A major obstacle to eradicating polio is that poliovirus from endemic countries can be reintroduced to polio-free countries. Environmental surveillance (ES) can detect poliovirus from sewage or wastewaters samples, even in the absence of patients with paralysis. ES is underused, in part because its sensitivity is unknown. We used two unique data sets collected during a natural experiment provided by the 2013 polio outbreak in Israel: ES data from different locations and records of supplemental immunization with the live vaccine. Data from the intersecting population between the two data sets (covering more than 63,000 people) yielded a dose-dependent relationship between the number of poliovirus shedders and the amount of poliovirus in sewage. Using a mixed-effects linear regression analysis of these data, we developed several quantitative tools, such as (i) ascertainment of the number of infected individuals from ES data for application during future epidemics elsewhere, (ii) evaluation of the sensitivity of ES, and (iii) determination of the confidence level of the termination of poliovirus circulation after an outbreak. These results will be valuable in monitoring future outbreaks with ES, and this approach could be used to certify poliovirus elimination or to validate the need for more containment efforts.