P. S. C. Rao

Sorption nonequilibrium during solute transport

Abstract The effects of pore-water velocity, solute hydrophobicity, and sorbent organic-carbon content on sorption nonequilibrium during solute transport were evaluated. Nonequilibrium transport was observed to increase with pore-water velocity, solute hydrophobicity, and sorbent organic-carbon content. Nonequilibrium transport of neutral organic compounds was not detected with low organic-carbon (TOC = 0.33 g kg−1) aquifer material, but was detected on higher organic sorbents from the unsaturated zone (TOC = 2.6 g kg−1) and the soil surface (TOC = 6.9 g kg−1). For solute-sorbent combinations yielding retardation factors > 2, nonequilibrium during transport was observed. After experimentally accounting for slow solute diffusion in the aqueous phase and isotherm nonlinearity as potential contributors to nonequilibrium solute transport, sorption nonequilibrium was attributed to slow solute diffusion within the organic-carbon matrix.

Determination of specific NAPL–water interfacial areas of residual NAPLs in porous media using the interfacial tracers technique

Abstract We present here an application of a new experimental method called the `interfacial tracers technique (IFTT), using tracer retardation by adsorption as the basis, to determine the specific NAPL–water interfacial areas ( a nw , cm 2 /cm 3 ) in packed columns of sand and glass beads containing residual saturation of non-aqueous phase liquid (NAPLs). Four NAPLs, two types of sand and uniform glass beads, and two interfacial tracers were used in the tracer experiments. The a nw values determined for residual NAPLs in sand or glassbead columns were in the range of 60 to 300 (with one exception of 1200) cm 2 /cm 3 , and are in agreement with the current understanding of immiscible fluid behavior in porous media. For a given NAPL, the a nw value was found to be about twice as large when the porous medium was NAPL wet first, compared to when the medium was first water wet. The interfacial tracer technique will be useful in assessing NAPL morphology, dissolution and mass transfer, and is useful in the characterization and remediation assessment at NAPL-contaminated waste sites.

Miscible fluid displacement stability in unconfined porous media:: Two-dimensional flow experiments and simulations

In situ flushing groundwater remediation technologies, such as cosolvent flushing, rely on the stability of the interface between the resident and displacing fluids for efficient removal of contaminants. Contrasts in density and viscosity between the resident and displacing fluids can adversely affect the stability of the displacement front. Petroleum engineers have developed techniques to describe these types of processes; however, their findings do not necessarily translate directly to aquifer remediation. The purpose of this laboratory study was to investigate how density and viscosity contrasts affected cosolvent displacements in unconfined porous media characterized by the presence of a capillary fringe. Two-dimensional flow laboratory experiments, which were partially scaled to a cosolvent flushing field experiment, were conducted to determine potential implications of flow instabilities in homogeneous sand packs. Numerical simulations were also conducted to investigate the differential impact of fluid property contrasts in unconfined and confined systems. The results from these experiments and simulations indicated that the presence of a capillary fringe was an important factor in the displacement efficiency. Buoyant forces can act to carry a lighter-than-water cosolvent preferentially into the capillary fringe during displacement of the resident groundwater. During subsequent water flooding, buoyancy forces can act to effectively trap the cosolvent in the capillary fringe, contributing to the inefficient removal of cosolvent from the aquifer.

Optimal estimation of residual non–aqueous phase liquid saturations using partitioning tracer concentration data

Stochastic methods are applied to the analysis of partitioning and nonpartitioning tracer breakthrough data to obtain optimal estimates of the spatial distribution of subsurface residual non–aqueous phase liquid (NAPL). Uncertainty in the transport of the partitioning tracer is assumed to result from small-scale spatial variations in a steady state velocity field as well as spatial variations in NAPL saturation. In contrast, uncertainty in the transport of the nonpartitioning tracer is assumed to be due solely to the velocity variations. Partial differential equations for the covariances and cross cpvariances between the partitioning tracer temporal moments, nonpartitioning tracer temporal moments, residual NAPL saturation, pore water velocity, and hydraulic conductivity fields are derived assuming steady flow in an infinite domain [Gelhar, 1993] and the advection-dispersion equation for temporal moment transport [Harvey and Gorelick, 1995]. These equations are solved using a finite difference technique. The resulting covariance matrices are incorporated into a conditioning algorithm which provides optimal estimates of the tracer temporal moments, residual NAPL saturation, pore water velocity, and hydraulic conductivity fields given available measurements of any of these random fields. The algorithm was tested on a synthetically generated data set, patterned after the partitioning tracer test conducted at Hill AFB by Annable et al. [1997]. Results show that the algorithm successfully estimates major features of the random NAPL distribution. The performance of the algorithm, as indicated by analysis of the “true” estimation errors, is consistent with the theoretical estimation errors predicted by the conditioning algorithm.

Kinetics of nitrogen mineralization in Costa Rican soils: model evaluation and pretreatment effects

Nitrogen mineralization kinetics of Costa Rican soils developed from volcanic ash were investigated. Two aspects of the aerobic incubation method proposed by Stanford and Smith (1972) were examined: the addition of—N nutrient leaching solution, and air-drying samples prior to incubation. Net cumulative N mineralized over 210 days was analyzed in terms of three kinetic models of N mineralization: single term, first-order (FO); two term, first-order (DFO); and combined first-order, zero-order (FOZ).Net cumulative N mineralization was significantly increased both by air-drying and by—N nutrient solution addition. The effect of air-drying was to induce a ‘flush’ of mineralization during the first 30 to 45 days of incubation, while the effect of the—N nutrient solution was more apparent after that time.The 16 mineralization curves generated were qualitatively similar. The FO model provided the worst fit with the data and deviated from it in a consistent manner. Both the DFO and the FOZ models fit the data closely but the precision of parameter estimation using the DFO model was relatively poor.The first-order term of the FOZ model was interpreted as accounting for pretreatment effects (e.g., air-drying) and the zero-order term as describing the ‘basal mineralization rate’ of the soil. The success of the zero-order term in describing mineralization over long times suggests that incubation methods cannot be used to isolate a ‘mineralizable fraction’ of soil organic N.

Estimation of spatially variable residual nonaqueous phase liquid saturations in nonuniform flow fields using partitioning tracer data

Estimates of spatially variable residual NAPL saturations SN are obtained in heterogeneous porous media using first temporal moments of breakthrough curves (BTCs) obtained from multilevel samplers during in situ partitioning tracer tests. An approach is adopted in which the distribution of the log NAPL/water volumetric ratio (Y = ln [SN/(1 − SN∥]) and log hydraulic conductivity (F = ln K) are treated as spatially correlated random fields. A nonlinear Gauss-Newton search technique is used to identify the spatial distribution of Y that minimizes the weighted sum of the deviation of the temporal moment predictions from their measured values and the deviation of the estimate of Y from its prior estimate obtained from the temporal moments of extraction well BTCs. Sensitivities required for the algorithm are obtained using a coupled flow and transport adjoint sensitivity method. In addition to obtaining optimal estimates for the spatial distribution of Y, the method also provides the estimation error covariance. The estimation error covariance can be used to evaluate the information that may be obtained from alternate pumping and monitoring configurations for tracer tests designed to detect NAPL in the subsurface. To this end, we tested the method using two different NAPL distributions (one with a random spatially correlated field and a second that was a block of NAPL) and three different pumping configurations (a double five-spot pattern, an inverted double five-spot pattern, and a line-drive pattern). The results show that measured temporal moments are more sensitive to Y in the double five-spot and inverted double five-spot patterns, and estimates produced in these configurations are slightly superior to those produced in the line-drive pattern.

Simulation of nitrogen in agro-ecosystems: Criteria for model selection and use

Available simulation models for describing nitrogen behavior in agro-ecosystems vary in two characteristics:(i) conceptual completeness in terms of the number of processes considered, and(ii) thelevel of detail at which each process is modeled. These model characteristics are determined by both the objectives that the model is designed to meet and the current state-of-the-art understanding of the various processes included in the model. The levels of conceptual completeness and detail in a model govern the potential applications for which the model may be used. Applications of models may be research-oriented, management-oriented, or planning-oriented. A model suitable for a given application should have an appropriate level of completeness and detail to accomplish the stated objective.Criteria to aid in the selection and evaluation of nitrogen simulation models for a particular application include: i) the availability of computational facilities, ii) the spatial and temporal scales of application, iii) the intended use of the simulations, iv) the availability of model input data, and v) the confidence regions associated with the model output.ResumenLos modelos de simulación del comportamiento en agro-ecosistemas difieren en dos características: (i)entereza o minuciosidad conceptual, en términos del número de procesos que considera, y (ii)el nivel de detalle en el cual cada proceso es modelado. Estas características del modelo son determinadas por los objetivos para los cuales el modelo es diseñado y por el grado acutal del conocimiento de los procesos incluídos en el modelo. Los niveles de entereza conceptual y de detalle en un modelo determinan las aplicaciones potenciales para las cuales dicho modelo puede ser utilizado. Las aplicationes pueden ser orientadas a la investigación, manejo o planificación. Un modelo adecuado para una aplicación determinada debería tener un nivel apropiado de entereza y detalle para lograr el objetivo establecido.Los criterios de ayuda en la selección y evaluación de los modelos de simulación del nitrógeno para una aplicación particular incluyen: (i) la disponibilidad de facilidades computacionales, (ii) las escalas espacial y temporal de aplicación, (iii) el uso deseado de las simulaciones, (iv) la disponibilidad de datos de entrada al modelo, y (v) los ámbitos de confianza asociadas con las salidas del modelo.

Simulation of nitrogen in agro-ecosystems: Criteria for model selection and use@@@Simulación de nitrógeno en agro-ecosistemas: Criterios para selección y uso del modelo

Available simulation models for describing nitrogen behavior in agro-ecosystems vary in two characteristics:(i) conceptual completeness in terms of the number of processes considered, and(ii) thelevel of detail at which each process is modeled. These model characteristics are determined by both the objectives that the model is designed to meet and the current state-of-the-art understanding of the various processes included in the model. The levels of conceptual completeness and detail in a model govern the potential applications for which the model may be used. Applications of models may be research-oriented, management-oriented, or planning-oriented. A model suitable for a given application should have an appropriate level of completeness and detail to accomplish the stated objective.Criteria to aid in the selection and evaluation of nitrogen simulation models for a particular application include: i) the availability of computational facilities, ii) the spatial and temporal scales of application, iii) the intended use of the simulations, iv) the availability of model input data, and v) the confidence regions associated with the model output.ResumenLos modelos de simulación del comportamiento en agro-ecosistemas difieren en dos características: (i)entereza o minuciosidad conceptual, en términos del número de procesos que considera, y (ii)el nivel de detalle en el cual cada proceso es modelado. Estas características del modelo son determinadas por los objetivos para los cuales el modelo es diseñado y por el grado acutal del conocimiento de los procesos incluídos en el modelo. Los niveles de entereza conceptual y de detalle en un modelo determinan las aplicaciones potenciales para las cuales dicho modelo puede ser utilizado. Las aplicationes pueden ser orientadas a la investigación, manejo o planificación. Un modelo adecuado para una aplicación determinada debería tener un nivel apropiado de entereza y detalle para lograr el objetivo establecido.Los criterios de ayuda en la selección y evaluación de los modelos de simulación del nitrógeno para una aplicación particular incluyen: (i) la disponibilidad de facilidades computacionales, (ii) las escalas espacial y temporal de aplicación, (iii) el uso deseado de las simulaciones, (iv) la disponibilidad de datos de entrada al modelo, y (v) los ámbitos de confianza asociadas con las salidas del modelo.