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PUMPING TEST IN A CONFINED AQUIFER: HOW TO DETECT A POORLY SEALED MONITORING WELL
Session: Groundwater / Eaux souterraines
Robert P. Chapuis, CGS (Canada)
Djaouida Chenaf, CGS (Canada)
When a well is monitoring a confined aquifer, its riser pipe must be perfectly sealed against the borehole wall. A poor seal produces a hydraulic short-circuit and preferential seepage in it. Then, the static water level in the monitoring well is not the aquifer piezometric level, which is unknown. During a pumping test, a poorly sealed monitoring well yields incorrect drawdown and recovery data. This paper explains how to detect short-circuiting and obtain the correct drawdown data, using the example of a test near Moncton, NB. The usual methods for interpreting the drawdown and recovery data ignored the possible short-circuiting: they yielded close values for transmissivity T, but storativity S values differing by 500%. The proposed method found that because of short-circuiting the static water level in the riser pipe was 124 cm below the aquifer piezometric level. Then, drawdown and recovery data were corrected and reanalyzed, which yielded new and close values for T and S, thus supporting the diagnosis of hydraulic short-circuiting.
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