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cost. In unconsolidated deposits, a common monitoring-well design (fig. 4) consists of a well screen and casing installed in a well bore with an annular space backfilled with filter packs and annular and surface seals. Specific aspects of NAWQA-design monitoring wells, however, can vary depending on requirements to meet specific data-collection objectives, site conditions encountered, and the drilling method used. For example, some Flowpath Studies will include investigation of ground-water quality as ground-water discharges to surface water, which might require installation of streambed piezometers that are steel, wire-wound well screens attached to the bottom of steel casing that are hand driven to the desired depth into the aquifer beneath the stream. In another situation, an aquifer might contain fine-grained sediment, which in order to prevent silting in of the screen, would require attachment of a riser pipe to the bottom of the well screen to serve as a collection area. Study Units that select or install wells in semiconsolidated deposits and rock must ensure that the wells meet the design criteria for the study component (table 3). Three possible de- signs consist of (1) an open borehole at the interval of interest, with well casing installed in the borehole above this interval (the annular spacing between the casing and borehole wall is sealed with grout) (fig. 5a); packers installed in the open borehole above and below the interval of interest to isolate part of the borehole for water-quality sampling (fig. 5b); or a well screen with filter pack installed at the interval of interest, with an annular seal installed above this in- terval (fig. 5c). Generally, all three of these designs meet the design criteria for Land-Use and Flowpath Studies (table 3) but have advantages and disadvantages that must be evaluated when selecting one of these three designs, a modification of one of these designs, or an alternative design (W. Lapham, U.S. Geological Survey, written commun., 1995--see footnote 1). For Flowpath Studies, clusters of monitoring wells usually will be installed. Clusters of monitoring wells are used when one well is considered inadequate in terms of characterizing the vertical distribution of hydraulic head or water quality. Several designs of well clusters suitable for water-level measurements and water-quality sampling are illustrated in figure 6 and include (1) monitoring wells with short screens, each installed in its own borehole (fig. 6a); (2) multiple monitoring wells, each with a short screen, installed in a single borehole, with an annular seal between each screened interval (fig. 6b); and (3) a single well, which contains a series of multiport samplers, installed in a single borehole, with each port separated by an annular seal, or by a packer (fig. 6c). The decision to use one design over another depends on a number of factors related to the objective of the Flowpath Study; the advantages of given well-cluster designs are discussed in another document (W. Lapham, U.S. Geological Survey, written commun., 1995--see footnote 1), and their use is described in greater detail in Jelinski (1990); LeBlanc and others (1991); Stites and Chambers (1991); Pickens and others (1978). Short-screened wells installed in separate boreholes (fig. 6a) is the design recommended for NAWQA studies. Spacing wells about 5 to 10 ft apart in a cluster generally maintains well integrity without comprising the intent of collecting data at a well cluster. 25