Reactive Solute Transport Research Articles

The Impact of Ice Sheet Geometry on Meltwater Ingress and Reactive Solute Transport in Sedimentary Basins

AbstractWe investigate the effect of ice sheet geometry on groundwater flow patterns and meltwater ingress in a hypothetical sedimentary basin. The simulation results indicate that meltwater ingress is much greater in 3D domains with a relatively narrow ice sheet extent compared to a wide ice sheet, or a simplified 2D model. In high permeability units (HPUs), the simulated meltwater penetration depth can reach up to 750 m in 3D domains compared to 400 m in a comparable 2D domain. In low permeability units (LPUs), very limited meltwater penetration occurs, indicating that ice sheet geometry and model dimensionality do not substantially affect water flow in these units. A conservative tracer, with a source located at a depth of 500 m in both HPUs and LPUs, illustrates that solutes can be transported to greater depths in HPUs for a narrow ice lobe scenario, in comparison to a wide ice sheet or the 2D approach. Tracer transport in LPUs is unaffected by ice sheet geometry. Similarly, simulation results indicate that the ingress of dissolved oxygen (O2) into HPUs is most substantial below a narrow ice lobe, while O2 ingress into LPUs is not affected by ice sheet geometry. The numerical experiments indicate that 3D analysis will give more comprehensive results for flow patterns and reactive solute transport subjected to glaciation/deglaciation cycles in the case of a narrow ice lobe, but also suggest that a 2D approach might provide an adequate representation for the case of a relatively wide ice sheet.

Decoupled Finite Particle Method With Normalized Kernel (DFPM‐NK): A Computationally Efficient Method for Simulating Solute Transport in Heterogeneous Porous Media

AbstractPrevious studies found that Finite Particle Method (FPM), an improved Smoothed Particle Hydrodynamics (SPH) method, yields more accurate solutions of the advection‐dispersion equation (ADE) than SPH method does when simulating solute transport in heterogeneous porous media. FPM however is computationally more expensive than SPH because FPM needs to solve a correction matrix equation for each particle. While decoupled FPM (DFPM) can reduce computational cost of FPM by using diagonal terms of the matrix equation, DFPM is less accurate than FPM due to discarding off‐diagonal terms of the matrix equation. This study develops the Decoupled Finite Particle Method with Normalized Kernel (DFPM‐NK) to improve computational accuracy of DFPM by using normalized kernels in DFPM. We evaluate computational performance of SPH, FPM, DFPM, and DFPM‐NK using two numerical experiments of ADE with non‐reactive and reactive solute transport in a heterogeneous aquifer. Results of the two experiments indicate that, among the four methods, SPH has the least computational time, but has the worst computational accuracy. DFPM‐NK is substantially more efficient than FPM, and has similar accuracy of FPM. On the other hand, DFPM‐NK and DFPM have similar computational cost, but DFMP‐NK is significantly more accurate than DFPM, especially for heterogeneous hydraulic conductivity fields. We thus recommend to use DFPM‐NK for computationally expensive ADE problems without sacrificing computational accuracy.

Novel Graphene‐Based Foam Composite As a Highly Reactive Filter Medium for the Efficient Removal of Gemfibrozil from (Waste)Water

AbstractGraphene‐based materials have emerged as alternative adsorbents, but their success in removing pharmaceutical contaminants has been limited due to degradation caused by restacking and limited control over their sizes and porosities. Driven by this issue, in the current study, to counteract the restacking behavior, graphene sheets are supported on a thread/rod‐like matrix structure in a boron nitride foam material, and a novel porous composite foam‐supported graphene is synthesized. The as‐prepared novel composite offers extraordinary features, such as high absorption kinetics, large available surface area, high porosity (>98%), ecofriendliness and cost‐effective synthesis, and excellent affinity to emerging pharmaceutical contaminants. When batch‐testing graphene‐based foam material and porous graphene nanosheets to remove gemfibrozil (GEM) from wastewater samples, rapid adsorption kinetics (<5 min) are exhibited by the graphene‐based foam. Column filter studies are conducted for both materials to test their performance in removing GEM from distilled water, synthetic graywater, and actual wastewater. Overall, the foam composite‐based filter marginally outperforms the sand‐supported graphene filter and significantly outperforms the unsupported graphene filter. A numerical MATLAB model is developed to simulate the reactive solute transport of GEM influent through the foam filter. Also, a formal sensitivity analysis is conducted to identify the key parameters influencing the model results.

Alfalfa reduces winter nitrate leaching relative to organic and conventional annual vegetable systems: Resin bag field measurements and modeling with HYDRUS-1D

Leaching of nitrate (NO₃⁻) to groundwater is a major concern in California, where groundwater NO₃⁻ levels often exceed public safe drinking water thresholds. The state has enacted legislation to implement monitoring programs and management plans that will minimize future NO₃⁻ loadings to groundwater based on modeled NO₃⁻ leaching; however, a need remains for empirical NO₃⁻ leaching data and assessment of model suitability to specific systems. Moreover, debate remains around the ability of different management practices like cover cropping, replacement of chemicals with organic inputs, and cultivating perennials to reduce NO₃⁻ leaching in California’s annual vegetable systems. We measured winter NO₃⁻ leaching over the wintery rainy season (October to March) within systems of the Century Experiment, a long-term cropping systems experiment in northern California evaluating effects of cover cropping, certified organic management, and alfalfa (<i>Medicago sativa</i> L.) incorporation in tomato (<i>Lycopersicon esculentum</i> Mill.)–maize (<i>Zea mays</i> L.) rotations. Anion exchange resin bags were installed at the bottom of the crop rooting zone (~65 cm) following tomato and prior to the onset of fall rains to adsorb leaching NO₃⁻ over the winter. Empirical resin bag NO₃⁻ leaching values were compared to modeled leaching results using HYDRUS-1D, which estimates water movement and reactive solute transport in soils. The rotation with alfalfa was the only system that reduced winter NO₃⁻ leaching (21.8 kg ha⁻¹), compared to conventional management (bare winter fallow after tomato) (47.1 kg ha⁻¹). Compared to conventional, certified organic management (44.7 kg ha⁻¹) and inclusion of a winter cover crop (58.2 kg ha⁻¹) had no significant impact on NO₃⁻ leaching. HYDRUS-1D model estimates for NO₃⁻ leaching were in good agreement with empirical field measurements in conventional and cover cropped systems, but less for certified organic and greater for the alfalfa systems. Results from this study show that perennial crops have potential to mitigate NO₃⁻ leaching losses across an agricultural landscape, and models like HYDRUS can provide useful estimates of NO₃⁻ leaching in some agricultural systems.

Experiment and Simulation of Non-Reactive Solute Transport in Porous Media.

To more accurately predict the migration behavior of pollutants in porous media, we conduct laboratory scale experiments and model simulation. Aniline (AN) is used in one-dimensional soil column experiments designed under various media and hydrodynamic conditions. The advection-dispersion equation (ADE) and the continuous-time random walk (CTRW) were used to simulate the breakthrough curves (BTCs) of the solute transport. The results show that the media and hydrodynamic conditions are two important factors affecting solute transport and are related to the degree of non-Fickian transport. The simulation results show that CTRW can more effectively describe the non-Fickian phenomenon in the solute transport process than ADE. The sensitive parameter in the CTRW simulation process is , which can reflect the degree of non-Fickian diffusion in the solute transport. Understanding the relationship of with velocity and media particle size is conducive to improving the reactive solute transport model. The results of this study provide a theoretical basis for better prediction of pollutant transport in groundwater.

The pesticide fate tool for groundwater vulnerability assessment within the geospatial decision support system LandSupport

The protection of groundwater resources from non-point-source pollutants, such as those coming from agricultural practices, is the focus of several European Directives, including the Water Framework Directive and the Pesticide Directive. Besides the environmental goals to be reached by the single EU member state, these directives clearly underline the role of experts in supporting planners and public authorities to fulfil these objectives. This work presents a new web-based, freely-available dynamical tool, named the pesticide fate tool, developed within the geospatial Decision Support system (DSS), LandSupport, for the assessment of groundwater vulnerability, specific for type of pollutant. The tool is based on the extended transfer function model, specifically expanded to consider the transport of reactive solutes, such as pesticides. The work describes the tool implementation for three case studies, with different spatial scales and pedo-climatic conditions: Valle Telesina, IT, Marchfeld, AT, and Zala County, HU. Principal inputs of the tool are: soil physical and hydrological properties, climate, groundwater table depth, type of crops and related pesticides. Results of the model are shown through the LandSupport GUI both as coloured maps, representing the relative concentration of pesticide at the arrival to the water table at the end of the simulation period, and as cumulative charts of the solute arrival at the depth of interest. The three case studies are shown as examples of application of the LandSupport DSS in supporting the Water and Pesticides directives, demonstrating that it represents a valuable instrument for public authorities, environmental planners, as well as agricultural extension services. For example, large differences are shown by soils in filtering the tetraconazole (99.9% vs 76%), a fungicide used in viticulture, or different percentage of arrival (0.32% and 0,01%) to the groundwater table are shown for two herbicides (Tribenuron and Florasulam) largely used to control annual dicotyledonous weeds.

Bed form-induced hyporheic exchange and geochemical hotspots

Small-scale bed form topographies control hyporheic exchange and biogeochemical processes within aquatic sediments, which ultimately affect water quality and nutrient cycling at the watershed scale. The impact of three-dimensional and small-scale bed form topographies on hyporheic exchange and solute mixing is investigated in the present work. The effect of bed form morphologies on the development of zones of enhanced reaction rates (i.e., hotspots) is also studied. A computational fluid dynamics model to simulate river flow over bed forms is combined with a subsurface flow and multicomponent reactive solute transport model. A wide variety of bed form topographies are generated using geometric models by varying parameters controlling curvature as well as bed form wavelength and amplitude. The results in this research suggest that out-of-phase bed forms generate more complex hyporheic flow patterns which reduce the efficiency of solute transformations. Higher phase shifts in bed form shapes result in overall higher average velocity, larger zones of enhanced pressure and reaction rates, and higher amounts of solute exchange. Moreover, the bed form shapes control the reaction process for a wide range of sediment conductivities. This study advances the understanding of the effects of complex and small scale morphological features on hyporheic exchange processes including the rate and spatio-temporal distribution of reaction hotspots.

Numerical Modeling of CO2 Sequestration within a Five-Spot Well Pattern in the Morrow B Sandstone of the Farnsworth Hydrocarbon Field: Comparison of the TOUGHREACT, STOMP-EOR, and GEM Simulators

The objectives of this study were (1) to assess the fate and impact of CO2 injected into the Morrow B Sandstone in the Farnsworth Unit (FWU) through numerical non-isothermal reactive transport modeling, and (2) to compare the performance of three major reactive solute transport simulators, TOUGHREACT, STOMP-EOR, and GEM, under the same input conditions. The models were based on a quarter of a five-spot well pattern where CO2 was injected on a water-alternating-gas schedule for the first 25 years of the 1000 year simulation. The reservoir pore fluid consisted of water with or without petroleum. The results of the models have numerous broad similarities, such as the pattern of reservoir cooling caused by the injected fluids, a large initial pH drop followed by gradual pH neutralization, the long-term persistence of an immiscible CO2 gas phase, the continuous dissolution of calcite, very small decreases in porosity, and the increasing importance over time of carbonate mineral CO2 sequestration. The models differed in their predicted fluid pressure evolutions; amounts of mineral precipitation and dissolution; and distribution of CO2 among immiscible gas, petroleum, formation water, and carbonate minerals. The results of the study show the usefulness of numerical simulations in identifying broad patterns of behavior associated with CO2 injection, but also point to significant uncertainties in the numerical values of many model output parameters.

A scalable parallel algorithm for reactive particle tracking

Modeling chemical reactions on the basis of probabilistic rules has been employed in Lagrangian schemes to simulate transport of solutes in porous media. Reactive random walk particle tracking allows reagents particle pairs to interact proportionally to their separation distance and the nature of the chemical process. However, most of the available algorithms result to be computationally prohibitive, since their efficiency depends on the number of pairs of particles interacting. In this work, a novel framework for modeling reactive solute transport is introduced, with the target of reducing the computational burden characteristic of existing approaches. Our method introduces an innovative optimal kernel function and permits the parallelization of a bimolecular reaction modeling scheme which leads to computational speedup. To test the performance of our method, we execute a computational speedup analysis with other approaches reported in the literature. The proposed method is successfully validated with exact analytical solutions. We illustrate the applicability of the method to simulate reactive transport in two-dimensional flow fields that are relevant for environmental applications. The computational time of the proposed methodology scales linearly with the number of particles employed in the simulation, feature that makes the algorithm suitable for simulating reactive transport with large number of particles.

Application of upscaling methods for fluid flow and mass transport in multi-scale heterogeneous media: A critical review

Physical and biogeochemical heterogeneity dramatically impacts fluid flow and reactive solute transport behaviors in geological formations across scales. From micro pores to regional reservoirs, upscaling has been proven to be a valid approach to estimate large-scale parameters by using data measured at small scales. Upscaling has considerable practical importance in oil and gas production, energy storage, carbon geologic sequestration, contamination remediation, and nuclear waste disposal. This review covers, in a comprehensive manner, the upscaling approaches available in the literature and their applications on various processes, such as advection, dispersion, matrix diffusion, sorption, and chemical reactions. We enclose newly developed approaches and distinguish two main categories of upscaling methodologies, deterministic and stochastic. Volume averaging, one of the deterministic methods, has the advantage of upscaling different kinds of parameters and wide applications by requiring only a few assumptions with improved formulations. Stochastic analytical methods have been extensively developed but have limited impacts in practice due to their requirement for global statistical assumptions. With rapid improvements in computing power, numerical solutions have become more popular for upscaling. In order to tackle complex fluid flow and transport problems, the working principles and limitations of these methods are emphasized. Still, a large gap exists between the approach algorithms and real-world applications. To bridge the gap, an integrated upscaling framework is needed to incorporate in the current upscaling algorithms, uncertainty quantification techniques, data sciences, and artificial intelligence to acquire laboratory and field-scale measurements and validate the upscaled models and parameters with multi-scale observations in future geo-energy research.

How to use solutions of Advection-Dispersion Equation to describe reactive solute transport through porous media

ResumenLas soluciones de la Ecuación de Advección-Dispersión son usadas frecuentemente para describir el transporte de solutos a través de medios porosos, considerando adsorción en equilibrio, de tipo lineal y reversible. Para indicar algunas sugerencias acerca de este tema, se hizo una revisión de las soluciones analíticas disponibles. Hay soluciones para Problemas con Condiciones de Frontera, de primer y tercer-tipo en la entrada así como de primer y segundo-tipo a la salida. Se analiza el comportamiento de las soluciones equivalentes, para sistemas finitos y semi-infinitos, observando que las soluciones de los sistemas semi-infinitos se aproximan a las correspondientes de los sistemas finitos conforme la condición de frontera de salida en el infinito se aproxima a la ubicación de medición del sistema finito. Solamente se presentan las soluciones analíticas con condiciones de frontera de segundo-tipo a la salida, ya que son iguales a las correspondientes soluciones analíticas con frontera de primer-tipo a la salida, para ambos tipos de condiciones de frontera de entrada usadas. Un análisis paramétrico, basado en el número de Peclet, muestra que todas las soluciones convergen cuando el número de Peclet es mayor que veinte. Los sistemas investigados deben tener un número de Peclet mayor que cinco para usar con confianza las soluciones de la Ecuación de Advección-Dispersión para describir el transporte de soluto en medios porosos.Palabras Clave: Ecuación de Advección-Difusión, Soluciones Analíticas, Transporte de Solutos Reactivos, Medios Porosos.AbstractThe solutions of Advection-Dispersion Equation are frequently used to describe solute transport through porous media when considering lineal and reversible equilibrium adsorption. To notice some warnings about this item, a review of analytical solutions available was done. There are solutions for Boundary Value Problems with first and third-type inlet boundary conditions as well as first and second-type outlet boundary condition. The behavior of equivalent solutions for finite and semi-infinite systems are analyzed, observing that semi-infinite system solutions approximates to the corresponding finite ones as the “infinite” outlet boundary condition approach to the finite measurement location. Because the analytical solutions with a first-type outlet boundary condition are equal to the corresponding analytical solutions with a second-type one, for both inlet boundary condition type used, only the latter is presented. A parametric analysis based on Peclet number shows that all solutions converge for Peclet number greater than twenty. Systems under research must have Peclet number greater than five to use confidently the solutions of Advection-Dispersion Equation to describe reactive solute transport through porous media.Keywords: Advection-Diffusion Equation, Analytical solutions, Reactive Solute Transport, Porous Media.

Modeling Reactive Solute Transport in Permafrost‐Affected Groundwater Systems

AbstractUnderstanding the interactions between ground freeze‐thaw, groundwater flow, and solute transport is imperative for evaluating the fate of contaminants in permafrost regions. However, predicting solute migration in permafrost‐affected groundwater systems is challenging due to the inherent interactions and coupling between subsurface mass and energy transport processes. To this end, we developed a numerical model that considers coupled groundwater flow, subsurface heat transfer, and solute transport, including water‐ice phase change, solute‐dependent porewater freezing, and temperature‐dependent solute reaction rates. As an illustrative example, we present simulations to investigate the potential for contamination from a municipal wastewater lagoon in the Canadian sub‐arctic. Two‐dimensional groundwater models assuming varying permafrost conditions were developed to evaluate possible contaminant migration scenarios associated with groundwater flow from the lagoon to a river, including the transport of conservative, degradable, and sorbing solutes. Model results reveal important transport mechanisms controlling the behavior of aqueous contaminants in permafrost landscapes as well as the hydrogeologic factors affecting reactive transport in cold regions. Seasonal freeze‐thaw episodically restricts connectivity of transport pathways, which attenuates both transport and reaction rates. However, elevated solute concentrations can depress the freezing temperature of porewater and produce thaw‐induced solute transport. Both thermally driven and solute‐enhanced thaw can decrease ice content in permafrost, which can have significant implications for solute migration. Therefore, thermo‐hydrogeologic process controlling reactive transport in cold‐region groundwater systems must be considered when quantitatively assessing the impact of changing environmental conditions on contaminant hydrogeology.

The Semi‐Analytical Solution for Non‐Equilibrium Solute Transport in Dual‐Permeability Porous Media

AbstractThis study presents a conceptual Dual‐Permeability Non‐Equilibrium (DPNE) model that accounts for both physical and chemical non‐equilibria to describe the reactive solute transport through a porous medium. A semi‐analytical solution of the DPNE model is derived in the Laplace domain, which is then numerically inverted to obtain concentrations in different domains at different times and depths. The derived semi‐analytical solution is validated using experimental data and existing analytical (for simple problems) and numerical solutions. The Global Sensitivity Analysis (GSA) is performed to identify model parameters with an impact on nonreactive tracer breakthrough curves (BTC) for a selected data set. A sensitivity‐based calibration of the DPNE model’s parameters is carried out to simulate a BTC with multiple inflection points. The DPNE model can better describe the experimental datasets than the model based on the dual‐porosity (MIM) concept with first‐order mass transfer. Since the DPNE model considers detailed sorption and mass transfer dynamics, it is suitable for process‐based investigations.

Simulation of reactive solute transport in the critical zone: a Lagrangian model for transient flow and preferential transport

Abstract. We present a method to simulate fluid flow with reactive solute transport in structured, partially saturated soils using a Lagrangian perspective. In this context, we extend the scope of the Lagrangian Soil Water and Solute Transport Model (LAST) (Sternagel et al., 2019) by implementing vertically variable, non-linear sorption and first-order degradation processes during transport of reactive substances through a partially saturated soil matrix and macropores. For sorption, we develop an explicit mass transfer approach based on Freundlich isotherms because the common method of using a retardation factor is not applicable in the particle-based approach of LAST. The reactive transport method is tested against data of plot- and field-scale irrigation experiments with the herbicides isoproturon and flufenacet at different flow conditions over various periods. Simulations with HYDRUS 1-D serve as an additional benchmark. At the plot scale, both models show equal performance at a matrix-flow-dominated site, but LAST better matches indicators of preferential flow at a macropore-flow-dominated site. Furthermore, LAST successfully simulates the effects of adsorption and degradation on the breakthrough behaviour of flufenacet with preferential leaching and remobilization. The results demonstrate the feasibility of the method to simulate reactive solute transport in a Lagrangian framework and highlight the advantage of the particle-based approach and the structural macropore domain to simulate solute transport as well as to cope with preferential bypassing of topsoil and subsequent re-infiltration into the subsoil matrix.

Reactive Solute Transport Through Two Contrasting Subterranean Estuary Exit Sites in the Ría de Vigo (NW Iberian Peninsula)

Subterranean estuaries (STEs), where continental groundwaters and saltwaters meet, are zones of intense biogeochemical reactivity. As such, STEs significantly modify groundwater-borne nutrient fluxes to the coastal zone. Thus, evaluating their reactive role is crucial to anticipate impacts of submarine groundwater discharge (SGD) over coastal ecosystems. Here, we studied the nitrogen biogeochemistry of two STEs with contrasting wave-exposure and redox conditions in Panxón and Ladeira beaches (Ría de Vigo, NW Iberian Peninsula). Seasonal surveys were performed at the permanently saturated zone of both beaches during low tide in February, May, July, and October 2019. Sediment was sampled and porewater samples collected using push-pull piezometers. Salinity, 222Rn and 226Ra activities were used to trace water circulation inside each beach. Porewater nitrate, ammonium, nitrite and dissolved oxygen were used to evaluate the role of these STEs as reactive sinks or sources of inorganic nitrogen. Our results showed a marked seasonal variability of water circulation inside both beaches, with strong salinity gradients in February and May and weakened circulation in July and October. The presence of a gravel layer in Panxón beach completely altered the typical structure of STEs by increasing porewater transport and mixing through the beach interior. As a result, Panxón beach profiles were highly enriched in nitrate and oxygen. Conversely, suboxic, and anoxic conditions were prevalent in Ladeira beach during the study period, with ammonium being the prevailing inorganic nitrogen form. High nitrate concentrations occurred associated to the tidal circulation cell during February and May, being the only effective mechanism of sediment oxygenation in Ladeira beach. Although nitrate reduction and production were observed in both STEs, comparison with averaged conservative mixing porewater profiles showed that Ladeira beach acted as a net nitrogen sink whereas Panxón beach acted as a net nitrogen source. The presence of a gravel layer oxygenates the interior of Panxón beach, thus limiting nitrate reduction and promoting the amplification of groundwater-borne nitrogen fluxes to the coast.

Long term assessment of the geochemical integrity of offshore wellbore cement– results from numerical modeling

Oil and gas wells across different offshore fields are currently experiencing plugging and abandonment. With this comes the risk associated with leakage of fluids from wellbores. One area of the long-term integrity of oil and gas wells that has received less attention is the geochemical assessments of cement and cement-rock integrity. This study investigates rock-fluid-cement interactions over long time scales using numerical models of coupled heat and fluid flow and reactive solute transport. Four different kinds of brine compositions from the Gulf of Mexico formations are used to simulate chemical interactions in wellbore plug, cement, and adjacent rock. Porosity changes are used as indicators of the geochemical integrity of the plugging and cement materials. Results demonstrate the impact of temperature, pH, and depth of cement placement in integrity predictions. At greater depths, higher temperatures and pressure will be encountered leading to the precipitation of gypsum and the dissolution of ettringite, calcite, and portlandite which adversely affects the integrity of the wells. The cement-rock interface is a zone of vulnerability and subject to greater loss of integrity. Other modeling parameters from our study which influenced integrity include reactive areas, diffusion coefficients, flux rates, boundary conditions, and the presence of mechanical damage. This study highlights the importance of brine specific geochemical assessments as well as the importance of thermodynamic properties when investigating geochemical wellbore integrity, and the approach used can be applied by practitioners for predictive analysis of geochemical longevity of cement and plugs in subsurface conditions.

Contrasting Raz–Rru stream metabolism and nutrient uptake downstream of urban wastewater effluent sites

Understanding how key stream ecosystem functions respond to wastewater treatment plant effluent is critical for assessing the ability of stream ecosystems to ameliorate anthropogenic nutrient loading and to effectively manage and restore impacted systems. We evaluated instream metabolism, reactive solute transport, and nutrient uptake along two 1.5 to 2-km-long reaches of a 2nd-order stream in the urbanized suburbs of Philadelphia, Pennsylvania, USA, each directly downstream of a wastewater treatment plant outfall. We compared metabolism based on resazurin–resorufin (Raz–Rru) with nutrient uptake and dissolved oxygen (DO) metabolism calculations. Plateau co-injections of the Raz–Rru metabolic tracer system and fluorescein provided integrated stream metabolism measurement. We sampled tracer concentrations hourly along longitudinal profiles and recorded them continuously, along with DO, at 2 discrete locations. The smaller reach 1, characterized by higher nutrient concentrations and canopy cover, had higher short-term transient storage and Raz uptake velocity. In contrast reach 2, with lower nutrient concentrations and less canopy cover, had higher nitrate and phosphorus uptake along with higher rates of gross primary productivity (GPP) and ecosystem respiration (ER). Temporal analysis indicated nitrate uptake increased over the afternoon at reach 2, whereas Raz uptake declined at both reaches. Our results suggest that nutrient uptake and GPP are sensitive to excessive nutrient concentrations and light in our system. In contrast, lower light and higher transient storage are likely driving larger, reach-scale spatial differences in Raz-based ER. Increasing nitrate uptake at reach 2, which lags behind diel DO concentrations, is likely the result of assimilatory N uptake coupled to GPP moderated by nitrification and denitrification, whereas decreasing rates of Raz transformation are likely related to diel variation in heterotrophic uptake. Our ability to resolve sub-daily changes in ER illustrates one of the key advantages of the Raz–Rru tracer system. However, our results also show the need for further investigation into the drivers of sub-daily ecosystem metabolism in streams as well as the mechanistic differences between DO- and Raz-based estimates of ER. Contrasting results from different measures of metabolic activity between reaches and over time highlight the complexity of metabolic processes in high-nutrient systems.

Modelling one-dimensional reactive transport of toxic contaminants in natural rivers

Solute transport models are widely employed to the predict spatio-temporal fate of contaminants in rivers. However, most previous studies have assumed that contaminants spilled are unreactive; they disregard the inherent reactivity of contaminants in water. The Reactive Solute Transport Model (RSTM) (Gooseff et al., 2005) includes lumped decay terms to reflect the decayability of the chemicals, but its applicability was still poor due to dependence on an optimization method. The purpose of this study is to perform reactive transport modelling considering chemicals’ reactivities for sorption, volatilization, and biodegradation. To this end, we manipulated the governing equations of the RSTM and suggested theoretical and empirical methods of estimating the key parameters of the reaction terms. The results showed, for example, that benzene lost 57.7% of its primary mass after being transported 4.54 km downstream due to its high volatility. Also, the arrival time of toluene was delayed by 10.4% due to adsorption.

Study of multispecies convection-dispersion transport equation with variable parameters

Multispecies pollutant migration is most natural phenomenon, frequently occurring in the polluted groundwater system and also evidently occurs in the nuclear repositories and sites having radioactive substances and chemical solvent. Modeling pollutant migration of reactive multispecies problem with variable transport parameters is quite a challenging issue, necessary for better sightedness of fate and transport of reactive solutes and their geochemical growth in the geological formations. At present most of the solutions for multispecies problem are corresponding to the constant parameters affecting the concentration distribution, not for variable transport parameters, because of the complexity of the modelled equation. This paper dealt with the comparative study of multispecies pollutant transport problem in porous structure to analyse the effect of spatially and temporally varying hydrodynamic dispersion coefficient and transport velocity terms in more general form. For mathematical elaboration, two-species transport problem is considered subjected to the concept of linear isotherm with both the parent and daughter species without using identical retardation factor and model is also incorporated with the first-order decay term under spatially varying initial conditions. Homotopy analysis method (HAM) is adopted to develop the series solution for the concentration segmentation of each species of the model. The advantage of this method is that it is capable to overcome the limitation of existing transform methods for some cases of complex modelled problems. The zeroth order deformation equation is derived by using HAM to generate the semi-analytical solution of the model under certain auxiliary linear operator. The impact of distinct retardation factor for different species and effect of diversity in dependency of modelled parameters over space and time are graphically depicted for both the species and illustrated that it has a valuable impact over multispecies pollutant migration. As there exist no analytical or numerical solution for such complex variable dependency of migration parameters over transport phenomenon, these solutions may be helpful to deliver a better significant about the concentration variation due to space and time dependent transport parameters, responsible for reactive multispecies migration, in more natural way.

Effect of bioclogging on the nitrate source and sink function of a hyporheic zone

Nitrogen dynamic in a hyporheic zone (HZ) is mediated by microbes in the form of biofilms attached to sediment grains. Previous studies have neglected the influence of microbial effects on hyporheic nitrate function despite the fact that biofilm-induced clogging can significantly reduce sediment permeability and affect both residence time distribution and biochemical reaction rates. In this study, to assess how the development of bioclogging affects the nitrate source and sink function of HZs, we established a numerical model by coupling water flow, reactive solute transport, and microbial metabolism to simulate nitrogen cycling in a dune-shaped streambed. The classic Damköhler number (DaO2) with an average reaction timescale was not appropriate to determine hyporheic function for the context of biofilm growth induced streambed permeability and biogeochemical reaction rates changing with time. We proposed a new Damköhler number (DaO2∗) with a correction factor (the ratio between current bacteria and initial bacteria) to identify hyporheic function while accounting for the dynamic process of microbial growth. Our results demonstrated that bioclogging layers with aerobic bacteria aggregation acted as a nitrate source system with DaO2∗ ≫ 1, whereas deeper streambed with negligible microbiota survival severed as a nitrate sink with DaO2∗ < 1 when VO2 = 1 h−1. The threshold value of net nitrate source or sink system was influenced by maximum reaction rate of oxygen, and it was in a general range of 1–50 (the range of VO2 was in 1–3 h−1). Threshold transition times from nitrate source to sink were evaluated from the spectrum of flow paths and had strong spatiotemporal variability due to microbial growth. The threshold times decreased over time and stabilized between 1.0 and 3.7 h (for VO2 = 1 h−1) in a dune-shaped streambed when biofilm layers formed.