Water quality and rare poisson events
by
Robert L Wolpert
Duke University ISDS

Many of the water treatment plants in the U.S. draw influent water from resevoirs, lakes, rivers, or streams whose waters are home to a variety of microbial pathogens. To protect public health the treatment plants use a variety of methods, such as filtration and chlorination, intended to reduce or eliminate microbial concentrations from delivered drinking water. The present study, intended to help assess the effectiveness of efforts to keep U.S. drinking water safe, is focussed on a single issue: learning about the concentrations in the influent waters of microbial pathogens. These concentrations may vary somewhat from those of the source water body, particularly in quiescent lakes or resevoirs, and will typically be much higher than any concentrations that remain following water treatment.

Estimating the concentrations is difficult because even quite small concentrations (a few hundredths of an oocysts per hectoliter) in drinking water would pose a public health risk; concentrations are so low even in the source waters that detection and measurement are challenging. We describe statistical methods and mathematical models developed to increase our ability to support reliable inferences and estimates about the concentrations of microbial pathogens such as cryposporidium in the source waters.

Date received: April 29, 2002