Falling-Head Permeability Tests in an Unconfined Sand Aquifer
Chapuis, RP
Department of Civil, Geological and Mining Engineering, Stn CV, Quebec, H3C 3A7, Canada

Dallaire, V
Department of Civil, Geological and Mining Engineering, Stn CV, Quebec, H3C 3A7, Canada

Gagnon, F
Department of Civil, Geological and Mining Engineering, Stn CV, Quebec, H3C 3A7, Canada

Marcotte, D
Department of Civil, Geological and Mining Engineering, Stn CV, Quebec, H3C 3A7, Canada

Chouteau, M
Department of Civil, Geological and Mining Engineering, Stn CV, Quebec, H3C 3A7, Canada

(Received 13 December 2005; accepted 9 August 2006)

Abstract

This paper examines the reliability of hydraulic conductivity estimates, k, obtained using equations of Hvorslev (1951) or Bouwer and Rice (1976) for falling-head tests in monitoring wells (MWs) having short screens at the bottom of an unconfined sand aquifer. Two issues are examined. First, the equations come from the theory of steady-state flow whereas a variable-head test means transient flow with a changing water table position. This first problem was investigated using a finite element analysis, taking into account the sand capillary retention curve and its saturated-unsaturated permeability. The main result was that the equations can still be used for variable-head tests. The effects of specific storage (elastic deformation of the solid matrix in saturated conditions) and delayed gravity drainage (frequently schematized by a specific yield) can be neglected for these tests. The second issue is how partial clogging or fine particle washing against the screen can influence the k value. This practical problem was investigated by using a surge block to develop MWs and then performing successive permeability tests to assess the effects of development. Before development, the tests provided k values in the range 1 to 12×10⁻² cm/s, the average being equal to 2/3 of the large-scale k value obtained using a pumping test under steady-state condition. After development, the tests provided k values that were increased by 50 % on average, and thus their mean value became almost equal to the pumping test k value.

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Author Chapuis RP, Dallaire V, Gagnon F, Marcotte D, Chouteau M Title Falling-Head Permeability Tests in an Unconfined Sand Aquifer Symposium , Committee on