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• Travel time moments for sorbing solutes in heterogeneous domains under nonuniform flow conditions

  X. Sánchez-Vila, Y. Rubin

  Water Resources Research (2003). Vol. 39 (4), pp. 1-13

  
  

  Abstract

   A methodology for evaluating the unconditional and conditional moments of travel time for a sorbing solute is presented. The approach is applicable for any flow configuration and for a wide range of mass transfer rate‐limited linear processes. The methodology is applicable to the general case of spatially variable hydrological and chemical parameters. The sorption model used to derive the temporal moments is that of a continuous distribution of mass rate coefficients [Haggerty and Gorelick, 1998]. Models such as instantaneous equilibrium, first‐order and two‐site sorption kinetics, among others, can be considered as particular cases of this general model. Using a deterministic approach, the low‐order moments of the breakthrough curves for reactive solutes can be obtained as a function of those for conservative tracers. Using a stochastic approach, the unconditional low‐order statistics of the travel time moments can be obtained. These moments depend on the statistics of two Lagrangian functions, the travel time for a conservative solute, and an integral of the variations of the chemical parameters weighted by the inverse local velocity along the trajectory. Finally, conditional temporal moments are derived. Moments can be conditioned to any type of information, hard or soft, hydraulic or geochemical. Conditioning is found to reduce uncertainty, characterized by a reduction in the variance of the travel time. The general results are particularized for both uniform in the mean and convergent flow conditions and for simple sorption models such as linear instantaneous equilibrium and first‐order kinetics. In all such cases, close‐form results, based on small perturbations expansions, are presented for the travel time moments.

  Abstract
   A methodology for evaluating the unconditional and conditional moments of travel time for a sorbing solute is presented. The approach is applicable for any flow configuration [...]

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• Conditional moments of the breakthrough curves of kinetically sorbing solute in heterogeneous porous media using multirate mass transfer models for sorption and desorption

  A. Lawrence, X. Sánchez-Vila, Y. Rubin

  Water resources research (2002). Vol. 38 (11), pp. 1-12

  
  

  Abstract

  A methodology is presented for evaluating the temporal moments of solutes undergoing linear rate‐limited mass transfer processes based on a Lagrangian approach to solute transport in heterogeneous media. The temporal moments of sorbing solutes are written as a function of those of conservative tracers. The general continuous diffusion rate model that has recently appeared in the hydrologic literature is used to model the rate‐limited mass transfer processes. The methodology is also applied to desorption from an initially uniformly contaminated aquifer, and the concentration expected value and variance are found quasi‐analytically. The conditional temporal moments of sorbing solutes can be written as a function of the conditional moments of conservative tracers. Conditioning results in a reduction of the variance of travel time. The amount of reduction depends on the chemical model selected

  Abstract
  A methodology is presented for evaluating the temporal moments of solutes undergoing linear rate‐limited mass transfer processes based on a Lagrangian approach to solute [...]

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• Using Geostatistical Analysis to Evaluate the Presence of Rotylenchulus reniformis in Cotton Crops in Brazil: Economic Implications.

  P. Farias, X. Sánchez-Vila, J. Barbosa, S. Vieira, L. Ferraz, J. Delfín

  Journal of nematology (2002). Vol. 34 (3), pp. 232-238

  
  

  Abstract

  In recent years, the productivity of cotton in Brazil has been progressively decreasing, often the result of the reniform nematode Rotylenchulus reniformis. This species can reduce crop productivity by up to 40%. Nematodes can be controlled by nematicides but, because of expense and toxicity, application of nematicides to large crop areas may be undesirable. In this work, a methodology using geostatistics for quantifying the risk of nematicide application to small crop areas is proposed. This risk, in economic terms, can be compared to nematicide cost to develop an optimal strategy for Precision Farming. Soil (300 cm(3)) was sampled in a regular network from a R. reniformis-infested area that was a cotton monoculture for 20 years. The number of nematodes in each sample was counted. The nematode number per volume of soil was characterized using geostatistics, and 100 conditional simulations were conducted. Based on the simulations, risk maps were plotted showing the areas where nematicide should be applied in a Precision Farming context. The methodology developed can be applied to farming in countries that are highly dependent on agriculture, with useful economic implications.

  Abstract
  In recent years, the productivity of cotton in Brazil has been progressively decreasing, often the result of the reniform nematode Rotylenchulus reniformis. This species can [...]

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• Convergent-flow tracer tests in heterogeneous media: combined experimental–numerical analysis for determination of equivalent transport parameters

  D. Fernàndez-García, X. Sánchez, T. Illangasekare

  Journal of Contaminant Hydrology (2002). Vol. 57 (1-2), pp. 129-145

  
  

  Abstract

  In modeling transport within naturally heterogeneous aquifers, it is usually assumed that the transport equations valid at local scales can also be applied at larger scales. At larger scales, the heterogeneous domain is represented by an equivalent homogeneous medium. Convergent-flow tracer tests constitute one of the most frequently used field tests to estimate effective input parameters of equivalent homogeneous aquifers. Traditionally, statistical approaches applied to groundwater flow and solute transport have provided tools to estimate these equivalent parameters. These approaches are based on a number of simplifications including the assumption that the point transmissivity values follow a multilog-normal random function. Several investigators have found that this assumption may not be valid in many field cases. In order to study the applicability of the equivalent homogeneous formulation in a nontraditional stochastic field, a number of experimental and numerical studies were conducted. The results are used to determine the apparent values of porosity and dispersivity that would be obtained if convergent-flow tracer tests were conducted in a deterministically generated heterogeneous transmissivity field displaying anisotropy in the correlation structure. It is shown that in this particular heterogeneous media, apparent porosity strongly depends on connectivity rather than on transmissivity. This dependence on connectivity questions the theoretical results obtained in continuum equivalent fields to estimate effective porosity.

  Abstract
  In modeling transport within naturally heterogeneous aquifers, it is usually assumed that the transport equations valid at local scales can also be applied at larger [...]

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• Simulation of Gas Dipole Tests in Fractures at the Intermediate Scale Using a New Upscaling Method

  H. Ramajo, S. Olivella, J. Carrera, X. Sánchez-Vila

  Transport in porous media (2002). Vol. 46 (2-3), pp. 269-284

  
  

  Abstract

  A high-level radioactive waste disposal site may lead to gas generation by different physical mechanisms. As these sites are to be located in areas with low water flow, any small amount of gas can lead to relative high gas pressures, so that multiphase flow analysis becomes relevant. The movement of gas and water through the system has two important implications. Firstly, water flow takes place in unsaturated conditions, and thus travel times of the radioactive particles transported are affected; and secondly, gas can also carry radioactive particles. Therefore, one of the key points in such studies is the time when gas would break through the biosphere under a number of different flow conditions. In fractured zones, gas would flow preferentially through the most conductive features. We consider a two-dimensional system representing an isolated fracture. In each point we assign a local porosity and permeability and a local pressure-saturation relationship. A dipole (injector-producer) gas flow system is generated and the variation in water saturation is studied. A simple method is proposed for obtaining upscaled values for several parameters involved in two-phase flow. It is based on numerical simulation on a block scale assuming steady-state conditions and absence of capillary pressure gradients. The proposed method of upscaling is applied to simulate a dipole test using a coarser grid than that of the reference field. The comparison between the results in both scales shows an encouraging agreement

  Abstract
  A high-level radioactive waste disposal site may lead to gas generation by different physical mechanisms. As these sites are to be located in areas with low water flow, any [...]

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• Geostatistical analysis of the spatial distribution of Rotylenchulus reniformis on cotton cultivated under crop rotation

  P. Farias, J. Barbosa, S. Vieira, X. Sánchez-Vila

  Russian Journal of Nematology (2002). Vol. 10 (9), pp. 1-9

  
  

  Abstract

  The spatial distribution of Rotylenchulus reniformis on cotton cultivated in crop rotation with sorghum-peanut-velvetbean was studied using geostatistics. The experimental field, which had been continuously cropped with cotton during 20 years, comprised two 32 x 48 m-grids, each divided in sixty-four 4 x 6 m sampling plots. For all crops, 300 cm 3 of soil samples were taken at the center of each plot at crop germination (Pi) and again at harvest (Pf), from which the numbers of nematodes were determined. The results revealedthat the spatial distribution of R. reniformis was highly aggregated and with the aid of the geostatistical techniques the nematode intensities were mapped and the risk areas accurately identified. Consequently, geostatistics is here considered a useful tool for planning nematode control strategies, particularly in Precision Agriculture.

  Abstract
  The spatial distribution of Rotylenchulus reniformis on cotton cultivated in crop rotation with sorghum-peanut-velvetbean was studied using geostatistics. The experimental [...]

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• Solute transport in heterogeneous media: the impact of anisotropy and non-ergodicity in risk assessment

  J. Delfín, X. Sánchez-Vila

  Stochastic environmental research and risk assessment (1999). Vol. 13 (5), pp. 365-379

  
  

  Abstract

  Conceptual model selection is a key issue in risk assessment studies. We analyze the effect of a number of conceptual aspects related to solute transport in two-dimensional heterogeneous media. The main issues addressed are non-ergodicity, anisotropy in the correlation structure of the transmissivity field, and dispersion at the local scale. In particular, we study the development of a solute plume when mean flow is oriented at an angle with respect to the principal directions of anisotropy. The study is carried out in a Lagrangian framework using Monte Carlo analysis. Of special interest is the evolution of individual plumes. A number of aspects are analyzed, namely the location of the center of mass for each plume and the different ways to compute the angles that the main axes of the plume develop with respect to the direction of the mean flow. Stochastic theories based upon ergodicity conclude that the plume gets oriented in the mean flow direction. In our non-ergodic simulations, the mean of the offset angles, for each individual plume in each particular realization, is offset from the mean flow direction towards the direction of maximum anisotropy. If, instead, the analysis is performed on the ensemble plume (superposition of all different simulations), it is then found oriented closer to the direction of the mean flow than the average offset angle for the different plumes considered separately. This last result adds an extra word of caution to the use of ensemble averaged values in solute transport studies. Serious implications for risk assessment follow from the conceptual model adopted. First, in any single realization there will a large uncertainty in locating the plume at any given time; second, real dilution would be less than what would be expected if the macrodispersion values obtained for ergodic conditions were applied; third, the volume that is affected by a non-zero concentration is smaller than that predicted from macrodispersion concepts; fourth, the orientation of the plume does not correspond to that of the mean flow; and fifth, accounting for local dispersion helps reducing uncertainty.

  Abstract
  Conceptual model selection is a key issue in risk assessment studies. We analyze the effect of a number of conceptual aspects related to solute transport in two-dimensional [...]

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• Sobre la interpretación de ensayos hidráulicos en acuíferos no homogéneos

  P. Meier, X. Sánchez-Vila, J. Carrera

  Boletín geológico y minero (2000). Vol. 111 (4), pp. 69-82

  
  

  Abstract

  La obtención de parámetros hidráulicos de un acuífero suele realizarse a partir de la interpretación de ensayos hidráulicos con condiciones de flujo radial convergente. El objetivo de este artículo es ver como las fórmulas habituales de la hidráulica de pozos se pueden extender para incluir explícitamente la heterogeneidad del medio, y más importante, que los valores que se obtienen de la interpretación de ensayos en medios heterogéneos son valores representativos del medio y con un claro significado e interés hidrogeológico. Para ello es preciso relacionar la transmisividad deducida de ensayos de bombeo con la heterogeneidad natural. Esto se ha realizado analítica y numéricamente. El resultado, que coincide con las observaciones de campo, es que cuando existen varios puntos de observación, la transmisividad (T) deducida por el método de Jacob tiende a ser la misma para todos los puntos de observación. Más importante, dicha transmisividad es, bajo condiciones muy generales, igual a la T efectiva, es decir, a la que controla el flujo natural bajo condiciones de gradiente uniforme. Esto apoya la metodología que han venido usando tradicionalmente los hidrogeólogos

  Abstract
  La obtención de parámetros hidráulicos de un acuífero suele realizarse a partir de la interpretación de ensayos hidráulicos con condiciones [...]

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• Gestión de las aguas subterráneas en zonas urbanas. conceptualización y modelización: aplicación a barcelona (españa)

  E. Vázquez-Suñé, X. Sánchez-Vila, J. Carrera

  Series de correlación geológica (1999). pp. 41-49

  
  

  Abstract

  Availability and quality of groundwater resources is seriously affected by urbanization in many large cities. This leads to significant social, environmental, economical, and political implications. Any urban hydrology study requires a detailed analysis of water fluxes flowing in and out of the system, including their magnitude and relative importance. Analyzing the quality of the resources is another key issue. In any real case study one has to face at least three aspects: (1) process identification, (2) aquifer characterization, and (3) water resources management. This methodology has been applied to the city of Barcelona. The outcome is a groundwater model that allows defining, characterizing, and quantifying the potential risk of aquifers, both on urban structures, and upon population. In this sense, it may become a management tool. This is an important step towards convincing city managers to consider groundwater as one of the topics to be taken into account in city planning.

  Abstract
  Availability and quality of groundwater resources is seriously affected by urbanization in many large cities. This leads to significant social, environmental, economical, [...]

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• A Numerical Study on the Relationship Between Transmissivity and Specific Capacity in Heterogeneous Aquifers

  P. Meier, J. Carrera, X. Sánchez-Vila

  Ground water (1999). Vol. 37 (4), pp. 611-617

  
  

  Abstract

  Specific capacity (Q/s) data are usually much more abundant than transmissivity (T) data. Theories which assume uniform transmissivity predict a nearly linear relationship between T and Q/s. However, linear dependence is seldom observed in field studies. Since hydrogeologic studies usually require T data, many hydrogeologists use linear regression analysis of T versus Q/s data to estimate T values where only Q/s data are available. In this paper we use numerical models to investigate the effects of aquifer heterogeneity on the relationship between Q/s and T estimates. The simulations of hydraulic tests in heterogeneous media show that estimates of T derived using Jacob's method tend to their late‐time effective value much faster than Q/s values. The latter are found to be more dependent upon local transmissivities near the well. This explains why the regression parameters for T versus Q/s data depend on heterogeneity and the‘lateness’of the test period analyzed. Since this effect is more marked in high T zones than in low T zones, we conclude that natural aquifer heterogeneity can explain the convex deviation from linearity often observed in the field. A further result is that the geometric mean of T estimates, obtained from short and intermediate time pumping tests, seems to systematically underestimate effective T (Teff) of heterogeneous aquifers. In the studied simulation cases, the median of the T values or the arithmetic mean yield better estimates for Teff

  Abstract
  Specific capacity (Q/s) data are usually much more abundant than transmissivity (T) data. Theories which assume uniform transmissivity predict a nearly linear relationship [...]

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