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Background: Water arsenic (As) sources beyond a rural household's primary well may be a significant source for certain individuals, including schoolchildren and men working elsewhere., Objective: To improve exposure assessment by estimating the fraction of drinking water that comes from wells other than the household's primary well in a densely populated area., Methods: We use well water and urinary As data collected in 2000-2001 within a 25 km 2 area of Araihazar upazila, Bangladesh, for 11,197 participants in the Health Effects of Arsenic Longitudinal Study (HEALS). We estimate the fraction of water that participants drink from different wells by imposing a long-term mass-balance constraint for both As and water., Results: The mass-balance model suggest that, on average, HEALS participants obtain 60-75% of their drinking water from their primary household wells and 25-40% from other wells, in addition to water from food and cellular respiration. Because of this newly quantified contribution from other wells, As in drinking water rather than rice was identified as the largest source of As exposure at baseline for HEALS participants with a primary household well containing ≤50 µg/L As., Significance: Dose-response relationships for As based on water As should take into account other wells. The mass-balance approach could be applied to study other toxicants., (© 2021. The Author(s), under exclusive licence to Springer Nature America, Inc.)