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#1 Posted : Thursday, July 30, 2009 1:29:39 PM(UTC)

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The Slug Test Method is one of a number of different methods that are used to evaluate the permeability (or hydraulic conductivity) of an aquifer. The procedure involves either adding or removing a measured quantity of water from a well rapidly, followed by making a rapid series of water-level measurements to assess the rate of water-level recovery (either rising-head or falling-head). These evaluations have advantages and disadvantages when compared with other methods.

Advantages of the slug test method include:
• Relatively low cost.
• Requires little time to conduct slug test(s).
• Involves removal of little or no water from the aquifer.

Disadvantages of the slug test method include:
• Only evaluates a small portion of the aquifer adjacent to the well bore.
• Does not provide an evaluation of portions of the aquifer not screened by the well being tested.
• May be profoundly influenced by gravel or sand pack material in the bore hole adjacent to the well screen.

The type of procedure that is used to evaluate a well is often determined by aquifer conditions. A bail-down evaluation may be conducted in aquifers that are poorly transmissive and require many minutes to recover after they have been bailed. However, it is difficult or impossible to test a highly transmissive aquifer by bail-down methods because water level recovery is so rapid. Assessments using transducer measurements and a data-logger may be used to evaluate hydraulic conductivity in situations where water-level recovery is either slow or rapid. In either situation, more accurate results are generally obtained when using an in-well transducer to collect periodic water-level versus time measurements.

Bail-Down Method
When conducting a bail-down test, the pre-test water-level is measured and noted. The same water-level reference measuring point should be used throughout testing. A bailer is then used to rapidly lower the water-level in the well for a period of one or more minutes. The exact time when bailing is stopped should then be noted and periodic water-level measurements collected to track the rate of water-level recovery. An electric tape measuring device (water-level meter) generally provides the best and most rapid measurements. The first measurement should be made as soon after bailing is stopped as possible. Subsequent measurements should be made at frequent (one-minute or so) intervals initially, and less frequent intervals as water-level recovery slows sufficient to define the recovery trend. Recovery measurements should be continued until a good percentage of total recovery is attained. If feasible, at least 75% of full recovery should be attempted.

Data-Logger (Transducer) Method
In materials of any hydraulic conductivity more accurate results are generally obtained when using an in-well transducer to collect periodic water-level versus time measurements. The transducer is placed in the well below the pre-test water-level a sufficient depth to permit testing (adding and/or removing a "slug" of water). An instrument (data-logger) records water-depth above the transducer before, during, and after the "slug" is introduced. The "slug" is introduced suddenly (either raising or lowering the water-level) and a series of water-level versus time measurements are made as the water-level changes toward an equilibrium situation. The measurements are collected automatically by the transducer and data-logger, usually at pre-programmed time intervals.
For the data-logger/transducer method of conductivity slug tests we have found that the rapid addition of a solid PVC cylinder to displace a known quantity of water in the well bore. Adding the cylinder causes an abrupt rise of water-level and rapid removal of the cylinder causes an abrupt drop in water-level in the well. Typically the cylinder is constructed of PVC tubing capped at each end and filled with sand for ballast. We have used 2-feet (for wells with limited standing water depth) and 5-feet long cylinders in slug tests.
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