The U.S. Geological Survey collected samples from 50 public supply wells during two distinct sampling episodes (2004-2005 and 2014) on the North-Central California coast. During each sampling episode, the samples were analyzed for 160 water-quality constituents, including water-quality indicators, nutrients, major and minor ions, trace elements, volatile organic compounds, and pesticide compounds. The paired-sample results were submitted to an evaluation method to determine whether or not step trends had occurred in the concentrations of each constituent. Quality-control samples were collected at a subset of the wells. The datasets presented here consist of a list and description of the 160 constituents, an Excel workbook which details the step trend evaluation method, and a summary of the quality-control data results.