National Water-Quality Assessment (NAWQA) Program
Design of the National Water-Quality
Occurrence and Distribution of Water-Quality Conditions
United States Geological Survey Circular 1112
By Robert J. Gilliom, William M. Alley, and Martin E. Gurtz
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
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.
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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