PUMPING TEST IN A CONFINED AQUIFER: HOW TO DETECT A POORLY SEALED MONITORING WELL

Robert P. Chapuis, Djaouida Chenaf

In the proceedings of: GeoQuébec 2015: 68th Canadian Geotechnical Conference & 7th Canadian Permafrost Conference

Session: Groundwater / Eaux souterraines

ABSTRACT: 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.

RÉSUMÉ: Un puits qui surveille une nappe captive doit avoir son tuyau parfaitement scellé contre la paroi du forage. Un mauvais scellement cause un court-circuit hydraulique avec écoulement préférentiel. Ainsi tuyau mal scellé produit des valeurs de rabattement, en pompage et remontée, qui sont incorrectes. Cet article explique comment détecter un court-circuit hydraulique et obtenir les bonnes valeurs du rabattement, avec près de Moncton, NB. -circuit : elles ont fourni des valeurs voisines pour la transmissivité T, mais des valeurs différant de 500% pour S-statique du tuyau était 124 cm plus bas que le niveau pirabattement en pompage et remontée ont été corrigées puis à nouveau analysées, ce qui a donné de nouvelles valeurs pour T et S, cette fois cohérentes entre elles, ce qui a confirmé le diagnostic de court-circuit hydraulique.

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