Sustaining the quality of the Nation's water resources and the health of our diverse ecosystems depends on the availability of sound water-resources data and information to develop effective, science-based policies. Effective management of water resources also brings more certainty and efficiency to important economic sectors. Taken together, these actions lead to immediate and long-term economic, social, and environmental benefits that make a difference to the lives of the almost 400 million people projected to live in the United States by 2050.In 1991, Congress established the National Water-Quality Assessment (NAWQA) Project to address where, when, why, and how the Nation's water quality has changed, or is likely to change in the future, in response to human activities and natural factors. Since then, NAWQA has been a leading source of scientific data and knowledge used by national, regional, State, and local agencies to develop science-based policies and management strategies to improve and protect water resources used for drinking water, recreation, irrigation, energy development, and ecosystem needs (htt ps://water .usgs.gov/ nawqa/ applications/ ). Plans for the third decade of NAWQA (2013-23) address priority water-quality issues and science needs identified by NAWQA stakeholders, such as the Advisory Committee on Water Information and the National Research Council, and are designed to meet increasing challenges related to population growth, increasing needs for clean water, and changing land-use and weather patterns.NAWQA is assessing the quality of groundwater used for public and domestic drinking-water supply. NAWQA obtains samples from public-supply wells, domestic wells, and shallow monitoring wells and analyzes those samples for a large number of chemical constituents. These data are used to assess the suitability of the resource for human consumption, as well as to evaluate changes in groundwater quality over a variety of time scales. Groundwater quality also is assessed at multiple scales: locally, regionally, and nationally. Groundwater-quality data collected by the NAWQA Project during each year are published in annual data series reports. This report, the fifth in the series, combines groundwater-quality data collected at 983 sites to provide a summary of groundwater quality in selected aquifers across the Nation during the sampling period. All NAWQA reports are available online at htt ps://water .usgs.gov/ nawqa/ bib/ .We hope this publication will provide you with insights and information to meet your waterresource needs and will foster increased citizen awareness and involvement in the protection and restoration of our Nation's waters. The information in this report is intended primarily for those interested or involved in resource management and protection, conservation, regulation, and policymaking at the regional and national levels.