Water quality of the Cambrian-Ordovician aquifer system was assessed primarily with data collected from 1995 through 2007 as part of the National Water-Quality Assessment (NAWQA) Program of the U.S. Geological Survey (USGS). Physical properties and concentrations of major ions, trace ele- ments, radionuclides, nutrients, pesticides, and volatile organic compounds were measured in raw water samples from as many as 155 wells. The distributions of constituent concentra- tions were related to various factors such as confining condi- tions, well type, land use, and groundwater age. Constituent concentrations were compared to water-quality benchmarks for human health, either U.S. Environmental Protection Agency (USEPA) Maximum Contaminant Levels (MCLs) or USGS Health-Based Screening Levels (HBSLs). Concentra- tions were also compared to USEPA Secondary Drinking Water Regulations (SDWRs) established for aesthetic quality or other non-health reasons. Data for raw water samples from other USGS sources and state agencies were combined with NAWQA data to help fill gaps in data coverage for mapping concentrations of selected water-quality constituents.