Design of the National Water-Quality Assessment Program; occurrence and distribution of water-quality conditions

The National Water-Quality Assessment Program assesses the status of and trends in the quality of the Nation's ground- and surface-water resources. The occurrence and distribution assessment component characterizes broad-scale water-quality conditions in relation to major contaminant sources and background conditions in each study area. The surface-water design focuses on streams. The ground-water design focuses on major aquifers, with emphasis on recently recharged ground water associated with human activities.