Evaluating Decadal Changes in Groundwater Quality: Groundwater-quality data were collected from 5,000 wells between 1988-2001 (first decadal sampling event) by the National Water-Quality Assessment Project. Samples are collected in groups of 20-30 wells with similar characteristics called networks. About 1,500 of these wells in 67 networks were sampled again approximately 10 years later between 2002-2012 (second sampling event) to evaluate decadal changes in groundwater quality. Between 2012 and 2018 (third sampling event), a subset of these networks was sampled again, allowing additional results to be displayed on the web page: Decadal changes in groundwater quality (https://nawqatrends.wim.usgs.gov/decadal/). This is the fourth iteration of data added to the website. With the additional data, it is possible to evaluate changes in water quality between the 2nd and 3rd sampling events for 21 additional networks (56 total), changes in water quality between the 1st and 3rd sampling events for 18 additional networks (45 total), and changes across all 3 sampling events for 18 additional networks (45 total). A total of 83 networks have been sampled at least twice. Samples were obtained from monitoring wells, domestic-supply wells, and some public-supply wells before any treatment on the system. 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 nutrients, metals, and pesticides and other organic contaminants in groundwater were evaluated in a total of 83 networks across the Nation by comparing changes between selected sampling events. 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 per decade. For chloride, which has a Secondary Maximum Contaminant level of 250 milligrams per liter, this would mean the change in concentration exceeded 12.5 milligrams per liter (mg/L), or 5 percent of the benchmark. 230 networks were sampled from 1988 to 2001 to assess the status of the Nation's groundwater quality. Each dot on the map on the "Learn more" tab of the Decadal mapper website, (https://nawqatrends.wim.usgs.gov/decadal/) represents the center point of a network of about 20 to 30 wells. Networks sampled in the first sampling event only are shown in green. There were 67 networks resampled from 2002 to 2012 to assess decadal changes in groundwater quality. Networks sampled from 2012 to 2018 and at least one previous sampling event are shown in orange and trend networks that have not yet been resampled in the third decadal sampling event are shown in blue. Networks sampled in the first and second sampling events but are no longer being sampled are shown in gray.