Methods and Uncertainty in Predictions of Arsenic Exposure and Health Outcomes for Private Well Users in Massachusetts

In the United States, most people get their drinking water from public water systems, whose quality is regulated by the Safe Drinking Water Act; however, an estimated 40 million people rely on unregulated private wells. In Massachusetts ∼500,000 people are estimated to rely on private wells, but records are incomplete, and public data on private well water quality is limited. We systematically explore the uncertainty and variability in estimated statewide health outcomes resulting from using different methods for estimating the number of users of private wells, groundwater arsenic concentrations, and arsenic toxicity. We use reported water quality data to approximate arsenic concentrations for private well users in the state and compare these methods to machine learning-based groundwater prediction maps. We compare the estimated cancer cases and adverse health effects for the different methods for assigning arsenic concentrations to wells, estimating locations of wells in Massachusetts, and the updated arsenic toxicity research. We find that updated arsenic toxicity research and uncertainty in groundwater arsenic concentrations have the greatest estimated impact on the cumulative number of cancer and noncancer outcomes. In Massachusetts, private well users have potentially 3-4 times greater risk of negative health impacts from arsenic compared to community water system users, with uncertainty largely driven by lack of contaminant concentration while in community water systems driven by uncertainty in toxicity.