sentences:  This affected the environment as some watercourses become seasonal and domestic users were subject to drought restrictions, for the first time for many years.This may have been a contributing factor in the outbreak of Cryptosporidium parvum in the Bushey area in March 1997 when Three Valleys had to ask 300,000 consumers in Hertfordshire to boil water.The source was never isolated though several water supply boreholes in the Chalk aquifer between St Albans and Bushey contained the pathogen.