Anomalous contaminant transport velocities in groundwater for species generally considered to be immobile are often attributed to the mechanism of colloid‐facilitated transport. In some of the field ...observations attributed to colloid facilitation, an extremely small fraction of the total contaminant mass introduced to the groundwater is detected downstream. In this study, a new model of colloid‐facilitated contaminant transport is proposed that explains this phenomenon as the variability of mobility of individual colloids in the population. The process of retardation via attachment and detachment of colloids on immobile surfaces is often modeled with time and space invariant parameters; here it is modeled assuming a diverse population of transport properties that account for the inherent variability of colloid size, surface charge and chemical properties, mineralogy, and the concomitant impact on colloid mobility. When the contaminant is assumed to irreversibly attach to or form colloids, the migration of the contaminant plume exhibits extremely non‐Fickian behavior. The plume's center of mass travels with a velocity governed by the groundwater velocity divided by the mean colloid retardation factor. However, small quantities of contaminant attached to a few highly mobile colloids travel at velocities up to the groundwater velocity, far exceeding the velocity of the centroid of the plume. This paper introduces the colloid diversity model, presents some sensitivity calculations for an idealized case, and discusses the implications of such a model on data needs, simulation of field observations, and model scaling.
We report here the genome sequence of an effective chromium-reducing bacterium, Bacillus cereus strain S612. The size of the draft genome sequence is approximately 5.4 Mb, with a G+C content of 35%, ...and it is predicted to contain 5,450 protein-coding genes.
In some cases, the accuracy and efficiency of a particle tracking model may be greatly enhanced by determining the time for a particle to travel a specified distance rather than the distance traveled ...during some time interval. For instance, it may be desirable to know when a particle reaches a boundary or interface or a location where the system properties change. In this work, we derive an analytical expression in the form of a probability function describing the distribution of random times necessary for a particle to diffusively travel a specified distance. Upon substitution of the specified distance and a random number for the probability, the expression may be solved for the necessary time. Comparison of the results from this equation with an empirical expression determined by James and Chrysikopoulos (Chem. Eng. Sci. 56(23)(2001)6535) shows excellent agreement. Recommendations for implementing the analytical solution into particle tracking algorithms with or without a deterministic velocity component are included. This general equation is amenable to use in modeling both fractured and porous media systems.
Closed-system experiments were conducted to investigate the decomposition of sodium dithionite in aqueous solutions under varying pH and starting concentrations to simulate the deployment of ...dithionite as an in-situ redox barrier. Co-determination of dithionite and its degradation products was conducted using UV–Vis spectrometry, iodometric titration, and ion chromatography. In unbuffered solutions, dithionite reacted rapidly, whereas in near-neutral solutions (pH ∼7), it persisted for ∼ 50 days and in alkaline solution (pH ∼9.5) for >100 days. These are the longest lifetimes reported to date, which we attribute to not only excluding oxygen but also preventing outgassing of H2S. Thoroughly constraining the reaction products has led to the following hypothesized reaction:
4 S2O42− + H2O → HS− + SO32−+2 SO42− + S4O62− + H+
which represents relatively rapid degradation at near-neutral pH values. At the more alkaline pH, and over longer time scales, the reaction is best represented by:
3 S2O42− + 3 H2O → 2HS- + SO32−+3 SO42−+ 4 H+
the following kinetic rate law was developed for the pH range studied:
dCidt=Si10−4.81{H+}0.24{S2O42-},
where dCidt is the rate of change of the ith chemical component in the simplified equation (mole L−1 s−1) and Si is the stoichiometric coefficient of the ith chemical. The kinetic rate law was used to calculate a pseudo first order half-life of 10.7 days for near-neutral pH and 33.6 days for alkaline pH. This work implies that if hydrogen sulfide is contained within the system, such as in the case of a confined aquifer below the water table, dithionite decomposes more slowly in alkaline aqueous solution than previously thought, and thus it may be more cost-effectively distributed in aquifers than has been previously assumed.
•Sodium dithionite concentrations in alkaline aqueous solutions were measured by UV–vis spectrometry for up to 105 days.•Analysis of degradation products revealed that sulfite, hydrogen sulfide, sulfate, and polythionates are present.•The closed system created in this study ensured no loss of hydrogen sulfide, which slowed the loss of dithionite.•The kinetic rate law developed yields a half-life of 10.7 days at near-neutral pH and 33.6 days at alkaline pH.
•Lithium cation exchange was stronger at 225°C and 300°C than at room temperature.•Lithium cation exchange approached equilibrium rapidly in column experiments.•Cation exchange occurred primarily in ...a thin (<0.05mm) “rim zone” on rock surfaces.•The above results support the use of lithium to interrogate fracture surface area.
Column transport experiments were conducted at 225°C and 300°C using a crushed amphibolite schist from Fenton Hill, NM to evaluate lithium ion as a cation-exchanging tracer to interrogate fracture surface area in enhanced geothermal systems. Lithium exchange proceeded to equilibrium rapidly, and Li+ selectivity doubled from 225°C to 300°C, with the selectivity at both temperatures being much greater than at room temperature. Also, cation exchange was deduced to be occurring primarily in a thin “rim zone” (<0.05mm) on the rock surfaces. These results are all encouraging for using lithium ion to interrogate fracture surface area in enhanced geothermal systems.
Legacy industrial waste has left groundwater plumes of hexavalent Cr (Cr(VI)) that requires treatment, predominantly by reduction to Cr(III) and subsequent precipitation. One promising technology is ...the injection of a strong reducing agent, sodium dithionite, to create an in-situ redox barrier, but implementation requires a comprehensive understanding of the reactions occurring with dithionite injection. Batch and column experiments were conducted with aquifer sediments to determine both the significant reactions and efficacy of sodium dithionite treatment for a groundwater plume of hexavalent chromium. The batch experiments demonstrate consumption of dithionite over the experiment (disappearance by 43 d) concurrent with leaching of about 1 mM Fe from the sediments. Reactions deduced from batch experiments were incorporated into a 1-D numerical model to simulate reactions occurring during the column injection. The treatment was able to successfully reduce approximately 30 pore volumes of groundwater containing 800 µg kg−1 Cr(VI). The experiments also demonstrate that although mineral forms of Fe are important phases in the reduction of Cr(VI), Fe alone cannot account for the entire reduction capacity imparted to the sediments. Rather, the correlation between Cr and reduced S retained in the columns, combined with Scanning Electron Microscopy (SEM) of treated sediments, suggest that formation of reduced S phases contributes to the prolonged reduction capacity.
Display omitted
•One mole of dithionite produced 0.35 moles Fe(II) from sedimentary Fe(III).•Sodium dithionite successfully treats groundwater containing 800 ppb Cr(VI).•Reduced S phases have an important role in the prolonged reduction of Cr(VI).
Diffusion cell experiments were conducted to measure nonsorbing solute matrix diffusion coefficients in forty-seven different volcanic rock matrix samples from eight different locations (with ...multiple depth intervals represented at several locations) at the Nevada Test Site. The solutes used in the experiments included bromide, iodide, pentafluorobenzoate (PFBA), and tritiated water (
3HHO). The porosity and saturated permeability of most of the diffusion cell samples were measured to evaluate the correlation of these two variables with tracer matrix diffusion coefficients divided by the free-water diffusion coefficient (
D
m/
D
⁎). To investigate the influence of fracture coating minerals on matrix diffusion, ten of the diffusion cells represented paired samples from the same depth interval in which one sample contained a fracture surface with mineral coatings and the other sample consisted of only pure matrix.
The log of (
D
m/
D
⁎) was found to be positively correlated with both the matrix porosity and the log of matrix permeability. A multiple linear regression analysis indicated that both parameters contributed significantly to the regression at the 95% confidence level. However, the log of the matrix diffusion coefficient was more highly-correlated with the log of matrix permeability than with matrix porosity, which suggests that matrix diffusion coefficients, like matrix permeabilities, have a greater dependence on the
interconnectedness of matrix porosity than on the matrix porosity itself. The regression equation for the volcanic rocks was found to provide satisfactory predictions of log(
D
m/
D
⁎) for other types of rocks with similar ranges of matrix porosity and permeability as the volcanic rocks, but it did a poorer job predicting log(
D
m/
D
⁎) for rocks with lower porosities and/or permeabilities. The presence of mineral coatings on fracture walls did not appear to have a significant effect on matrix diffusion in the ten paired diffusion cell experiments.
The purpose of this study was to determine the attenuation potential and retardation of uranium in sediments taken from boreholes at the Smith-Ranch Highland in-situ recovery (ISR) site. Five column ...experiments with four different sediments were conducted to study the effects of variable mineralogy and alkalinity on uranium breakthrough. Uranium transport was modeled with PHREEQC using a generalized composite surface complexation model (GC SCM) with one, two, and, three generic surfaces, respectively. Reactive surface areas were approximated with PEST using BET derived surface areas to constrain fitting parameters. Uranium breakthrough was delayed by a factor of 1.68, 1.69 and 1.47 relative to the non-reactive tracer for three of the 5 experiments at an alkalinity of 540 mg/l. A sediment containing smectite and kaolinite retained uranium by a factor of 2.80 despite a lower measured BET surface area. Decreasing alkalinity to 360 mg/l from 540 mg/l increased retardation by a factor of 4.26. Model fits correlated well to overall BET surface area in the three columns where clay content was less than 1%. For the sediment with clay, models consistently understated uranium retardation when reactive surface sites were restricted by BET results. Calcite saturation was shown to be a controlling factor for uranium desorption as the pH of the system changes. A pH of 6 during a secondary background water flush remobilized previously sorbed uranium resulting in a secondary uranium peak at twice the influent concentrations. This study demonstrates the potential of GC SCM models to predict uranium transport in sediments with homogenous mineral composition, but highlights the need for further research to understand the role of sediment clay composition and calcite saturation in uranium transport.
•Uranium attenuation potential and retardation assessed for ISR.•Calcite saturation is a controlling factor for uranium desorption.•General surface complexation models predicted uranium transport for homogenous mineral composition soils.
► Developing different conceptualizations of the sorption processes. ► Modeling neptunium and uranium transport and sorption in fractured rock. ► Estimating the flow and reactive transport ...parameters. ► Identifying sorption processes with four model discrimination criteria.
Identification of chemical reaction processes in subsurface environments is a key issue for reactive transport modeling because simulating different processes requires developing different chemical–mathematical models. In this paper, two sorption processes (equilibrium and kinetics) are considered for modeling neptunium and uranium sorption in fractured rock. Based on different conceptualizations of the two processes occurring in fracture and/or matrix media, seven dual-porosity, multi-component reactive transport models are developed. The process models are identified with a stepwise strategy by using multi-tracer concentration data obtained from a series of transport experiments. In the first step, breakthrough data of a conservative tracer (tritium) obtained from four experiments are used to estimate the flow and non-reactive transport parameters (i.e., mean fluid residence time in fracture, fracture aperture, and matrix tortuosity) common to all the reactive transport models. In the second and third steps, by fixing the common non-reactive flow and transport parameters, the sorption parameters (retardation factor, sorption coefficient, and kinetic rate constant) of each model are estimated using the breakthrough data of reactive tracers, neptunium and uranium, respectively. Based on the inverse modeling results, the seven sorption-process models are discriminated using four model discrimination (or selection) criteria, Akaike information criterion (
AIC), modified Akaike information criterion (
AICc), Bayesian information criterion (
BIC) and Kashyap information criterion (
KIC). These criteria suggest the kinetic sorption process for modeling reactive transport of neptunium and uranium transport in both fracture and matrix. This conclusion is confirmed by two chemical criteria, the half reaction time and Damköhler number criterion.
PDF
