The lowering of the arsenic drinking water standard from 50 to 10 μg/L is expected to have a disproportionate impact on groundwater-based community water supplies. Such distribution systems often have multiple entry points each of which may correspond to either a single well or a well field. If the wells within the distribution system vary in their arsenic concentrations, some control over the arsenic concentration in the water supply may be achieved by blending prior to entry; some level of blending will also occur within the distribution system. In this study, we examine the spatial variation of arsenic concentrations in wells within a groundwater-based community water supply in Hanford, California. We show that, in this community, arsenic concentrations in wells and within the distribution system can vary substantially from location to location. We also show that arsenic concentrations within the piping network (and thus at the tap) are temporally variable, depending on the pumping rules applied and the system demand. Thus, samples taken at the well heads may not be representative of arsenic exposure to consumers. In particular, the effects of blending water from wells of different quality as well as temporal changes resulting from activation/deactivation of pumps in response to changes in demand must be considered. In this study, we quantify these effects on water quality using an EPANET model of the Hanford, CA distribution system as an example of spatial and temporal variation in health risk due to arsenic-contaminated well waters. Copyright ASCE 2004.