Filter Packs And Annular Seals

Filter packs placed around the well screen allow ground-water to flow freely into the well while keeping fine par ticles from entering the well. Two types of filter packs are used in monitoring wells: naturally developed filter packs and artificially introduced filter packs. Naturally developed filter packs are produced in situ when the fine-grained materials around the screen are removed during the well development process. Environmental engineers construct artificial filter packs by backfilling the annular space surrounding the screen with a granular, relatively inert material such as clean silica sand.

In an artificially filter-packed well, the filter material can be selected for optimum efficiency of well operation, but the procedure of introducing the filter pack is time consuming and expensive. Furthermore, bridging can prevent complete filling around the well screen, and the filter pack material can introduce contaminants into the aquifer; a leaching test can determine whether this contamination is a problem. Naturally developed filter packs are, on the other hand, simpler, less expensive, and do not introduce new contaminants into the aquifer. However, well development for these filter packs is more difficult, and success is less assured.

Engineers can use a tremie pipe or a reverse circulation method to place the artificial filter pack. The tremie pipe method allows funneling of the material directly into the interval around the well screen. In a reverse circulation method, a mixture of sand and water is fed into the an-nulus around the screen, and the water entering the screen is pumped up to the surface through the casing. The engineer progressively pulls back the temporary casing (for hollow-stem augers) to expose the screen as the filter pack material builds up around the well screen.

Artificially introduced filter packs usually extend from the bottom of the screen to at least 3 to 5 feet above the bottom of the screen. This extension accounts for settlement of the filter pack material and allows a sufficient buffer zone between the well screen and the annular seal above.

After the filter pack is placed around the well screen, the engineer seals the annular space between the well casing and the formation to prevent upward or downward movement of water and contaminants along this pathway. In addition, the engineer places a surface seal of concrete around the protective casing to prevent surface drainage into the borehole. The annular seal is usually composed of bentonite or neat cement (Williams and Evans 1987). Bentonite is readily available and inexpensive but can cause constituent interference due to ion exchange. Neat cement is also readily available and inexpensive, but channeling between the casing and seal can develop due to temperature changes during the curing process (U.S. EPA 1993).

The engineer places the sealing mixture in the annular space using a side-discharge tremie pipe through which the grout is pumped from the surface. Complete sealing of the annular space is necessary to avoid potential bridging of the grout with formation material (Campbell and Lehr 1975).

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