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
6 41






footer




 

 

 

 

 Home | About Us | Network | Services | Support | FAQ | Control Panel | Order Online | Sitemap | Contact

quality web hosting

 

Our partners: PHP: Hypertext Preprocessor Cheap Web Hosting JSP Web Hosting Ontario Web Hosting  Jsp Web Hosting

Cheapest Web Hosting Java Hosting Cheapest Hosting

Visionwebhosting.net Business web hosting division of Vision Web Hosting Inc.. All rights reserved