USGS

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

Flowpath Studies

The primary objectives of Flowpath Studies are to characterize the spatial and temporal distribution of water quality in relation to ground-water flow in shallow ground-water systems for particular settings, increase understanding of the natural processes and human influences in these settings that affect the evolution of ground-water quality along flowpaths through the saturated zone, and evaluate the degree and water-quality significance of interaction between ground water and streams for selected settings. Ideally, many of the Flowpath Studies provide a perspective on the potential significance of poor-quality shallow ground water to regional aquifers and streams.

Most Flowpath Studies are within Land-Use Study areas where the areal distribution of shallow ground-water quality is broadly characterized. Because of the low spatial density of sampling in many Land-Use Studies, Flowpath Studies frequently include additional areal sampling of shallow ground water in the vicinity of the transect.

Selection of Flowpath Study Sites

Typically, one to two Flowpath Studies are undertaken in each Study Unit during the first NAWQA cycle. Principal considerations for selecting and prioritizing Flowpath Study sites are as follows:

Transect Design and Sampling Strategy

Ideally, well transects extend along a ground-water flowpath. Most flowpath transects either begin or terminate at a stream. As an example, a typical transect in terrain with a gaining stream is shown in figure 9. This "typical" transect illustrates some of the general concepts and level of detail envisioned for a Flowpath Study. Variations of the design are made to adjust to differences in scales and complexities of ground-water flow in different terrains. Four to six clusters of two to three wells are located along the transect in figure 9, for a total of 10 to 12 wells. Generally, in each cluster, one well is screened beginning 1 to 2 m below the lowest anticipated position of the water table, and the additional wells are screened between 5 to 75 m below the water table. Typically, well screens do not exceed 2 m in length to ensure that water samples are representative of a "point" in the ground-water flow system. To best define the hydrologic system, one well cluster is located at the ground-water divide and another at the bank or in the bed of the gaining surface-water body. Additional streambank wells include wells on the streambank opposite the well transect and wells screened directly below the streambed. The samples from these wells provide the best measure of the quality of ground water that interact with the stream. Where streambed wells cannot be maintained, an option is to collect a one-time series of ground-water samples below the streambed by driving a well point, or minipiezometer (Lee and Cherry, 1978), into the streambed and collect water samples and water-level information at several depths.

Figure 9

(Click on image for a larger version, 21K)

Figure 9. Typical idealized transect design for a Flowpath Study associated with a gaining stream.

In general, well transects are located in unconsolidated earth materials, and the wells are constructed as part of the study. Occasionally, a well transect is located so that one or two existing wells can be used. The depth of the deepest screened interval in a transect well and the length of a transect vary considerably among different hydrogeologic settings. Ideally, the deepest transect wells are screened to the depth at which water samples show little or no human influence on water quality. Usually, the lengths of well transects, which range from one to several kilometers, depend on the scale of the flow system.

Each Study-Unit team designs a specific sampling and analytical strategy for each transect site to meet study objectives. Typically, hydraulic measurements associated with a well-transect site include 2 years of monthly head measurements in all wells, three to four measurements of stream base flow at different seasons of the year (for transects with streams) and, if feasible, seepage investigations near the terminus of the transect during the annual low-flow period. Water-quality sampling associated with each Flowpath Study typically consists of at least one sample from all wells analyzed for the national target analytes (table 5 and table 6) and isotopes (such as tritium) or chlorofluorocarbons that allow an estimate of water age; monthly or seasonal samples from some wells for selected analytes, particularly for wells screened near the water table; and samples of base flow in the stream associated with the well transect that are analyzed for selected analytes.

Flowpath Studies during the initial phase of NAWQA are viewed as the potential beginning of long-term studies that will lead to more intensive studies. Moreover, networks established as part of some Flowpath Studies may form an important element of a long-term trend network.


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