Transport In Subsurface Environment Research Articles

Intraparticle sorption and desorption of antibiotics

Intraparticle domains are the critical locations for storing contaminants and retarding contaminant transport in subsurface environments. While the kinetics and extent of antibiotics sorption and desorption in subsurface materials have been extensively studied, their behaviors in intraparticle domains have not been well understood. This study investigated the sorption and desorption of antibiotics (ATs) in the intraparticle domains using quartz grains and clay, and antibiotic tetracycline (TC) and levofloxacin (LEV) as examples that are commonly present in groundwater systems. Batch experiments coupled with the analyses using various microscopic and spectroscopic techniques were performed to investigate the sorption and desorption kinetics, and to provide insights into the intraparticle sorption and desorption of TC and LEV. Results indicated that both TC and LEV with different physiochemical properties can migrate into intraparticle domains that were consistent with sorptive diffusion. The rate and extent of the sorption are a function of intraparticle surface area and properties, pore volume and connectivity, and ionic properties of the ATs. The sorptive diffusion led to the slow desorption of both TC and LEV after their sorption, apparently showing an irreversible desorption behavior (with desorption percentage about 1.86–20.51%). These results implied that intraparticle domains can be important locations for storing ATs, retarding ATs transport, and may serve as a long-term secondary source for groundwater contamination.

Experimental Study of the Adsorption of Nitrogen and Phosphorus by Natural Clay Minerals

Nitrogen and phosphorus are commonly recognized as causing eutrophication in aquatic systems, and their transport in subsurface environments has also aroused great public attention. This research presented four natural clay minerals (NCMs) evaluated for their effectiveness of NH4 + and PO4 3- adsorption from wastewater. All the NCMs were fully characterized by X-ray diffraction (XRD), Fourier transform infrared spectroscopy (FTIR), scanning electron microscopy (SEM), BET analysis, and adsorption kinetics and isotherms to better understand the adsorption mechanism-property relationship. The results show that the adsorption efficiency of the four NCMs for phosphate was better than that for ammonia nitrogen. The removal rate of phosphate was higher than 65%, generally in the range of 80%-90%, while the removal rate of ammonia nitrogen was less than 50%. The adsorption kinetic behavior followed the pseudo-second-order kinetic model. The ammonia nitrogen adsorption isotherm was in good agreement with the Freundlich isotherm equilibrium model, and the phosphate adsorption isotherm matched the Langmuir model. Among all the NCMs studied, bentonite (7.13 mg/g) and kaolinite (5.37 mg/g) showed higher adsorption capacities for ammonia nitrogen, while zeolite (0.21 mg/g) and attapulgite (0.17 mg/g) showed higher adsorption capacities for phosphate. This study provides crucial baseline knowledge for the adsorption of nitrogen and phosphate by different kinds of NCMs.

Transport of Colloidal Particles in Microscopic Porous Medium Analogues with Surface Charge Heterogeneity: Experiments and the Fundamental Role of Single-Bead Deposition.

Understanding colloid transport in subsurface environments is challenging because of complex interactions among colloids, groundwater, and porous media over several length scales. Here, we report a versatile method to assemble bead-based microfluidic porous media analogues with chemical heterogeneities of different configurations. We further study the transport of colloidal particles through a family of porous media analogues that are randomly packed with oppositely charged beads with different mixing ratios. We recorded the dynamics of colloidal particle deposition at the level of single grains. From these, the maximum surface coverage (θmax = 0.051) was measured directly. The surface-blocking function and the deposition coefficient (kpore = 3.56 s-1) were obtained. Using these pore-scale parameters, the transport of colloidal particles was modeled using a one-dimensional advection-dispersion-deposition equation under the assumption of irreversible adsorption between oppositely charged beads and colloids, showing very good agreement with experimental breakthrough curves and retention profiles at the scale of the entire porous medium analogue. This work presents a new approach to fabricate chemically heterogeneous porous media in a microfluidic device that enables the direct measurement of pore-scale colloidal deposition. Compared with the conventional curve-fitting method for deposition constant, our approach allows quantitative prediction of colloidal breakthrough and retention via coupling of direct pore-scale measurements and an advection-dispersion-deposition model.

Assessment of artificial neural network models based on the simulation of groundwater contaminant transport

The characterization of contaminant transport in subsurface environments is a pre-requisite for sustainable groundwater use and management. Various analytical and numerical models are generally utilized for such characterization. Analytical models are used for solving simple and idealized pollution transport problems, while numerical models deal with real-world pollution transport simulations. However, in situations where any of the aquifer parameters (such as geological properties, boundary conditions, initial conditions, etc.) are not explicitly defined, use of these models becomes redundant. Simulation of pollution transport under such scenarios becomes challenging. To tackle such problems, researchers generally make use of artificial neural network (ANN) models. Existing literature review reveals that feed-forward backpropagation (FFBNN) models are the most commonly used training method while the applicability of other ANN models has not been appropriately explored. In this study, various ANN models, encompassing supervised and unsupervised neural networks, like, cascade-forward backpropagation (CFBNN), FFBNN, radial basis function (RBFNN), exact radial basis function (ERBFNN) and generalized regression (GRNN), have been developed, and their performances are compared for transport simulation of a conservative pollutant in a two-dimensional hypothetical aquifer. The models reported have pollution injection rates and locations of the sources as input data and the pollution concentrations measured in some water supply wells (or monitoring wells) are considered as target data. Stability of the developed models has been validated by sensitivity analyses. This study reveals that alternative ANN models (other than the ones already reported in literature) can be reliable for simulation of pollutant transport in groundwater systems.

Retracted Article: Influence of aqueous sulfide on speciation of U(vi) adsorbed to nanomagnetite

Heterogeneous reduction of U(vi) by structural Fe2+ and S2− is a key process influencing its fate and transport in subsurface environment.

Impacts of leachates from livestock carcass burial and manure heap sites on groundwater geochemistry and microbial community structure.

We investigated the impacts of leachates from a swine carcass burial site and a cow manure heap on the geochemical and microbiological properties of agricultural water samples, including leachate, groundwater from monitoring wells and background wells, and stream water. The leachate from the livestock burial site showed extremely high electrical conductivity, turbidity, and major ion concentrations, but low redox potential and dissolved oxygen levels. The groundwater in the monitoring wells adjacent to both sites showed severe contamination from the leachate, as indicated by the increases in EC, turbidity, Cl-, and SO42-. Bacteria from the phylum Firmicutes and Bacteriodetes and Archaea from the phylum Euryarchaeota were the major phyla in both the leachates and manure heap. However, the class- or genus-level components of these phyla differed markedly between the leachate and manure heap samples. The relative abundance of Firmicutes decreased from 35% to 0.3~13.9% in the monitoring wells and background wells at both sites. The Firmicutes in these wells was unlikely to have originated from the transportation of leachate to the surrounding environment because Firmicutes genera differed drastically between the leachate and monitoring wells. Meanwhile, sulfate-reducing bacteria (SRB) from the livestock carcass burial site were detected in the monitoring wells close to the leachate. This was likely because the release of carcass decomposition products, such as organic acids, to adjacent areas improved the suitability of the local environments for SRB, which were not abundant in the leachate. This study highlights the need to better understand microbial community dynamics along groundwater flow paths to evaluate bacterial transport in subsurface environments and provides new insights into the effective management of groundwater quality at both farm and regional scales.

The method of distributions for dispersive transport in porous media with uncertain hydraulic properties

AbstractPredictions of solute transport in subsurface environments are notoriously unreliable due to aquifer heterogeneity and uncertainty about the values of hydraulic parameters. Probabilistic framework, which treats the relevant parameters and solute concentrations as random fields, allows for quantification of this predictive uncertainty. By providing deterministic equations for either probability density function or cumulative distribution function (CDF) of predicted concentrations, the method of distributions enables one to estimate, e.g., the probability of a contaminant's concentration exceeding a safe dose. We derive a deterministic equation for the CDF of solute concentration, which accounts for uncertainty in flow velocity and initial conditions. The coefficients in this equation are expressed in terms of the mean and variance of concentration. The accuracy and robustness of the CDF equations are analyzed by comparing their predictions with those obtained with Monte Carlo simulations and an assumed beta CDF.

A preliminary study on sulfate reduction bacteria behaviors in groundwater by sulfur and carbon isotopes: a case study in Jiaozuo City, China.

Inorganic pollutants in groundwater, such as sulfate and nitrate, have been a serious problem in China for decades. These pollutants are difficult to be removed because of their high solubility and ease of transport in subsurface environment. It had been found that microorganism could be one of the most feasible methods for inorganic pollutant elimination. During the process of degradation, some microorganisms can utilize sulfur and nitrogen in sulfate and nitrate forms, respectively, as energy sources. Meanwhile, significant variations of sulfur stable isotope ratios happened. Therefore sulfur isotope can be used as a good indicator for pollutant degradation and microbial activities. Shallow groundwater (SGW), deep groundwater (DGW), and surface water (SFW) were investigated in alluvial plain in Jiaozuo City, China. The results of hydrochemical analysis indicated that K(+), Na(+), and HCO3(-) were dominant ions in DGW, Mg(2+) and HCO3(-) were dominant ions in SGW, and Ca(2+) and HCO3 (-) were dominant in SFW except for LR sample. A wide variation of δ (34)SSO4 values ranging from + 7.3 to +23.6‰ had been observed for all water samples, with a mean value of +20.7, +12.6 and +10.0‰ for DGW, SGW, and SFW respectively. At the same time, δ(13)C values of dissolved inorganic carbon (DIC) ranged from -12.4 to -5.7‰, with a mean value of -7.5, -9.0, and -9.6‰ for DGW, SGW, and SFW, respectively. The microbial degradation processes resulted in significant sulfur isotope fractionations in DGW. Organic carbon was utilized by bacteria and transferred into inorganic carbon, leading to negative fractionation of carbon isotopes. Thus the variations in stable isotope ratios of sulfur and carbon in groundwater can be used as good indicators for understanding of the relationship between bacteria behaviors and sulfate degradation.

Transport of kinetically sorbing solutes in heterogeneous sediments with multimodal conductivity and hierarchical organization across scales

Solute transport in subsurface environments is controlled by geological heterogeneity over multiple scales. In reactive transport characterized by a low Damkohler number, it is also controlled by the rate of kinetic mass transfer. A theory for addressing the impact of sedimentary texture on the transport of kinetically sorbing solutes in heterogeneous porous formations is derived using the Lagrangian-based stochastic methodology. The resulting model represents the hierarchical organization of sedimentary textures and associated modes of log conductivity (K) for sedimentary units through a hierarchical Markov Chain. The model characterizes kinetic sorption using a spatially uniform linear reversible rate expression. Our main interest is to investigate the effect of sorption kinetics relative to the effects of K heterogeneity on the dispersion of a reactive plume. We study the contribution of each scale of stratal architecture to the dispersion of kinetically sorbing solutes in the case of a low Damkohler number. Examples are used to demonstrate the time evolution and relative contributions of the auto- and cross-transition probability terms to dispersion. Our analysis is focused on the model sensitivity to the parameters defined at each hierarchical level (scale) including the integral scales of K spatial correlation, the anisotropy ratio, the indicator correlation scales, and the contrast in mean K between facies defined at different scales. The results show that the anisotropy ratio and integral scales of K have negligible effect upon the longitudinal dispersion of sorbing solutes. Furthermore, dispersion of sorbing solutes depends mostly on indicator correlation scales, and the contrast of the mean conductivity between units at different scales.

Characterization of Environmental Nano- and Macrocolloid Particles Extracted from Selected Soils and Biosolids

Environmental nanoparticles found in soil systems and biosolids may pose a considerable risk to groundwater quality as contaminant carriers. Little effort has been invested in the characterization of natural nanocolloids compared to corresponding macrocolloids. This study involved physicochemical, mineralogical, and morphological characterizations of nanocolloids and macrocolloids fractionated from three Kentucky soils and one biosolid. Particle size and morphology were investigated using scanning/transmission electron microscopy and dynamic light scattering. Mineralogical composition was determined by X-ray diffraction and thermogravimetric and Fourier-transform infrared spectroscopy analyses. Zeta potentials and cation exchange capacities assessed surface charge and chemical reactivity. The estimated average hydrodynamic diameter of nanoparticles was nearly twice the ideal 100 nm range, apparently due to irregular particle shapes and partial aggregation. Nanoparticles were also found attached to surfaces of macrocolloids, forming macro-nano aggregates and obscuring some of their physical and chemical characteristics. However, nanocolloids exhibited greater surface reactivity, likely due to their smaller size, poor crystallinity, and morphological shape distortions. In spite of some behavior modification due to nanoaggregation phenomena, nanocolloids appeared to be much more potent vectors of contaminant transport in subsurface environments than their macrosize fractions. Nevertheless, their heterogeneous nature brings to light important considerations in addressing pollution prevention and remediation challenges.

Herbicide distribution and transport in subsurface environment of a tomato field in the Three Gorges Reservoir area, China

The presence of pesticides via non-point source (NPS) pollution might be a threat to human health and aquatic ecosystems. This study aims to characterize quantitatively the distribution and transport of Linuron in a coupled canopy-soil system by using modelling techniques, viz. an integrated pesticide transport modelling system to simulate three-phase pesticide fate and transport in a tomato field of the Three Gorges Reservoir area. Results indicate that spatio-temporal distributions of Linuron levels in surface soil are dominated by its properties, rainfall, irrigation and its applications. The occurrence and distribution of Linuron in subsurface and deep soil are closely related to the cumulative and delayed effects. Runoff and erosion also play important roles in the environmental transport and fate of Linuron. The results may be useful for taking effective pesticide management measures to control pesticide NPS pollution at watershed scales.

Lab-scale simulation of the fate and transport of nano zero-valent iron in subsurface environments: Aggregation, sedimentation, and contaminant desorption

Heavy metal removal using nano zero-valent iron (NZVI) has drawn growing attention due to the ease of application and high removal efficiency. However, uncertainties regarding its fate and transport in subsurface environments have raised concerns that require further exploration. In this study, aggregation, sedimentation, and Cr/As desorption of three types of NZVIs were investigated under various conditions. It was found that the aggregation behavior of the NZVIs differed from one another in regard to reaction time and ionic strength, associated with the respective critical size for sedimentation. Sedimentation of NZVIs was positively related to the concentrations and average particle sizes. The sedimentation kinetics of NZVI followed two concomitant processes, i.e., (1) direct sedimentation of larger particles, and (2) initial aggregation and then sedimentation of smaller particles. When loaded with Cr/As, NZVIs tended to deposit faster, possibly due to the precipitation of Cr/As onto the nanoparticle surfaces resulting in larger particle sizes. Moreover, desorption of Cr/As from Cr/As loaded NZVIs was detected in the presence of typical groundwater ions, as well as natural organic matter, and poses a potential risk to the subsurface environment. The desorption of Cr was linearly related to the release of iron ions, while As desorption was mitigated when the immobilization of Cr increased.

Special Section on Mass Transport and Environmental Risk Assessment

Special Section on Mass Transport and Environmental Risk Assessment

ChemInform Abstract: In situ Technologies for Remediation of Organic Chemicals-Contaminated Sites: A Critical Review

The paper identifies the basic site assessment information required to define and understand the release incident and the organic chemicals released in the subsurface environment. It evaluates various remediation technologies, based on the characteristics of concerned pollutants and site geologic properties, that may be applicable to cleaning up organic chemicals from surface spills or leaking underground storage tanks. A systematic discussion of organic chemicals transport in subsurface environment enables the user to: develop a conceptual understanding of site conditions before initiation of extensive field investigation, define remediation goals, evaluate technologies capable of meeting remediation goals, and identify monitoring requirements during and after remediation.

A modified Lagrangian-volumes method to simulate nonlinearly and kinetically sorbing solute transport in heterogeneous porous media

Transport in subsurface environments is conditioned by physical and chemical processes in interaction, with advection and dispersion being the most common physical processes and sorption the most common chemical reaction. Existing numerical approaches become time-consuming in highly-heterogeneous porous media. In this paper, we discuss a new efficient Lagrangian method for advection-dominated transport conditions. Modified from the active-walker approach, this method comprises dividing the aqueous phase into elementary volumes moving with the flow and interacting with the solid phase. Avoiding numerical diffusion, the method remains efficient whatever the velocity field by adapting the elementary volume transit times to the local velocity so that mesh cells are crossed in a single numerical time step. The method is flexible since a decoupling of the physical and chemical processes at the elementary volume scale, i.e. at the lowest scale considered, is achieved. We implement and validate the approach to the specific case of the nonlinear Freundlich kinetic sorption. The method is relevant as long as the kinetic sorption-induced spreading remains much larger than the dispersion-induced spreading. The variability of the surface-to-volume ratio, a key parameter in sorption reactions, is explicitly accounted for by deforming the shape of the elementary volumes.

Fate and transport of chlormequat in subsurface environments

Chlormequat (Cq) is a plant growth regulator used throughout the world. Despite indications of possible effects of Cq on mammalian health and fertility, little is known about its fate and transport in subsurface environments. The aim of this study was to determine the fate of Cq in three Danish subsurface environments, in particular with respect to retardation of Cq in the A and B horizons and the risk of leaching to the aquatic environment. The study combines laboratory fate studies of Cq sorption and dissipation with field scale monitoring of the concentration of Cq in the subsurface environment, including artificial drains. For the laboratory studies, soil was sampled from the A and B horizons at three Danish field research stations-two clayey till sites and one coarse sandy site. Adsorption and desorption were described by means of the distribution coefficient (K (d)) and the Freundlich adsorption coefficient (K (F,ads)). The dissipation rate was estimated using soil sampled from the A horizon at the three sites. Half life (DT(50)) was calculated by approximation to first-order kinetics. A total of 282 water samples were collected at the sites under the field monitoring study- groundwater from shallow monitoring screens located 1.5-4.5 m b.g.s. at all three sites as well as drainage water from the two clayey sites and porewater from suction cups at the sandy site, in both cases from 1 m b.g.s. The samples were analysed using LC-MS/MS. The field monitoring study was supported by hydrological modelling, which provided an overall water balance and a description of soil water dynamics in the vadose zone. The DT(50) of Cq from the A horizon ranged from 21 to 61 days. The Cq concentration-dependant distribution coefficient (K (d)) ranged from 2 to 566 cm(3)/g (median 18 cm(3)/g), and was lowest in the sandy soil (both the A and B horizons). The K (F,ads) ranged from 3 to 23 (microg(1 - 1/n ) (cm(3))(1/n) g(-1)) with the exponent (1/n) ranging from 0.44 to 0.87, and was lowest in the soil from the sandy site. Desorption of Cq was very low for the soil types investigated (<10%w). Cq in concentrations exceeding the detection limit (0.01 microg/L) was only found in two of the 282 water samples, the highest concentration being 0.017 microg/L. That sorption was highest in the clayey till soils is attributable to the composition of the soil, the soil clay and iron content being the main determinant of Cq sorption in both the A and B horizons of the subsurface environment. Cq was not detected in concentrations exceeding the detection limit in either the groundwater or the porewater at the sandy site. The only two samples in which Cq was detected were drainage water samples from the two clayey till sites. The presence of Cq here was probably attributable to the hydrogeological setting as water flow at the two clayey till sites is dominated by macropore flow and less by the flow in the low permeability matrix. In contrast, water flow at the sandy site is dominated by matrix flow in the high permeability matrix, with negligible macropore flow. Given the characteristics of these field sites, Cq adsorption and desorption can be expected to be controlled by the clay composition and content and the iron content. Combining these observations with the findings of the sorption and dissipation studies indicates that the key determinant of Cq retardation and fate in the soil is sorption characteristics and bioavailability. The leaching risk of Cq was negligible at the clayey till and sandy sites investigated. The adsorption and desorption experiments indicated that absorption of Cq was high at all three sites, in particular at the clayey till sites, and that desorption was generally very limited. The study indicates that leaching of Cq to the groundwater is hindered by sorption and dissipation. The detection of Cq in drainage water at the clayey till sites and the evidence for rapid transport through macropores indicate that heavy precipitation events may cause pulses of Cq. The present study is the first to indicate that the risk of Cq leaching to the groundwater and surface water is low. Prior to any generalisation of the present results, the fate of Cq needs to be studied in other soil types, application regimes and climatic conditions to determine the Cq retardation capacity of the soils. The study identifies bioavailability and heavy precipitation events as important factors when assessing the risk of Cq contamination of the aquatic environment. The possible effects of future climate change need to be considered when assessing whether or not Cq poses an environmental risk.

Effect of bacterial adsorption on low frequency electrical properties of clean quartz sands and iron‐oxide coated sands

Low frequency electrical measurements (0.1–1000 Hz) were conducted to investigate the adsorption effect of Pseudomonas aeruginosa cells onto clean quartz sands and iron‐oxide coated sands. The clean quartz sands showed a gradual increase in the microbial adsorption to mineral grains, concurrent with an increase of 13% in the imaginary conductivity component (σ″). However, iron‐oxide coated sands (20–100% by weight) showed a rapid increase in microbial adsorption with σ″ reaching a maximum of 37 % for the 80–100% iron coated sands. No significant changes were observed in the real conductivity component (σ′) due to microbial adsorption. A power law dependency was observed between the adsorbed cells and σ″. We suggest that the polarization results from the increase in the surface roughness and surface area of the grain due to bacteria sorption. These results suggest that low frequency electrical measurements can play an important role in assessing microbial transport in subsurface environments.

Microbial nanowires: Is the subsurface “hardwired”?

The Earth's shallow subsurface results from integrated biological, geochemical, and physical processes. Methods are sought to remotely assess these interactive processes, especially those catalysed by micro‐organisms. Using saturated sand columns and the metal reducing bacterium Shewanella oneidensis MR‐1, we show that electrically conductive appendages called bacterial nanowires are directly associated with electrical potentials. No significant electrical potentials were detectable in columns inoculated with mutant strains that produced non‐conductive appendages. Scanning electron microscopy imaging revealed a network of nanowires linking cells‐cells and cells to mineral surfaces, “hardwiring” the entire length of the column. We hypothesize that the nanowires serve as conduits for transfer of electrons from bacteria in the anaerobic part of the column to bacteria at the surface that have access to oxygen, akin to a biogeobattery. These results advance understanding of the mechanisms of electron transport in subsurface environments and of how microorganisms cycle geologic material and share energy.

Influence of biofilms on the movement of colloids in porous media. Implications for colloid facilitated transport in subsurface environments

Colloid transport through porous media can be influenced by the presence of biofilms. Sterile and non-sterile sand columns were investigated using Laponite RD as model colloid and a highly mucoid strain of Pseudomonas aeruginosa as model biofilm former. Laponite RD was marked specifically by fluorescent complexes with rhodamine 6G. Breakthrough curves (BTCs) were used as parameters for determination of colloid transport characteristics. In the sterile columns, the colloid was mobile (collision efficiencies from 0.05 to 0.08) both after the presence of Na + and Ca 2+ ions followed by deionised water influent. In the biofilm-grown column, the same treatment did not result in colloid retention in the case of Na + exposure, but in altered or enhanced colloid transport. In the case of Ca 2+ ions exposure, colloid retention increased with biofilm age. After 3 weeks, almost complete retention was observed. Similar observations were made in columns packed with material from slow sand filtration units. These data reveal the complex interactions between biofilms, cations and colloid transport. Changes in the electrolyte composition of water percolating the subsurface can frequently occur and will result in different colloid transport characteristics with regard to the dominating species of ions and the relative abundance of microbial biofilms. This has to be considered when modelling colloid transport through the subsurface.

Introduction to special section on Colloid Transport in Subsurface Environments

The Water Resources Research special section on Colloid Transport in Subsurface Environments presents new knowledge that is critical to solving problems related to groundwater pollution by microbial pathogens and hazardous chemicals. This introduction to the special section surveys fourteen manuscripts that advance current understanding of the transport of biocolloids (e.g., bacteria, viruses, and protozoa), mineral colloids, and colloid‐associated contaminants in the vadose zone and in groundwater. These papers present new techniques for elucidating mechanisms that govern colloid mobility, propose mathematical models appropriate for quantifying colloid and colloid‐associated contaminant transport, and report pore‐scale and column‐scale observations requisite for evaluating these models. Together, the papers of this special section illuminate the complexity of the colloid transport problem and describe progress toward understanding this complexity.