LT2 Cryptosporidium data: what do they tell us about Cryptosporidium in surface water in the United States?

Beginning in 2006 a United States Federal regulation required public water suppliers using surface water serving more than 10,000 population to analyze for Cryptosporidium in at least 24 consecutive monthly samples from each surface water source. In July 2012, the U.S. EPA released the resulting data consisting of ca. 45,000 records. No Cryptosporidium were found in 93% of samples and no Cryptosporidium were found in any samples analyzed from over half of 1670 locations sampled. Nevertheless, at 250 locations representing every region of the U.S., Cryptosporidium were found in sufficient numbers of samples to provide a picture of their occurrence nationwide. Data from about 100 sites reporting the highest numbers were examined in detail. Although analysis of matrix spikes was required for quality control, the results do not permit estimating organism concentrations. The data reported at each of the individual sample locations were analyzed in the form of cumulative probability distributions to describe key risk-related features of median level and variability. Taken as a whole, the data describe a spectrum of median Cryptosporidium occurrence in surface waters of the U.S. ranging from ca. 0.005 to ca. 0.5 oocysts/L. The variability at individual sites ranged from ca. 1 to 15 r.s.d. Based on the LT2 positive data, comparison to measurements of other water quality parameters, and independent means of estimating organism production from watersheds reported in the literature, the hypothesis is offered that Cryptosporidium may be found in surface water anywhere worldwide continuously and within the spectrum defined above.