Sydney 1998: lessons from a drinking water crisis

Abstract

From July to September 1998, Sydney, Australia, experienced a drinking water crisis after high concentrations of Cryptosporidium and Giardia were detected in the city's water supply and distribution systems. Three boil-water advisories were issued, the Sydney Water Inquiry was formed to investigate the incident's origin, and outside laboratories, experts, and auditors were called in to assess the original analytical procedures and results. This study reviewed the information submitted to the inquiry and found evidence to confirm that the contamination did occur and explain its causes. The results of the Cryptosporidium and Giardia microbiological analyses are in agreement with the body of limnological evidence collected from across the supply system. This finding suggests that the potable supply was contaminated when flood-waterborne Cryptosporidium and Giardia short-circuited the storage reservoir and entered the treatment plant in a series of pulses that also produced rapid fluctuations in water chemistry. Limnological data from the reservoir showed how the barrier was circumvented. A simple mass balance model demonstrated that cysts and oocysts also crossed the filtration barrier periodically. Prior to the 1998 water crisis, the Sydney Water Corporation had assumed that the barriers provided by a storage reservoir with three years' average retention time and a modern filtration plant would eliminate the risk of microbial contamination to its supply. The water crisis highlighted some of the unforeseen contamination risks to surface water supplies and emphasized the benefits of maintaining multiple barriers against contaminant transport through the system.