During a drought of historic proportions in 1995, the Mineries Pool Outflow stream (the largest stream entering St. Cuthbert’s Swallet) and a nearby spring stopped flowing. The Pool is supplied by water draining an upland marsh in an area that has seen more than 1600 years of lead mining and lead smelting. When flow of the Pool Outflow resumed, a huge surge of calcium sulphate appeared in its water. From a mean concentration of 10.1 × 10-5 M, and a previous maximum of 17 × 10-5 M, in 40 days the sulphate concentrations rose to 91 × 10-5 M. The event lasted approximately 140 days. In 1996 there was another severe drought, but the streams did not dry up, and the sulphate levels remained normal. Ion chromatography was used to determine sulphate and nitrate concentrations in the streams. The survey was an example of the application of a scheme of analysis developed by Knights and Stenner, in which ion balances were used to analyse the reliability of the analytical data.