The high atmospheric pressure systems associated with the Spring, usually mean dry weather; It takes about 10 days of no rain for the peat layer to start to dry out. Then it’s like turning a tap off - suddenly the burns cease to flow. After five weeks the water runs out.

Subsequent short, sharp, heavy rainfall will often run off the dried peat surface with out penetrating the turf, or at least saturating the peat right through to the rock. The rivers will be in spate, lochs will fill up, but ultimately once past, the burns will remain inactive. The peat's saturation being still incomplete. A regular, light rainfall is needed to keep the water cycle going.

As distillers, it is the condensing water that is the first water supply to be affected – the 300 litres per second pumped into the still house for cooling the spirit vapour. The water comes from holding ponds at Burnside, fed by the Bruichladdich Burn.

The last to be affected is the 'process water' – the water the whisky is actually made from at mashing. This soft, peaty, non-mineralised water, comes from a shallow loch two miles up in the hills behind the distillery; it is piped directly from there in to the boiler house. With at least 36,000 litres a day removed by pipe and evaporation from the loch, the level quickly starts to fall.

An analysis of 2007's January to July rainfall shows that, there were two distinct, extended periods of low rainfall - March and June:

The four week period at the mid January to mid February period did not last quite long enough to be a problem. The heavy rainfall in first week of June was over too short a period to make any real difference and disappeared in spate. It is into the 6th week of no rain that the water levels start to become a problem for distilling, and in 2007 we had to stop briefly in both late March and late June.

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