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

Ground-Water Study Design

The study design for ground water focuses on assessing the water-quality conditions of major aquifers in each Study Unit with emphasis on the quality of recently recharged ground water associated with present and recent human activities. Although stream-water quality is highly variable through time, ground-water quality is determined primarily by chemical characteristics that tend to vary more spatially than temporally. Thus, the general approach to the ground-water assessment focuses on spatial characterization. Furthermore, the initial emphasis is on the chemical characteristics of ground water. The need to understand microbial processes at regional scales, however, is increasingly evident (Chapelle and others, 1993), and this aspect of ground-water quality may receive greater emphasis as Study-Unit Investigations evolve.

Overview of Approach

Ground-water quality is assessed by three primary study components (table 10):

Study-Unit Surveys assess the water quality of the major aquifer systems of the 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, contaminant occurrence and transport, and surface- and ground-water interaction.

The sampling design is based on the need to examine ground-water quality at a range of spatial scales. Study-Unit Surveys are used, in conjunction with an analysis of available data, to broadly characterize ground-water quality across the Study Unit. Land-Use and Flowpath Studies are done at intermediate and more local scales, respectively, to build an understanding of causal relations and processes. The Land-Use and Flowpath Studies are directed, for the most part, toward the effects of human activities on ground-water quality. The three study components are phased in over different sequences and time frames in different Study Units; the sequence depends on the availability and quality of existing data and the size and complexity of the ground-water system. Generally, the first intensive study phase in each Study Unit includes two to four Land-Use Studies and two to four Flowpath Studies. The most important aquifer zones are sampled during the first phase of the Study-Unit Survey if existing data are inadequate.

The Study Units allocate different proportions of study resources to the three study components; the allocation depends on the types and extent of available water-quality data for the Study Unit, the complexity and extent of aquifer units and Environmental Settings, the types and locations of existing wells available to sample in the Study Unit, and whether new wells must be installed. In allocating resources among the different study components, each Study-Unit Investigation attempts to achieve a balance between broad-scale assessment and spatially focused studies aimed at understanding causal relations.

Table 10. Components and attributes of the National Water-Quality Assessment ground-water sampling design.

Study component

Feature Study-Unit Survey Land-Use Studies Flowpath Studies

General objective To supplement existing data in providing broad overview of ground-water quality within each Study Unit To examine natural and human factors that affect the quality of shallow ground water that underlies key types of land use To examine ground-water quality along inferred flowpaths and interactions of ground water with surface water

Spatial domain Ground-water resource throughout Study Unit Uppermost part of ground- water system in specified land-use settings Shallow flow systems in specified settings

Selection of areas Aquifer system divided typically into 3-5 subunits on the basis of physiographic and hydrogeologic features Each land-use setting represents a combination of a land-use type and a hydrogeologic subunit
Typically, 2-4 Land-Use Studies per Study Unit
Typically, 1-2 Flowpath Studies per Study Unit
Typically, located in an indicator basin for surface-water sampling design
Upper part of flowpath generally lies within one of land-use settings examined in Land-Use Studies
Generally, unconsolidated shallow aquifers

Number of wells sampled Typically, 20-30 wells in each subunit
General goal for spatial density is one well per 100 km²
Typically, 20-30 wells in each land-use setting Typically, 10-12 wells along flowpath and 10 wells for areal sampling

Well-selection strategy Spatially distributed "random" sampling

Primarily existing wells
Spatially distributed "random" sampling

New or existing wells
Wells distributed at multiple depths along flowpath and areally in vicinity of flowpath
New wells to extent possible

Temporal sampling strategy Each well typically sampled once Each well typically sampled once Variable; multiple samples from most wells

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Table 10. Components and attributes of the National Water-Quality Assessment ground-water sampling design.
	Study component
Feature	Study-Unit Survey	Land-Use Studies	Flowpath Studies
General objective	To supplement existing data in providing broad overview of ground-water quality within each Study Unit	To examine natural and human factors that affect the quality of shallow ground water that underlies key types of land use	To examine ground-water quality along inferred flowpaths and interactions of ground water with surface water
Spatial domain	Ground-water resource throughout Study Unit	Uppermost part of ground- water system in specified land-use settings	Shallow flow systems in specified settings
Selection of areas	Aquifer system divided typically into 3-5 subunits on the basis of physiographic and hydrogeologic features	Each land-use setting represents a combination of a land-use type and a hydrogeologic subunit Typically, 2-4 Land-Use Studies per Study Unit	Typically, 1-2 Flowpath Studies per Study Unit Typically, located in an indicator basin for surface-water sampling design Upper part of flowpath generally lies within one of land-use settings examined in Land-Use Studies Generally, unconsolidated shallow aquifers
Number of wells sampled	Typically, 20-30 wells in each subunit General goal for spatial density is one well per 100 km²	Typically, 20-30 wells in each land-use setting	Typically, 10-12 wells along flowpath and 10 wells for areal sampling
Well-selection strategy	Spatially distributed "random" sampling Primarily existing wells	Spatially distributed "random" sampling New or existing wells	Wells distributed at multiple depths along flowpath and areally in vicinity of flowpath New wells to extent possible
Temporal sampling strategy	Each well typically sampled once	Each well typically sampled once	Variable; multiple samples from most wells

Design of the National Water-Quality Assessment Program:

Occurrence and Distribution of Water-Quality Conditions

Ground-Water Study Design

Overview of Approach

Design of the National Water-Quality
Assessment Program: