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):

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
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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