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

Study-Unit Survey

The primary objective of the Study-Unit Survey is to provide a broad assessment of the water-quality conditions of the most important present and future ground-water resources of each Study Unit. The large areal and depth dimensions of this resource require that the Study-Unit Survey rely primarily on sampling existing wells and, wherever possible, on existing data collected by other agencies and programs. In parts of some Study Units, existing data on presently used ground-water resources are more complete in many respects than NAWQA studies can produce.

Subunit Definition and Prioritization

The first step in the design of a Study-Unit Survey is the division of the ground-water resource into aquifer subunits, generally 3-5 per Study Unit. The subunits serve as a first-order subdivision of the Study Unit into aquifer zones that are expected to be homogeneous in water-quality characteristics compared with the Study Unit as a whole (Alley, 1993a). This subdivision is based mainly on identification of major hydrogeologic settings. Shallow aquifer systems are further subdivided on the basis of physiographic characteristics, which include soil characteristics, landforms, discharge and recharge areas, and drainage characteristics; however, land use or other human influences are not the primary criteria. Some thick aquifers are subdivided into two or more subunits on the basis of depth; however, some grouping of aquifers with similar characteristics usually is necessary to achieve a manageable number of subunits.

A key challenge in defining aquifer subunits is to group a number of different factors that could affect ground-water quality to produce a small number of carefully chosen units. A detailed subdivision is done first, which is followed by grouping of the initial subdivisions to designate the three to five subunits for the Study-Unit Survey.

Because of the large spatial dimensions of ground-water resources, completion of the Study-Unit Survey in some Study Units will take place in phases over multiple NAWQA cycles. Although this approach means that some subunits might not be sampled for many years, careful prioritization ensures that the most important parts of the resource are assessed first. Prioritization of subunits is based on present water use, potential for future water use, likelihood of contamination, and potential for change.

Well Selection and Sampling Strategy

For each aquifer subunit, the well-selection process and sampling strategy are designed to achieve a preliminary assessment from a combination of new and existing data. As a general planning guideline, an areal sampling density of at least one well per 100 km2 is desired, and at least 20 wells are selected in each subunit. The areal density sampling goal may not always be met in some larger Study Units, and some subunits with extensive existing data may not be sampled during the first cycle of NAWQA.

Perhaps the most significant factor that affects the utility of Study-Unit Surveys is that locations of existing wells are biased relative to the ground-water resource. Different types of wells (domestic, public water supply, and so forth) are likely to be biased in different ways. In general, each Study-Unit Survey uses as few different types of wells as needed to obtain adequate spatial (areally and with depth) distribution of ground-water samples from the aquifer system. A description of the type(s) of wells sampled is included as part of the description of the "target population" of the Study-Unit Survey.

Wells are selected for sampling in a subunit by using a grid-based random sampling approach (Scott, 1990; Alley, 1993b). Where feasible, candidate wells are identified by a systematic inventory of existing wells in each subunit. The inventory results in maps that display the lengths and three-dimensional distribution of the screened parts of available wells and well locations relative to the water table, land surface, and aquifer(s) of interest. The grid-based random sample is selected from suitable candidate wells. Where an initial well inventory is not feasible, potential sites are randomly selected on a grid system, and suitable wells are identified near each site to achieve a distribution of sampling wells through the subunit.

Sample Analyses

All samples collected for the Study-Unit Survey are analyzed for the same field characteristics and dissolved constituents that are analyzed for samples from Basic and Intensive Fixed Sites (table 6 and table 7). Depending on Study-Unit and National Synthesis priorities, all or part of the samples also are analyzed for other constituents. In particular, most Study-Unit Survey samples will be analyzed for volatile organic compounds.

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