Approximately 43 million people (about 14 percent of the U.S. population) rely on domestic wells as their source of drinking water. Unlike community water systems, which are regulated by the Safe Drinking Water Act, there is no comprehensive national program to ensure that the water is tested to ensure that is it safe to drink. A study published in 2009 from the National Water-Quality Assessment Program of the U.S. Geological Survey that assessed water-quality conditions from 2,100 domestic wells within 48 states reported that more than one in five (23 percent) of the sampled wells contained one or more contaminants at a concentration greater than a human-health benchmark. In addition, there are many activities, e.g., resource extraction, climate change-induced drought, and changes in land use patterns that could potentially affect the quality of the ground water source for domestic wells. The Health Studies Branch (HSB) of the National Center for Environmental Health, Centers for Disease Control and Prevention, created a Clean Water for Health Program to help address domestic well concerns. The goals of this program are to identify emerging public health issues associated with using domestic wells for drinking water and begin to develop a plan to address these issues. As part of this effort, HSB in cooperation with the U.S. Geological Survey has created models to estimate the probability of arsenic occurring at various concentrations in domestic wells in the U.S. Similar work has been done by public health professionals on a state and regional basis. We will present preliminary results of the project, including estimates of the domestic well population that is likely to have arsenic greater than 10 micrograms per liter. Nationwide, we estimate this to be just over 2 million people. Logistic regression model results showing probabilities of arsenic greater than 10 micrograms per liter in domestic wells in the U.S., based on data for arsenic concentrations in domestic wells across the U.S. will be described, as well as the use of data on domestic well use by county in the U.S., to estimate the affected population.