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VADOSE FLUTe

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VADOSE GAS SAMPLING

The FLUTe liner is often installed in the vadose zone to seal the hole against flow and to stabilize the hole wall. Several kinds of installation procedures are used. In most cases, the liner is everted into the hole
(see everting liner mechanism). For other situations, the liner is lowered into the hole, as when the hole is supported by a temporary casing, and then filled with sand as the casing is withdrawn. In all cases, the flexible liner provides a seal against the hole wall.

The sampling geometry is shown in the drawing.

The spacer material on the outside of the liner serves to define the interval of the hole from which the sample will be drawn. As the pressure is reduced in the sampling tube at the surface, the pressure is reduced in the spacer material interstices. This low-pressure draws pore gas into the spacer and hence into the tube to the surface via the port through the liner. If the spacer is relatively short, the pressure field near the spacer is essentially a spherical 1-D flow field centered on the spacer. For longer spacers, the flow field is more like a 1-D cylindrical flow field. The assumption for both geometries is that the medium is homogeneous and isotropic.
As more pore gas is removed from the spacer, the larger is the volume from which the sample is drawn in the formation. Typically, the tube volume is purged of its gas and the sample is collected thereafter.
Because the tubing is gathered in interior sleeves of the liner, it is relatively easy to emplace many sampling ports in the hole.

Transducers can be attached at the surface to the sample tubes to monitor the pore pressure history at each port. This is useful for design of a soil vapor extraction remediation system.
These systems are particularly useful for monitoring of gas flow near landfills in the vadose zone. FLUTe gas sampling systems are often installed in driven casing systems to hundreds of feet and filled with sand upon the withdrawal of the casing.

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ABSORBER INSTALLATIONS ON A LINER

The drawing shows how segments of absorbers attached to the liner are rolled out against the hole wall. A variety of absorbent coverings have been used. Some were only short annular surrounds on the liner, attached to the liner with buttons. Others were patches of absorbent material. The most common is a continuous covering with wicking barriers of coarse nonabsorbent material dividing the tubular absorber into short segments. The wicking barriers prevent the absorber from transporting contaminants very far along the hole. In that way, each section is a local sample of the available pore liquids.

In many cases, the absorbers become quite wet. As the liner is inverted from the hole, the absorbent material is rolled to the interior of the inverting liner. This prevents the absorber from contacting the hole wall at any location other than where it was initially placed. The absorber travels to the surface, well protected inside the inverted liner.

At the surface, the liner is often everted into a long, flat tubular sleeve of plastic film. The absorber is then disconnected from the end of the liner. The liner is then inverted from out of the absorber. The absorbent covering is left flat in the tubular film, with little or no contact with the air. Elastic bands can then be wrapped on the tubular plastic to form "sausage link-like sections" of absorber sealed in plastic for analysis. If the contaminant was colored, it can be easily seen staining a white absorber. If the contaminant is radioactive, it can be scanned for a profile of the activity in the hole. The NAPL FLUTe system of color reactive mapping of NAPLs is a special kind of absorber.  Another useful absorber system is the FACT (FLUTe Activated Carbon Technique) which contains an activated carbon felt for wicking, by diffusion, contaminants from the pore space of the formation in both the vadose and saturated zones.

The absorbent covering is most easily emplaced in the vadose zone in a stable hole. Air or water can be used as the pressurizing fluid. If a liquid is used, it should be tagged with a tracer/dye to assure that if any is absorbed in the cover, it is not mistaken for a pore liquid. As with other everting liners, absorber liners can be everted into passages in any direction, even vertically upward and around bends. Absorber FLUTe systems have been installed, using the duet technique, in a hole already sealed and supported by another liner (e.g., a 4 in. absorber liner installed into a hole sealed by a 6 in. liner). In most cases, the first larger liner was a Vadose FLUTe in use for gas sampling.

Some absorbent covers have been instrumented with wire pairs, like a gypsum block, to monitor the resistance change with water absorption to identify the passage of wetting fronts or to determine when the absorber has reached equilibrium saturation prior to removal.

The absorber collection of vadose pore liquids has been done the most by Lawrence Livermore National Laboratory since 1991. Carl Keller and Brian Travis wrote a paper on the utility of absorber collection of vadose fluids. Comparisons of laboratory measurements were made with vadose flow model calculations for a variety of saturations and materials. That paper is in the proceedings of the NGWA 7th Outdoor Action Conference in Las Vegas, 1993. With the use of well-characterized absorbers, the weight gain of the absorber can be an indirect measure of the capillary tension of the formation.

Some vadose systems pressurized with a small air pump powered by a solar panel have been in use for many years.