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  • 1.
    Cheng, Hua
    KTH, School of Architecture and the Built Environment (ABE), Land and Water Resources Engineering.
    Hydrodynamic control of retention in heterogeneous aquifers and fractured rock2005Doctoral thesis, comprehensive summary (Other scientific)
    Abstract [en]

    In this thesis, fluid flow and solute transport in heterogeneous aquifers and particularly in frac-tured rock have been investigated using Lagrangian Stochastic Advective-Reaction (LaSAR) framework. The heterogeneity of the aquifer structure or fracture configuration, as well as the various reaction/retention processes have been considered in the modelling approach. Advection and retention processes are considered to be the dominant transport processes. Monte-Carlo simulation results for transport of nonreactive tracers in 2D generic heterogeneous aquifers indicate that the travel time τ can be well approximated by a lognormal distribution up to a relative high degree of heterogeneity of the aquifers. Comparison between the Monte-Carlo simulation results and the results of first-order approximation reveals that the analytical solutions of the statistical moments of τ are valid only when the variability of the aquifer properties is small. For reactive tracers, Monte-Carlo simulations have been conducted by accounting for spatial variability of both hydraulic conductivity and one sorption parameter simultaneously. The simulation results indicate that the reaction flow path μ is a nonlinear function of distance for shorter distance, linear function for longer distance, and also that μ and τ are well correlated over the considered parameter range. The parameter β, which is purely determined by the flow condi-tions, quantifies the hydrodynamic control of retention processes for transport of tracers in frac-tures. Numerical simulations have been performed to study the statistical properties of the pa-rameter β, travel time τ and flow rate Q in a single heterogeneous fracture and in a sequence of fractures. The results of Monte-Carlo simulations indicate that the parameter β and τ are corre-lated with a power-law relationship β ∼ τm. The correlation between β and the flow rate Q have also been studied and an inverse power-law relationship β ∼ Q-m is proposed. The establishment of these relationships provides a link between the parameter β and measurable parameters τ (or Q).

    The LaSAR approach has been applied for prediction, evaluation and interpretation of the results of a number of tracer tests (TRUE-1, TRUE Block Scale and TRUE Block Scale Continuation) conducted by SKB at the Äspö site for tracer transport in fractures. The breakthrough curves may be predicted reasonably well, provided that the retention parameters, boundary conditions and hydraulic properties of the domain are given. The evaluation of TRUE tests indicates that the retention occurs mainly in the rim zone on site characterization time scales, while on the per-formance assessment time scale, diffusion and sorption in the unaltered rock matrix are likely to become dominant retention mechanisms.

  • 2.
    Cheng, Hua
    KTH, School of Architecture and the Built Environment (ABE), Land and Water Resources Engineering.
    Impact of internal heterogeneity for tracer transport in fractured rockManuscript (preprint) (Other (popular science, discussion, etc.))
  • 3.
    Cheng, Hua
    et al.
    KTH, School of Architecture and the Built Environment (ABE), Land and Water Resources Engineering, Water Resources Engineering.
    Cvetkovic, Vladimir
    KTH, School of Architecture and the Built Environment (ABE), Land and Water Resources Engineering, Water Resources Engineering.
    Evaluation of Colloid Transport Experiments in a Quarried Block2009In: SCIENTIFIC BASIS FOR NUCLEAR WASTE MANAGEMENT XXXII / [ed] Hyatt NC; Pickett DA; Rebak RB, 2009, Vol. 1124, p. 519-524Conference paper (Refereed)
    Abstract [en]

    Colloid tracer experiments were performed in a single, heterogeneous fracture contained in a quarried block (QB) under the configuration of dipole tracer tests. The experiments were first performed using bentonite and 100 nm latex colloids, as well as conservative tracer iodide and bromide, under conditions of different flow rates in order to identify the flow rates that favour colloid transport. The tracer experiments were later expanded to include experiments with different colloid sizes and longer transport distances. The aims of the present study are to identify the processes that affect colloid transport in the QB fracture and to estimate the retention parameters for the different sized colloids. We model the measured breakthrough curves (BTCs) using an advection-retention approach. The key feature of the advection-retention model is that advective transport and retention processes are related in a dynamic manner through the flow equation. Two Lagrangian random variables, tau and beta, that depend solely on flow conditions, control the retention processes. Here tau is the nonreactive travel time and beta is related to tau but also depends on the local aperture value. We assume the water residence time distribution g(tau) to be inverse-gaussian. The first two moments of g(tau) were obtained by calibrating the measured BTCs of conservative tracers. We then model the colloid BTCs using g(tau) and take into account the retention processes. The modelling results indicate that dominating retention processes include first-order linear kinetic attachment/detachment on the fracture surface, and mass loss (removal) by filtration/sedimentation. Diffusion into the rock matrix is of a much lesser importance.

  • 4.
    Cheng, Hua
    et al.
    KTH, Superseded Departments, Land and Water Resources Engineering.
    Cvetkovic, Vladimir
    KTH, Superseded Departments, Land and Water Resources Engineering.
    Hydrodynamic control of tracer retention in heterogeneous rock fractures2003In: Water resources research, ISSN 0043-1397, E-ISSN 1944-7973, Vol. 39, no 5, p. 1130-1139Article in journal (Refereed)
    Abstract [en]

    We investigate the statistical properties of a Lagrangian random variable beta[T/L], which has been shown to quantify hydrodynamic impact on retention [Cvetkovic et al., 1999], using Monte Carlo simulations of flow and transport in a single fracture. The "local cubic law'' of water flow is generalized to a power law Qsimilar tob(n), where Q is the flow rate, b is the half aperture, and nless than or equal to3. Simulations of flow and particle transport are carried out assuming "local cubic law'' (n=3) and "local quadratic law'' (n=2), and for two typical flow configurations: uniform flow and radially converging flow. We find that beta is related to tau as betasimilar totau(m), where m is dependent on the power n and the configuration of flow and transport. Simulation results for uniform flow indicate that betasimilar totau(n/n-1) for a small source section; as the source section increases, we have the convergence to betasimilar totau. For radially converging flow, we find betasimilar totau for a small source section and a convergence to beta=const for an increasing source section. Simulation results for both flow configurations are consistent with the results for a homogeneous fracture. The results for a homogeneous fracture provide reasonable bounds for simulated beta. The correlation between beta and Q is relatively weak for all cases studied.

  • 5.
    Cheng, Hua
    et al.
    KTH, Superseded Departments, Land and Water Resources Engineering.
    Cvetkovic, Vladimir
    KTH, Superseded Departments, Land and Water Resources Engineering.
    Impact of temperature increase on nuclide transport in crystalline rock on the near field scale2004In: Coupled Thermo-Hydro-Mechanical-Chemical Processes in Geo-Systems: Fundamentals, Modelling, Experiments and Applications, Elsevier , 2004, 2, p. 413-418Chapter in book (Other academic)
    Abstract [en]

    The TRUE (“Tracer Retention Understanding Experiments”) programme at the Äspö Hard Rock Laboratory (Winberg et al., 2000, 2002) has since 1995 generated a unique database for quantifying retention of radionuclides in crystalline rock on the 5-30m scales. Temperature conditions in the TRUE analysis were about 15°C. In this study, we consider the effect of temperature increase to ca 60°C consistent with conditions after say 1000 years in the rock volume surrounding a KBS-3 type of repository, anticipated to persist over a relatively long time. Temperature elevation will decrease the mean aperture of a conducting fracture by approximately 30%, which in turn will enhance retention. Furthermore, diffusion in the rock matrix will increase at elevated temperatures by approximately factor 4, further enhancing retention. Sorption coefficients are assumed to be unchanged since there is still considerable uncertainty regarding sorption and its change with increasing temperature. We show that elevated temperature on the near field scale enhances nuclide retention, thereby providing an additional safety margin.

  • 6.
    Cvetkovic, Vladimir
    et al.
    KTH, Superseded Departments, Land and Water Resources Engineering.
    Cheng, Hua
    KTH, Superseded Departments, Land and Water Resources Engineering.
    Contaminant transport in aquifers with spatially variable hydraulic and sorption properties1998In: Proceedings of the Royal Society of Edinburgh. Section A Mathematics, ISSN 0308-2105, E-ISSN 1473-7124, Vol. 454, p. 2173-2207Article in journal (Refereed)
  • 7.
    Cvetkovic, Vladimir
    et al.
    KTH, School of Architecture and the Built Environment (ABE), Land and Water Resources Engineering, Water Resources Engineering.
    Cheng, Hua
    KTH, School of Architecture and the Built Environment (ABE), Land and Water Resources Engineering, Water Resources Engineering.
    Evaluation of single-well injection-withdrawal tests in Swedish crystalline rock using the Lagrangian travel time approach2011In: Water resources research, ISSN 0043-1397, E-ISSN 1944-7973, Vol. 47, p. W02527-Article in journal (Refereed)
    Abstract [en]

    A series of 10 single-well injection-withdrawal (SWIW) tests are evaluated with two tracers each: uranine and cesium (Cs). An evaluation tool for SWIW tests in crystalline rock is presented on the basis of the Lagrangian travel time approach, whereby probabilities of tracer particle residence times are computed for key stages of the test cycle. Calibration results for three transport parameters and each breakthrough curve are presented. We show that estimates of the controlling retention parameter group psi [1/root T] are robust for Cs but highly uncertain for uranine. The estimated retention for Cs is larger for the Laxemar-Simpevarp site compared to the Forsmark site. Deviations from the -3/2 asymptotic breakthrough curve slope observed in a few of the tests at Forsmark are possibly due to a thin fracture coating that has been identified in mineralogical studies at some locations of the site.

  • 8.
    Cvetkovic, Vladimir
    et al.
    KTH, School of Architecture and the Built Environment (ABE), Land and Water Resources Engineering, Water Resources Engineering.
    Cheng, Hua
    KTH, School of Architecture and the Built Environment (ABE), Land and Water Resources Engineering.
    Sorbing tracer experiments in a crystalline rock fracture at Aspo (Sweden): 3. Effect of microscale heterogeneity2008In: Water resources research, ISSN 0043-1397, E-ISSN 1944-7973, Vol. 44, no 12Article in journal (Refereed)
    Abstract [en]

    We study the impact of microscale (10(-3) -10(-2) m) heterogeneity in material and structural properties on sorbing tracer transport in a single crystalline fracture of the TRUE-1 tests (mesoscale, 5 m) at Aspo (Sweden). The analysis is based on the microscale characterization results as presented in part 1 of this series. Our main objective in this last part of the series is to provide an independent interpretation (or "prediction'') of the effective parameters as estimated from calibration in part 2 by combining the data presented in part 1 with analytical and numerical transport modeling. We show here that the independent information from microscopic characterization can be used for "predicting'' the effective diffusion time t(d) reasonably well; a discrepancy is to be expected given the uncertainties of microscale retention properties, in particular of the sorption coefficient.

  • 9.
    Cvetkovic, Vladimir
    et al.
    KTH, Superseded Departments, Land and Water Resources Engineering.
    Cheng, Hua
    KTH, Superseded Departments, Land and Water Resources Engineering.
    Transport of reactive tracers in rock fractures1999In: Journal of Fluid Mechanics, ISSN 0022-1120, E-ISSN 1469-7645, Vol. 378, p. 335-336Article in journal (Refereed)
  • 10.
    Cvetkovic, Vladimir
    et al.
    KTH, Superseded Departments, Land and Water Resources Engineering.
    Cheng, Hua
    KTH, Superseded Departments, Land and Water Resources Engineering.
    Wen, X. -H.
    Analysis of nonlinear effects on tracer migration in heterogeneous aquifers using Lagrangian travel time approach1996In: Water resources research, ISSN 0043-1397, E-ISSN 1944-7973, Vol. 32, no 6, p. 1671-1680Article in journal (Refereed)
    Abstract [en]

    Advective transport in a heterogeneous two-dimensional aquifer is simulated and quantified by the statistical moments of tracer travel time and transverse displacement. These moments depend on the statistics of the Lagrangian velocity given as a function of space rather than time. A comparison is made with first-order results that appear to be robust for log transmissivity variance at least up to 1. Lagrangian statistics for travel time clearly expose a few essential features of nonlinear transport: nonstationary distribution and the increasing integral scale of the Lagrangian velocity. These reflect the tendency of streamlines to concentrate into relatively few flow paths ('preferential flow') for increasing log transmissivity variance. Two simple empirical relationships are identified that in combination with first-order results for the flow capture the main features of nonlinear transport. The proposed methodology can readily be generalized for analyzing advective transport under more complex flow conditions and for establishing relatively simple analytical models for estimating solute mass flux in heterogeneous aquifers.

  • 11.
    Cvetkovic, Vladimir
    et al.
    KTH, School of Architecture and the Built Environment (ABE), Land and Water Resources Engineering, Water Resources Engineering.
    Cheng, Hua
    KTH, School of Architecture and the Built Environment (ABE), Land and Water Resources Engineering.
    Widestrand, H.
    Byegard, J.
    Winberg, A.
    Andersson, P.
    Sorbing tracer experiments in a crystalline rock fracture at Aspo (Sweden): 2. Transport model and effective parameter estimation2007In: Water resources research, ISSN 0043-1397, E-ISSN 1944-7973, Vol. 43, no 11Article in journal (Refereed)
    Abstract [en]

    [1] Transport and retention of sorbing tracers in a single, altered crystalline rock fracture on a 5 m scale is investigated. We evaluate the results of a comprehensive field study ( referred to as Tracer Retention Understanding Experiments, first phase ( TRUE- 1)), at a 400 m depth of the Aspo Hard Rock Laboratory ( Sweden). A total of 16 breakthrough curves are analyzed, from three test configurations using six radioactive tracers with a broad range of sorption properties. A transport- retention model is proposed, and its applicability is assessed based on available data. We find that the conventional model with an asymptotic power law slope of - 3/ 2 ( one- dimensional diffusion into an unlimited rock matrix) is a reasonable approximation for the conditions of the TRUE- 1 tests. Retention in the altered rock of the rim zone appears to be significantly stronger than implied by retention properties inferred from generic ( unaltered) rock samples. The effective physical parameters which control retention ( matrix porosity and retention aperture) are comparable for all three test configurations. The most plausible in situ ( rim zone) porosity is in the range 1% - 2%, which constrains the effective retention aperture to the range 0.2 - 0.7 mm. For all sorbing tracers the estimated in situ sorption coefficient appears to be larger by at least a factor of 10, compared to the value inferred from through- diffusion tests using unaltered rock samples.

1 - 11 of 11
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