Vertical heterogeneities of hydraulic aquitard parameters: preliminary results from laboratory andin situmonitoring

Abstract

A borehole is developed in a shallow multi-layered aquifer and used to derive the porosity, specific storage and hydraulic conductivity of the aquitard. Local values of hydrodynamical parameters are estimated from petrophysical analysis of core samples, and the empirical relationship between porosity and permeability. Vertical diffusivity is determined from the response of the aquitard to a loading cyclic signal using pressure records at different depths. Hydraulic conductivities deduced from the petrophysical analysis ranged from 10 -8 to 10 -10 m s -1and are comparable with those of facies of marine/lacustrine clay observed in samples. The permeability values calculated based on diffusivity are within the range 10 -9 to 10 -11 m s -1 with a quasi-systematic bias of one order of magnitude. These values are average for a larger part of the aquitard and correspond to an integrated value. The methodology retained for the aquitard characterization is discussed with emphasis on the implications for the management of a complex aquifer system. Key words aquitard; pressure propagation; permeability; specific storage; porosity Hétérogénéités verticales des paramètres hydrodynamiques d'un aquitard: résultats préliminaires à partir de mesures en laboratoire et in situRésumé Un puits est implanté dans un aquifère multi-couches peu profond et utilisé pour mesurer les valeurs de porosité, de coefficient d'emmagasinement spécifique et de conductivité hydraulique des formations semiperméables. Les valeurs ponctuelles des paramètres hydrodynamiques sont estimées à partir de mesures pétro-physiques sur les échantillons carottés durant le forage, et grâce à des modèles empiriques porosité-perméabilité. La diffusivité verticale est déterminée à partir de la réponse de l'aquitard aux signaux cycliques de surcharge à partir des enregistrements de pression atmosphérique, à différentes profondeurs. Les valeurs de conductivité hydraulique obtenues à partir des analyses pétrophysiques sont comprises entre 10 -8 et 10 -10 m s -1 et sont cohérentes avec les faciès argileux marins/lacustres observés sur les échantillons. La perméabilité calculée à partir des valeurs de diffusivité se situe dans une gamme comprise entre 10 -9 et 10 -11 m s -1 avec un décalage quasisystématique d'un ordre de grandeur. Ce sont des valeurs moyennes pour un plus grand volume de l'aquitard, correspondant à une valeur intégrée. La méthode retenue ici pour caractériser l'aquitard est discutée, l'accent étant mis sur les implications pour la gestion d'un système aquifère complexe.

Chloride Dispersion across Silt Deposits in a Glaciated Bedrock River Valley

“…Leakage can exist across an aquitard, which can strongly affect the drawdown response of an underlying confined aquifer during groundwater pumping ( Jacob, 1946). Aquitard hydrogeology has been well documented, with the majority of the research concentrated on clay-rich aquitards (Desaulniers et al, 1981;Hendry et al, 2004;Hiscock and Tabatabai Najafi, 2011;Larroque et al, 2013;Machado et al, 2007;Wiese and Nützmann, 2011). Having a greater permeability than clay-rich aquitards, silty aquitards have a greater average linear velocity and therefore greater potential for contaminants to reach the aquifer faster.…”

Palaeohydrogeology and Transport Parameters Derived from 4He and Cl Profiles in Aquitard Pore Waters in a Large Multilayer Aquifer System, Central Australia

“…Aquitard properties, such as hydraulic conductivity, and their variability can be determined by fluid pressure measurements within the aquitard using in situ vibrating wire pressure transducers [6,7], although hydraulic conductivity measurements can increase with the scale of measurement [8,9]. When combined with the gradient of fluid pressure using Darcy's law, the vertical flow direction and rate can be determined.…”

Geophysical Characterization of Serpentinite Hosted Hydrogeology at the McLaughlin Natural Reserve, Coast Range Ophiolite

“…In these Regions S and W, water (i.e., surface and ground water) is stored year round regardless of changes in seasonal climate (Figures and ) in part due to the formation with higher clay content, which typically shows lower hydraulic conductivities (e.g. Larroque et al, ; Yu et al, ) (Figure ). These observations revealed that the water table at both CROMO sites is not laterally continuous but perched by the surrounding lithology.…”