USGS

National Water-Quality Assessment (NAWQA) Program

Design of the National Water-Quality
Assessment Program:

Occurrence and Distribution of Water-Quality Conditions

United States Geological Survey Circular 1112
By Robert J. Gilliom, William M. Alley, and Martin E. Gurtz

Abstract

The National Water-Quality Assessment Program of the U.S. Geological Survey is designed to assess the status of and trends in the quality of the Nation's ground- and surface-water resources and to link the status and trends with an understanding of the natural and human factors that affect the quality of water. The study design balances the unique assessment requirements of individual hydrologic systems with a nationally consistent design structure that incorporates a multiscale, interdisciplinary approach. The building blocks of the program are Study-Unit Investigations in 60 major hydrologic basins (Study Units) of the Nation. The Occurrence and Distribution Assessment is the largest and most important component of the first intensive study phase in each Study Unit.

The goal of the Occurrence and Distribution Assessment is to characterize, in a nationally consistent manner, the broad-scale geographic and seasonal distributions of water-quality conditions in relation to major contaminant sources and background conditions. The national study design for surface water focuses on water-quality conditions in streams, using the following interrelated components:

Sampling designs for all three components rely on coordinated sampling of varying intensity and scope at Integrator Sites, which are chosen to represent water-quality conditions of streams with large basins that are often affected by complex combinations of land-use settings, and at Indicator Sites, which are chosen to represent water-quality conditions of streams associated with specific individual Environmental Settings. The national study design for ground water focuses on water-quality conditions in major aquifers, with emphasis on recently recharged ground water associated with present and recent human activities, by using the following components:

In selected locations, ground-water studies are codesigned with streamwater-quality studies to investigate interactions between ground and surface waters. Overall, the broad range of coordinated spatial and temporal strategies employed for surface-water and ground-water assessments is designed to describe the most important aspects of water quality in a consistent manner for the wide range of hydrologic environments of the Nation.


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