Understanding the fate of Cryptosporidium and Giardia in storage reservoirs: a legacy of Sydney's water contamination incident

High levels of Cryptosporidium oocysts and Giardia cysts (C&G) were detecte d in the raw and filtered water supply in Sydney, Australia, in July and Au gust 1998. This paper describes the results of daily monitoring in the Sydn ey water supply catchments between December 1998 and May 1999, as one respo nse to that incident. C&G were most abundant in the largest and most developed catchment. The lev els of contamination in Sydney's raw water supply were of similar magnitude to raw waters in the USA and Europe, but the frequency of contamination wa s much lower. Physical variables of increased stream flow and turbidity were associated w ith the presence of C&G. The abundance of Giardia cysts was highly correlat ed with the number of Cryptosporidium oocysts. Other microbial indicators o f faecal contamination (faecal coliforms and C. perfringens) were not usefu l indicators of either C or G. Both organisms occurred episodically in brief events, which spread rapidly through the water supply system, following stream rises in the catchment. L ake Burragorang was a poor barrier to the transfer of C&G into the bulk sup ply. C&G disappeared from Lake Burragorang within-4 weeks after each event. This loss rate was more rapid than predicted from settling theory. We obse rved settling rate of 5-10 m day(-1), and postulate that sedimentation was accelerated by agglomeration of oo/cysts with other suspended particles. A design for monitoring programmes and management action in lakes to preven t C&G entering the supply to water treatment plants is suggested.