Miscible Displacement Experiments Research Articles

Effect of wildfires on mobility of metribuzine herbicide in agricultural soil

Metribuzine has been used extensively for controlling wide range of broadleaf weeds and grasses, which can lead to severe contamination of soil and groundwater. Wildfires are an important process in environment, which can be severe for many areas and could have a significant impact on the properties of the soil. The objective of this study was to investigate the effect of wildfires for the fate and transport of atrazine for agricultural soil. Miscible displacement experiments were used for the objectives. Results showed that the average distribution coefficient (Kd) obtained from metribuzine was 0.28 and 0.13 L/Kg for soils prior to and after wildfire, respectively. Results indicate that organic matter content of soil has most likely played major role on sorption of metribuzine and exhibited rate-limited sorption-desorption. In addition, sorption of metribuzine onto soil after wildfire was significantly lower which could lead to more contamination risk of groundwater resources. The results from the present study would also help in designing of effective herbicide management strategies for contaminated sites.

Insights into the synergistic effects between deposited Fe oxides and dissolved organic matter in influencing perfluorooctanoic acid transport in saturated porous media

Perfluorooctanoic acid (PFOA) transport in the subsurface environment is relevant to drinking water safety, while the compounding effects of soil components on PFOA migration are poorly understood. Laboratory miscible-displacement experiments were conducted using saturated sand columns to explore how metal oxide surfaces and dissolved organic matter (DOM) jointly affect PFOA transport in porous media. Retardation factors indicated that Fe oxide coating inhibited PFOA migration due to electrostatic interaction. However, PFOA recovery rates changed insignificantly, decreasing by less than 4 % when the proportion of Fe oxide-coated sand reached 50 %. DOM (1 mg/L humic acid) in the pore water slightly decreased PFOA recovery rates (by about 10 %) in quartz sand, indicating the effect of hydrophobic interaction on PFOA migration. When the PFOA solution containing 1 mg/L humic acid was injected into the column containing Fe oxide-coated sand, PFOA recovery was significantly decreased by nearly 20 %, and the retardation factor was more than doubled. This could be attributed to the stronger hydrophobic effect provided by the higher DOM deposition on the Fe oxide surface. These results, supported by SEM-EDS, zeta potential, and model fitting data, highlight the microscopic mechanisms by which interactions between metal oxides and DOM influence PFOA transport. However, this inhibitory effect disappeared at higher humic acid concentrations (20 mg/L), indicating the risk of PFOA re-migration when the DOM concentration greatly exceeds the adsorption capacity of the media for it. The findings of this work have implications for predicting or controlling the environmental risks of PFOA in soil and groundwater.

Effect of the phosphate and mineralogical composition on the movement and mineralization of glyphosate in clay soils

Glyphosate is one of the most used herbicides worldwide. In rice paddy fields, it is usually applied for weed control during the pre-planting stage. Phosphate fertilizers may enhance herbicide displacement in the soil matrix. The objective of this study was to assess the effect of monoamoniun phosphate and the mineralogical composition on the movement and mineralization of glyphosate in clay soils (CS1; CS2 and CS3) in Colombia. Glyphosate miscible displacement experiments were performed in disturbed soil columns, with and without the addition of phosphate after the application of a pulse of N-(phosphonomethyl-14C) glycine. Simultaneously, 14C-glyphosate mineralization was measured indirectly by quantifying the amount 14C–CO2 released daily. At the end of the experiment, the columns were divided into six horizontal sections and glyphosate-bound residues were determined in the soil. The addition of phosphate decreased glyphosate retention time (in CS1 and CS2) and increased the total leached amount only in CS1 soil. Overall, more than 95% of the applied glyphosate was retained in the soil columns. Glyphosate mineralization half-life adjusted to a bi-exponential model, implying that one fraction degrades rapidly due to being more bioavailable, and the other fraction presents a slow rate of degradation and, that although high contents of kaolinite clays are important in the adsorption and translocation of the herbicide, the presence of calcites and divalent cations modify this process, favoring the persistence of the molecule in the soil. Glyphosate partitions into an easily degradable fraction and a more recalcitrant fraction adsorbed to kaolinite clays, calcites, and divalent cations. This fraction is less available for biodegradation thus favoring glyphosate persistence in soil.

New turbulent flow and eddy dispersion mechanism to interpret dispersivity in homogeneous soils in miscible displacement experiments

New turbulent flow and eddy dispersion mechanism to interpret dispersivity in homogeneous soils in miscible displacement experiments

Miscibility of light oil and flue gas under thermal action

The miscibility of flue gas and different types of light oils is investigated through slender-tube miscible displacement experiment at high temperature and high pressure. Under the conditions of high temperature and high pressure, the miscible displacement of flue gas and light oil is possible. At the same temperature, there is a linear relationship between oil displacement efficiency and pressure. At the same pressure, the oil displacement efficiency increases gently and then rapidly to more than 90% to achieve miscible displacement with the increase of temperature. The rapid increase of oil displacement efficiency is closely related to the process that the light components of oil transit in phase state due to distillation with the rise of temperature. Moreover, at the same pressure, the lighter the oil, the lower the minimum miscibility temperature between flue gas and oil, which allows easier miscibility and ultimately better performance of thermal miscible flooding by air injection. The miscibility between flue gas and light oil at high temperature and high pressure is more typically characterized by phase transition at high temperature in supercritical state, and it is different from the contact extraction miscibility of CO2 under conventional high pressure conditions.

Sorption and transport of atrazine for stable manure-amended agricultural soil from Kahramanmaraş, Turkey

ABSTRACT This study investigated how adding stable manure affected the transport of atrazine within agricultural soils from Kahramanmaraş, Turkey. Based on the batch and miscible-displacement experiments, it was found that atrazine sorption was nonlinear, following a Freundlich isotherm, and was rate-limited and reversible for both the non-amended and amended soil. The addition of manure resulted in higher retardation of atrazine, with R values of 10.1 to 19.1, compared to values of 3.2 to 4.3 for the non-amended soil. The KF values for stable-manure amended soil calculated from batch experiment was three times higher than non-amended soil. The transportation of atrazine shows non-ideal behavior, characterized by significant elution tailing at low concentrations due to the organic carbon content of the soil. This effect was more prominent in soil with a 10% stable-manure amendment. By incorporating non-linear sorption, a two-site transport modeling approach provides a reasonably accurate representation of the atrazine breakthrough curves. The distinct sorption-desorption characteristics of atrazine have significant implications for risk assessment of atrazine-contaminated soil and should be considered in the regulation, management, and remediation of such contaminated sites.

Study on Enhanced Oil Recovery Mechanism of CO2 Miscible Flooding in Heterogeneous Reservoirs under Different Injection Methods.

The mechanism by which oil recovery can be enhanced by different injection methods of CO2 miscible flooding to alleviate the influence of heterogeneous reservoirs was studied to optimize the injection methods, further improving oil recovery. According to the reservoir heterogeneity characteristics of the TI oil formation group in Lunnan Oilfield, a 1 m double-layer long core was designed and prepared for four CO2 miscible displacement experiments with different injection methods. By analyzing the variation in the injection-production parameters, the displacement effects of different injection methods were compared, and the mechanism of enhanced oil recovery (EOR) was summarized. The results indicate the following. ① The displacement efficiency of the different injection methods lies in the following order. Alternate CO2-water injection, continuous CO2 flooding, cyclic CO2 flooding, and alternate CO2-hydrocarbon gas injection. ② The recovery of crude oil via CO2 miscible flooding in heterogeneous reservoirs relies on both convective diffusion and miscible mass transfer. Convective diffusion depends mainly on control of the displacement pressure differential and the plugging of preferential seepage channels in high-permeability areas, while miscible mass transfer depends mainly on the swept range of the convective diffusion and the degree of miscibility between CO2 and crude oil. ③ To improve the recovery efficiency of CO2 miscible flooding in heterogeneous reservoirs, it is necessary to choose an injection method that optimizes these two aspects.

Modeling the competitive sorption and transport of Ni(II) and Zn(II) in soils: Comparing two multicomponent approaches

The mobility of contaminants in soil is controlled by sorption reactions which can be affected by the presence of other solutes that compete for sorption sites. The ability to model such effects is necessary for evaluating the environmental risk of a given contaminant. In this study, the competitive sorption and transport of nickel (Ni) and zinc (Zn) in Olivier and Windsor soils was investigated using batch equilibration and miscible displacement experiments. During batch experiments, the sorption of Ni and Zn was mutually reduced in multicomponent systems, indicating that the metal cations compete for sorption sites. When applied concurrently, the retardation of both ions decreased and peak effluent concentrations increased relative to single ion experiments, demonstrating that competition increased the mobility of both ions during miscible displacement experiments. A novel Freundlich-type multicomponent isotherm (CDI) and its kinetic analog (CDIT) were developed and compared to the commonly used SRS isotherm and SRS-based kinetic approach (SRST) in describing the experimental data. The CDI provided a superior description of the competitive batch data, especially at low surface coverage, and may therefore be more applicable to multicomponent sorption than the SRS. The Olivier and Windsor transport data were best described by the CDIT and SRST, respectively, however, both models generally described the data well. Since both approaches gave comparable descriptions of the transport data while the CDI outperformed the SRS in describing the batch data, the CDI/CDIT may be more generally applicable to multicomponent systems and warrants further study.

Crossover from diffusive to convective regimes during miscible displacements in 2D porous media

Miscible displacements are involved in many applications, including enhanced oil recovery and groundwater remediation. When a less viscous and miscible fluid displaces another more viscous one, an unstable front occurs, and its evolution is controlled by the interplay between diffusion and convection. Much of the research has focused on the instability of the mixing zone front, but the crossover of flow regimes during miscible flooding remains unclear. Here, we conduct unstable miscible displacement experiments in rectangular 2D microfluidic models with controlled pore-scale disorder under various flow rates. By quantifying the features of mixing zone and sweep efficiency, we observe and characterize the crossover from diffusive regime to convective regime during flooding. We further study the effects of the flow rate and pore-scale disorder on the crossover time. We show that the increase in flow rate results in an earlier crossover by promoting the convection in the horizontal direction. Moreover, the increasing pore-scale disorder also results in an earlier crossover by inducing a more unstable mixing zone front. We then perform scaling analysis of the crossover time by introducing the coefficient of variation that represents the heterogeneity of the flow geometry. We establish a unified relationship between the scaled crossover time and the Péclet number, which provides a useful approach to predict the crossover time under various conditions. Our work reveals the crossover between flow regimes and improves our understanding of the front evolution during unstable miscible displacement.

The Effect of Viscosity Ratio and Peclet Number on Miscible Viscous Fingering in a Hele-Shaw Cell: A Combined Numerical and Experimental Study

The results from a series of well characterised, unstable, miscible displacement experiments in a Hele-Shaw cell with a quarter five-spot source-sink geometry are presented, with comparisons to detailed numerical simulation. We perform repeated experiments at adverse viscosity ratios from 1 to 20 and Peclet numbers from 10\(^4\) to 10\(^6\) capturing the transition from 2D to 3D radial fingering and experimental uncertainty. The open-access dataset provides time-lapse images of the fingering patterns, transient effluent profiles, and meta-information for use in model validation. We find the complexity of the fingering pattern increases with viscosity ratio and Peclet number, and the onset of fingering is delayed compared to linear displacements, likely due to Taylor dispersion stabilisation. The transition from 2D to 3D fingering occurs at a critical Peclet number that is consistent with recent experiments in the literature. 2D numerical simulations with hydrodynamic dispersion and different mesh orientations provide good predictions of breakthrough times and sweep efficiency obtained at intermediate Peclet numbers across the range of viscosity ratios tested, generally within the experimental uncertainty. Specific finger wavelengths, tip shapes, and growth are hard to replicate; model predictions using velocity-dependent longitudinal dispersion or simple molecular diffusion bound the fingering evolution seen in the experiments, but neither fully capture both fine-scale and macroscopic measures. In both cases, simulations predict sharper fingers than the experiment. A weaker dispersion stabilisation seems necessary to capture the experimental fingering at high viscosity ratio, which may also require anisotropic components. 3D models with varying dispersion formulations should be explored in future developments to capture the full range of effects at high viscosity ratio and Peclet number.

A screening model for quantifying PFAS leaching in the vadose zone and mass discharge to groundwater

A growing body of site investigations have demonstrated that vadose zones serve as significant long-term sources of PFAS to groundwater. Quantifying PFAS leaching in the vadose zone and mass discharge to groundwater is therefore critical for characterizing, managing, and mitigating long-term contamination risks. Mathematical models representing the PFAS-specific transport and retention processes, including surfactant-induced flow, and rate-limited, nonlinear adsorption at solid–water and air–water interfaces, have been recently developed. While these advanced models provide fundamental insights into the primary processes controlling the long-term retention of PFAS, they are less suitable for screening-type applications due to significant computational cost and the requirement for detailed input parameters. To address this knowledge gap, we develop a simplified model by assuming steady-state infiltration and linear solid-phase and air–water interfacial adsorption; a two-domain model is used to represent kinetic solid-phase adsorption. We derive novel analytical solutions for the simplified model allowing for arbitrary initial conditions. The newly derived analytical solutions are then validated by application to miscible-displacement experiments under a wide range of conditions and by comparisons to a state-of-the-art comprehensive model under both experimental and field conditions applicable to PFAS-contamination sites. Overall, the simplified analytical model provides an efficient and accurate screening-type tool for quantifying long-term PFAS leaching in the vadose zone.

Sucralose as an oxidative-attenuation tracer for characterizing the application of in situ chemical oxidation for the treatment of 1,4-dioxane.

In situ chemical oxidation (ISCO) has become a widely used soil and groundwater remediation method. Oxidative-attenuation tracers can be used to provide real-time, explicit delineation of contaminant mass-transfer and transformation behavior during an ISCO remediation project. The objective of this study was to evaluate the potential of employing sucralose, a widely used artificial sweetener, as an oxidative-attenuation tracer to characterize the remediation efficiency of 1,4-dioxane (dioxane) by persulfate-based ISCO. Batch and miscible-displacement experiments were conducted to examine the degradation rate and transport behavior of sucralose compared to that of dioxane. Comparable magnitudes and rates of degradation were observed for sucralose and dioxane in batch-reactor experiments with soil and persulfate. The breakthrough curves of sucralose and dioxane transport in a soil-packed column were coincident. The retardation factors were 1.1 for both compounds, indicating limited sorption for both sucralose and dioxane by the soil. Limited degradation was observed in the miscible-displacement experiments, consistent with the short residence time compared to the half-lives of sucralose and dioxane. Persulfate transport and decomposition behavior in the soil-packed columns was similar in the presence of sucralose or dioxane. A simulated tracer test was conducted to illustrate the application of sucralose as an oxidative-attenuation tracer at the pilot scale. These results demonstrate the potential of sucralose as an oxidative-attenuation tracer to support the robust design of ISCO applications for dioxane. The oxidative-attenuation tracer test method is anticipated to be an effective approach for characterizing mass-removal behavior of other emerging contaminants with appropriate selection of tracer.

Air-water interfacial areas relevant for transport of per and poly-fluoroalkyl substances

Per and polyfluoroalkyl substances (PFAS) present in the soil pose a long-term threat to groundwater. Robust characterization and modeling of PFAS retention and transport in unsaturated systems requires an accurate determination of the magnitude of air-water interfacial area (AWIA). Multiple methods are available for measuring or estimating air-water interfacial area, including x-ray microtomography (XMT), various aqueous and gas-phase interfacial tracer-test (ITT) methods, and thermodynamic-based estimation methods. AWIAs determined with the different methods can vary significantly. Therefore, it is critical to determine which measurement methods are relevant for application to PFAS retention and transport. This is achieved by employing AWIAs determined with different methods to simulate the results of miscible-displacement experiments reported in the literature for the transport of perfluorooctanoic acid (PFOA) in an unsaturated quartz sand. Measured PFOA breakthrough curves were successfully predicted using AWIA values measured by aqueous ITT methods. Conversely, AWIAs measured with the XMT method and estimated with the thermodynamic method under-predicted the magnitude of retardation and could not successfully simulate the measured transport data. These results indicate that the ITT method appears to provide the most appropriate AWIA values for robust characterization and modeling of PFAS transport in unsaturated systems. The long-term impact of employing different AWIA values on PFOA leaching in the vadose zone was simulated for a representative AFFF application scenario. The predicted timeframes for PFOA migration to groundwater varied from 3 to 6 to 20 years depending on which AWIA was used in the simulation. These relatively large differences would result in significantly different risk-assessment outcomes. These results illustrate that it is critical to employ the AWIA that is most representative of PFAS retention for accurate predictions of PFAS leaching in the vadose zone.

The impact of multiple-component PFAS solutions on fluid-fluid interfacial adsorption and transport of PFOS in unsaturated porous media

The objective of this research was to investigate the impact of multiple-component PFAS solutions on the retention of PFOS during transport in unsaturated porous media. Surface tensions were measured to characterize the impact of co-PFAS on the surface activity of PFOS. Miscible-displacement experiments were conducted to examine the air-water interfacial adsorption of PFOS during transport in single and multi-PFAS systems. Literature data for transport of PFOS in NAPL-water systems were also investigated for comparison. A mathematical model incorporating surfactant-induced flow, nonlinear rate-limited sorption, nonlinear rate-limited fluid-fluid interfacial adsorption, and competitive adsorption at the fluid-fluid interface was used to simulate the transport of PFOS. The results indicate that the presence of co-PFAS had no measurable impact on solid-phase sorption of PFOS during transport under the extant conditions of the experiments. Conversely, the air-water interfacial adsorption of PFOS was decreased by the presence of co-PFAS during transport under unsaturated-flow conditions for relatively high input concentrations. The multiple-component Langmuir model could not predict the competitive adsorption behavior observed during transport. Conversely, competitive interactions were not observed for transport with a lower input concentration. The results indicate that the retention and transport of individual PFAS in mixtures may in some cases be impacted by the presence of co-PFAS due to competitive fluid-fluid interfacial adsorption effects. Reduced retention due to competitive interfacial-adsorption interactions has the potential to decrease PFOS retardation during transport, thereby increasing migration rates in sources zones and enhancing groundwater-pollution risks. SynopsisThe impact of PFAS mixtures on the retention and transport of PFOS in unsaturated porous media is examined with a series of experiments and mathematical modeling.

The Co-Transport of PFAS and Cr(VI) in porous media

PFAS and Cr are present at some sites as co-contaminants. The objective of this research was to investigate the co-transport behavior of per- and polyfluoroalkyl substances (PFAS) and hexavalent chromium (Cr(VI)) in porous media. Miscible-displacement experiments were conducted using two soils and an aquifer sediment with different geochemical properties. Perfluorooctane sulfonic acid (PFOS) and perfluorooctanoic acid (PFOA) were employed as model PFAS. The retardation of PFOS was decreased in the presence of Cr(VI). Conversely, the transport and retardation of PFOA was not affected by the presence of Cr(VI). The reduction of PFOS retardation caused by Cr(VI) is likely due to sorption competition for both organic-carbon and inorganic (metal-oxides and clay minerals) domains. The relative contributions of the three soil constituents to PFOS sorption and the potential for competition between PFOS and Cr(VI) is a function of the geochemical composition of the porous media (i.e., organic carbon, metal-oxides and clay minerals). The PFAS had minimal impact on the retention and transport of Cr(VI). To our knowledge, the results presented herein represent the first reported data for PFOS and Cr(VI) co-transport in porous media. The results of this study indicate that the presence of Cr(VI) has the potential to increase the migration potential of PFOS in soil and groundwater, which should be considered when characterizing electroplating facilities, leather tanning facilities, and other co-contaminated sites.

Transport of GenX in Saturated and Unsaturated Porous Media.

The objective of this research was to investigate the retention and transport behavior of GenX in five natural porous media with similar median grain diameters but different geochemical properties. Surface tensions were measured to characterize surface activity. Miscible-displacement experiments were conducted under saturated conditions to characterize the magnitude of solid-phase adsorption, while unsaturated-flow experiments were conducted to examine the impact of air-water interfacial adsorption on retention and transport. The results from surface-tension measurements showed that the impact of solution composition is greater for the ammonium form of GenX than for the acid form, due to the presence of the NH4 counterion. The breakthrough curves for the experiments conducted under saturated conditions were asymmetrical, and a solute-transport model employing a two-domain representation of nonlinear, rate-limited sorption provided reasonable simulations of the measured data. The magnitudes of solid-phase adsorption were relatively small, with the highest adsorption associated with the medium containing the greatest amount of metal oxides. The breakthrough curves for the experiments conducted under unsaturated conditions exhibited greater retardation due to the impact of adsorption at the air-water interface. The contributions of air-water interfacial adsorption to GenX retention ranged from ∼24% to ∼100%. The overall magnitudes of retardation were relatively low, with retardation factors < ∼3, indicating that GenX has significant migration potential in soil and the vadose zone. To our knowledge, the results presented herein represent the first reported data for solid-water and air-water interfacial adsorption of GenX by soil. These data should prove useful for assessing the transport and fate behavior of GenX in soil and groundwater.

Effect of anisotropy on solute transport in degraded fen peat soils

AbstractPeat soils are heterogeneous, anisotropic porous media. Compared to mineral soils, there is still limited understanding of physical and solute transport properties of fen peat soils. In this study, we aimed to explore the effect of soil anisotropy on solute transport in degraded fen peat. Undisturbed soil cores, taken in vertical and horizontal direction, were collected from one drained and one restored fen peatland both in a comparable state of soil degradation. Saturated hydraulic conductivity (K s) and chemical properties of peat were determined for all soil cores. Miscible displacement experiments were conducted under saturated steady state conditions using potassium bromide as a conservative tracer. The results showed that (1) the K s in vertical direction (K sv) was significantly higher than that in horizontal direction (Ksh), indicating that K s of degraded fen peat behaves anisotropically; (2) pronounced preferential flow occurred in vertical direction with a higher immobile water fraction and a higher pore water velocity; (3) the 5% arrival time (a proxy for the strength of preferential flow) was affected by soil anisotropy as well as study site. A strong correlation was found between 5% arrival time and dispersivity, K s and mobile water fraction; (4) phosphate release was observed from drained peat only. The impact of soil heterogeneity on phosphate leaching was more pronounced than soil anisotropy. The soil core with the strongest preferential flow released the highest amount of phosphate. We conclude that soil anisotropy is crucial in peatland hydrology but additional research is required to fully understand anisotropy effects on solute transport.

Simulating PFAS transport influenced by rate-limited multi-process retention

The transport of per- and poly-fluoroalkyl substances (PFAS) in the vadose zone is complicated by the fact that multiple mass-transfer processes can contribute to their retention and retardation. In addition, PFAS transport at some sites can be further complicated by the presence of organic immiscible liquids (OIL). Mass-transfer processes are inherently rate limited and, therefore, have the potential to cause nonideal transport of PFAS. The objectives of this research were to: (1) develop a solute-transport model that explicitly accounts for multiple retention processes, including adsorption at air-water and OIL-water interfaces, adsorption by the solid phase, and diffusive mass-transfer between advective and nonadvective domains, and (2) apply the model to measured transport data to delineate which processes are rate limited and contribute to observed nonideal transport. Breakthrough curves for transport of two PFAS and one hydrocarbon surfactant in sand obtained from prior miscible-displacement experiments exhibited nonideal transport. The multiprocess model effectively simulated the measured transport data. The results of the analyses indicate that adsorption at the air-water and OIL-water interface can generally be treated as effectively instantaneous for transport in porous media. The rate limitations associated with solid-phase adsorption and diffusive mass transfer between advective and nonadvective domains were of greater significance.

Mobility of 2,4-Dichlorophenoxyacetic Acid, Glyphosate, and Metribuzine Herbicides in Terra Rossa-Amended Soil: Multiple Approaches with Experimental and Mathematical Modeling Studies

2,4-Dichlorophenoxyl acetic acid (2,4-D), glyphosate, and metribuzine are used extensively as an herbicide in the vicinity of Antalya, Turkey. The objective of this study was to investigate the mobility of such herbicides in terra rossa-amended soil by multiple approaches with batch, miscible-displacement, and mathematical modeling studies. The results indicated that the sorption isotherms exhibited predominantly linear behavior over a wide range (0–100 mg/L) of selected herbicides. The average distribution coefficient (Kd) obtained from linear sorption isotherm was 0.95, 1.20, and 44.15 L/kg for 2,4-D, metribuzine, and glyphosate, respectively. After hydrogen peroxide treatment for eliminating organic carbon content in the soil, the average distribution coefficient (Kd) obtained from the linear sorption isotherm was 0.096, 0.118, and 4.37 L/kg for 2,4-D, metribuzine, and glyphosate, respectively. Miscible-displacement experiments also showed that the sorption of 2,4-D, metribuzine, and glyphosate was described by linear and rate-limited processes in the soil. Transport of metribuzine and 2,4-D was slightly but glyphosate was extensively delayed compared to non-reactive tracer experiments in the soil. Hydrogen peroxide treatment experiments also indicated that the organic carbon content was responsible for the non-ideal sorption behavior of such herbicides. The rapid desorption can be attributed to soft carbon fraction (humic/fluvic acid and lipids), whereas slower desorption can be responsible by hard carbon fraction (black carbon, kerogen) of soils that led to chemically non-ideal behavior. Two-site transport modeling analyses, including linear sorption and rate-limited sorption–desorption, provided a reasonably good match to the herbicide breakthrough curves indicating mathematical modeling results were generally consistent with the results obtained from the batch and column experiments. As a result, sorption of 2,4-D and metribuzine onto terra rossa-amended soil was low which could lead to contamination risk of karstic groundwater resources overlying the Antalya travertine plateau in the region. On the other hand, glyphosate exhibits much more sorption affinity in the soil and leaching risk is limited as with more delay. As a result, investigating the mobility of such herbicides in top soils overlying the karstic aquifer systems in Antalya would be very important to identify the contamination risk of groundwater resources. The results from the present study would also help in the design of effective herbicide management strategies in Antalya, Turkey.

Nonideal Transport and Extended Elution Tailing of PFOS in Soil

The objective of this research was to examine the influence of nonideal sorption/desorption on the transport of polyfluorinated alkyl substances (PFASs) in soil, with a specific focus on characterizing and quantifying potential extended, mass-transfer-limited elution behavior. Perfluorooctane sulfonic acid (PFOS) was used as a representative PFAS, and miscible-displacement experiments were conducted with two soils comprising contrasting geochemical properties. The influence of nonlinear, rate-limited, hysteretic, and irreversible sorption/desorption on transport was investigated through experiments and model simulations. The breakthrough curves measured for PFOS transport in the two soils were asymmetrical and exhibited extensive elution tailing, indicating that sorption/desorption was significantly nonideal. The widely used two-domain sorption kinetics model could not fully simulate the observed transport behavior, whereas a multirate model employing a continuous distribution of sorption domains was successful. The overall results indicated that sorption/desorption was significantly rate-limited and that nonlinear, hysteretic, and irreversible sorption/desorption had minimal impact on PFOS transport. Comparison of PFOS transport data to data reported for two hydrophobic organic contaminants (HOCs) showed that the HOCs exhibited much more extensive elution tailing, likely reflecting differences in sorption/desorption mechanisms. The projected influence of rate-limited sorption/desorption on PFOS transport at the field scale was investigated through simulation. The results of the study suggest that rate-limited sorption/desorption may affect the field-scale transport of PFOS and other PFAS for systems influenced by transient or short-residence-time conditions and in some cases could possibly increase the amount of flushing required to reduce PFOS concentrations to levels below those associated with human-health concerns.