Predicting Health Risks from Intrusion into Drinking Water Pipes over Time

Abstract

This exploratory study uses corrosion rates in aging metal pipes to predict the number of holes in drinking water supply systems over time. Using this estimate of the number of holes, it is possible to estimate the volume and number of pathogens introduced by negative-pressure events by Monte Carlo simulation. The probability of an intrusion volume greater than 10 L increases from less than 1% in a 25-year-old pipe to greater than 70% in a 150-year-old pipe. Surprisingly, however, health risks associated with three pathogens increase little with increasing pipe age. Some pathogens, like Giardia, are so abundant that almost any intrusion volume presents a risk, virtually independent of pipe age. Other organisms, such as Cryptosporidium, are present at such low concentrations that the risk remains low. The only evidence of significant increase in health risk (rotavirus) was predicted to occur early in the service life of the pipe (i.e., fewer than 50 years), and many pipes are much older. Because there is little evidence health risk due to intrusion increasing over time, it would seem that controlling intrusion risk is unlikely to be the key factor in developing pipe repair and replacement strategies. A simple model for assessing the overall health risks associated with intrusion is provided, suggesting the number of negative-pressure events and pipe residence time are important risk factors.