Reactive Transport Model Research Articles

A One-Dimensional Reactive Transport Model for Microbially Induced Calcium Carbonate Precipitation (MICP) in Phreeqc

A One-Dimensional Reactive Transport Model for Microbially Induced Calcium Carbonate Precipitation (MICP) in Phreeqc

Integrating Enzyme-Based Kinetics in Reactive Transport Models to Simulate Spatiotemporal Dynamics of Biomarkers during Chlorinated Ethene Degradation.

Biomarkers such as functional gene mRNA (transcripts) and proteins (enzymes) provide direct proof of metabolic regulation during the reductive dechlorination (RD) of chlorinated ethenes (CEs). Yet, current models to simulate their spatiotemporal variability are not flexible enough to mimic the homologous behavior of RDase functional genes. To this end, we developed new enzyme-based kinetics to model the concentrations of CEs together with the transcript and enzyme levels during RD. First, the model was calibrated to existing microcosm data on RD of cis-DCE. The model mirrored the tceA and vcrA gene expression and the production of their enzymes in Dehalococcoides spp. Considering tceA and vcrA as homologous instead of nonhomologous improved fitting of the mRNA time series. Second, CEs and biomarker patterns were explored as a proof of concept under groundwater flow conditions, considering degraders occurring in immobile and mobile states. Under both microcosm and flow conditions, biomarker-rate relationships were nonlinear hysteretic because tceA and vcrA acted as homologous genes. The mobile biomarkers additionally undergo advective-dispersive transport, which increases the nonlinearity and makes the observed patterns even more challenging to interpret. The model offers a thorough mechanistic description of RD while also allowing simulation of spatiotemporal dynamic patterns of various key biomarkers in aquifers.

Stable silicon isotope fractionation reflects the routing of water through a mesoscale hillslope

Concentration - isotope Ratio - Discharge (C-R-Q) relationships offer a promising means of disentangling the underlying hydrologic, geochemical, and ecological factors that produce variations in stream solute chemistry across a variety of critical zone systems. However, natural environments are both temporally and spatially complex, and prevailing interpretations of these C-R-Q patterns remain difficult to validate. Here we employ three replicate artificially constructed hillslopes at the Landscape Evolution Observatory (LEO) in Tucson, Arizona as simplified analogs to natural catchments. LEO allows us to record silicon stable isotope (δ30Si) signatures of fluid discharge under a controlled irrigation schedule in a system devoid of vegetation. This unique meso‑scale experiment enables, for the first time, evaluation of metalloid stable isotope signatures at the scale of natural hillslopes constrained to known fluid transit time distributions (TTDs) and limited to fractionation in association with secondary mineral formation. We report δ30Si in discharge samples collected over three randomized storm events of varying intensity. The data reflect consistent enrichment in the fluid phase between +1.00 and +2.07 ‰ across the three hillslopes, despite highly variable irrigation scenarios, reflecting substantial loss of SiO2 from solution due to secondary mineral precipitation. We compare results from an isotope-enabled Reactive Transport Model (RTM) and the discharge measurements from LEO to constrain the contributions of both characteristic watershed TTDs and fractionation pathways in emergent δ30Si signatures. Our study confirms and expands upon recent work in natural systems attributing intra-site variability in silicon stable isotope signatures to the routing of fluid through catchments.

Reconstruction of ancient sulfate‒methane transition depths using the concentrations and sulfur isotopic compositions of authigenic sulfide minerals in marine sediments

On the basis of a one-dimensional transport-reaction model, we propose a novel approach to identify the relationship between the depth of the sulfate‒methane transition (SMT) and the stratigraphic distribution of authigenic sulfide minerals in terms of both concentration and sulfur isotopic composition. We apply this approach to Ocean Drilling Program Sites 994 and 995 at the Blake Ridge, offshore southeastern North America. Within the present-day sulfate reduction zone, our numerical simulation suggests that sulfide mineralization is basically at a steady state, the concentrations of which depend largely on the reactivity of sedimentary organic carbon. The observed extreme 34S-depletion of sulfide minerals (e.g., δ34S values as low as −45‰ VCDT) can occur without the process of sulfur disproportionation. We then derive a series of theoretical profiles by changing the SMT depth from 4 m to 30 m below the seafloor and construct a contour plot of the theoretical SMT depth as a function of the concentration and sulfur isotopic composition of solid-phase sulfides after complete mineralization under steady-state conditions. Below the present-day SMT, six stratigraphic locations of the ancient SMT are identified and their depths below each contemporaneous seafloor are interpolated. Theoretical estimates suggest that the SMT gradually subsided from 8.4 to ca. 24 m below the contemporaneous seafloor (mbcsf) over the past 643 to 6.6 ka. At present, the SMT has been occurring steadily at depths of ca. 21 to 22 mbcsf for at least 2.9 kyr. This is the first time that the evolution of the ancient SMT depth has been quantitatively reconstructed, by which the attainment of knowledge can be used to research the early diagenetic evolution history of the pore water sulfur species and will arouse much interest and concern from other researchers.

Multiphase flow and reactive transport benchmark for radioactive waste disposal

AbstractCompacted bentonite is part of the multi-barrier system of radioactive waste repositories. The assessment of the long-term performance of the barrier requires using reactive transport models. Here we present a multiphase flow and reactive transport benchmark for radioactive waste disposal. The numerical model deals with a 1D column of unsaturated bentonite through which water, dry air and $${\hbox {CO}_{2{(g)}}}$$ CO 2 ( g ) may flow and with the following reactions; aqueous complexation, calcite and gypsum dissolution/precipitation, cation exchange and gas dissolution. INVERSE-FADES-CORE V2, $$\hbox {DuMu}^X$$ DuMu X , TOUGHREACT and iCP were benchmarked with 6 test cases of increasing complexity, starting with conservative tracer transport under variably unsaturated conditions and ending with water flow, gas diffusion, minerals and cation exchange. The solutions of all codes exhibit similar trends. Small discrepancies are found in conservative tracer transport due to differences in hydrodynamic dispersion. Computed $${\hbox {CO}_{2{(g)}}}$$ CO 2 ( g ) pressures agree when a sufficiently refined grid is used. Small discrepancies in $${\hbox {CO}_{2{(g)}}}$$ CO 2 ( g ) and pH are found near the no-flow boundary at early times which vanish later. Discrepancies are due differences in the formulations used for gas flow at nearly water-saturated conditions. Computed $${\hbox {CO}_{2{(g)}}}$$ CO 2 ( g ) pressures show a fluctuation between $$10^{-4}$$ 10 - 4 and $$10^{-3}$$ 10 - 3 years which slows down the in-diffusion of $${\hbox {CO}_{2{(g)}}}$$ CO 2 ( g ) . This fluctuation is associated with chemical reactions involving $${\hbox {CO}_{2}}$$ CO 2 . There are discrepancies in solute concentrations due to differences in the Debye–Hückel (DH) formulation. They are overcome when all codes use the same DH formulation. The results of this benchmark will contribute to increase the confidence on multiphase reactive transport models for radioactive waste disposal.

Biofilm Growth in Porous Media Well Approximated by Fractal Multirate Mass Transfer With Advective‐Diffusive Solute Exchange

AbstractBiofilm growth in porous media changes not only the hydrodynamic properties of the medium (reduction in porosity and permeability, and increase in dispersivity), but also the transport itself (breakthrough curves display increasingly fast first arrivals and long tails). These features are well reproduced by multicontinuum models (Multi‐Rate Mass Transfer, MRMT) which can be used to describe reactive transport in heterogeneous porous media and facilitate the simulation of reactions that are localized within biofilms. Here, we present a conceptual and numerical model of biochemical reactive transport with dynamic biofilm growth based on MRMT formulations. Mass exchange between mobile water and immobile biofilm aggregates is represented by a memory function, which simplifies definition of MRMT parameters. We successfully tested this model on two sets of laboratory data and found that (a) a basic model based on the growth of uniformly sized biofilm aggregates fails to reproduce laboratory tracer tests and rate of biofilm growth, while a fractal growth model, which we obtain by integrating the memory functions of biofilm aggregates with a power law distribution, does; (b) the biofilm memory function evolves as the biofilm grows; and (c) the early time portion of eluted volume tracer breakthrough curves are independent of flow rate, whereas the tail becomes heavier when the flow rate is decreased, which implies that both advection controlled and diffusion controlled mass exchange coexist in biofilms. These findings imply that porous media biofilms are essentially different from those developing in human tissues or open spaces.

Numerical simulation of a base metal deposit related to a fossil geothermal system

Fossil and active geothermal systems that produce ore deposits are sites of complex physicochemical processes and a favorable combination of factors related to the amount of metal-bearing fluid that flows through the system, ore fluid metal concentrations, depositional efficiency, and the duration of ore deposition. Of all these factors, the length of the mineralizing event is one of the least understood aspects of ore genesis.We used fluid inclusion data, chemical compositions of base metal sulfides, and fluid flow rates to constrain a reactive-transport model of a fossil geothermal system - the Patricia Zn-Pb-Ag deposit in northern Chile. The Patricia deposit consists of quartz and base metal sulfide veins of hydrothermal origin with structural control, hosted in a volcanic succession with intense propylitic alteration. The fluid inclusions are liquid-rich, with homogenization temperatures ranging from 250 to 150 °C and salinities between 22 and 1 wt% NaCl equiv., with an early fluid mixing trend and no evidence of boiling in the system. Sulfide mineralogy indicates intermediate sulfidation conditions.To identify the most relevant geochemical and transport parameters controlling the formation of this fossil geothermal system >1000 simulations were performed using the reactive-transport code TOUGHREACT. The paragenesis of the deposit is mimicked by a model of successive stages of fluid circulation consistent with the observed mineral assemblage distribution, the fluid inclusion data, and the estimated resources in the deposit.The entire geothermal activity of the system was modeled considering 10,000 years of fluid-rock interaction, with periods of circulation of metal-barren fluids followed by metal-rich fluids driving the ore formation. In the initial model, base metal solubility with predominant chloride complexing suggests that the most efficient ore-forming mechanism for the Patricia deposit was the result of the interaction of two different fluids, one fluid transporting metals and another fluid transporting reduced sulfur, mixing in a rock volume of high permeability. Mass balance estimations with this model give a period of 3500 to 5000 years for the ore stage duration in which all the ore resources of the Patricia deposit could have been precipitated by fluid mixing.In a second model, the previous estimates for the duration of the main ore stage were used to simulate the fluid-rock interaction during the ore stage for 3500 years. The results indicated the importance of the permeability of the host rock enhanced by fractures to concentrate the volume of the mineralization and the role of the hydrothermal alteration assemblage in controlling the circulating fluid acidity. A higher efficiency in forming sulfide minerals appears to coincide with pH values ranging from 5.1 to 5.3.The results of both models are validated by replicating the system evolution, reproducing the same mineral alteration assemblage, the expected base metal resource distribution, and similar amounts of ore resources to those of the Patricia deposit: a total of 52,602 tons of lead and 157,731 tons of zinc. The models indicate that the hydrothermal event might be two times longer than the ore stage.

Unravelling Coupled Hydrological and Geochemical Controls on Long-Term Nitrogen Enrichment in a Large River Basin.

Many groundwater and surface water bodies around the world show a puzzling and often steady increase in nitrogen (N) concentrations, despite a significant decline of agricultural N inputs. This study uses a combination of long-term hydrogeochemical and hydraulic monitoring, molecular characterization of dissolved organic matter (DOM), column experiment, and reactive transport modeling to unravel the processes controlling N-reactive transport and mass budgets under the impacts of dynamic hydrologic conditions at a field site in the central Yangtze River Basin. Our analysis shows that the desorption of ammonium (NH4+) from sediments via cation exchange reactions dominates N mobilization and aqueous N concentrations, while the mineralization of organic N compounds plays only a minor role. The reactive transport modeling results illustrate the important role of cation exchange reactions that are induced by temporary NH4+ input and cation concentration changes under the impact of both seasonal and long-term hydrologic variations. Historically, cation exchangers have acted as efficient storage devices and mitigated the impacts of high levels of NH4+ input. The NH4+ residing on cation exchanger sites later acts as a long-term N source to waters with the delayed desorption of sediment-bound NH4+ induced by the change of hydrologic conditions. Our results highlight the complex linkages between highly variable hydrologic conditions and NH4+ partitioning in near-surface, river-derived sediments.

Trace element partitioning between natural barite and deep anoxic groundwaters: Implications for radionuclide retention in host rocks of nuclear waste repositories

Safety assessments for deep geological repositories involve risk calculations for the release of radionuclides like Ra, U, Pu and trivalent actinides from the storage containers to the groundwater. The retention of radionuclides through water-mineral interaction along the groundwater flow path could be a crucial factor in case of a repository failure. Barite (BaSO4) assumes significance in this context, as it has the potential to (re)crystallize and incorporate significant quantities of radioactive elements under relevant physico-chemical conditions.The assessment of mineral-fluid partition coefficients provides a means to evaluate the uptake potential of elements into the mineral. Usually, partition coefficients are determined under well-defined and controlled experimental conditions in laboratories. However, these results have shown discrepancies to partitioning coefficients determined from natural systems.Furthermore, effects like diagenesis or changes in the chemical fluid parameters might lead to a secondary alteration of the phases and affecting the retention ability.Here we investigate the incorporation of trace elements in natural barite from a borehole at 415 m depth in the Äspö Hard Rock Laboratory (Sweden). High-resolution LA-ICP-MS enables the quantitative determination of field-based partition coefficients through the integrated pixel average of selected zones in element distribution maps, combined with existing fluid concentration data. Similarities between the solid solution systems (Ra,Ba)SO4 and (Sr,Ba)SO4 allowed the combination of Sr partitioning data with density-functional theory simulations for an estimation of the partition coefficient value for Ra in natural barite. Values in the 10−2 range were determined, showing a deviation from those reported in previous experimental studies in the 10° range.Moreover, lanthanum serves as an analogue element for the radioactive trivalent actinides. The partition coefficient values for La in natural barite were determined in the range of 10−2 and 10−1, aligning well with experimental partition coefficients. Although density functional theory simulations cannot directly convert a La partition coefficient into a partition coefficient for trivalent actinides, it is assumed that these elements exhibit comparable behavior. Besides primary growth zonation, La also exhibits strong secondary enrichment, probably resulting from groundwater mixing and late fluid-mediated element transport through connected intracrystalline pore space oriented at cleavage plane systems. Additional SIMS analysis provides insights into the temporal variation of the sulfate source at the sampling site reflecting the influence of different waters during mineral growth.This study demonstrates a discrepancy between natural and synthetic PRa for barite and emphasizes that secondary processes significantly impact radionuclide retention in barite. Including these effects in reactive transport models will improve the reliability and applicability of integrative risk models.

Understanding the nonlinear reactive transport model in porous catalysts

This paper discusses the steady-state nonlinear reaction-diffusion processes in porous catalysts. This model is based on a nonlinear second-order differential equation that includes a nonlinear term associated with the Michaelis-Menten and non-Michaelis-Menten kinetics of the reaction. The nonlinear equations can be approximately solved using the Taylors series, modified Taylors series and Akabri-Ganji technique to obtain the concentration of dissolved species. The influence of the half-saturation parameter and the characteristic reaction rate on concentration is explored. Sensitivity analysis of parameters is discussed. Our analytical findings were compared with numerical solutions.

Acceleration of Numerical Modeling of Uranium In Situ Leaching: Application of IDW Interpolation and Neural Networks for Solving the Hydraulic Head Equation

The application of In Situ Leaching (ISL) has significantly boosted uranium production in countries like Kazakhstan. Given that hydrodynamic and chemical processes occur underground, mining enterprises worldwide have developed models of reactive transport. However, modeling these complex processes demands considerable computational resources. This issue is particularly significant in the context of numerical analyses of mining processes or when modeling production scenarios in uranium mining by the ISL technique, given that a substantial portion of computational resources is allocated to solving the hydraulic head equation. This work aims to explore the applicability of PINNs to accelerate hydrodynamic simulations of the ISL process. The solution of the Poisson equation is accelerated by generating an initial approximation for the iterative method through the application of the Inverse Distance Weighting (IDW) interpolation and PINNs. The impact of various factors, including the computational grid and the spacing between wells, on both the accuracy and efficiency of initial approximation and the overall solution of the elliptic equation are explored. Employing the hydraulic head distribution obtained through PINNs as the initial approximation led to a significant reduction in computation time and a decrease in the number of iterations by a factor of 2.8 to 7.10.

Reactive transport model of the long-term geochemical evolution in a HLW repository in granite at the disposal cell scale: Variants, sensitivities, and model simplifications

The assessment of the long-term performance of the engineered barrier systems of high-level radioactive waste (HLW) repositories requires the use of reactive transport models. Montenegro et al. (2023) presented a non-isothermal reactive transport model of the long-term geochemical evolution of a HLW disposal cell in a granitic host rock corresponding to a generic reference concept. The model accounted for the vitrified waste, the carbon-steel canister, the bentonite buffer and the reference granitic rock. Here we extend their model by considering model variants (V), sensitivity cases (SC) and model abstractions (MA). Variants V1, V2 and V3 consist of considering MX-80 bentonite instead of FEBEX bentonite (V1), a larger groundwater flux through the granite (V2) and the Czech reference crystalline rock as a host rock (V3). Cases SC1 and SC2 consider a decrease of the silica concentration threshold value in the glass dissolution rate (SC1) and an earlier canister failure time (SC2), respectively. Runs MA1 to MA4 consider smectite as an unreactive mineral phase (MA1), the porosity feedback effect on chemical and transport parameters (MA2), a time-varying corrosion rate (MA3), and a coarser finite element grid (MA4), respectively. Model results of V1 show a larger pH, a smaller precipitation of magnetite, siderite and greenalite and a slightly smaller dissolution of ISG and smectite than the base run of Montenegro et al. (2023). Model predictions are very sensitive to the increase in the groundwater flow through the granitic host rock (V2). However, predictions are not sensitive to the chemical composition of the granite porewater (V3). The decrease in the silica saturation threshold from 1·10−3 to 5·10−4 mol/L in SC1 leads to a significant decrease in glass dissolution. Glass dissolution after 50,000 years in SC2 (earlier canister failure) is much larger than that of the base run. Model results are not sensitive to considering smectite as an unreactive mineral phase (MA1). However, model results are very sensitive to the porosity feedback effect (MA2). A 60% volume fraction of Fe(s) remains uncorroded after 50,000 years when a variable corrosion rate is considered in MA3. In this case the precipitation of corrosion products is much smaller than that of the base run. The general patterns of the numerical results in MA4 (coarser grid) are similar to those of the base case.

A New Efficient Approach to Model Stiff Biogeochemical Reactive Transport Scenarios Across Groundwater Systems

AbstractBiogeochemical reactive transport models (RTMs) are key for understanding the evolution of the quality of groundwater systems and their interaction with anthropogenic activities. The inherent stiffness of these models, within which bio‐geochemical reactions and transport processes take place simultaneously across diverse time scales, poses significant computational challenges. The development of current RTMs is characterized by a tradeoff between accuracy and computational efficiency. Our study introduces a novel approach grounded on Computational Singular Perturbation (CSP) with the aim of efficiently solving stiff biogeochemical RTMs in groundwater systems. We integrate the CSP concept and algorithm with a reactive transport model associated with a groundwater system. Our results document that this yields a significant improvement in terms of efficiency while maintaining accuracy. For demonstration and evaluation purposes, we apply the approach to a collection of typical groundwater biogeochemical RTMs including H2O2 production/consumption, Cr(VI) adsorption‐desorption equilibrium, and denitrification processes within riparian aquifers. The new approach is then evaluated against traditional apparent rate (AR) and Equilibrium‐kinetic (EK) methods. Our results reveal that the new approach effectively identifies fast species and simplifies reaction networks, thus significantly reducing stiffness and computational costs while maintaining remarkable accuracy. Overall, our approach offers a robust and efficient solution for modeling stiff biogeochemical processes in groundwater systems. Its successful application to diverse reaction networks highlights its potential for broad implementation in environmental and engineering contexts, paving the way for accurate and computationally feasible groundwater quality assessments.

Foraminiferal denitrification and deep bioirrigation influence benthic biogeochemical cycling in a seasonally hypoxic fjord

Benthic macro- and micro-biota often play significant roles in controlling the biogeochemical dynamics in sediments. Their activity can be influenced by oxygen availability and impacted by the rise in global hypoxia in coastal regions over the last decades. To understand how these organisms interact with coastal hypoxia and influence sediment biogeochemistry, we undertook a study of early diagenesis in Bedford Basin, a seasonally hypoxic fjord on the West Atlantic coast in Nova Scotia, Canada, using a combination of observations and reaction-transport modeling. We observed that the seafloor was a source of ammonium and sink of nitrate with average fluxes of 2.2 ± 1.8 and −0.9 ± 0.7 mmol m−2 d−1 respectively. The diffusive oxygen uptake was 14 ± 4.6 mmol m−2 d−1 and the total organic carbon content in collected sediment cores was 5–7 % with a C/N ratio of ∼10. The pyrite content increased steadily from 0.5 wt% Fe at surface to ∼2 wt% Fe at 20 cm depth. Hydrogen sulfide was negligible down to 25 cm depth most of the time. The sediment was inhabited by tube-forming polychaete Spiochaetopterus sp. that formed tubes up to ∼30 cm in length. The living foraminiferal assemblage in the top 5 cm sediment was found to be dominated (>85 %) by nitrate-storing and denitrifying benthic foraminifera Stainforthia fusiformis. These observations were used to develop and constrain a biogeochemical reaction-transport model. The model results suggest that the observed decrease in porewater concentrations of ammonium and dissolved inorganic carbon below 5 cm depth, was due to deep bioirrigation by tubeworms, accounting for almost 50 % of the benthic efflux. The model further revealed that the deep bioirrigation along with bioturbation and iron cycling prevented accumulation of free sulfide in the top 25 cm sediment despite oxygen penetration depths of ∼1 mm. Modelled organic carbon and nitrogen deposition was 25.2 and 2.9 mmol m−2 d−1 with burial efficiencies of 23 % and 17 %, respectively. The model indicated a total denitrification rate of 1.3 mmol N m−2 d−1 that was largely (∼70 %) driven by benthic foraminifera. This study reports the first evidence of foraminiferal denitrification in western Atlantic coastal sediments, and suggests that eukaryote mediated denitrification is an important driver of sediment N-loss in seasonally hypoxic environments, a process that has been traditionally assumed to be carried out by prokaryotic microbes.

Soil gas gradient method for estimating natural source zone depletion rates of LNAPL and specific chemicals of concern

This paper presents a simplified approach for the soil gas gradient method for estimating natural source zone depletion (NSZD) rates of specific contaminants of concern (COCs) at sites contaminated by light non-aqueous phase liquids (LNAPL). Traditional approaches to quantify COC-specific NSZD rates often rely on numerical or analytical reaction-transport models that require detailed site-specific data. In contrast, the proposed method employs simple analytical solutions, making it more accessible to practitioners. Specifically, it requires only the maximum soil gas concentration, the effective diffusion coefficient, and the diffusive reaction length calculated from vertical soil gas concentration profiles. The simplified approach was validated against a reactive transport numerical model reported in the literature, showing consistent results within the same order of magnitude for BTEX NSZD rates at a gasoline spill site in South Carolina. Further validation using a larger dataset involved comparing NSZD rate estimates for benzene and total petroleum hydrocarbons (TPH) against those obtained using BioVapor, utilizing empirical soil gas data from the USEPA Petroleum Vapor Intrusion Database. Results demonstrated a strong correlation between NSZD rates and maximum soil gas concentrations, allowing the development of a rapid screening approach based only on the measured soil gas concentrations and literature values for diffusion coefficients and diffusive reaction lengths. This approach aligned well with previous modeling studies and was consistent with literature values for TPH NSZD rates. Overall, both the simplified and screening approaches offer practical, easy-to-use tools for evaluating temporal variability in natural attenuation rates, supporting baseline assessments and ongoing performance evaluations of remediation at LNAPL sites.

Assessment of intermediate level radioactive waste disposal cell evolution in granitic host rock applying abstracted reactive transport models

Assessment of intermediate level radioactive waste disposal cell evolution in granitic host rock applying abstracted reactive transport models

Impact of chemical aging on pyrogenic carbon colloid facilitated transport and transformation of Cr (VI) in anoxic environments

Frequent wildfires have accumulated the pyrogenic carbon (PyC) colloids in the environment, where they undergo environmental aging processes. The altered properties of aged PyC colloids may affect their ability to facilitate transformation and transport of contaminants in post-fire environments, posing unknown threats to ecological security. This study investigated the effect of chemical aging on the PyC colloid-facilitated transformation and transport of chromium (Cr) using batch experiments, column experiments, and transport model simulations. Results showed that aged PyC colloids exhibited weaker electron-donating capacity, and the reduction of Cr (VI) to Cr (III) decreased from 37.6 % to 13.5 % with the increasing aging time. Although PyC colloid transport increased with aging time, the PyC colloid-facilitated Cr (III) transport decreased because of the weakened reduction of Cr (VI). The transport of PyC colloid-facilitated Cr (III) was weaker at low pH. The reactive solute transport model well simulated the aged PyC colloid-facilitated transformation and transport of Cr (VI). Our findings highlight the significance of aging processes and environmentally relevant conditions in influencing the PyC colloid-facilitated transformation and transport of Cr, which is crucial for assessing risks of wildfire-driven Cr pollution and the potential of PyC for in-situ pollution control.

Reactive transport modelling of in-situ CO2 mineralization in basalt formations

Reactive transport modelling of in-situ CO2 mineralization in basalt formations

A 3D convolutional neural network model with multiple outputs for simultaneously estimating the reactive transport parameters of sandstone from its CT images

Porosity, tortuosity, specific surface area (SSA), and permeability are four key parameters of reactive transport modeling in sandstone, which are important for understanding solute transport and geochemical reaction processes in sandstone aquifers. These four parameters reflect the characteristics of pore structure of sandstone from different perspectives, and the traditional empirical formulas cannot make accurate predictions of them due to their complexity and heterogeneity. In this paper, eleven types of sandstone CT images were firstly segmented into numerous subsample images, the porosity, tortuosity, SSA, and permeability of the subsamples were calculated, and the dataset was established. The 3D convolutional neural network (CNN) models were subsequently established and trained to predict the key reactive transport parameters based on subsample CT images of sandstones. The results demonstrated that the 3D CNN model with multiple outputs exhibited excellent prediction ability for the four parameters compared to the traditional empirical formulas. In particular, for the prediction of tortuosity and permeability, the 3D CNN model with multiple outputs even showed slightly better prediction ability than its single-output variant model. Additionally, it demonstrated good generalization performance on sandstone CT images not included in the training dataset. The study showed that the 3D CNN model with multiple outputs has the advantages of simplifying operation and saving computational resources, which has the prospect of popularization and application.

Disinfectant control in drinking water networks: Integrating advection–dispersion–reaction models and byproduct constraints

Effective disinfection is essential for maintaining water quality standards in distribution networks. Chlorination, as the most used technique, ensures safe water by maintaining sufficient chlorine residuals but also leads to the formation of disinfection byproducts (DBPs). These DBPs pose health risks, highlighting the need for chlorine injection control (CIC) by booster stations to balance safety and DBPs formation. Prior studies have followed various approaches to address this research problem. However, most of these studies overlook the changing flow conditions and their influence on the evolution of the chlorine and DBPs concentrations by integrating simplified transport-reaction models into CIC. In contrast, this paper proposes a novel CIC method that: (i) integrates multi-species dynamics, (ii) allows for a more accurate representation of the reaction dynamics of chlorine, other substances, and the resulting DBPs formation, and (iii) optimizes for the regulation of chlorine concentrations subject to EPA mandates thereby mitigating network-wide DBPs formation. The novelty of this study lies in its incorporation of time-dependent controllability analysis that captures the control coverage of each booster station. The effectiveness of the proposed CIC method is demonstrated through its application and validation via numerical case studies on different water networks with varying scales, initial conditions, and parameters.