Propagation of pressure change through thick clay sequences: an example from Liverpool Plains, NSW, Australia

In-situ hydraulic conductivity and specific storage measurements are derived from an analysis of pore-water pressure changes in a nest of piezometers installed in a 40-m-thick succession of smectitic clay on the Liverpool Plains of northern New South Wales, Australia. The cumulative response to the rainfall events that typically occurs during winter or early spring is propagated through the clay with measurable loss of amplitude and increasing phase lag. Five major rainfall events occurred over the four years of detailed monitoring. The phase lag at the base of the clay varied between 49 and 72 days. Barometric efficiency (BE) measurements for the clay sequence (BE = 0.07) and the underlying confined aquifer (BE = 0.10) were used, with a known porosity of 0.567, to derive specific storage values of 3.710 super(-5) and 6.810 super(-6)m super(-1) respectively. Vertical hydraulic conductivity (K sub(v)) of the clay sequence derived from observed amplitude and phase changes, resulted in an average value of 2.810 super(-9)m/s. These in-situ-derived values indicate that previous estimates of vertical hydraulic conductivity of the clays, made on core samples, are unrealistically high. The instantaneous response to individual rainfall events transmitted through the clay succession (tidal efficiency of 0.93) is also described.Original Abstract: Des mesures in-situ de conductivite hydraulique et d'emmagasinement specifique sont obtenues a partir de l'analyse de la variation des pressions interstitielles telles que mesurees au droit d'un nid de piezometres interceptant une epaisseur de 40m d'argile smectique dans les plaines de Liverpool, region nord du New South Wales, Australie. La reponse cumulative aux evenements pluviometriques qui surviennent typiquement durant l'hiver ou le debut du printemps, se propage dans les argiles suite a une diminution d'amplitude (des pressions interstitielles) et a un accroissement du pas de temps.. Cinq evenements pluviometriques sont survenus lors d'un suivi detaille effectue sur quatre (4) annees. Le pas de temps a la base de l'argile a varie de 49 a 72 jours. Les mesures d'efficience barometrique (EB) de l'assemblage argileux (BE = 0.07) et de la nappe aquifere confinee sous-jacente (BE = 1.10) ont ete utilisees, avec une porosite connue de 0,567, afin d'obtenir des valeurs d'emmagasinement specifique de respectivement 3.710 super(-5) et 6.610 super(-6)m super(-1). La conductivite hydraulique verticale (Ky) de l'assemblage argile