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Slug Test Method (Continued)  



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  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 permeability (or 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.

Calculation of Hydraulic Conductivity

A number of methods have been developed to calculate hydraulic conductivity (or permeability) from slug test data.   We usually use the Method of Bouwer and Rice (1976) to obtain hydraulic conductivity from raw slug test data.   This method was originally published in an article entitled "Slug Test Procedure to Evaluate Hydraulic Conductivity of an Aquifer Applicable to Fully or Partially Penetrating Wells in Unconfined Aquifers".   Computer programs for aquifer evaluation by slug test and other methods are most commonly used to interpret the raw slug test data.   

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