Distribution of Cryptosporidium oocysts and Giardia cysts in water above and below the normal limit of detection

Analysis of water samples for Cryptosporidium oocysts and Giardia cysts is a specialised and demanding pursuit. Understanding and evaluating data resulting from such analyses is equally specialised and complicated by the most common result—not finding any of the target organisms. Coming to an accurate conclusion regarding such monitoring results has been hampered by a lack of pertinent information presented in the context of current monitoring requirements. The work reported here presents laboratory data demonstrating an appropriate skewed distribution model statistical framework. It is shown that the Poisson model provides for understanding how Cryptosporidium oocysts and Giardia cysts are distributed in water at typical ambient concentrations that are near or most commonly below the limit of detection of the most widely used analytical procedure, USEPA Method 1623. From three to six replicate 50-L volumes of particle-free water were seeded with Cryptosporidium oocysts and Giardia cysts each at concentrations of ca. 0.2/L, 1–2/L, and 6–8/L. The seeded 50-L volumes were analysed in five 10-L aliquots to determine the number of oocysts and cysts in each. The data conformed to the Poisson distribution. This supports the interpretation that analysis of 10-L surface water samples resulting in not finding any target organisms is the result of their presence below the limit of detection. This interpretation strongly suggests that analysing fewer larger volume samples would provide more useful information.