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The primary filter pack (also commonly called a sand or gravel pack) is material that fills the annulus around and just above the well screen to retain and stabilize material from the adjacent screened unit (fig. 4). A filter pack has a greater grain size than that of the aquifer material in the vicinity of the screen. Filter-pack grain size and gradation are designed to sta- bilize the hydrologic unit adjacent to the screen and permit only the finest grains to enter the screen during development, resulting in relatively sediment-free water for sampling after development. The primary filter pack should consist of relatively inert material such as quartz, contain no limestone or other calcareous materials such as shell fragments, and contain no organic material such as wood fragments or lignite. Alternatively, filter-pack material of known chemistry (ASTM, 1992) can be used, such as glass beads. The primary filter pack commonly is extended up the annulus to a minimum of 5 ft above the top of the screen (Hardy and others, 1989), if a secondary filter pack is impractical., The primary filter pack must not intersect multiple water-bearing units, nor cross confining units that otherwise would not be screened (table 3). Intersection of such units can result in an arti- ficial, vertical, hydraulic connection along the annulus between these units, and can affect the chemistry of the ground water being sampled. The secondary filter pack (fig. 4) is a finer grained material than the primary filter pack, placed in the annulus between the primary filter pack and the overlying annular seal, or be- tween different types of annular seals (ASTM, 1992, p. 124). The purpose of the secondary filter pack is to prevent material used for the annular seal from infiltrating and clogging the filter pack and from affecting ambient water chemistry. The secondary filter pack should con- sist of inert material, consistent with that of the primary filter pack. A length of secondary filter pack of about 1 to 2 ft is recommended (Hardy and others, 1989, p. 16; ASTM, 1992, p. 129, figs. 2 and 3). Annular seal(s) are installed from above the secondary filter pack or the extended pri- mary filter pack to near land surface, in order to seal the annular space between the casing. and borehole wall (fig. 4). These seals prohibit vertical flow of water between aquifers and prevent cross-contamination of aquifers by contaminants. They also protect against infiltration of water and contaminants from the surface. A 3- to 5-ft plug should be placed above the extended primary or secondary filter pack (ASTM, 1992). The plug is formed from a hydrated material such as bentonite or cement that acts as a sealant. The choice of a sealant material must minimize possible effects on the con- stituents to be analyzed from the well (table 10). Penetration of the sealant into the underlying filter pack should be limited to less than a few inches (Hardy and others, 1989). 37