Constant-Head Permeability Tests Performed in Monitoring Wells at Laboratory Scale Lu Zhang, Robert P. Chapuis & Vahid Marefat Department CGM, Ecole Polytechnique, Montreal, Quebec, Canada ABSTRACT Two types of permeability tests have been performed in ten wells monitoring a confined aquifer, installed in a large sand box. Electronic transducers have been used to register total pressure and atmospheric pressure versus time. The paper presents first the method used to calibrate the sensors, in order to assess their zero offset and its influence on the test data. Then, it presents the results of the variable-head tests and those of constant-head tests performed using a peristaltic pump, thus as constant flow rate tests until stabilization of the water level in the well riser pipe. In addition, several types of constant-head tests have been performed: (i) single flow rate as for groundwater sampling; (ii) different flow rates, with or without rest periods between the periods of constant flow rate. The paper presents and discusses the hydraulic conductivity values that have been obtained from the field data for each type of test, and the influence of seepage direction through the slots of screens having a low open area. RÉSUMÉ Deux types d’essais de perméabilité ont été réalisés dans dix puits de surveillance d’un aquifère à nappe captive, dans une grande cuve de sable. Des capteurs électroniques ont été utilisés pour enregistrer la pression totale et la pression atmosphérique en fonction du temps. L’article présente d’abord la méthode utilisée pour calibrer les capteurs, de façon à évaluer l’erreur sur leur zéro et son influence sur les données d’essai. Il présente ensuite les résultats des essais à niveau variable et ceux des essais à niveau constant, réalisés avec une pompe péristaltique, donc comme des essais à débit constant jusqu’à stabilisation du niveau d’eau dans le tuyau du puits. De plus, différents types d’essais à niveau constant ont été réalisés : (i) débit unique comme pour un échantillonnage; (ii) plusieurs débits, avec ou sans période de récupération entre les étapes à débit constant. L’article présente et discute les valeurs de conductivité hydraulique obtenues des données d’essais in situ pour chaque type d’essai, et l’influence de la direction de l’écoulement dans les fentes des crépines de faible surface ouverte. 1 INTRODUCTION The hydraulic properties of confined aquifers are critical for many hydrogeological and geotechnical problems. The hydraulic conductivity k may be assessed by several types of field permeability tests, which may provide different values. Monitoring wells installed in a large sand- box are used herein to compare the k values of a confined aquifer obtained by four methods: variable-head test, constant-head permeability test and step-drawdown test with or without rest between the pumping steps. All tests are performed within a saturated material where the pore water pressure is greater than the atmospheric pressure. The step-drawdown tests are regularly used to assess the well installation performance, but in this paper they are used to obtain other k values to make comparisons. Step- drawdown tests are also frequently used for groundwater sampling. For all tests, the water level position versus time has been registered by a pressure transducer (PT) and an atmospheric pressure transducer (APT). The drawdown for a pumping test and the change in water column for a permeability test, are often obtained by the following direct subtraction: current PT reading at current time t minus the initial PT reading for the water level at rest before the test. However, this direct subtraction does not take into account the change in air pressure during the test. This paper examines how this change in barometric pressure, during a test, influences the test data accuracy and the resulting k value. This paper first describes the calibration method for different pairs of PTs and APTs to assess their accuracy and then, it examines the influence of a variable barometric pressure. The variable-head permeability tests were started with a water level sudden change in the well riser casing by quick insertion or discharge of water, and the subsequent water level response was recorded over time. The falling- head test was conducted with an extension transparent tube added on the top of the well riser casing. The PT and APT were synchronized and programmed to take readings with a 2-seconds interval because the water recovery was rapid. The variable-head test data were analyzed using the method of Hvorslev (1951) and the velocity graph method (Chapuis, 1998). The constant-head permeability tests were performed by pumping at a constant rate using a peristaltic pump, and registering the water level position versus time. The step-drawdown tests have used several constant pumping rates, which were either successive or separated by rest periods to return to initial equilibrium. The step-drawdown test data were analyzed using the method of Lefranc to obtain the k values. The different k obtained with different testing methods are compared and discussed.