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
GLOSSARY OF STUDY COMPONENTS
- Basic Fixed Sites--
- Sites on streams at which
streamflow is measured and samples are collected to assess the broad-scale
spatial and temporal character and transport of inorganic constituents of
streamwater in relation to hydrologic conditions and environmental
settings. Data from these sites are the primary source of information for
meeting water-column assessment objectives for temperature, salinity,
suspended sediment, major ions and metals, nutrients, and organic carbon.
Three types of water-column sampling activities--continuous monitoring,
fixed-interval sampling, and extreme-flow sampling--are conducted for 2
years during each cycle of investigation. Reach assessments of ecological
conditions and bed-sediment and tissue sampling also are conducted at these
sites. Study Units usually have 8-12 Basic Fixed Sites.
- Bed-Sediment and Tissue Studies--
- Assessment of
concentrations and distributions of trace elements and hydrophobic organic
contaminants in streambed sediment and tissues of aquatic organisms to
identify potential sources and to assess spatial distribution. Studies are
divided into two phases--the Occurrence Survey and the Spatial Distribution
Survey.
- Case Studies--
- Detailed studies of selected
contaminants in selected hydrologic systems to address specific questions
that concern the characteristics, causes, and governing processes of
water-quality degradation.
- Ecological Studies--
- Studies of biological
communities and habitat characteristics to evaluate the effects of physical
and chemical characteristics of water and hydrologic conditions on aquatic
biota and how biological and habitat characteristics differ among
environmental settings in Study Units. Ecological Studies have three main
components--Fixed-Site Reach Assessments, Intensive Ecological Assessments,
and Ecological Synoptic Studies.
- Ecological Synoptic Studies--
- Short-term
investigations of specific ecological characteristics within all or part of
a Study Unit to provide improved spatial resolution and representativeness
compared with fixed-site sampling, and to evaluate the spatial distribution
of selected ecological characteristics in relation to causative factors,
such as land uses or contaminant sources and instream habitat conditions.
These studies supplement information derived from the more comprehensive
data collected at Basic and Intensive Fixed Sites by targeting specific and
more narrowly defined seasonal and habitat conditions for ecological
characterization at more locations. One to two Ecological Synoptic Studies
are constructed in most Study Units.
- Environmental Framework--
- Natural and human-related
features of the land and hydrologic system, such as geology, land use, and
habitat, that provide a unifying framework for making comparative
assessments of the factors that govern water-quality conditions within and
among Study Units.
- Environmental Setting--
- Land areas characterized by
a unique, homogeneous combination of natural and human-related factors,
such as row-crop cultivation on glacial-till soils.
- Fixed-Site Reach Assessments--
- One-time assessments
of biological communities and habitat conditions of a stream reach at all
Basic and Intensive Fixed Sites.
- Flowpath Studies--
- Assessments of the spatial and
temporal distribution of ground-water quality in relation to ground-water
flow and land use in shallow aquifer systems for selected Environmental
Settings. Flowpath Studies contribute to understanding the natural
processes and human factors that control the evolution of ground-water
quality along flowpaths through the saturated zone and to evaluate the
degree and water-quality significance of interaction between ground water
and streams. Typically, one to two Flowpath Studies are undertaken in each
Study Unit during the first NAWQA cycle, with most in Land-Use Study
areas.
- Indicator Sites--
- Stream sampling sites located at
outlets of drainage basins with homogeneous land use and physiographic
conditions. Basins are chosen to be as large and representative as
possible while still encompassing primarily one Environmental Setting
(typically 50-500 km2).
- Integrator Sites--
- Stream sampling sites located
down-stream of drainage basins that are large and complex and often contain
multiple Environmental Settings. Most Integrator Sites are on major
streams with drainage basins that include a substantial portion of the
Study Unit area (typically 10-100 percent).
- Intensive Ecological Assessments--
- Annually repeated
Fixed-Site Reach Assessments for at least three reaches for 3 or more years
at selected Basic and Intensive Fixed Sites.
- Intensive Fixed Sites--
- Basic Fixed Sites with
increased sampling frequency during selected seasonal periods and analysis
of dissolved pesticides for 1 year. Most Study Units have one to two
integrator Intensive Fixed Sites and one to four indicator Intensive Fixed
Sites.
- Land-Use Studies--
- Investigations of the
concentrations and distribution of water-quality constituents in recently
recharged ground water (generally less than 10 years old) associated with
the most important regionally significant Environmental Settings of land
use and hydrogeologic conditions in each Study Unit. For each study,
usually a combination of 20-30 shallow existing and observation wells are
sampled. Two to four studies typically are completed in each Study Unit
during the first cycle of NAWQA.
- National Synthesis--
- Synthesis of results from all
Study Units with information from other programs, agencies, and researchers
to produce regional and national assessments for priority water-quality
issues.
- Occurrence and Distribution Assessment--
- Assessment
of the broad-scale geographic and seasonal distributions of water-quality
conditions for surface and ground water of a Study Unit in relation to
major contaminant sources and background conditions. This assessment is
the largest and most important component of the first intensive study phase
in each Study Unit.
- Occurrence Survey--
- The first phase of study of
trace elements and hydrophobic organic contaminants in streambed sediment
and tissues of aquatic organisms to determine which target constituents are
common and important to water-quality conditions in each Study Unit.
Typically, all Basic and Intensive Fixed Sites and 5-10 additional sites
are sampled.
- Retrospective Analysis--
- The review and analysis of
existing water-quality data to provide a historical perspective on the
water quality in the Study Unit, to assess strengths and weaknesses of
available information, and to evaluate initial implications for
water-quality management and study design.
- Spatial Distribution Survey--
- Extension of the
Occurrence Survey for bed sediments and tissues to improve geographic
coverage with particular emphasis on assessment of priority constituents
identified in the Occurrence Survey. Occurrence Survey results, combined
with existing data, govern the analytical strategy and the geographic
distribution of sampling sites. The combined data from the two phases of
sampling, typically collected from 20-30 sites, provide a basic description
of spatial distribution of trace elements and hydrophobic organic
contaminants for each Study Unit and support initial evaluation of sources
and biological availability for selected constituents.
- Study Unit--
- A major hydrologic system of the
United States in which NAWQA studies are focused. Study Units are
geographically defined by a combination of ground- and surface-water
features and usually encompass more than 10,000 km2 of land
area. The NAWQA design is based on an assessment of 60 Study Units,
which collectively cover a large part of the Nation, encompass the
majority of population and water use, and include diverse hydrologic
systems that differ widely in natural and human factors that affect
water quality.
- Study-Unit Investigation--
- The systematic study of a
NAWQA Study Unit. These investigations consist of four main components:
Retrospective Analysis, Occurrence and Distribution and Trend and Change
Assessments, and Case Studies. Study Units are organized into three groups
of 20 that are studied on a rotational schedule, with 3-year intensive
study periods repeated once every 9 years.
- Study-Unit Survey--
- Broad assessment of the
water-quality conditions of the major aquifer systems of each Study Unit.
The Study-Unit Survey relies primarily on sampling existing wells and,
wherever possible, on existing data collected by other agencies and
programs. Typically, 20-30 wells are sampled in each of three to five
aquifer subunits.
- Trend and Change Assessment--
- Decadal scale trends
and changes in water-quality conditions will be assessed by using a
combination of existing historical data, periodic intensive assessments,
and selected long-term monitoring strategies.
- Water-Column Studies--
- Investigations of physical
and chemical characteristics of stream water, which include suspended
sediment, dissolved solids, major ions and metals, nutrients, organic
carbon, and dissolved pesticides, in relation to hydrologic conditions,
sources, and transport. These studies also involve selected studies of
other water-quality conditions and include selective investigations of
hydrophobic organic contaminants or trace elements where their importance
is indicated by results of Bed-Sediment and Tissue Studies. Water-Column
Studies have three main components--Basic and Intensive Fixed-Site
Assessments and Water-Column Synoptic Studies.
- Water-Column Synoptic Studies--
- Short-term
investiga-tions of specific water-quality conditions during selected
seasonal or hydrologic periods to provide improved spatial resolution for
critical water-quality conditions compared to fixed-site sampling. For the
period and conditions sampled, they assess the spatial distribution of
selected water-quality conditions in relation to causative factors, such as
land use and contaminant sources, through mass-balance analysis of sources
and transport. During the first 3-year intensive study period, two to
three Water-Column Synoptic Studies are included in most Study-Unit
Investigations.
Continue to Design of the National Water-Quality
Assessment Program: Occurrence and Distribution of Water-Quality
Conditions, or return to Contents
