[1] We describe the upscaled groundwater flow and solute transport characteristics of two-dimensional hydraulic conductivity fields with three fundamentally different spatial textures and consider the conditions under which physical mobile-immobile domain mass transfer occurs in these fields. All three fields have near-identical lognormal univariate conductivity distributions, as well as near-identical isotropic spatial covariance functions. They differ in the pattern by which high-or low-conductivity regions are connected: the first field has connected high-conductivity structures; the second is multivariate logGaussian and, hence, has connected structures of intermediate value; and the third has connected regions of low conductivity. We find substantially different flow and transport behaviors in the three different fields. Flow and transport in the multivariate log-Gaussian field are consistent with stochastic theory. The field with connected high-conductivity paths has an effective conductivity greater than the geometric mean and large variations in fluid velocity. It produces significant mass transfer behavior (i.e., tailing) when the conductivity variance is large and, depending on the system parameters, this mass transfer is driven by either diffusion or advection. In the field with connected low-conductivity regions, the effective conductivity is below the geometric mean and transport is well characterized by the advection-dispersion model with a dispersivity smaller than that in the multivariate log-Gaussian field. Thus, physical mobile-immobile domain mass transfer may occur in smooth hydraulic conductivity fields with univariate log-Gaussian density functions if the variability in conductivity is sufficient and the high values are more connected than modeled by the multivariate log-Gaussian distribution.

Solute transport displaying mass transfer behavior (i.e., tailing) occurs in many aquifers and soils. Spatial patterns of hydraulic conductivity may play a role because of both advection and diffusion through isolated low conductivity areas. We demonstrated such processes in laboratory experiments designed to visualize solute transport through a thin chamber (40 cm x 20 cm x 0.64 cm thick) packed with glass beads and containing circular emplacements of smaller glass beads with lower conductivity. The experiments used three different contrasts of conductivity between the large-bead matrix and the emplacements, targeting three different regimes of solute transport: low contrast, targeting macrodispersion; intermediate contrast, targeting advection-dominated mass transfer between the high-conductivity regions and the emplacements; and high contrast, targeting diffusion-dominated mass transfer. Use of a strong light source, a high-resolution CCD camera, and a colorimetric dye produced images with a spatial resolution of about 400 microm and a concentration range of approximately 2 orders of magnitude. These images confirm the existence of the three different regimes, and we observed tailing driven by both advection and diffusion. Outflow concentration measured by spectrophotometer achieved 3 orders of magnitude in concentration range and showed good agreement with known models in the case of dispersion and diffusive mass transfer, with estimated parameters close to a priori predictions. Existing models for diffusive mass transfer did notfitthe breakthrough curves from the intermediate-contrast chamber, but a model of slow advection through cylinders did. Thus, both breakthrough curves and chamber images confirm that different contrasts in small-scale K lead to different regimes of solute transport and thus require different models of upscaled solute transport.

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[1] Groundwater ages estimated from environmental tracers can help calibrate groundwater flow models. Groundwater age represents a mixture of traveltimes, with the distribution of ages determined by the detailed structure of the flow field, which can be prone to significant transient variability. Effects of pumping on age distribution were assessed using direct age simulation in a hypothetical layered aquifer system. A steady state predevelopment age distribution was computed first. A well field was then introduced, and pumpage caused leakage into the confined aquifer of older water from an overlying confining unit. Large changes in simulated groundwater ages occurred in both the aquifer and the confining unit at high pumping rates, and the effects propagated a substantial distance downgradient from the wells. The range and variance of ages contributing to the well increased substantially during pumping. The results suggest that the groundwater age distribution in developed aquifers may be affected by transient leakage from low-permeability material, such as confining units, under certain hydrogeologic conditions.

Environmental tracers are used to estimate groundwater ages and travel times, but the strongly heterogeneous nature of many subsurface environments can cause mixing between waters of highly disparate ages, adding additional complexity to the age-estimation process. Mixing may be exacerbated by the presence of wells because long open intervals or long screens with openings at multiple depths can transport water and solutes rapidly over a large vertical distance. The effect of intraborehole flow on groundwater age was examined numerically using direct age transport simulation coupled with the MultiNode Well Package of MODFLOW. Ages in a homogeneous, anisotropic aquifer reached a predevelopment steady state possessing strong depth dependence. A nonpumping multi-node well was then introduced in one of three locations within the system. In all three cases, vertical transport along the well resulted in substantial changes in age distributions within the system. After a pumping well was added near the nonpumping multi-node well, ages were further perturbed by a flow reversal in the nonpumping multi-node well. Results indicated that intraborehole flow can substantially alter groundwater ages, but the effects are highly dependent on local or regional flow conditions and may change with time.Résumé Les traceurs environnementaux sont habituellement utilisés pour estimer les âges des eaux souterraines et les temps de résidence. Cependant, la nature hautement hétérogène de nombreux environnements souterrains peut engendrer des mélanges entre des eaux d'âges très disparates, complexifiant par-là même le processus d'estimation des âges. La présence de puits peut exacerber le phénomène de mélange : de longues sections en trou nu ou crépinées exploitant plusieurs niveaux productifs distincts peuvent transporter rapidement l'eau et les solutés sur une grande distance verticale. Les conséquences de flux intraforages sur les ages des eaux souterraines ont été étudiées numériquement en couplant les simulations directes de temps de résidence avec le "Multi-Node Well Package" de MODFLOW. Dans un aquifère homogène et anisotrope, les âges ont atteint un régime permanent étroitement dépendant de la profondeur. Un puits multinoeud au repos a ensuite été inséré dans l'une des trois zones du système. Dans les trois cas, les transports verticaux par le puits ont entraîné des modifications substantielles des distributions des âges dans le système. Enfin, après ajout d'un puits en pompage à proximité du puits au repos, les âges ont été perturbé davantage, par une inversion du flux dans le puits au repos. Les résultats ont montré que les flux intraforages peuvent modifier substantiellement les âges des eaux souterraines, mais leurs effets sont hautement dépendants des conditions locales ou régionales d'écoule-ment, et peuvent de surcroît changer dans le temps.Resumen Los trazadores ambientales pueden usarse para estimar el tiempo de viaje y las edades del agua subterránea pero la naturaleza fuertemente heterogénea de muchos ambientes s...

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