Mass transfer between immobile and mobil• zones is a consequence of simultaneous processes. We develop a "multirate" modeithat allows modeling of smallscale variation in rates and types of mass transfer by using a series of first-order equations to represent each of the mass transfer processes. The multirate model is incorporated into the advective-dispersive equation. First, we compare the multirate model to the standard first-order and diffusion models of mass transfer. The spherical, cylindrical, and layered diffusion models are all shown to be specific cases of the multirate model. Mixtures of diffusion from different geometries and first-order rate-limited mass transfer can be combined and represented exactly with the multirate model. Second, we develop solutions to the multirate .equations under conditions of no flow, fast flow, and radial flow to a pumping well. Third, using the multirate model, it is possible to acCUrately predict rates of mass transfer in a bulk sample of the Borden sand containing a mixture of different grain sizes and diffusion rates. Fourth, we investigate the effects on aquifer remediation of having a heterogeneous mixture of types and rates of mass transfer. Under some circumstances, even in a relatively homogeneous aquifer such as at Borden, the mass transfer process is best modeled by a mixture of diffusio n rates.

[1] Groundwater consumption by phreatophytes is a difficult-to-measure but important component of the water budget in many arid and semiarid environments. Over the past 70 years the consumptive use of groundwater by phreatophytes has been estimated using a method that analyzes diurnal trends in hydrographs from wells that are screened across the water table (White, 1932). The reliability of estimates obtained with this approach has never been rigorously evaluated using saturated-unsaturated flow simulation. We present such an evaluation for common flow geometries and a range of hydraulic properties. Results indicate that the major source of error in the White method is the uncertainty in the estimate of specific yield. Evapotranspirative consumption of groundwater will often be significantly overpredicted with the White method if the effects of drainage time and the depth to the water table on specific yield are ignored. We utilize the concept of readily available specific yield as the basis for estimation of the specific yield value appropriate for use with the White method. Guidelines are defined for estimating readily available specific yield based on sediment texture. Use of these guidelines with the White method should enable the evapotranspirative consumption of groundwater to be more accurately quantified.Citation: Loheide, S. P., II, J. J. Butler Jr., and S. M. Gorelick (2005), Estimation of groundwater consumption by phreatophytes using diurnal water table fluctuations: A saturated-unsaturated flow assessment, Water Resour. Res., 41, W07030,

. We argue here that there is a high probability that earthquakes will be triggered by injection of large volumes of CO 2 into the brittle rocks commonly found in continental interiors. Because even small-to moderate-sized earthquakes threaten the seal integrity of CO 2 repositories, in this context, large-scale CCS is a risky, and likely unsuccessful, strategy for significantly reducing greenhouse gas emissions.carbon sequestration | climate change | triggered earthquakes

Numerical models that solve governing equations for subsurface fluid flow and transport are commonly applied to analyze quantitatively the effects of heterogeneity. These models require maps of spatially variable hydraulic properties. Because complete three‐dimensional information about hydraulic properties is never obtainable, numerous methods have been developed to interpolate between data values and use geologic, hydrogeologic, and geophysical information to create images of aquifer properties. Image creation approaches fall into three general categories: structure‐imitating, process‐imitating, and descriptive. Structure‐imitating methods rely on one or more of the following to constrain the geometry of spatial patterns in geologic media: correlated random fields, probabilistic rules, and deterministic constraints developed from facies relations. Structure‐imitating methods include spatial statistical algorithms and geologically based sedimentation pattern‐matching approaches. Process‐imitating models include aquifer model calibration methods and geologic process models. Aquifer model calibration methods use governing equations for subsurface fluid flow and transport to relate hydraulic properties to heads and solute information through history and steady state data matching. Geologic process models combine fundamental laws of conservation of mass and momentum with sediment transport equations to simulate spatial patterns in grain size distributions. At the sedimentary basin scale, multiprocess models include thermomechanical mechanisms of basin subsidence. Descriptive methods couple geologic observations with facies relations to divide an aquifer into zones of characteristic hydraulic properties. All approaches are capable of reproducing heterogeneity over a range of scales and considering some types of geologic information. Some approaches are strictly spatial while some are linked to the time evolution of sedimentation. Some approaches can be conditioned on measurements. Recent advances aimed at infusing geologic information into images of the subsurface include extracting more information from sedimentological facies models, incorporating qualitative geologic information into random field generators and simulating depositional processes. Classes of research missing from the literature include multiprocess models that incorporate diagenesis and three‐dimensional surface water flow, hybrid methods that combine features of existing approaches, and approaches that can make use of all available geologic, geophysical, and hydrologic data.

Abstract. We present a model of solute transport that explains the large-scale behavior of the solute tracer-test plumes at the Macrodispersion Experiment (MADE) site as the result of advection and rate-limited mass transfer between mobile and small-scale immobile domains. This model does not consider the process of dispersion and yet provides an alternative explanation of the evolution of the observed concentration profiles.Compared to the macrodispersion model, the mass transfer model better represents the change in mobile dissolved mass with time, the peak of the concentration profile, and the profile asymmetry. Specifically, unlike the macrodispersion model, the mass transfer model explains the facts that the observed mass of the plume was greater than the injected mass in early snap shots of the plume and less than the injected mass at late times. We suggest that the injected mass advects through the mobile domain and diffuses into and out of the immobile domain. The immobile domain consists of a combination of low-permeability zones on the scale of centimeters to decimeters (the Darcy-scale immobile domain), and intragranular porosity, dead-end pores, and surface sorption (the pore-scale immobile domain). We suggest that the mobile domain was sampled preferentially when water was extracted. Therefore, at early times, relatively clean water in the immobile domain was not sampled and incorrectly assumed to contain high solute concentrations. Similarly, the mass at late times was underestimated because solute trapped in the pore-scale immobile domain was not extracted during sampling and therefore ignored. The combination of advection and slow mass transfer is consistent with the fact that the peak of the plume migrated only -5 m by the termination of the experiment, as well as the different behavior of bromide and tritium tracers. IntroductionTwo natural gradient tests have been conducted at the Columbus Air Force Base in northeastern Mississippi (Figure 1) We contend that a combination of physical nonequilibrium mass transfer (molecular diffusion into and out of low permeability areas) and chemical sorption can explain much of the behavior of the plumes at the Mississippi site. Declining mobile mass and extreme asymmetric spreading are both typical of transport subject to rate-limited mass transfer when the capacity coefficient (ratio of immobile to mobile mass at equilibrium) is large and when the timescale of mass transfer is similar to the timescale of the experiment. Thus, we suggest that the mass transfer model provides an alternative to the macrodispersion model that may better explain the key features of large-scale spreading of the solute plumes. We present our model of plume evolution and analyze the behavior of the solute mass, advection, and spreading. The paper is organized as follows: we introduce the MADE site and the controversy regarding processes that dominate plume evolution (this section); we present the mass transfer model (section 2); we apply the mass transfer model to the MADE plumes ...

Groundwater exploitation is a major cause of land subsidence, which in coastal areas poses a flood inundation hazard that is compounded by the threat of sea-level rise (SLR). In the lower Mekong Delta, most of which lies <2 m above sea level, over-exploitation is inducing widespread hydraulic head (i.e., groundwater level) declines. The average rate of head decline is ∼0.3 m yr −1 , based on time-series data from 79 nested monitoring wells at 18 locations. The consequent compaction of sedimentary layers at these locations is calculated to be causing land subsidence at an average rate of 1.6 cm yr −1 . We further measure recent subsidence rates (annual average, 2006-10) throughout the Delta, by analysis of interferometric synthetic aperture radar (InSAR), using 78 ALOS PALSAR interferograms. InSAR-based subsidence rates are 1) consistent with compaction-based rates calculated at monitoring wells, and 2) ∼1-4 cm yr −1 over large (1000s of km 2 ) regions. Ours are the first mapped estimates of Delta-wide land subsidence due to groundwater pumping. If pumping continues at present rates, ∼0.88 m (0.35-1.4 m) of land subsidence is expected by 2050. Anticipated SLR of ∼0.10 m (0.07-0.14 m) by 2050 will compound flood inundation potential. Our results suggest that by mid-century portions of the Mekong Delta will likely experience ∼1 m (0.42-1.54 m) of additional inundation hazard.

[1] Cross-well electrical resistivity tomography (ERT) was used to monitor the migration of a saline tracer in a two-well pumping-injection experiment conducted at the Massachusetts Military Reservation in Cape Cod, Massachusetts. After injecting 2200 mg/L of sodium chloride for 9 hours, ERT data sets were collected from four wells every 6 hours for 20 days. More than 180,000 resistance measurements were collected during the tracer test. Each ERT data set was inverted to produce a sequence of 3-D snapshot maps that track the plume. In addition to the ERT experiment a pumping test and an infiltration test were conducted to estimate horizontal and vertical hydraulic conductivity values. Using modified moment analysis of the electrical conductivity tomograms, the mass, center of mass, and spatial variance of the imaged tracer plume were estimated. Although the tomograms provide valuable insights into field-scale tracer migration behavior and aquifer heterogeneity, standard tomographic inversion and application of Archie's law to convert electrical conductivities to solute concentration results in underestimation of tracer mass. Such underestimation is attributed to (1) reduced measurement sensitivity to electrical conductivity values with distance from the electrodes and (2) spatial smoothing (regularization) from tomographic inversion. The center of mass estimated from the ERT inversions coincided with that given by migration of the tracer plume using 3-D advective-dispersion simulation. The 3-D plumes seen using ERT exhibit greater apparent dispersion than the simulated plumes and greater temporal spreading than observed in field data of concentration breakthrough at the pumping well.Citation: Singha, K., and S. M. Gorelick (2005), Saline tracer visualized with three-dimensional electrical resistivity tomography: Field-scale spatial moment analysis, Water Resour. Res., 41, W05023,

Petrophysical relations are derived to predict porosity and hydraulic conductivity from grain size distributions considering particle packing in sediment mixtures. First, we develop a fractional packing model for porosity that considers the fraction of intrapore fines that occur as the fines content increases. Then, a fractional packing Kozeny-Carman relation for hydraulic conductivity is developed by examining which particle sizes dominate the pore structure, and which averaging procedure best represents the mean grain diameter in any given sediment mixture. The relations developed here perform well for a wide range of sediment mixtures regardless of confining pressure. Graphs are presented that show hydraulic conductivity versus weight fraction of fines for mixtures of coarse-and fine-grained sediment commonly observed in the field, such as clayey gravel and silty sand. These graphs show that the wide range of hydraulic conductivity values reported for sediment mixtures can display a 5 order of magnitude variation over a few percent fines. Finally, a field scale application using grain size distributions from a quantitative depositional model shows that these petrophysical relations successfully predict more than 90% of hydraulic conductivity values to within 1 order of magnitude over 7 orders of magnitude of spatial variability. Introduction Qualitative relations between the porous media hydraulic properties of porosity and permeability and the petrographicproperties of grain diameter, shape, sorting, and packing are well established [Chilingar, 1964; Wolf and Chilingarian, 1976; Beard and Weft, 1973; Blatt et al., 1980]. Laboratory and field experiments have shown that grain diameter and sorting are the key petrographic factors affecting porosity and permeability in unconsolidated siliciclastic sediments [Chilingarian and Wolf, 1976; Blatt et al., 1980; Wolf and Chilingarian, 1976; Beard and Weft, 1973]. Less well established are quantitative relations between key petrographic factors and hydraulic properties. It is known that estimation of subsurface material propert, ies made with quantitative petrophysical relations is often subject to much uncertainty [Taylor et al., 1987]. Two basic approaches have been applied to transform grain size distributions to hydraulic properties: (1) average hydraulic properties can be based on average grain diameters using tabulated values or nomographs; and (2) hydraulic properties can be calculated from grain size distributions using quantitative relations developed either empirically or from the theory of flow through porous media. The grain size approach requires tables of hydraulic conductivity and porosity versus average grain size [Freeze and Cherry, 1979; Marsily, 1986; Domenico and Schwartz, 1990]. Such tables do not consider sorting, porosity variations, and intrapore finegrained materials and cements. To include sorting, nomographs have been developed that plot porosity and permeability versus grain size and sorting for coarse to very fine sands [Masch and Denny, 19...