Retardation Coefficient Research Articles

Particle mechanics model for the effects of shear on solute retardation coefficient in rock fractures

Damage on rock fracture surfaces during shear process changes the mechanical and hydrological properties of the fractures, and therefore affects solute migration in fractured rocks. Laboratory experiments on this issue are rarely reported in the literature due to technical difficulties in measuring asperity damage and gouge generation. To conceptually investigate the effects of rock fracture surface damage on solute sorption during shear, we present a retardation coefficient model considering the effect of wear. The particle mechanics model was employed to investigate the effects of gouge generation (abrasive wear) and microcrack development in the damaged zones, on the solute retardation coefficient in rock fractures. The results from demonstration examples show that the shear process significantly increases the retardation coefficients, by offering more sorption surfaces in the factures due to gouge generation (wear), microcracking and crushing of gouge particles. Conceptually three damage zones are classified to characterize the various wear impacts on the solute transport in single fractures. Outstanding issues of the present model and suggestions for future study are also presented.

画像処理による油輸送の定量化手法の開発とホットウォーターフラッシング実験への適用

Hot water flushing experiments in a two-dimensional sandbox (35 cm × 20 cm × 1 cm) were conducted using dyed salad oil in single and two-layered porous formations under saturated conditions. Based on a calibration relation between the dye concentration and the corresponding pixel brightness an image processing technique in conjunction with spatial moments was developed to quantify not only the spatial distribution of oil but the apparent retardation and dispersion coefficients both in longitudinal and lateral directions. The increase of flooding water temperature resulted in accelerated downward movement of oil because of the reduced viscosity at elevated temperatures. The results also revealed that the variation of the apparent lateral dispersion coefficient depended on the oil saturation within a pore space and exhibited an increasing tendency in a smaller size porous material due to the difference of intrinsic permeability.

Fate of N-Nitrosodimethylamine in recycled water after recharge into anaerobic aquifer

Laboratory and field experiments were undertaken to assess the fate of N-nitrosodimethylamine (NDMA) in aerobic recycled water that was recharged into a deep anaerobic pyritic aquifer, as part of a managed aquifer recharge (MAR) strategy. Laboratory studies demonstrated a high mobility of NDMA in the Leederville aquifer system with a retardation coefficient of 1.1. Anaerobic degradation column and (14)C-NDMA microcosm studies showed that anaerobic conditions of the aquifer provided a suitable environment for the biodegradation of NDMA with first-order kinetics. At microgram per litre concentrations, inhibition of biodegradation was observed with degradation half-lives (260±20 days) up to an order of magnitude greater than at nanogram per litre concentrations (25-150 days), which are more typical of environmental concentrations. No threshold effects were observed at the lower ng L(-1) concentrations with NDMA concentrations reduced from 560 ng L(-1) to <6 ng L(-1) over a 42 day 14C-NDMA aerobic microcosm experiment. Aerobic (14)C-NDMA microcosm studies were also undertaken to assess potential aerobic degradation, likely to occur close to the recharge bore. These microcosm experiments showed a faster degradation rate than anaerobic microcosms, with a degradation half-life of 8±2 days, after a lag period of approximately 10 days. Results from a MAR field trial recharging the Leederville aquifer with aerobic recycled water showed that NDMA concentrations reduced from 2.5±1.0 ng L(-1) to 1.3±0.4 ng L(-1) between the recharge bore and a monitoring location 20 m down gradient (an estimated aquifer residence time of 10 days), consistent with data from the aerobic microcosm experiment. Further down gradient, in the anaerobic zone of the aquifer, NDMA degradation could not be assessed, as NDMA concentrations were too close to their analytical detection limit (<1 ng L(-1)).

Determination of 137Cs and 85Sr transport parameters in fucoidic sand columns and groundwater system

Abstract The determination is based on the evaluation of experimentally obtained breakthrough curves using the erfc-function. The first method is founded on the assumption of a reversible linear sorption/desorption isotherm of radionuclides on solid phase with constant distribution and retardation coefficients, whereas the second one is based on the assumption of a reversible non-linear sorption/desorption isotherm described with the Freundlich equation, i.e., with non-constant distribution and retardation coefficients. Undisturbed cores of 5 cm in diameter and 10 cm long were embedded in the Eprosin-type cured epoxide resin column. In this study the so-called Cenomanian background groundwater was used as transport medium. The groundwater containing radionuclides was introduced at the bottom of the columns at about 4 mL h−1 constant flow-rate. The results have shown that in the investigated fucoidic sands: (i) the sorption was in principle characterized by linear isotherms and the corresponding retardation coefficients of 137Cs and 85Sr, depending on the type of sample, were approximately 13 or 44 and 5 or 15, respectively; (ii) the desorption was characterized by non-linear isotherms, and the retardation coefficients of the same radionuclides ranged between 23–50 and 5–25, respectively. The values of the hydrodynamic dispersion coefficients of these radionuclides varied between 0.43–1.2 cm2 h−1.

A novel explicit approach to model bromide and pesticide transport in connected soil structures

Abstract. The present study tests whether an explicit treatment of worm burrows and tile drains as connected structures is feasible for simulating water flow, bromide and pesticide transport in structured heterogeneous soils at hillslope scale. The essence is to represent worm burrows as morphologically connected paths of low flow resistance in a hillslope model. A recent Monte Carlo study (Klaus and Zehe, 2010, Hydrological Processes, 24, p. 1595–1609) revealed that this approach allowed successful reproduction of tile drain event discharge recorded during an irrigation experiment at a tile drained field site. However, several "hillslope architectures" that were all consistent with the available extensive data base allowed a good reproduction of tile drain flow response. Our second objective was thus to find out whether this "equifinality" in spatial model setups may be reduced when including bromide tracer data in the model falsification process. We thus simulated transport of bromide for the 13 spatial model setups that performed best with respect to reproduce tile drain event discharge, without any further calibration. All model setups allowed a very good prediction of the temporal dynamics of cumulated bromide leaching into the tile drain, while only four of them matched the accumulated water balance and accumulated bromide loss into the tile drain. The number of behavioural model architectures could thus be reduced to four. One of those setups was used for simulating transport of Isoproturon, using different parameter combinations to characterise adsorption according to the Footprint data base. Simulations could, however, only reproduce the observed leaching behaviour, when we allowed for retardation coefficients that were very close to one.

Determination of Water Saturation in Relatively Dry Porous Media Using Gas‐Phase Tracer Tests

Soil desiccation (drying), induced by dry air injection and moist air extraction, is a potentially robust remediation process to slow migration of inorganic or radionuclide contaminants through the vadose zone. The application of gas‐phase partitioning tracer tests has been proposed as a means to estimate initial water volumes and to monitor the progress of the desiccation process at pilot tests and field sites. In this study, tracer tests were conducted in porous medium columns with various water saturations using SF6 as the conservative tracer and trichlorofluoromethane and difluoromethane as the water‐partitioning tracers. For porous media with minimal silt or organic matter fractions, tracer tests provided reasonable saturation estimates for saturations close to zero. For sediments with significant silt or organic matter fractions, however, tracer tests only provided satisfactory results when the water saturation was greater than 0.1 to 0.2. For drier conditions, the apparent tracer retardation increased due to air–soil sorption, which is not included in traditional retardation coefficients derived from advection–dispersion equations accounting only for air–water partitioning and water–soil sorption. Based on these results, gas‐phase partitioning tracer tests may be used to determine initial water volumes in sediments, provided that the initial water saturations are sufficiently large. Tracer tests are not suitable for quantifying moisture content in relatively dry sediments, however, especially if significant amounts of organic matter or silt are present.

Fate of N-nitrosomorpholine in an anaerobic aquifer used for managed aquifer recharge: A column study

The fate of N-nitrosomorpholine (NMOR) was evaluated at microgram and nanogram per litre concentrations. Experiments were undertaken to simulate the passage of groundwater contaminants through a deep anaerobic pyritic aquifer system, as part of a managed aquifer recharge (MAR) strategy. Sorption studies demonstrated the high mobility of NMOR in the Leederville aquifer system, with retardation coefficients between 1.2 and 1.6. Degradation studies from a 351 day column experiment and a 506 day stop-flow column experiment showed an anaerobic biologically induced reductive degradation process which followed first order kinetics. A biological lag-time of less than 3 months and a transient accumulation of morpholine (MOR) were also noted during the degradation. Comparable half-life degradation rates of 40–45 days were observed over three orders of magnitude in concentration (200 ng L −1 to 650 μg L −1). An inhibitory effect on microorganism responsible to the biodegradation of NMOR at 650 μg L −1 or a threshold effect at 200 ng L −1 was not observed during these experiments.

Pollutant Transport in Geomedia Using X-ray Computed Tomography

Computed tomography (CT) measurement systems have been introduced as a method to measure fluid transport at the macropore-scale to estimate transport parameters. The objective of this study was to use x-ray CT methods to measure transport of iodophenol solution in geomedia columns and estimate spatial distributions of chemical retardation. An initial CT-measured breakthrough curve experiment was conducted with potassium iodide to spatially estimate pore-water velocity and dispersivity in the geomedia core, with a subsequent transport experiment with iodophenol to estimate the spatial distribution of chemical retardation. A geomedia core was removed from the surface horizon of Sarpy loamy sand (Typic Udispamment). Spatial distributions followed a fractal pattern with dimension values similar among pore-water velocity, dispersivity and retardation coefficient parameters. The retardation coefficient estimated using CT methods was similar compared to the dynamic fluid experiment indicating this new method may be appropriate for pollutant transport analysis. This study illustrates that the CT method is useful for evaluating solute transport on a macropore-scale for porous materials.

Behaviour and fate of nine recycled water trace organics during managed aquifer recharge in an aerobic aquifer

The fate of nine trace organic compounds was evaluated during a 12 month large-scale laboratory column experiment. The columns were packed with aquifer sediment and evaluated under natural aerobic and artificial anaerobic geochemical conditions, to assess the potential for natural attenuation of these compounds during aquifer passage associated with managed aquifer recharge (MAR). The nine trace organic compounds were bisphenol A (BPA), 17β-estradiol (E2), 17α-ethynylestradiol (EE2), N-nitrosodimethylamine (NDMA), N-nitrosomorpholine (NMOR), carbamazepine, oxazepam, iohexol and iodipamide. In the low organic carbon content Spearwood sediment, all trace organics were non-retarded with retardation coefficients between 1.0 and 1.2, indicating that these compounds would travel at near groundwater velocities within the aquifer. The natural aerobic geochemical conditions provided a suitable environment for the rapid degradation for BPA, E2, iohexol (half life < 1 day). Lag-times for the start of degradation of these compounds ranged from < 15 to 30 days. While iodipamide was persistent under aerobic conditions, artificial reductive geochemical conditions promoted via the addition of ethanol, resulted in rapid degradation (half life < 1 days). Pharmaceuticals (carbamazepine and oxazepam) and disinfection by-products (NDMA and NMOR) did not degrade under either aerobic or anaerobic aquifer geochemical conditions (half life > 50 days). Field-based validation experiments with carbamazepine and oxazepam also showed no degradation. If persistent trace organics are present in recycled waters at concentrations in excess of their intended use, natural attenuation during aquifer passage alone may not result in extracted water meeting regulatory requirements. Additional pre treatment of the recycled water would therefore be required.

Transport of 125I−, 137Cs+ and 85Sr2+ in granitoidic rock and soil columns

Transport of 125I−, 137Cs+ and 85Sr2+ radionuclides in crushed granitoidic rocks and homogenized soils was studied. Two simple methods for calculation of breakthrough curves in flow column experiments with groundwater as transport medium have been described. The first method, so called non-linear approach, is derived on the assumption of a reversible non-linear sorption isotherm described with Freundlich equation, i.e., with non-constant distribution and retardation coefficients. The second method, so-called linear approach, is applied for reference only, and is based on the assumption of a reversible sorption characterized with linear sorption isotherm, i.e., with constant distribution and retardation coefficients. Both methods model the experimental breakthrough curves with the integrated form of the simple 1-D advection–dispersion equation (ADE) expressed analytically for pulse application of radiotracer to the liquid phase before entering the columns. The integrated form of the ADE equation was modified by the so-called peak position and peak height correction coefficients the advantage of which consists among others in the elimination of the influence of starting concentration. The comparison of both approaches has shown that fitting by means of non-linear approach has given rather reliable values of the transport parameters and calculated dependences, especially in a case of 137Cs system characterized evidently with non-linear type of sorption isotherms. As for 125I−, the sorption capacity of all solid samples studied is nearly on the zero level and 125I− is practically not retarded, and from this point of view it behaves as non-interacting component. In addition, it was found that the modified ADE gives rather better results than the classical one.

Caracterização hidrodispersiva de dois solos da região irrigada do Vale do São Francisco

A caracterização hidrodispersiva de um Latossolo e de um Neossolo Quartzarênico, ambos da região do Vale do São Francisco, foi realizada em laboratório por meio de ensaios de deslocamento de líquidos miscíveis em colunas de solo deformado. Os ensaios compreenderam o deslocamento de um pulso de 1,0 volume de poros da solução de KBr nas vazões de 2,4.10-5; 4,8.10-5; 7,2.10-5; 9,6.10-5 e 1,2-10-4 m³ h-1 e nas concentrações de 11,89; 59,49 e 118,99 g L-1, e três repetições para cada ensaio, sob condições de saturação e fluxo em regime estacionário. Por meio do programa CXTFIT 2.0, que ajusta a solução analítica da equação do modelo convectivo-dispersivo (CDE) aos pontos da curva experimental de eluição, foi possível obter os parâmetros hidrodispersivos do fator de retardamento (R) e o coeficiente de dispersão hidrodinâmica (D). O modelo CDE realizou um bom ajuste aos pontos das curvas de eluição, com valores dos coeficientes de determinação variando de 98 a 99% para ambos os solos; os valores de R ficaram próximos a 1,0 nos dois solos, indicando que o KBr não interage com os mesmos e a dispersividade do Neossolo é maior que a do Latossolo devido a distribuição do tamanho dos poros ser maior no Neossolo.

Modeling of transport of radionuclides in beds of crushed crystalline rocks under equilibrium non-linear sorption isotherm conditions

Abstract A simple method for fitting the values of the experimental breakthrough curves in the form of pulse response obtained in dynamic flow column experiments is presented. It is based on the equation obtained by the analytical solution of a 1-D advection-dispersion equation (ADE) under defined conditions (equilibrium dynamics, linear sorption isotherm, constant bed height, pulse input), where the concentration (or activity) dependence on the number of pore volumes is expressed explicitly. It is shown that the method can be used in the case of validity of a non-linear Freundlich sorption isotherm if the experimental data are fitted by means of a Newton-Raphson multidimensional non-linear regression procedure in which the regression function consists of the above mentioned ADE equation and of the equation for a retardation coefficient including the first derivative of the Freundlich equation. Values of four parameters, namely, Freundlich equation parameters (nF, kF), Peclet number (Pe) and integration constant (k int) are sought in the course of the regression procedure. Applicability of the method is demonstrated in the evaluation of breakthrough curves of the 137Cs and 85Sr obtained by dynamic flow column experiments in beds of five different types of crushed crystalline rocks, where synthetic groundwater spiked with both radionuclides is used.

Mathematical modelling of immobilization of radionuclides 137Cs and 60Co in concrete matrix

Transport phenomena involved in the leaching of a radioactive material from a cement composite matrix are investigated using an empirical method employing a polynomial equation. To assess the safety for disposal of radioactive waste-cement composition, the leaching of 137Cs and 60Co, from a waste composite into a surrounding fluid has been studied. Leaching tests were carried out in accordance with a method recommended by IAEA. Determination of retardation factors, K F and coefficients of distribution, k d, using a simplified mathematical model for analyzing the migration of radionuclides, have been developed. Transport phenomena involved in the leaching of a radioactive material from a cement composite matrix are investigated using an empirical method employing a polynomial equation. In our experiment we have analized mechanism of 137Cs and 60Co leaching values during a period of 60 days. Results presented in this paper are examples of results obtained in a 25-year mortar and concrete testing project, which will influence the design of the engineered trenches system for a future central Serbian radioactive waste storage center.

Glyphosate transport through weathered granite soils under irrigated and non-irrigated conditions — Barcelona, Spain

The transport of Glyphosate ([N-phosphonomethyl] glycine), AMPA (aminomethylphosphonic acid, CH 6NO 3P), and Bromide (Br −) has been studied, in the Mediterranean Maresme area of Spain, north of Barcelona, where groundwater is located at a depth of 5.5 m. The unsaturated zone of weathered — granite soils was characterized in adjacent irrigated and non-irrigated experimental plots where 11 and 10 boreholes were drilled, respectively. At the non irrigated plot, the first half of the period was affected by a persistent and intense rainfall. After 69 days of application residues of Glyphosate up to 73.6 μgg − 1 were detected till a depth of 0.5 m under irrigated conditions, AMPA, analyzed only in the irrigated plot was detected till a depth of 0.5 m. According to the retardation coefficient of Glyphosate as compared to that of Br − for the topsoil and subsoil (80 and 83, respectively) and the maximum observed migration depth of Br − (2.9 m) Glyphosate and AMPA should have been detected till a depth of 0.05 m only. Such migration could be related to the low content of organic matter and clays in the soils; recharge generated by irrigation and heavy rain, and possible preferential solute transport and/or colloidal mediated transport.

A Three-Scale Model of pH-Dependent Flows and Ion Transport with Equilibrium Adsorption in Kaolinite Clays: II Effective-Medium Behavior

In part I (Lima et al., Transp Porous Media, 2009), a three-scale model governing the movement of an aqueous saline solution containing four monovalent species (Na+, H+, Cl−, OH−) in kaolinite clays was derived. Unlike purely macroscopic approaches, the novelty of the formulation relied on the double averaging of the nanoscopic electro- chemistry of particle/electrolyte solution interface ruled by the electrical double layer coupled with protonation/deprotonation reactions. The passage from the nano to the micro (pore)-scale gave rise to ion-sorbed concentrations and slip velocity at the solid/fluid interface which are coupled with the microscopic Stokes problem and Nernst–Planck equations governing the hydrodynamics and ion transport in the micropores. Application of a formal homogenization procedure led to macroscopic governing equations with effective electro-chemical parameters, such as retardation coefficients, electro-osmotic permeability, and electric conductivity. In this study, we reconstruct the constitutive laws of the macroscopic coefficients by solving the nano and microscopic closure problems. New generalized isotherms for Na+ and H+ − OH− sorption are build-up based on a perturbation approach and the limitations of classical Freundlich isotherm for modeling ion sorption at the solid/fluid interface are discussed. The macroscopic governing equations are discretized by the finite volume method and numerical simulations of a transient electroosmosis experiment for desalination of a clay sample by electrokinetics are presented.

A NEW MODEL FOR DESCRIBING EVOLUTION AND CONTROL OF DISASTER SYSTEM INCLUDING INSTANTANEOUS AND CONTINUOUS ACTIONS

A new model for describing the disaster system including instantaneous and continuous action synchronously has been developed. The model is composed of three primary parts, that is, the impact from its causative disaster events, stochastic noise of disaster node and self-healing function, and every part is modeled concretely in terms of their characteristics in practice. Some key parameters, namely link appearance probability, retardation coefficient, ultimate repair capacity of government, dynamical modes considering different disaster evolving chains, and the positions of link with the specific performance in disaster network system are involved. Combined with a case study, the proposed model is applied to a certain disaster evolution system, and the influence law of different parameters on disaster evolution process, in disaster networks with instantaneous-action and/or continuous-action, is presented and compared. The results indicate that the destructive impact in the networks by link in continuous action is far greater an order of magnitude than that in instantaneous action. If a link in continuous action emerges in the disaster network system, properties of the causative event for the link, link appearance probability and its position in the network all have a notable influence to the severity of the disaster network. In addition, some peculiar phenomena are also commendably observed in the disaster evolution process based on the model, such as the multipeaks emerging in the destroyed rate number curve for some crisis nodes caused by their various inducing paths together with the relevant retardation coefficients, the existence of the critical value for ultimate repair capacity to recover the disaster node, and so on.

Fate of nine recycled water trace organic contaminants and metal(loid)s during managed aquifer recharge into a anaerobic aquifer: Column studies

Water quality changes associated with the passage of aerobic reverse osmosis (RO) treated recycled water through a deep anaerobic pyritic aquifer system was evaluated in sediment-filled laboratory columns as part of a managed aquifer recharge (MAR) strategy. The fate of nine recycled water trace organic compounds along with potential negative water quality changes such as the release of metal(loid)s were investigated in large-scale columns over a period of 12 months. The anaerobic geochemical conditions provided a suitable environment for denitrification, and rapid (half-life <1–25 days) degradation of the endocrine disrupting compounds (bisphenol A, 17β-estradiol, 17α-ethynylestradiol), and iodipamide. However, pharmaceuticals (carbamazepine and oxazepam), disinfection by-products ( N-nitrosodimethylamine, N-nitrosomorpholine) and iohexol did not degrade rapidly (half-life > 100 days). High retardation coefficients (R) determined for many of the trace organics (R 13 to 67) would increase aquifer residence time and be beneficial for many of the slow degrading compounds. However, for the trace organics with low R values (1.1–2.6) and slow degradation rates (half-life > 100 days), such as N-nitrosodimethylamine, N-nitrosomorpholine and iohexol, substantial biodegradation during aquifer passage may not occur and additional investigations are required. Only minor transient increases in some metal(loid) concentrations were observed, as a result of either pyrite oxidation, mineral dissolution or pH induced metal desorption, followed by metal re-sorption downgradient in the oxygen depleted zone.

Transport and sorption of 85Sr and 125I in crushed crystalline rocks under dynamic flow conditions

Transport and sorption of water-soluble 85Sr2+ and 125I− in the columns with beds of crushed crystalline rocks from synthetic groundwater has been studied under dynamic flow conditions. Samples of crystalline rocks: diorite-I, diorite-II, gabbro, granite and tonalite, having the grain size between 0.25 and 0.80 mm, were used. Plastic syringes of 8.8 cm length and 2.1 cm in diameter were applied as columns. The synthetic groundwater was pumped downward through the columns with a seepage velocity of about 0.2 cm/min and the given radioactive nuclide was added into the water stream individually in a form of a short pulse. In case of 85Sr, desorption from diorite-I was also studied using an artificial acid rainfall and then, the longitudinal distribution of the residual 85Sr activity along the bed was measured. Retardation, distribution and hydrodynamic dispersion coefficients were determined by the evaluation of respective breakthrough curves. A corrected integral form of a simple advection–dispersion equation was derived and used for fitting the experimental data. The K d-parameters resulting from dynamic experiments were also compared with the results of static sorption experiments.

Assessment of synthetic zeolite Na A–X as sorbing barrier for strontium in a radioactive disposal facility

The retention of strontium onto synthetic zeolite Na A–X, candidate as backfill material in a near surface disposal facility, was studied using batch and column techniques. In order to investigate the sorption mechanism, the kinetic data were tested using pseudo-first-order, pseudo-second-order, homogenous particle, and intraparticle models. The suitability of the geochemical conditions was preliminarily assessed by conducting equilibrium sorption studies at different pH ranging from 2.0 to 9.0. To optimize the design of the barrier, the effect of the initial concentration on the retention of Sr 2+ onto the proposed material at three temperatures was investigated. The sorption equilibrium data were analyzed using non-linear Freundlich, Langmuir, and D–R models to evaluate the sorption characteristics and the thermodynamic parameters such as changes in free energy (Δ G), enthalpy (Δ H), and entropy (Δ S) were calculated. To assess the effect of the utilized method to determine the retardation coefficients on the predicted concentration, a simple pulse analytical model was used.

Evaluation of Solute Transport through a Fracture by Considering the Spatial Distributions of Retardation Effect in Grain Scale

The retardation effect of radionuclides (RNs) is a key factor to comprehend the performance of the geological disposal system. Since natural rock includes various minerals, the physical and chemical properties are not spatially uniform. However, in general, the performance assessments ignore such heterogeneity. This study focused on the grain size of each mineral in granite, to consider the spatial distribution of retardation coefficients. In a grain size, the retardation coefficient was assumed to be uniform in this study. Through such heterogeneous media, the transport rates of cesium-135 ( 2.1× 106 y in half-life) have been numerically examined, using two-dimensional advection-dispersion equations. Here, the sorption data of each mineral in granite were quoted from the past report. The conclusions are summarized as follows: (1) Since the flow test limits the sorption onto biotite (taking the highest sorption-coefficient in the minerals of granite) in the flow paths, the flow test showed less sorption, ...