Use QMRA to quantify the risk of GI associated with consuming untreated private well water and compare to Aim 1. Using approaches similar to other QMRA models of risk from well water consumption,45–47 (link) a multi-pathogen QMRA model will be developed using the untreated well water quality data collected from the subcohort of 180 households (270 samples). The following pathogens will be included in the QMRA: adenovirus, enterovirus, norovirus, diarrheagenic E. coli, Campylobacter, Salmonella, Shigella, Cryptosporidium, Giardia. The model will be developed before completing the final analysis of Aim 1 to allow for true unblinded comparison of the results. Individual models will be developed for each pathogen detected using established pathogen-specific dose-response curves. The concentration of pathogens found in the well water in our study will be fit to probability distributions and are the key inputs into the mathematical models. The model endpoint will be the annual number of cases/child-year for each pathogen. The results of the different pathogen-specific models will be combined to produce a total number of cases/child-year of GI expected in the 908 children studied in the trial. The final estimates will be reported as means with 90% probability intervals (PI) around the mean generated using Monte Carlo simulation (10 000 iterations using @Risk (Palisade, USA)). The PIs from the QMRA will be compared with the 90% CIs of intervention effect from Aim 1 to assess whether these intervals intersect.