Groundwater quality data were collected in 5,000 wells between 1988-2001 by the National Water-Quality Assessment Project. About 1,500 of these wells were sampled again between 2002-2012 to evaluate decadal changes in groundwater quality. Monitoring wells, domestic supply wells, and some public supply wells were included in this study. All water was collected prior to treatment. Groundwater samples used to evaluate decadal change were collected from networks of wells with similar characteristics. Some networks, consisting of domestic or public supply wells, were used to assess changes in the quality of groundwater used for drinking water supply. Other networks, consisting of monitoring wells, assessed changes in the quality of shallow groundwater underlying key land use types such as agricultural or urban lands. Networks were chosen based on geographic distribution across the Nation and to represent the most important water-supply aquifers and specific land-use types. Decadal changes in concentrations of pesticides, nutrients, metals, and organic contaminants in groundwater were evaluated in 67 networks across the Nation and displayed on an interactive web mapping tool. Decadal changes in median concentrations for a network are classified as large, small, or no change in comparison to a benchmark concentration. For example, a large change in chloride concentrations indicates that the median of all differences in concentrations in a network is greater than 5 percent of the chloride benchmark. For chloride, this would mean the change in concentration exceeded 12.5 mg/L.