Longitudinal Multiyear Characteristics of Cryptosporidium and Giardia Concentrations in Surface Water: Application to Risk Characterization

The waterborne outbreak potential of Cryptosporidium and Giardia is recognized by public water supply agencies, motivating monitoring of surface sources to meet applicable drinking water regulations. In this study, data from 10 or more years of continuous monitoring at several sites were compiled and analyzed to describe patterns of occurrence and to outline data analysis procedures to better inform watershed management and treatment system management for control of these pathogens. Data were analyzed to identify parameters that enable risk characterization, the median and standard deviation for annual data sets. These values, monitored for a sequence of years, indicate trends related to watershed processes and provide guidance for watershed management. The scope of annual variations, in combination with tracking concentrations corresponding to short-term (seasonal) water quality variations, provides essential information for treatment system management. The patterns of Cryptosporidium and Giardia occurrence over 10 or more years demonstrates their continuous presence in virtually all surface water sources. The results provide a direct means of quantifying risk relative to other years at a single site or to occurrence similarly described elsewhere. Reflection on the results shows that inference of absence from negative findings is no longer acceptable. Clear understanding of the data presented here provide a basis for monitoring that is both more effective and more efficient, providing real value for the substantial investment that monitoring requires.

Long-term, $10+\text{-}\mathrm{year}$ data for concentrations of Cryptosporidium and Giardia describe their continuous presence and variations over time. Given the high cost of monitoring, a clear understanding of monitoring objective is essential. A continuing record provides a basis for comparing concentration-based risks over time and comparisons with concentrations monitored at other locations. This provides a foundation for watershed management and for treatment system management in relation to risk. The description of monitoring and data analysis principles provides guidance for the development of value-oriented monitoring of these waterborne pathogens and, ultimately, an efficient and effective monitoring program that will satisfy health-based and regulatory monitoring needs.