Identifying water purveyor in this manner can be labour intensive, and not all homes receive water from a publicly monitored water system. Therefore, we also explored whether relative tap water levels of these contaminants could be determined by instead using geographic information systems (GIS) methods to link residences to water quality monitoring data. We hypothesized that this method might be feasible because water sources are geographically referenced in these data, and some contaminants are geographically dispersed (i.e. water contamination in a region may reflect levels in the home). We focused on arsenic and nitrate because they tend to be regionally dispersed, are prevalent in our region, are often the focus of etiologic studies of environmental contaminants, and have been regulated since the inception of the SDWA.