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:
- Water-Column Studies assess physical and chemical characteristics,
which include suspended sediment, major ions and metals, nutrients,
organic carbon, and dissolved pesticides, and their relation to
hydrologic conditions, sources, and transport.
- Bed-Sediment and Tissue Studies assess trace elements and hydrophobic
organic contaminants.
- Ecological Studies evaluate the relations among physical, chemical, and
biological characteristics of streams.
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:
- Study-Unit Surveys assess the water quality of the major aquifer
systems of each Study Unit by sampling primarily existing wells.
- Land-Use Studies use observation wells and selected existing wells to
assess the quality of recently recharged shallow ground water associated
with regionally extensive combinations of land use and hydrogeologic
conditions.
- Flowpath Studies use transects and groups of clustered, multilevel
observation wells to examine specific relations among land-use
practices, ground-water flow, and contaminant occurrence and transport and
interactions between ground and surface water.
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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