should be analyzed for cation exchange capacity and volatile organic compounds (VOCs) to

determine whether it will interact with analytes of concern in the ground water.

Commercially available pea gravel may be acceptable for use in gravel aquifers; however, to

meet the Agency's requirement that the filter pack be chemically inert, the pea gravel itself

should not be chemically active or coated with a chemically active metal oxide. Filters

constructed from fabric should not be used as they tend to plug and may be chemically

reactive.

The Agency recommends that filter pack material be well rounded and of uniform

grain size. Aller et al. (1989) provide the following summary of methods for selecting the

size of filter pack materials:

"Although design techniques vary, all use the filter pack ratio to establish size

differential between the formation materials and filter pack materials. Generally this

ratio refers to either the average (50 percent retained) grain size of the formation

material or the 70 percent retained size of the formation material. For example,

Walker (1974) and Barcelona et al. (1985a [1985b in this document]) recommend

using a uniform filter pack grain size that is 3 to 5 times the 50 percent retained size

of the formation materials. Driscoll (1986) recommends a more conservative approach

by suggesting that for fine grained formations, the 50 percent retained size of the

finest formation sample be multiplied by a factor of 2 to exclude the entrance of fine

silts, sands, and clays into the monitoring well. The United States Environmental

Protection Agency (1975) recommends that filter pack grain size be selected by

multiplying the 70 percent retained grain size of the formation materials by a factor

between 4 and 6. A factor of 4 is used if the formation is fine and uniform; a factor

of 6 is used if the formation is coarser and non uniform. In both cases, the uniformity

coefficient of the filter pack materials should not exceed 2.5 and the gradation of the

filter material should form a smooth and gradual size distribution when plotted. The

actual filter pack used should fall within the area defined by these two curves.

According to Williams (1981), in uniform formation materials, either approach to filter

pack material sizing will provide similar results; however, in coarse, poorly sorted

formation materials, the average grain size method may be misleading and should be

used with discretion."

Filter pack material should be installed in a manner that prevents bridging and

particle size segregation. Filter pack material installed below the water table should generally

be tremied into the annular space. Allowing filter pack material to fall by gravity (free fall)

into the annular space is only appropriate when wells are relatively shallow, when the filter

pack has a uniform grain size, and when the filter pack material can be poured continuously

into the well without stopping.

At least two inches of filter pack material should be installed between the well screen

and the borehole wall

.

The filter pack should extend at least two feet above the top of the

November 1992

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