Although the current ash cloud sitting over the UK is not likely to have any deleterious effects at ground level It is worth a thought that any serious fall of ash into uncovered reservoirs in the UK could cause water quality problems.
It was the contamination of water supplies with Hydrofluoric Acid that caused the widespread livestock deaths that led to famine in Iceland in 1783.
Of course boreholes draw their water from deep underground and would therefore not be contaminated by airborne volcanic pollution.
Worth a thought?