No full-text available
To read the full-text of this research,
you can request a copy directly from the authors.
A Natural Gradient Experiment on Solute Transport in a Sand Aquifer 4. Sorption of Organic Solutes and Its Influence on Mobility
Water Resources Research
Abstract
Laboratory investigations were conducted to determine whether the observed field retardation of bromoform, carbon tetrachloride, tetrachloroethylene, 1,2-dichlorobenzene, and hexachloroethane at the Borden field site could be explained by the linear, reversible, equilibrium sorption model. The five halogenated organic solutes, which have octanol-water partition coefficients ranging from 200 to 4000, were the same as those used in the field study. The sorbent, a medium sand containing 0.02% organic carbon, was excavated 11.5 m from the experimental well field at the Borden site. Sorption isotherms were linear in the aqueous concentration range from 1 to 50 μg/L and could be described by a single distribution coefficient Kd. The experimentally determined Kd exceed those predicted by the hydrophobic sorption model that accounts only for partitioning into organic matter, by factors ranging from 1.7 for hexachloroethane to 10 for tetrachloroethylene. Retardation factors inferred from the laboratory determined distribution coefficients fell within the range estimated from spatial sampling data in the field experiment.
... /M). Tale modello può essere usato per descrivere i fenomeni di adsorbimento di inquinanti reattivi inorganici polari presenti in basse concentrazioni , per i quali, data l'abbondanza di siti attivi rispetto al quantitativo di particelle di soluto presenti, non si raggiungono le condizioni di saturazione (Robin et al., 1991) e per composti organici di bassa polarità in suoli ricchi di materia organica non alterata e con con basso potenziale di adsorbimento (Curtis et al., 1986; Lion et al.,1989; Allen-King et al., 2002). ...
... in cui K d è il coefficiente di distribuzione o di ripartizione (L 3 /M). Tale modello può essere usato per descrivere i fenomeni di adsorbimento di inquinanti reattivi inorganici polari presenti in basse concentrazioni , per i quali, data l'abbondanza di siti attivi rispetto al quantitativo di particelle di soluto presenti, non si raggiungono le condizioni di saturazione (Robin et al., 1991) e per composti organici di bassa polarità in suoli ricchi di materia organica non alterata e con con basso potenziale di adsorbimento (Curtis et al., 1986; Lion et al.,1989; Allen-King et al., 2002). ...
... The sorption of organic compounds slows down their migration in environment. In soils, organic ions are preferentially adsorbed on clay minerals and/or oxides (Borisover and Davis, 2015;Gu et al., 1996;Jardine et al., 1989) and neutral organic compounds are often absorbed by soil organic matter (Borisover and Graber, 1997;Curtis et al., 1986;Weber et al., 1991). The migration of organic molecules was mainly investigated in many oxide-rich soils and model materials, e.g. ...
... Values of TCE and PCE K d in aquifer sediments from previously reported studies have been shown to be greater than those estimated from key parameters (i.e., f oc , K oc ), consistent with the findings of this study (Ball and Roberts 1991;Curtis et al. 1986;Piwoni and Banerjee 1989). Most significantly, the high TCE and PCE K d values calculated based on experimental measurement of retardation factors for A/M Area sediments suggests a need to be aware of the possibility of underestimation of the time required for remediation of these sediments based on use of estimated values of K d or values that have been historically recommended (Looney et al. 1987). ...
The DOE Savannah River Site (SRS) is evaluating the potential applicability of the monitored natural attenuation (MNA) process as a contributor to the understanding of the restoration of its unconfined groundwater aquifer known to be contaminated with the chlorinated hydrocarbon compounds trichloroethylene (TCE) and tetrachloroethylene (PCE). This report discusses the results from aqueous desorption experiments on SRS aquifer sediments from two different locations at the SRS (A/M Area; P-Area) with the objective of providing technically defensible TCE/PCE distribution coefficient (Kd) data and data on TCE/PCE reversible and irreversible sorption behavior needed for further MNA evaluation.
... The PC probably represents a similarity of these OCPs in behaviour or mode of interaction with other components of the sediment, such as soil organic matter. The phenomenon behind such interactions relies on the effect of hydrophobicity on the rate at which the molecule diffuses through the polymeric structure of natural organic matter within soil particles/aggregates (Curtis et al. 1986;Brusseau and Rao 1989). This is also supported by the absence of TOC and BC in the PC. ...
The persistence of Organochlorine pesticides (OCPs) and Black Carbon (BC) was studied in sediment samples from river Yamuna, a major tributary of the Ganges (one of the largest, most populated and intensively used rivers in Asia). High levels (21.41 to 139.95 ng g-1) of sum of 20 OCPs (∑20OCPs) have been detected with β-HCH as the predominant component. ∑HCH and ∑DDT constituted ~86% of ∑20OCPs. Lindane, DDT and technical grade HCH were recently used. Toxicological studies with reference to fresh water sediment quality guidelines showed alarming levels of γ-HCH and DDT. DDD, DDE, β-HCH, α-HCH, Endrin, Heptachlor epoxide and Chlordane exceeded some of the guideline levels. High levels of BC (0.46 ± 0.23 mg g-1) constituted 1.25 to 10.56% of TOC. BC was correlated with isomers of HCH, p,p’-DDT and Methoxychlor; while TOC with ∑20OCPs, γ-HCH, Endosulfan Sulfate and Methoxychlor. PCA enabled better understanding of the relationship between various OCPs, BC and TOC. The association of BC with various OCPs indicated its importance in retention of some OCPs into fluvial sediments; exhibiting the potential to reduce their bioavailability. The study is unique to report the role of BC in persistence of OCPs in fluvial sediments.
... On the contrary of preferential flow, matrix flow is relatively slower water movement through finer pores in soil. Sorption between solutes and soil materials affects their mobility in vadose zone (Curtis et al., 1986;McCarthy and Zachara, 1989). For acidic organic compound, pH of soil and water can influence on sorbility and sorption capacity (Calvet, 1989;Delle Site, 2001). ...
Sulfonamide antibiotics is one group of important pharmaceuticals in the veterinary medicine market. Discharge of antibiotics from wastewater treatment plants lead to their presence in river and groundwater. In agricultural regions, application of animal manure on crop lands is a major contamination path in the environmental matrices. Contamination by veterinary antibiotics should be managed with long-term perspectives.
Most of past studies focused on the detection and monitoring of the veterinary antibiotics in agricultural land and water bodies. However, the studies provided only little quantitative knowledge on transport and fluxes of pharmaceuticals in the agricultural soils. The main purpose of this study is to investigate the transport processes of veterinary sulfonamide antibiotics (Sulfadimethoxine, Sulfamethazine, and Sulfamethoxazole) in agricultural soils in Haean catchment located in South Korea. In particular, we emphasized in detail: i) sorption and transport of the sulfonamides in structured and homogeneous soil columns, ii) distribution of the applied antibiotics in runoff and soil layer on sloped fields, iii) comparing the transport processes between sulfadimethoxine and sorbable/unsorbable tracers.
In the batch sorption study, sorption of the all three sulfonamides was non-linear, and the Freundlich isotherm provided the best fits to the measured data. For all target sulfonamides, their sorbility to the soils decreased with increasing pH value of aqueous solutions in range of 4.0-8.0. An increasing sorption tendency was obtained in the following order: Sulfadimethoxine>Sulfamethoxazole> Sulfamethazine. Transport of the antibiotics in soils with different soil structures and pH conditions was quantitatively investigated with a numerical modelling program, HYDRUS-1D. The soil column studies using a conservative tracer, bromide and the antibiotics showed that an important transport mechanism in the natural soil columns can be preferential flow through macropore. A comparison of water flow and solute transport in natural and disturbed soil columns revealed that the transport processes were affected by the soil structure. The transport of the sulfonamides was typical non-equilibrium process, and their mobility was reduced with decreasing pH value.
In the field campaigns, loss of the antibiotics varied between two sloped fields, and was related to the slope of the surface and precipitation. Heavy rainfalls during Monsoon season from 24 June, 2011 to 10. July, 2011 generated a huge amount of overland flow as well as seepage water in the soil layer. In accordance with the field measurements and modelling with HydroGeoSphere, water fluxes were higher for the more sloped site during the entire monsoon season. Among the sulfonamides, the mobility of sulfadimethoxine in the soil layers was relatively low. This reflects that the mobility of the sulfonamides was strongly dependent on their sorbility on soil materials.
In the tracer experiments with Brilliant Blue FCF, image analysis for horizontal cross-sections of the soil columns showed that dye coverage was almost 100% in the upper 6 cm. Below the fully dye covered depth, the ratio of dye coverage decreased more rapidly for disturbed soils due to their homogeneous soil structure. Both lateral and vertical dispersion of bromide were higher compared to Brilliant Blue FCF and sulfadimethoxine, which demand more time to reach flow regions than bromide. This reflects that water flow does not coincide with transport of the sorbable solutes, Brilliant Blue FCF and the sulfonamide due to adsorption to soil materials.
In summary, this thesis provided valuable knowledge about the transport phenomena of veterinary pharmaceuticals in agricultural soils. Through coupling of laboratory experiments, field campaigns and numerical modelling studies, major transport processes in soils can be quantitatively described. In this thesis, we present several recommendations, aiming at reducing the risk of pharmaceuticals in groundwater and surface water. In order to reduce residual amounts of the contaminants in the environment, direct application of animal faeces and urine to agricultural lands should be restricted. Preferential flow in macropores leads to fast movement of contaminants to deeper soil layers, which results in a high risk of groundwater pollutions. We suggests regular ploughing in order to remove rest of roots creating preferential flow paths. One important sources of the antibiotics in surface waters is runoff and soil erosion from agricultural lands. To reduce their input into surface waters, we recommend establishing buffer zones between agricultural fields and streams.
... 21) and(4.22) can be found inDeng et al. (2013). ...
This dissertation addresses the transport of reactive solutes in groundwater. The goal is to better link reactive transport processes to what is known about sedimentary architecture. New forms of Lagrangian-based models for the processes of retardation and dispersion are derived through linking the models to hierarchical and multiscale sedimentary architecture. This allows for a fundamental understanding of how these processes arise from the hierarchical architecture of sedimentary facies, and allows for a quantitative decomposition of these processes into facies-related contributions at different scales within the hierarchy.
The main focus is on reactive transport characterized by a high Damkohler number, DN, in which the transport processes are controlled by equilibrium sorption. Reactive transport characterized by a low DN, in which transport processes are controlled by the rate of kinetic mass transfer, is also considered.
For the high DN case, the reactive plume behavior is assumed to be controlled by linear-equilibrium sorption and the heterogeneity in both the log permeability, Y=ln(k), and the log sorption distribution coefficient, Ξ=ln(Kd). Heterogeneity in Y and Ξ arises from sedimentary processes and is structured by the consequent sedimentary architecture. The spatial auto- and cross covariances for the relevant attributes are linear sums of terms corresponding to the probability of transitioning across stratal facies types defined at different scales. Unlike previous studies that used empirical relationships for the spatial covariances, here the model parameters are developed from independent measurements of physically quantifiable attributes of the stratal architecture (i.e., proportions and lengths of facies types, and univariate statistics for Y and Ξ). Nothing is assumed about Y - Ξ point correlation; it is allowed to differ by facies type. However, it is assumed that Y and Ξ variance is small but meaningful, and that pore-scale dispersion is negligible. The time-dependent retardation and dispersion are functions of the effective ranges of the cross-transition probability structures (i.e., the ranges of indicator correlation structures) for each relevant scale of stratal hierarchy. The well-documented perchloroethene (PCE) tracer test conducted at the Borden research site is used to illustrate the models. The models were parameterized with univariate statistics for Y, Ξ of (PCE), and proportions and lengths of lithologic facies types defined at two scales within a two-level hierarchical classification. The model gives a viable explanation for the observed time-dependent retardation and dispersion of the PCE plume, and thus the processes can be explained by the equilibrium sorption and the heterogeneity in permeability and sorption coefficient. The plume velocity and the effective retardation stabilized at a large-time limit after the plume centroid had traveled a distance that encompassed the effective ranges of the cross-transition probability structure. By quantitatively decomposing the retardation and dispersion into facies-related contributions, it was shown that the retardation and the time-dependent rate of spreading were mostly defined by the cross-transition probability correlation structure imparted by the proportions and sizes of the larger-scale facies types. The weak Y - Ξ cross-correlation was shown to not significantly affect the retardation, however, it did significantly impact the spreading rate.
In unconsolidated sedimentary deposits like the Borden aquifer, texturally defined facies types can be effectively used to characterize heterogeneity in both hydraulic and reactive attributes. However, in bedrock flow systems it is the type of reactive minerals and their spatial distributions which exert the strongest control on reactivity. Therefore, instead of texturally defined facies, reactive mineral facies can be used to characterize heterogeneity in reactive attributes. Lagrangian-based model development is extended for analyzing the time-dependent retardation and dispersion in hierarchical porous media with multimodal reactive mineral facies.
A model for the transport of kinetically sorbing solutes in heterogeneous porous formations is also derived, for the case of a low DN. The effect of sorption kinetics on the dispersion of a reactive plume are evaluated relative to the effect of heterogeneity in Y. In an example illustration, the contribution of each scale of stratal architecture to the dispersion of kinetically sorbing solutes is evaluated.
... The equilibrium aqueous-phase concentration in the presence of NAPL is generally given by solubility, and the equilibrium Pinder and Abriola, 1986]. Sorption to the solid phase can be determined from the distribution coefficient [e.g., Curtis et al., 1986]. Under ideal conditions, the equilibrium aqueous concentration of a NAPL constituent is given by its fractional solubility; however, nonideal behavior can occur when the constituent is only a very small fraction of the NAPL [McCray and Dugan, 2002]. ...
Toxic organic contaminants may enter the subsurface as slightly soluble and volatile non-aqueous-phase-liquids (NAPLs) or as dissolved solutes resulting in contaminant plumes emanating from the source zone. A large body of research published in Water Resources Research has been devoted to characterizing and understanding processes controlling the transport and fate of these organic contaminants and the effectiveness of natural attenuation, bioremediation and other remedial technologies. These contributions include studies of NAPL flow, entrapment, and interphase mass transfer that have advanced from the analysis of simple systems with uniform properties and equilibrium contaminant phase partitioning to complex systems with pore- and macro-scale heterogeneity and rate-limited interphase mass transfer. Understanding of the fate of dissolved organic plumes has advanced from when biodegradation was thought to require oxygen, to recognition of the importance of anaerobic biodegradation, multiple redox zones, microbial enzyme kinetics, and mixing of organic contaminants and electron acceptors at plume fringes. Challenges remain in understanding the impacts of physical, chemical, biological and hydrogeological heterogeneity, pore-scale interactions, and mixing on the fate of organic contaminants. Further effort is needed to successfully incorporate these processes into field-scale predictions of transport and fate. Regulations have greatly reduced the frequency of new point source contamination problems; however, remediation at many legacy plumes remains challenging. A number of fields of current relevance are benefiting from research advances from point-source contaminant research. These include geologic carbon sequestration, nonpoint source contamination, aquifer storage and recovery, the fate of contaminants from oil and gas development, and enhanced bioremediation. This article is protected by copyright. All rights reserved.
... One possible reason for the difference between the measured and the observed R is that equilibrium was not fully reached in measurements of K d . The R reported by Curtis et al. [1986] is from measurements allowing 3 days to reach equilibrium. The K d measurements reported in Allen-King et al. [1998,2006] and Ritzi et al. [2013, see Allen-King et al., 2015 for methods] also allowed 3 days for equilibration. ...
This series of papers addresses the transport of reactive solutes in groundwater. In part 1, the time-dependent effective retardation factor, inline image, of reactive solutes undergoing equilibrium sorption is linked to hierarchical stratal architecture using a Lagrangian-based transport model. The model is based on hierarchical expressions of the spatial covariance of the log distribution coefficient, inline image and the spatial cross covariance between inline image and the log permeability, inline image. The spatial correlation structure in these covariance expressions is the probability of transitioning across strata types of different scales, and they are parameterized by independent and quantifiable physical attributes of sedimentary architecture including univariate statistics for Y, inline image, and the proportions and lengths of facies. Nothing is assumed about Y- inline image point correlation; it is allowed to differ by facies type. The duration of the time-dependent change in inline image is a function of the effective ranges of the cross-transition probability structures (i.e., the ranges of indicator correlation structures) for each scale of stratal architecture. The plume velocity and the effective retardation stabilize at a large-time limit after the plume centroid has traveled a distance that encompasses the effective ranges of these cross-transition probability structures. The well-documented perchloroethene (PCE) tracer test at the Borden research site is used to illustrate the model. The model gives a viable explanation for the observed PCE plume deceleration, and thus the observed inline image can be explained by the process of linear equilibrium sorption and the heterogeneity in k and Kd. In part 2 [Soltanian et al., 2015a], reactive plume dispersion, as quantified by the particle displacement variance is linked to stratal architecture using a Lagrangian-based transport model.
... The PC probably represents a similarity of these OCPs in behaviour or mode of interaction with other components of the sediment, such as soil organic matter. The phenomenon behind such interactions relies on the effect of hydrophobicity on the rate at which the molecule diffuses through the polymeric structure of natural organic matter within soil particles/aggregates (Curtis et al. 1986;Brusseau and Rao 1989). This is also supported by the absence of TOC and BC in the PC. ...
... The Amituk Lake data suggest that the partitioning of OCs on particles is related more to the fraction of clay than to organic carbon content of Ž . particles Freitas, 1994 in agreement with observations in other low organic carbon systems such Ž as groundwater aquifers Curtis et al., 1986; . Grundl and Small, 1993 . ...
... One possible reason for the difference between the measured and the observed R is that equilibrium was not fully reached in measurements of K d . The R reported by Curtis et al. [1986] is from measurements allowing 3 days to reach equilibrium. The K d measurements reported in Allen-King et al. [1998,2006] and Ritzi et al. [2013, see Allen-King et al., 2015 for methods] also allowed 3 days for equilibration. ...
This series of papers addresses the transport of reactive solutes in groundwater. In part 1, the time-dependent effective retardation factor of reactive solutes undergoing equilibrium sorption is linked to hierarchical stratal architecture using a Lagrangian-based transport model. The model is based on hierarchical expressions of the spatial covariance of the log distribution coefficient, Ξ = ln(Kd), and the spatial cross-covariance between Ξ and the log permeability, Y = ln(k) . The spatial correlation structure in these covariance expressions is the probability of transitioning across strata types of different scales, and they are parameterized by independent and quantifiable physical attributes of sedimentary architecture including univariate statistics for Y, Ξ , and proportions and facies lengths. Nothing is assumed about Y-Ξ point correlation; it is allowed to differ by facies type. The duration of the time-dependent change in is a function of the effective ranges of the cross-transition probability structures (i.e. the ranges of indicator correlation structures) for each scale of stratal architecture. The plume velocity and the effective retardation stabilize at a large-time limit after the plume centroid has traveled a distance that encompasses the effective ranges of these cross-transition probability structures. The well-documented perchloroethene (PCE) tracer test at the Borden research site is used to illustrate the model. The model gives a viable explanation for the observed PCE plume deceleration, and thus the observed time-dependent effective retardation factor can be explained by the process of linear equilibrium sorption and the heterogeneity in k and Kd . In part 2 (Soltanian et al., 2015), reactive plume dispersion, as quantified by the particle displacement variance is linked to stratal architecture using a Lagrangian-based transport model.
... One possible reason for the difference between the measured and the observed R is that equilibrium was not fully reached in measurements of K d . The R reported by Curtis et al. [1986] is from measurements allowing three days to reach equilibrium. . The K d measurements reported in Allen King et al. [1998,2006] and in Ritzi et al. [2013, see Allen-King et al., in review, for lab methods] also allowed three days for equilibration. ...
This series of papers addresses the transport of reactive solutes in groundwater. In part 1, the time-dependent effective retardation factor of reactive solutes undergoing equilibrium sorption is linked to hierarchical stratal architecture using a Lagrangian-based transport model. The model is based on hierarchical expressions of the spatial covariance of the log distribution coefficient, Ξ = ln(Kd), and the spatial cross-covariance between Ξ and the log permeability, Y = ln(k) . The spatial correlation structure in these covariance expressions is the probability of transitioning across strata types of different scales, and they are parameterized by independent and quantifiable physical attributes of sedimentary architecture including univariate statistics for Y, Ξ , and proportions and facies lengths. Nothing is assumed about Y-Ξ point correlation; it is allowed to differ by facies type. The duration of the time-dependent change in is a function of the effective ranges of the cross-transition probability structures (i.e. the ranges of indicator correlation structures) for each scale of stratal architecture. The plume velocity and the effective retardation stabilize at a large-time limit after the plume centroid has traveled a distance that encompasses the effective ranges of these cross-transition probability structures. The well-documented perchloroethene (PCE) tracer test at the Borden research site is used to illustrate the model. The model gives a viable explanation for the observed PCE plume deceleration, and thus the observed time-dependent effective retardation factor can be explained by the process of linear equilibrium sorption and the heterogeneity in k and Kd . In part 2 (Soltanian et al., 2015), reactive plume dispersion, as quantified by the particle displacement variance is linked to stratal architecture using a Lagrangian-based transport model.
... Also, different solutes containing varying types of organic matter can demonstrate linear (n=1) or highly nonlinear (n as low as 0.38) sorption isotherms (Allen-King et al. 1996;Xing and Pignatello 1997;Huang et al. 1997;Chiou and Kile 1998;Kleineidam et al. 1999;Xia and Ball 1999;Xia and Ball 2000;Chiou et al. 2000;Huang et al. 1997). In addition, the nonlinear theory of sorption isotherms in the subsurface has shown that chlorinated solvent sorption is nonlinear (Freundlich exponent <1; Curtis et al. 1986;Ball and Roberts 1991a, b). Further, the subsurface soil and solvent compounds like tetrachloroethylene and trichlorobenzene exhibited nonlinear sorption isotherms with Freundlich exponent <1 (McGinley et al. 1996). ...
... The PC probably represents a similarity of these OCPs in behaviour or mode of interaction with other components of the sediment, such as soil organic matter. The phenomenon behind such interactions relies on the effect of hydrophobicity on the rate at which the molecule diffuses through the polymeric structure of natural organic matter within soil particles/aggregates (Curtis et al. 1986;Brusseau and Rao 1989). This is also supported by the absence of TOC and BC in the PC. ...
The present study assesses the persistence and variation of organochlorine pesticides (OCPs) and their regulation by total organic carbon (TOC) and black carbon (BC) in freshwater sediment. Sediment samples from the Yamuna River, a major tributary of the Ganges (one of the most populated and intensively used rivers in Asia), had high levels of Σ20OCPs (21.41 to 139.95 ng g(-1)). β-Hexachlorocyclohexane (β-HCH) was the most predominant component. ΣHCH and Σdichloro-diphenyl-trichloroethane (DDT) constituted ~86 % of Σ20OCPs. Isomer ratios indicated fresh usage of lindane, DDT and technical-grade HCH. Toxicological comparison with freshwater sediment quality guidelines showed γ-HCH and DDT at high levels of concern. β-HCH, α-HCH, endrin, heptachlor epoxide, dichloro-diphenyl-dichloroethane (DDD), dichloro-diphenyl-dichloroethylene and chlordane were above some of the guideline levels. TOC and BC had mean concentrations of 1.37 ± 0.51 % and 0.46 ± 0.23 mg g(-1), respectively. BC constituted 1.25 to 10.56 % of TOC. We observed low to moderate correlations of BC with isomers of HCH, p,p'-DDT and methoxychlor while of TOC with Σ20OCPs, γ-HCH, endosulfan sulfate and methoxychlor. Principal component analysis enabled correlating and clustering of various OCPs, BC and TOC. OCP distribution was related with pH, electrical conductivity, soil moisture and finer fractions of sediment. OCPs with similarity in properties that determine their interactions with carbonaceous components of sediment clustered together. A number of factors may, thus, be involved in the regulation of interactive forces between BC and OCPs. BC in this study may be more important than TOC in the retention of some OCPs into fluvial sediments, thereby reducing their bioavailability. The finding is probably the first of its kind to report and emphasises the role of BC in the persistence of OCPs in fluvial sediments.
... In this case formation properties are averaged over depth and are regarded as functions of the horizontal dimension only [Rubin, 1990]. Steady, uniform, and two-dimensional flow prevail at the natural gradient tracer experiment in the sand aquifer that Was carried out at the Borden site [Mackay et al., 1986; Freyberg, 1986; Roberts et al., 1986; Curtis et al., 1986; Sudicky, 1986]. Freyberg [1986] found that the motion of the plume and its center of mass is essentially horizontal. ...
Monte Carlo studies of flow and transport in two-dimensional synthetic
conductivity fields are employed to evaluate first-order flow and
Eulerian transport theories. Hydraulic conductivity is assumed to obey
fractional Brownian motion (fBm) statistics with infinite integral scale
or to have an exponential covariance structure with finite integral
scale. The flow problem is solved via a block-centered finite difference
scheme, and a random walk approach is employed to solve the transport
equation for a conservative tracer. The model is tested for mass
conservation and convergence of computed statistics and found to yield
accurate results. It is then used to address several issues in the
context of flow and transport. The validity of the first-order relation
between the fluctuating velocity covariance and the fluctuating log
conductivity is examined. The simulations show that for exponential
covariance, this approximation is justified in the mean flow direction
for log conductivity variance, σf2, of the
order of unity. However, as σf2 increases,
the relation for the transverse velocity component deviates from the
fully nonlinear Monte Carlo results. Eulerian transport models neglect
triplet correlation functions that appear in the nonlocal macroscopic
flux. The relative importance of the triplet correlation term for
conservative chemicals is examined. This term appears to be small
relative to the convolution flux term in mildly heterogeneous media. As
σf2 increases or the integral scale grows,
the triplet correlation becomes significant. In purely convective
transport the triplet correlation term is significant if the
heterogeneity is evolving. The exact nonlocal macroscale flux for the
purely convective case significantly differs from that of the
convective-dispersive transport. This is in agreement with recent
theoretical analysis and numerical studies, and it suggests that
neglecting local-scale dispersion may lead to large errors. Localization
errors in the flux term are evaluated using Monte Carlo simulations. The
nonlocal in time model significantly differs from the fully nonlocal
model. For small variance and integral scale there is a slight
difference between the fully localized flux and the fully nonlocal
convolution flux. This is also in agreement with recent theories that
suggest that moments through the second for the two models are nearly
identical for conservative tracers. The fully localized model does not
perform well in the purely convective cases.
The heterogeneity of subsurface media and variability of hydraulic conductivity controls groundwater flow and contamination transport in aquifers. Here, several complementary approaches are developed to used heat to better constrain subsurface flow and reconstruct the subsurface heterogeneity. A typical approach for reconstruction of hydraulic conductivity values and their spatial distribution is hydraulic tomography relying on the inversion of hydraulic head data. In this work, we first assess the advantages of groundwater flux data (inferred by using Active Fiber Optic Distributed Temperature Sensors (A-FO DTS)) in a synthetic hydraulic tomography test. Next, we design and implement the first hydraulic tomography experiment measuring groundwater head and flux data simultaneously in a shallow granular aquifer at Saint-Lambert site close to Québec city, Canada. The results from numerical studies reveal that flux data outperforms the head data for a small number of observations while for the high number of observations, the reconstructed heterogeneity is independent of the data type. The results from the hydraulic tomography experiment show the advantage of including a high number of measured flux obtained by A-FO DTS leads to a reduction of estimation uncertainty. Finally, the passive DTS is used to monitor groundwater flow in an artesian fractured borehole at Ploemeur site in Brittany. The results unveil the potential of passive DTS for capturing the groundwater dynamics in fractured media.
In the design of barriers for containment of petroleum products it is essential to know the conditions for contaminant transport. In this work, a batch test method was used to determine the adsorption coefficients (Kd) of benzene, toluene, and 2-fluorotoluene (a tracer for toluene) on three soils. For Ottawa sand using soil to water ratios of 0.10-0.30 g/mL, Kd values were 2.5-1.2 mL/g (benzene), 11.3-3.6 mL/g (toluene), and 10.9-3.7 mL/g (2- fluorotoluene), respectively. Using organophilic clay at similar soil to water ratios, the Kd values were 40.9-50.0 mL/g (benzene), 154-129 mL/g (toluene), and 157-114 mL/g (2-fluorotoluene), respectively. Kd values for bentonite were 37.6-0.14 mL/g (benzene), 60.3-16.5 mL/g (toluene), and 51.2-33.6 mL/g (2-fluorotoluene) using soil to water ratios in the range 0.01-0.05. In general, for a given mixture, toluene was two to five times more adsorptive than benzene, indicating that hydrophobicity was an important factor in their adsorption. The diffusion coefficients in material comprised of 3% organophilic clay, 12% bentonite, and 85% Ottawa sand ranged from 0.48 × 10⁻⁶ to 2.5 × 10⁻⁶ cm²/s at 20°C. These values are lower than those measured for natural clay with low organic carbon content.
This thesis consists of two parts. The goal of first section is to better understand the influence of heterogeneity on contaminant transport. This objective is approached by quantifying the heterogeneity that may have affected transport of the Stanford-Waterloo tetrachloroethene (PCE) plume. Specifically, I determine the statistical distribution of the PCE sorption distribution coefficient (Kd) at Canadian Forces Base Borden, identify the spatial distribution of Kd, and determine the cross-correlation structure between PCE Kd and permeability (k). The results show that Kd is not lognormally distributed, and that variance is insufficient to fully describe heterogeneity. Skewness may be an important aspect of the ln Kd distribution because the samples constituting the high Kd tail are clustered spatially into predominantly one lens. The cross-correlation in this section of the aquifer is both very weak positive and negative, the sign depending on the spatial location of the samples. The primary goal of the second section is to determine if geologic processes separate sediment into distinct zones of chemical reactivity. If such a correlation exists, the sedimentology of site may be used to constrain the prediction of subsurface chemical heterogeneity. An intermediate step to achieving this goal is to determine if characteristic Kd values can be assigned to lithofacies; qualitative lithofacies information may be used to generate quantitative model input if such a relationship exists. The results suggest that distinct Kd distributions can be linked to lithofacies and that chemical reactivity does correlate to depositional processes. The observation that sorption and permeability are related to different aspects of facies, coupled with the observation that cross-correlation between sorption and permeability varies spatially, suggests that synthetic sorption and permeability fields should be generated independently and be constrained by sedimentology of the site.
In this chapter, the analytical models that were presented previously are used to simulate situations that might be encountered in the “real world.” Two example scenarios are looked at: (1) a groundwater remediation problem and (2) a large-scale natural gradient contaminant transport experiment. Of course, due to the simplifications that are needed to create models that can be solved analytically, the scenarios that are analyzed are very far from real-world conditions. Nevertheless, the modeling exercise is useful in that it provides insights into transport behavior that will be helpful to decision-makers who are dealing with groundwater contamination problems.
Sorption und Desorption leichtflüchtiger, halogenierter Kohlenwasserstoffe wurden mit Hilfe frontaler Gaschromatographie untersucht. Aquifermaterial mit nur geringen Anteilen an organischem Kohlenstoff oder Glasperlen wurden in HPLC-Säulen gepackt, die on-line an einen Flammen-Ionisationsdetektor angeschlossen waren, um die Extraktion flüchtiger organischer Verbindungen bei der Bodenluftabsaugung in der vadosen Zone zu simulieren. Das untersuchte Aquifermaterial umfaßte einen schluffigen Sand mit 0.15% oc aus dem Santa Clara Valley, Kalifornien und einem Feinsand mit 0.02% oc aus Borden, Ontario. Die Entfernung der flüchtigen Verbindungen von feuchtem Material verlief nur sehr langsam. Nach dem Austausch von ca. 30 Porenvolumen wurde der Schadstoffaustrag unabhängig vom Volumenstrom. Bei wasserfreiem (ofentrockenem) Material blieb der Schadstoffaustrag über die gesamte Extraktionszeit proportional dem Volumenstrom. Die Zeit um 90% Trichlorethen auszutragen, war bei feuchten Proben deutlich höher als bei trockenem Material, obwohl dieses gleichzeitig eine weitaus größere Sorptionskapazität aufwies. Die Ergebnisse deuten darauf hin, daß der Transport solcher Verbindungen im intrapartikulären Bereich auf molekulare Diffusion im Wasser zurückgeht. Bereits ein Wassergehalt, der sich allein durch die Adsorption von Wasser bei einer relativen Luftfeuchtigkeit von annähernd 100% einstellt, reicht aus, um diese Ungleichgewichtsbedingungen hervorzurufen. Nach diesen Ergebnissen werden Sanierungsmaßnahmen, wie die Absaugung von Bodenluft, oft unter Ungleichgewichtsbedingungen durchgeführt. Die Dekontamination der Feststoffphase verläuft sehr viel langsamer als die Abnahme der Schadstoffkonzentration in der mobilen Phase — der Bodenluft. Dies wird vor allem dann zutreffen, wenn poröse Komponenten wie zum Beispiel Fragmente sedimentärer Gesteine verbreitet sind.
The sequential, microbial reductive dechlorination of tetrachloroethylene and other chlorinated alkenes under methanogenic conditions was studied in liquid and soil slurry systems. A field contaminated soil was used and the effect of sorption on the reductive dechlorination rates was elucidated. As a result of microbial activity and enhanced reductive dechlorination, the extent of soil-bound contaminant release was five-fold more than in the soil slurry controls. The reductive dechlorination rates in the soil slurry system were between one and two orders of magnitude lower than those achieved in a soil-free culture. Therefore, the soil-bound contaminants exhibited lower bioavailability when compared to liquid-phase chloroalkenes. This study demonstrated the need for a continuous supply of electron donors to sustain an active primary metabolism (e.g., methanogenesis), as well as to supply the required electrons for the reductive dechlorination process. A very small fraction (less than 0.01%) of the total reducing power used for both the methanogenesis and dechlorination processes was actually channeled towards the latter process. The results of this study indicate tha development and/or enhancement of subsurface, methanogenic activity could effectively result in the biotransformation of soil-bound chloroalkenes.
Although laboratory studies and studies of uncontrolled releases are essential elements in our quest to understand the fate and transport of organic contaminants in the subsurface, both types of studies have inherent limitations. Laboratory studies, though well controlled, do not recreate the complexity and variability of natural systems. Studies of uncontrolled releases, by definition, do not have the experimental controls (for instance, knowledge of the location of contaminant source in time and space) necessary to quantitatively elicit fate and transport processes. Thus, controlled field experiments are vital links, bridging the gap between laboratory and application, and helping us to understand fate and transport processes in natural systems, as well as helping us bring theory to bear on remediation of uncontrolled releases.
The transport and fate of contaminants in subsurface systems has become one of the major research areas in the environmental/hydrological/earth sciences. This interest has been instigated by concerns associated with the effects of human activities on the environment. Numerous examples may be cited that illustrate the adverse impact of human activities on the subsurface environment, such as contamination of soil and groundwater by chemicals associated with industrial and commercial operations, service stations, waste disposal facilities, and agricultural production. An overview of the impact of such activities on the quality of the soil environment in the Australasia-Pacific Region is presented in Chapters 14 to 25.
Several sorption nonequilibrium models have been developed to gain a better understanding of solute transport in porous media, among which are those that assume a single-rate behavior. In this study, two commonly used single-rate models were fitted to computer-simulated breakthrough data from hypothetical column experiments in which multirate sorption kinetics exist at the pore scale. The objective was to determine how the sorption distribution coefficient (K) predicted using these models depends on the conditions under which the data were obtained. Simulated cases covered a range of experimental conditions and involved compounds with different sorption characteristics and different degrees of sorption rate heterogeneity. Results revealed that, for a system with a multirate sorption behavior, the true K value is under-predicted if the parameter estimation is determined by curve fitting a single-rate model. The extent of deviation between the fitted and true K increases with the decrease in residence time and increase in sorption rate heterogeneity. Functional relationships were developed between the relative reduction in K and solute residence time. Analysis using the relationships developed suggests that a major potential cause of the previously reported discrepancy between batch- and column-determined K could be attributed to the use of single-rate models for parameter prediction.
Realistic models for transport of organic chemicals in soil require accurate predictions of the adsorption-desorption kinetics. In this study, a physically based two-compartment one-rate (TCOR) sorption model was evaluated by comparison of model simulations to measured adsorption-desorption isotherms of naphthalene (C 10 H 8 ) for five different soils on a short-term (48 h) and a longer-term (504 h) time scale. Two soils exhibited minor adsorption-desorption nonsingularity (labeled Type I soils), two soils pronounced nonsingularity (Type II soils), and the fifth soil pronounced nonsingularity only on the longer-term time scale (Type I/II soil). The TCOR sorption model fitted, measured adsorption-desorption isotherms well on both short-term and longer-term time scales. However, the TCOR sorption model parameters varied for each soil between short-term and longer-term data, especially for Type II soils. The uniqueness of the TCOR model fit was tested by varying the number of desorption data used, resulting in markedly changed parameter values. The TCOR sorption model was evaluated by using model parameters obtained from short-term data to predict longer-term sorption results. The TCOR prediction of adsorption-desorption behavior was good for Type I soils, satisfactory for the Type I/II soil, and poor for Type II soils. Model parameters obtained from short-term and longer-term experiments were used to predict independently measured adsorption kinetics, showing decreasing prediction accuracy with increasing sorption nonsingularity. The results imply that the TCOR sorption model description of the diffusion process at the grain scale is oversimplified, and that sorption nonsingularity is not well explained by kinetic factors alone.
Increasing efforts have been spent in the past two decades on field measurement of soil physical properties and on the development of transport models. Here a summary is given of the characteristics of field soils gleaned from such measurements. The principle feature of field soils is their variability over many length and time scales. Issues involved in measurements in a heterogeneous environment are considered. These include the interaction of measurement scale with perceived variability, soil-imposed weighting of measurements, the Heisenberg uncertainty principle, the use of correlations, inverse measurement techniques, and soil macrostructure and biological activity. Some criteria for field measurements are identified. Predictive frameworks for interpreting and using field data are discussed. These range from the fully deterministic to stochastic theories. The successes and limitations of these are considered. Finally, some remaining problem areas are identified.
Sorption and desorption of volatile, halogenated hydrocarbons from aquifer material are investigated using a frontal gas chromatography technique. Soil columns packed with low organic carbon (oc) aquifer materials or glass beads were connected on-line to a flame ionization detector in order to simulate soil air extraction of volatile organic compounds from the vadose zone. One of the aquifer materials studied was a silty sand with 0.15% oc from the Santa Clara Valley, California. The second aquifer material used was a fine grained sand containing 0.02% oc from Borden, Ontario. Removal of the volatile organic compounds from the wet aquifer solids occurred under highly nonequilibrium conditions. Contaminant flux was independent of the flow rate after the displacement of approximately 30 pore volumes. For water free (oven-dried) material the flux of sorbate was proportional to the flow rate of carrier gas over the entire time of the extraction. The time to recover 90% of the TCE was significantly higher for the water containing samples than for the dry solids despite the fact that the dry solids sorbed much more strongly. These findings suggest that mass transfer limitations are due to diffusion through the aqueous phase in the intraparticle domain. A water content achieved by adsorption at 100% relative humidity is sufficient to establish the nonequilibrium conditions. Our findings suggest that remediation of contaminated soil using soil air extraction techniques is typically operated under nonequilibrium conditions. Removal of the solid-bound contaminants is much slower than the concentration decrease in the mobile phase. This is especially true when porous solids such as sedimentary rock fragments are abundant.
Concepts of reactive contaminant transport in porous media that involve mass transfer processes, such as sorption, and chemical and microbial transformation are introduced. These reactive transport concepts are quantitatively described within the context of hydrogeologic heterogeneity. In particular, the degree of fate and transport of a single reactive contaminant in the subsurface subjected to certain spatially heterogeneous physical or chemical processes is addressed. This paper presents some results from recent research in the area of reactive contaminant transport that has attempted to create models that integrate these previously disparate viewpoints by including both physical and chemical heterogeneity.
The solubility and sorption behavior of naphthalene in the presence of dissolved organic acid was studied. The presence of dissolved organic acids at lower concentrations had only a little effect on naphthalene sorption. However, when the concentration exceeds a certain concentration, a significant inhibition was observed. Moreover, the inhibition enhanced with the increasing concentrations of dissolved organic acids. The change trend on solubility is similar to the sorption of naphthalene, that is, the solubility of naphthalene at lower concentration increased slowly and significant at higher concentration of dissolved organic acids, and followed the order butyric acid > proponic acid > acetic acid > formic acid. In addition, the change of structure of sediment is an important factor that can affect the naphthalene sorption. This suggests the change of solubility of naphthalene and the change of structure of sediment in dissolved organic acids solution both exert effect on naphthalene sorption.
Soils* and sediments are often thought to be filters for agrochemicals and wastes. For an efficient management of this pollutant attenuation function in subsurface percolating water, a complete understanding of the behaviour of pollutants in soils and sediments is imperative. Physical, degradation (biotic and abiotic) and geochemical processes are briefly reviewed from the perspective of their influence on pollutant transfer and retardation. It is concluded that the attenuation of trace metal and halogenated organic pollutants is controlled by local hydraulic and biogeochemical properties and that interfaces between lithological or hydrogeological compartments are most likely key areas controlling the subsurface hydrocycle. Dilution by dispersion is only significant in the case of point source and single event contaminations. The kinetics and reversibility of biogeochemical processes need further investigation.
Einleitung Trockene Deposition Luft Boden Austausch von Schadstoffen zwischen Atmosphäre, Vegetation und Pedosphäre Adsorption im Boden Transport im Grundwasser
Anthropic activities can greatly modify natural geochemical cycles of toxic heavy metals. Their accumulation in the pedosphere and lithosphere is of great concern since, contrary to organic xenobiotic compounds, they are not subject to any degradation. It is now widely recognized that it is not the total amount or concentration of a given heavy metal which is most important for understanding its environmental behaviour but rather that the key to the flux of a metallic element is its speciation.
In recent years pollution of the unsaturated zone by petroleum products has aroused concern particularly with regard to ground water quality. The research carried out in this field has dealt mainly with the biological degradation of petroleum products and their residues in soil as well as with their transport through porous media as a liquid immiscible with water or through the gaseous phase.
The main aim of this study was to determine the sorption and biodegradation parameters of trichloroethene (TCE) and tetrachloroethene (PCE) as input data required for their fate and transport modelling in a Quaternary sandy aquifer. Sorption was determined based on batch and column experiments, while biodegradation was investigated using the compound-specific isotope analysis (CSIA). The aquifer materials medium (soil 1) to fine (soil 2) sands and groundwater samples came from the representative profile of the contaminated site (south-east Poland). The sorption isotherms were approximately linear (TCE, soil 1, K
d = 0.0016; PCE, soil 1, K
d = 0.0051; PCE, soil 2, K
d = 0.0069) except for one case in which the best fitting was for the Langmuir isotherm (TCE, soil 2, K
f = 0.6493 and S
max = 0.0145). The results indicate low retardation coefficients (R) of TCE and PCE; however, somewhat lower values were obtained in batch compared to column experiments. In the column experiments with the presence of both contaminants, TCE influenced sorption of PCE, so that the R values for both compounds were almost two times higher. Non-significant differences in isotope compositions of TCE and PCE measured in the observation points (δ13C values within the range of −23.6 ÷ −24.3 ‰ and −26.3 ÷−27.7 ‰, respectively) indicate that biodegradation apparently is not an important process contributing to the natural attenuation of these contaminants in the studied sandy aquifer.
Sorption of five chlorinated benzenes have been determined for a lake sediment. Sorption isotherms are linear over the tested aqueous concentrations ranges (three orders of magnitude), which supports the sorption partitioning model. Sorption coefficients of the chlorobenzenes are compared to the results of other sorption studies on the basis of sorption coefficient ratios and are consistent with literature data.In this study emphasis is on the experimental problems in determining sorption coefficients. Sediment extractions and experimental conditions such as equilibrium period, water-sediment separation and water/sediment ratio may influence sorption results. These possible experimental artefacts are investigated in this study in order to have better insight in the influence of the experimental artefacts and consequently in the spread of the experimental sorption coefficients.
Permanganates have been used for in situ chemical oxidation (ISCO) projects since the 1990s. Unfortunately, there has been very little research performed on the phenomenon of concentration rebound after ISCO. Most research on ISCO has focused on demonstrating effectiveness, estimating kinetics, or quantifying the effects of reaction products. Only one study has demonstrated that a correlation between concentration rebound and hydrogeological parameters exists. Our study uses a numerical solution to an advection‐dispersion‐reaction equation to quantify a correlation between the rate of concentration rebound and molecular diffusivity in pure water. It accomplishes this by simulating a variety of sites contaminated with chlorinated ethenes that also had an ISCO with permanganate. Each simulation included advection, two‐dimensional dispersion, oxidation, concentration rebound, natural oxidant demand, and retardation. Five sites were suitable for simulation and eight cells were delineated within the five sites. These cells allowed for a variety of soils, contaminants, injection methods (i.e. frequency, depth, mass of oxidant, duration, etc…), time scales, spatial scales, and hydrogeological variables to be examined. A robust correlation (R2 = 0.92) was identified with a regression analysis between the molecular diffusion coefficient in pure water and the rate of concentration rebound.
An extended radial pore effective diffusion model is presented, describing the (microbial) decontamination of oil contaminated soil in a slurry. The aim of the model is to estimate the biodegradation rates of nonpolar contaminants in soil slurries. In the model, the soil is divided into three separate fractions, each of which contributes independently to the oil concentration in the slurry. The sand fraction is not inhibiting microbial degradation of the oil. The organic matter fraction limits the oil transport rate from the soil particles to the microorganisms. The clay fraction exhibits an extremely low transport rate for the nonpolar contaminants, about 1000 times lower than the organic matter fraction. The two types of clay used in this research showed comparable sorption coefficients. The model was tested on the decrease of hexachlorocyclohexane concentrations in two different types of soil. The diesel oil concentration in a sieve fraction of a contaminated soil, containing high amounts of clay and organic matter, could be modeled well with only one fitting parameter by using the previously measured sorption coefficients for hexadecane.
The results of several experiments reported in the literature indicate
that the assumptions of porous media homogeneity and instantaneous
sorption are generally invalid for transport of sorbing solute at the
field scale. In addition, it is probable that the "nonideal" transport
observed is caused by more than one factor. The purpose of this work was
to evaluate the ability of a one-dimensional, multifactor nonideality
model to represent the nonideal transport of sorbing solutes at the
field scale. For this model the two-domain approach is used to represent
heterogeneity, and sorption is represented as being essentially
instantaneous for a portion of the sorbent and rate limited for the
remainder. Data (breakthrough curves) obtained from four field
experiments reported in the literature were used to test the performance
of the model. With one exception, data from laboratory or field
experiments were used to identify values for input parameters; the
exception concerned an assumption of a uniform distribution of sorbent
between the advective and nonadvective pore water domains. Specification
of input parameters independent of the data being simulated allowed the
model to be used in a predictive mode. On the basis of the good match
between predictions produced with the model and the data, it appears
that the model provides a valid representation of sorption dynamics and
solute transport for field-scale systems influenced by heterogeneity and
rate-limited sorption. The model should be useful in situations where
the extensive input data required for more complex models (e.g.,
stochastic) are not available.
The long-term behavior of five organic solutes during transport over a period of 2 years in ground water under natural gradient conditions was characterized quantitatively by means of moment estimates. Total mass was conserved for two of the organic compounds, carbon tetrachloride and tetrachloroethylene, while the total mass declined for three other compounds, bromoform, 1,2-dichlorobenzene, and hexachloroethane. The declines in mass for the latter three compounds are interpreted as evidence of transformation of the compounds. Retardation factors for the organic solutes, relative to chloride, ranged from 1.5 to 9.0, being generally greater for the more strongly hydrophobic compounds. The retardation is attributed to sorption. The apparent retardation factor increased markedly for all compounds over the duration of the experiment, by as much as 150%. Results from temporal and spatial sampling were in good agreement when compared at the same scale of time and distance.
A large-scale field experiment on natural gradient transport of solutes in groundwater has been conducted at a site in Borden, Ontario. Well-defined initial conditions were achieved by the pulse injection of 12 m3 of a uniform solution containing known masses of two inorganic tracers (chloride and bromide) and five halogenated organic chemicals (bromoform, carbon tetrachloride, tetrachloroethylene, 1,2-dichlorobenzene, and hexachloroethane). A dense, three-dimensional array of over 5000 sampling points was installed throughout the zone traversed by the solutes. Over 19,900 samples have been collected over a 3-year period. The tracers followed a linear horizontal trajectory at an approximately constant velocity, both of which compare well with expectations based on water table contours and estimates of hydraulic head gradient, porosity, and hydraulic conductivity. The vertical displacement over the duration of the experiment was small. Spreading was much more pronounced in the horizontal longitudinal than in the horizontal transverse direction; vertical spreading was very small. The organic solutes were retarded in mobility, as expected.
Soil-water equilibrium data suggest that the transfer of nonionic chemicals from water to soil may be described in terms of
a hypothesis of solute partitioning in the soil organic matter. This concept allows estimation of soil-water distribution
coefficients either from solvent-water partition coefficients or aqueous solubilities.
Equations describing the transport of ion-exchanging solutes governed by
local chemical equilibrium through a saturated porous medium are well
established in the literature. Concentration profiles resulting from the
numerical solution of the general multispecies equations typically
exhibit unusual and complicated features such as multiple fronts and
plateau zones. This paper presents an analytical framework, based upon
the theory of chromatography, which permits a priori characterization of
certain key concentration profile features. The cases studied include
both homovalent and heterovalent exchange in binary and ternary systems.
In order to test its validity, the chromatographic analysis is applied
to a field project involving direct injection of advanced treated
municipal effluent into an aquifer. All of the major features observed
in the available field data are accurately predicted by the
chromatographic theory.
The three-dimensional movement of a tracer plume containing bromide and chloride is investigated using the data base from a large-scale natural gradient field experiment on groundwater solute transport. The analysis focuses on the zeroth-, first-, and second-order spatial moments of the concentration distribution. These moments define integrated measures of the dissolved mass, mean solute velocity, and dispersion of the plume. Moments are estimated from the point observations using quadrature approximations tailored to the density of the sampling network. The estimators appear to be robust, with acceptable sampling variability. Estimates of the mass in solution for both bromide and chloride demonstrate that the tracers behaved conservatively, as expected. Analysis of the first-order moment estimates indicates that the experimental tracer plumes traveled along identical trajectories. The horizontal trajectory is linear and aligned with the hydraulic gradient. The vertical trajectory is curvilinear, concave upward. The total vertical displacement is small, however, so that the vertical component of the mean solute velocity vector is negligible. The estimated mean solute velocity is identical for both tracers (0. 091 m/day) and is spatially and temporally uniform for the first 647 days of travel time.
Temperature and water effect on the adsorption of parathion ( O, O ‐diethyl O‐p ‐nitrophenyl phosphorothioate) by soils was studied using ¹⁴ C‐tagged parathion. The parathion was applied in aqueous solutions and in hexane over a concentration range of 1–10µg/ml. It was found that the parathion is adsorbed by soils from aqueous solution without any appreciable thermic effect. In a dry soil‐hexane‐parathion system the slightly polar parathion molecules efficiently compete with the apolar hexane molecules for adsorption sites. In hydrated soil‐hexane‐parathion systems there was no parathion adsorption. Generally as the soil water content increases, parathion adsorption decreases. The water‐temperature composite effect in a soil‐hexane‐parathion system was also studied. The rise of temperature in the soilhydrated parathion‐hexane system favors the endothermic process of water desorption, resulting in greater free adsorption sites, and consequently, in increasing parathion adsorption.
Vapor‐phase desorption of soil‐applied lindane (γ‐BHC) was studied by measuring vapor density of lindane as affected by soil‐water content, temperature, and lindane concentration. Water content had no effect on vapor density until the soil was dried to a water content below one molecular layer of water. When more than a monomolecular layer of water was present in Gila silt loam, vapor density increased with temperature and lindane concentration until a saturated vapor density, equal to that of lindane without soil was reached at 50, 55, and 62 ppm lindane at 20, 30, and 40C, respectively. Isotherms relating soil lindane concentration to relative vapor density were similar to isotherms relating soil lindane concentration to solution concentration indicating that Henry's Law, p = kc , can be used to predict pesticide behavior in the soil‐water system. Desorption isotherms for lindane on Gila silt loam and bentonite fitted the Freundlich equation. Freundlich constants for vaporphase lindane were similar to those reported previously by others for adsorption of lindane from the solution phase by soil materials.
Freundlich adsorption isotherms were obtained for gamma‐ and beta‐isomers of 1,2,3,4,5,6‐hexachlorocyclohexane adsorbed from aqueous and hexane solvents on Ca‐Staten peaty muck, Ca‐bentonite Ca‐Venado clay, and silica gel in the temperature range of 10 to 40C. Comparison of normal and “corrected” isotherms revealed a significant effect on adsorption due to the influence of temperature on solubility. The 1/ n constants of the Freundlich equation increased with temperature according to the theory of dilute solutions. When the solubility effect is removed, the gamma isomer has greater adsorbability than the beta isomer, possibly because the former possesses a dipole moment whereas the latter does not. At constant solution fugacities, the gamma isomer on Staten peaty muck competed somewhat more effectively with hexane than with water.
A theoretical approach, based upon the assumption of predominance of solvophobic interactions, was formulated to quantitatively describe the sorption and transport of hydrophobic organic chemicals (HOC) from aqueous and aqueous-organic-solvent mixtures. In the theoretical approach, solvent-sorbate interactions (solubility) are specifically considered in order to predict sorbate-sorbent interactions (sorption). For HOC sorption from a single solvent, the HOC sorption coefficient was shown to increase log-linearly with the hydrocarbonaceous surface area (HSA) of the sorbate. For HOC sorption from aqueous-organic binary solvent mixtures, the sorption coefficient is predicted to decrease exponentially as the fraction of organic cosolvent increases. This is a direct consequence of increased HOC solubility in the binary solvent. Because sorption and mobility of HOC are inversely related, a decrease in sorption coefficient leads to an enhanced HOC mobility as the fraction of organic cosolvent is increased. A preliminary verification of the theory was performed by an analysis of published data for (i) HOC sorption by soils and sediments from water, (ii) HOC retention by reversed-phase chromatographic sorbents during isocratic elution with methanol-water binary solvent mixture, and (iii) HOC mobility on soil-TLC plates eluted with ethanol-water mixtures.
An empirical relationship between the octanol/water partition coefficients (K(ow)) and sediment sorption coefficients (K(oc)) was determined for nine chloro-s-triazine and dinitroaniline compounds. Results were compared with earlier work that reported an excellent correlation between K(ow) and K(oc) for polynuclear aromatic and chlorinated hydrocarbons. The more polar character of the present compounds, however, reduced the precision of estimating sorption from octanol/water partition coefficients. A fit of the combined data sets for a total of 19 compounds of widely varied properties (five orders of magnitude variation in solubility) yielded a relationship that should be useful for predicting simple partition coefficients K(p)) in a number of modeling and assessment applications. Reasonable estimates of K(p) or K(oc) can be obtained from a single determination of the octanol/water partition coefficient and knowledge of the organic carbon content of the sediment.
We examined the statistical structure of two sets of soil sorption isotherm data. Nickel sorption isotherms with Puye soil differed depending upon whether 0.01 M CaSO4 or 0.0067 M CaCl2 was the background electrolyte. Isotherms with 21 replicates at each of six initial Ni concentrations were prepared for each background matrix. The data for each matrix could be described by both linear (R2 > 0.90) and linearized Freundlich (R2 > 0.98) isotherm equations. The variance structure and the normality of both untransformed (linear) and log-transformed (Freundlich) data sets were evaluated using a comparative table of variances and normal probability plots. Only the log-transformed data satisfied the homogeneity of variance and normality assumptions required for application of standard parametric statistics. Statistical comparisons using the log-transformed data indicated that significant differences between Cl- and SO42- isotherms could be explained on the basis of differences in free Ni2+ activity in the two systems. This study illustrates the importance of knowing the statistical structure of the data prior to making valid objective comparisons among isotherms.
The equilibrium or Henry's constant, H//c, and the mass transfer rate constant, K//La, are the major factors determining the extent of transfer during bubble aeration and in countercurrent packed columns. This study delineates the limits of validity of Henry's law for nonionizing halogenated solutes and presents data on the effects of temperature and the addition of model cosolvents on the Henry's constant. The experimental results are compared with predictions from a semi-empirical thermodynamic model and from vapor pressure and solubility data. The Henry's constant of the organic compounds studied increases by a factor of approximately 1. 6 with each 10 degree C rise in temperature, representing a very strong effect. Thus, temperature is likely to be a key parameter affecting the extent of removal of volatile organic compounds in gas-liquid contacting processes.
An understanding of sorption processes is an important key to describing pollutant fate in an aquatic system because sorption may alter significantly physical transport and chemical reactivity of pollutants. The sorption of uncharged organic chemicals to natural aquatic sorbents is dominated by “hydrophobic interactions.” For composite particulates (i.e., sediments/soils), organic matter is the primary sorbing constituent. Sorption partition coefficients, indexed to organic carbon (Koc), are relatively invariant for natural sorbents. Koc’s can be estimated from other physical properties of pollutants (aqueous solubility or octanol/water partition coefficients). Hydrophilic contributions to sorption tend to occur with one or both of the following conditions: (1) High sorbate polarity; and (2) low organic carbon content of the sorbent, especially with coincident high clay content. Although a priori estimation techniques comparable to hydrophobic sorption are not presently available, hydrophilic contributions relative to Koc can be estimated based upon chemical class and sorbent composition. Although sorption to sediment or soils is frequently viewed as a rapid process in environmental modeling, true sorption equilibrium may require weeks to months to achieve, with pollutant uptake and release kinetics highly dependent on molecular size, sorbent cohesive properties, and solids concentration.
Methods were developed for estimating the equilibrium sorption behavior of hydrophobic pollutants. At low pollutant concentration (aqueous phase concentration less than half the solubility), sorption isotherms were linear, reversible, and characterized by a partition coefficient, Kp. Partition coefficients normalized to organic carbon, KOC ( organic carbon), were highly invariant over a set of sediments and soils collected from throughout the nation. Equations for estimating KOC from water solubility (including crystal energy) and octanol/water partition coefficients were developed. The predictive equations were tested on literature sorption data and found to estimate measured KOC's generally within a factor of two.
Equilibrium isotherms for the adsorption of herbicides, diuron, and 2,4,5-T on whole soil and various-size separates of Webster soil (Typic Haplaquolls) were measured. The Freundlich adsorption coefficient, K, for diuron varied by a factor of about 7 among the different particle-size fractions. However, the sorption coefficient, K, normalized with respect to soil organic carbon (OC) content, denoted as K(oc) = (K/OC), varied only within a factor of 1.5. For 2,4,5-T herbicide, the K(oc) values for the silt and clay-size fractions, as well as the whole soil, were essentially identical and about 3 times larger than that for the sand-size fraction. The results suggested that the soil particle-size separates may be grouped into two size classes: 'fine' (≤ 50 μm) and 'coarse' (2,000-50 μm). For a given pesticide, a specific K(oc) value may be assigned to each class.
The effect of sediment concentration variations on polychlorinated biphenyl (PCB) sorption to natural lake sediment (Saginaw Bay, Mich.) and montmorillonite clay has been investigated. In studies utilizing tritiated 2,4,5,2',4',5'-hexachlorobiphenyl (HCBP), partition coefficient values for adsorption isotherms (π(a)) were found to increase as sediment concentrations (m) were decreased from m = 1000 mg/L to m = 50 mg/L. HCBP adsorption to montmorillonite (m = 1000 mg/L, π(a) = 2900 L/kg; m = 50 mg/L, π(a) = 10,600 L/kg) appeared to be more sensitive to sediment concentration than did adsorption to the Saginaw Bay sediment (m = 1000 mg/L, π(a) = 9900 L/kg; m = 50 mg/L, π(a) = 17,100 L/kg). Evidence suggests that these variations are not the result of nonlinear isotherms. Although variations in solution chemical composition and kinetic effects were found to affect partitioning, neither factor appeared to adequately account for the magnitude of the observed sediment concentration effect. In experiments in which sediment concentrations were decreased (m = 1000 mg/L to 10 mg/L) while equilibrium aqueous HCBP concentrations were maintained at approximately constant levels, π(a) values for adsorption increased. The behavior was in conformity with the isotherm results. The observed behavior might be due to direct solid phase interactions between suspended particles. The occurrence of such interactions in natural waters could have potentially significant implications for efforts to predict sediment-water PCB distributions.
Three methods for the determination of dissolved organic carbon in seawater were compared. Samples were analysed using persulphate oxidation, high-temperature combustion, and ultraviolet photo-oxidation. The dissolved organic carbon content of the seawater samples ranged from 0.6 to 1.6 mg C/I. This study shows that results of high-temperature oxidation and photo-oxidation procedures differ by less than 5%, whereas results with persulphate oxidation are about 15% less than those obtained with the high-temperature oxidation. The relative merits of each of the oxidation techniques for the determination of organic matter in seawater are discussed.
A solution for the problem of miscible displacement, or for the chromatographic problem involving a linear isotherm and dispersion, is given and adapted to a least squares calculation. The procedure enables utilization of all the data from column breakthrough experiments. The method is illustrated by a study of the velocity dependence of the dispersion coefficient. When applied to data not fulfilling the theoretical postulates, the method produces empirical descriptions of column behavior, a representation of column performance in terms of equivalent column volume, and a single additional parameter. The theory gives excellent, and undoubtedly fortuitous, agreement for a case of slow isotopic exchange.
Sorption of anthracene and two herbicides (diuron and atrazine) by soils from aqueous solutions and binary solvent mixtures consisting of methanol-water and acetone-water was measured. These data were used to evaluate recently proposed solvophobic theory for describing sorption of hydrophobic molecules from mixed solvents. As predicted by the theory, the sorption coefficient (K/sup m/) decreased exponentially with increasing fraction of the organic cosolvent (f/sup c/) in the binary solvent mixtures. The slope of the ln K/sup m/ vs. f/sup c/ plot, designated as sigma/sup c/, was unique to each sorbate-solvent combination and was independent of the soil (sorbent). Thus, the organic cosolvent effects on sorption could be specified by a single parameter that combines the coefficients characterizing solvent and sorbate properties. The sigma/sup c/ value was shown to be directly proportional to the solvent-sorbate interfacial free energy( ..delta gamma../sup c/) and the hydrocarbonaceous molecular surface area (HSA) of the sorbate.
If precautions are taken to eliminate or account for nonsettling (or nonfilterable) microparticles or organic macromolecules that remain in the aqueous phase during laboratory sorption tests, the observed partition coefficients (Kp or Koc) for a group of model hydrophobic organic compounds (PCBs) are found to remain constant over a wide range of solid-to-solution ratios. Further, the partition coefficients for either sorptive uptake or desorptive release are indistinguishable and confirm the reversible nature of hydrophobic sorption. It is proposed that descriptions of the 'speciation' of hydrophobic compounds in natural waters should include not only dissolved and sorbed-to-sediment fractions but also a component sorbed to nonsettling microparticles or organic macromolecules.
Laboratory batch and column experiments have been conducted to elucidate the sorption behavior of nonpolar organic compounds (e.g., halogenated alkenes and benzenes) in a river water-groundwater infiltration system. For the low concentrations typical of the environment, sorption equilibria can be described by the equation S = KPC, where S = concentration in the solid phase, Kp = partition coefficient, and C = concentration in the liquid phase. For a variety of sorbents, it was found that the partition coefficient Kpz of a particular compound Z can be estimated from its 1-octanol/ water partition coefficient KOWZ and from the organic-carbon (OC) content fOC (fraction organic carbon) of the sorbents if fOC is greater than 0.001: log Kpz = 0.72 lo KOwz + log fOC(s) + 0.49. Sorption was found to be reversible, but sorption kinetics may have an effect on material transport over the range of flow velocities encountered in aquifers. The conclusions of this study are applicable to a wide range of sorption and transport problems including artificial groundwater recharge and leaching of pollutants from landfills.
In considering reversible sorption processes in ground-water flow systems, it is important to determine whether the ground water is in steady-state or dynamic chemical evolution. In steady-state chemical evolution, the sorbed site populations at any point along the flow path are in equilibrium with the way in which ground water will evolve chemically due to mineral dissolution/precipitation, redox reactions, etc., during flow when it reaches this point. In dynamic chemical evolution it is not. In the steady-state case, a location in the flow system can be characterized by a constant distribution coefficient over time for an ionic species involved in reversible sorption reactions. In the dynamic case, the distribution coefficient can be expected to vary at a location in a flow system over time. Subsurface contaminant migration problems which confront hydrogeologists invariably are of the dynamic chemical evolution type and the notion of a constant distribution coefficient or Kd to describe reversible sorption processes cannot be entertained.
The extent of reversibility of PCB bonding to sediments has been characterized in studies on the partitioning behavior of a hexachlorobiphenyl isomer. Linear non-singular isotherms have been observed for the adsorption and desorption of 2.4.5.2′,4′,5′ hexachlorobiphenyl (HCBP) to 1100 ppm sediment suspensions. Partition coefficients, π(1 kg−1), for desorption from lake sediments (Saginaw Bay. Lake Huron. Michigan) are substantially greater (πd ∼ 20.000–35.000) than those obtained for adsorption (π4 ∼ 9000–14,000). HCBP was found to be more weakly adsorbed to montmorillonite (πa − 3000, πd ∼ 9000) and kaolinite (πa ∼ 1000, πd ∼ 3000) clay samples than to the natural sediment samples. Desorption results (πd) for Saginaw Bay sediments were quite similar to π values (15,000–35.000) calculated from field measurements of aqueous and particulate PCB concentrations. For Saginaw Bay sediments and clay minerals partitioning appeared to be correlated both to sediment surface area and to sediment organic content. A regression analysis using both of these variables explained ∼ 90% of the observed variations. HCBP adsorption at 40 C (πa ∼ 14,000) was significantly greater than at 1° C (πa ∼ 6500) resulting in a calculated enthalpy of adsorption of +3.3 kcal mol−1. Non-singular isotherm behavior was not found to be readily attributable to microbiological, kinetic or experimental effects. Evidence from consecutive desorption studies suggests that while HCBP adsorption may ultimately be reversible. release from sediments appeared to involve desorption along two distinct isotherms. These results have been interpreted in terms of possible similarities between the sorption properties observed in the distilled water systems of the present study and PCB bonding processes in natural water systems.
Mechanistic roles of soil humus and soil minerals and their contributions to soil sorption of nonionic organic compounds from aqueous and organic solutions are illustrated. Parathion and lindane are used as model solutes on two soils that differ greatly in their humic and mineral contents. In aqueous systems, observed sorptive characteristics suggest that solute partitioning into the soil-humic phase is the primary mechanism of soil uptake. By contrast, data obtained from organic solutions on dehydrated soil partitioning into humic phase and adsorption by soil minerals is influenced by the soil-moisture content and by the solvent medium from which the solute is sorbed.
The sorption of hydrophobic compounds (aromatic hydrocarbons and chlorinated hydrocarbons) spanning a concentration range in water solubility from 500 parts per trillion (ppt) to 1800 parts per million (ppm) on local (North Georgia) pond and river sediments was investigated. The sorption isotherms were linear over a broad range of aqueous phase pollutant concentrations. The linear partition coefficients (Kp) were relatively independent of sediment concentrations and ionic strength in the suspensions. The Kp ś were directly related to organic carbon content for given particle size isolates in the different sediments. On an organic carbon basis (Koc = Kp/fraction) organic carbon), the sand fraction (> 50 μm particle size) was a considerably less effective sorbent (50–90% reduction in Koc) than the fines fraction (> 50 μm particles). Differences in sorption within the silt and clay fractions were largely related to differences in organic carbon content. Reasonable estimates of Kocś can be made from octanol/water distribution coefficients, which are widely catalogued or easily measured in the laboratory.
The results of a number of laboratory studies are presented to demonstrate an inverse relationship between concentration of adsorbing solids and partition coefficient. Various functional forms which define the relation are developed and correlated with the data. A power-law dependence of partition coefficient on concentration of solids is shown. The significance of this relationship in assessing the fate of hydrophobic pollutants in natural water systems is discussed.
Equilibrium isotherms for the simultaneous uptake of binary nonionic organic compounds from water on soil indicated no competitive effect between the two solutes. This observation supports the hypothesis that partition to the soil organic phase is the primary process for sorption of nonionic organic compounds from water on soil. The partition process between soil organic matter and water was analyzed by using the conventional solution concept for solutes in water and the Flory-Huggins treatment for solutes in the polymeric humic phase. Sorption determined for 12 aromatic compounds on a Woodburn soil shows that the extent of solute insolubility in water (S) is the primary factor affecting the soil organic matter-water partition coefficient (K(om)) and that the effect of solute incompatibility with soil organic matter is significant but secondary. This explains the commonly observed correlations of log K(om) vs. log S and log K(om) vs. log K(ow) (octanol-water).
Results from laboratory equilibration studies indicate that sediment-adsorbed 2,4,5,2',4',5'-hexachlorobiphenyl (HCBP) fractions may be comprised of both reversibly and strongly bound or resistant components. This evidence suggests that for many environmental modeling efforts it may be inappropriate to treat this and other PCB isomer adsorption reactions as either completely reversible or completely irreversible. The initial refinement of such models requires a means of estimating the relative magnitudes of the respective sorbed PCB fractions. To this end, a computational method has been derived to allow prediction of the magnitude of the reversible and more strongly adsorbed HCBP fractions from conventional isotherm data. The derivation, which is based upon experimentally observed linear HCBP isotherms, has also been applied to other organic molecules of similar isotherm character. This methodology provides an initial quantitative approximation of the strongly bound, resistant, sediment fractions while utilizing relatively simple experimental adsorption-desorption data. Results suggest that for montmorillonite clay, kaolinite clay, and natural lake sediment (Saginaw Bay, MI) linear isotherms effectively describe the relationships between both adsorbed HCBP fractions and the aqueous HCBP concentration. The implications of these and other model predictions for the cycling of PCB's in natural waters are discussed.
The sorption of pyrene, 3-methylcholanthrene, 7,12-dimethylbenz(a)anthracene, and dibenzanthracene on 14 soil and sediment samples exhibiting a wide range of physicochemical properties was studied. The equilibrium Freundlich constants and linear partition coefficients for each compound were highly correlated with the organic carbon content of the soils/sediments tested. No other significant correlations with soil/sediment properties were observed. The sorption constants when normalized to organic carbon content of the substrate predicted the octanol-water partition coefficients for the compounds tested. A highly significant negative correlation was observed between log substrate and log water solubility for these compounds. Experimental values of the water solubility and octanol-water partition coefficients for the four compounds are reported.
Partition coefficients are commonly used to quantify the distribution of organic pollutants between the aqueous and particulate phases of natural aquatic systems. The magnitude of the partition coefficient for a specific pollutant in a particular system has been shown to be related to the octanol-water partition coefficient of the pollutant and the organic carbon content of the solid phase onto which sorption occurs. The present work demonstrates that a third variable is important - the concentration of the sorbing solid phase in the system. Laboratory partitioning experiments were performed over a range of solids concentrations by using three different Lake Michigan sediment samples as sorbents and four hydrophobic priority pollutants (chlorobenzene, naphthalene, 2,5,2'-trichlorobiphenyl, and 2,4,5,2',4',5'-hexachlorobiphenyl) as sorbates. The results indicate a significant increase in partitioning as solids concentration decreases. A predictive relationship is developed, and potential mechanistic explanations are explored.
Adsorption at the water-sediment interface has been correlated with the organic carbon content of natural sediment samples and with water-octanol partitioning equilibria. In deriving a new model for sorption in sediment, it is assumed that adsorption is due to humin-kerogen polymers associated with the clay component in the sediment. The model includes solubility parameter theory applied to solute-gel-liquid interactions and a theory of the liquid-polymer interactions that control gel swelling. Partially swollen gels are expected to exhibit impeded diffusion. These concepts are able to explain observations of limited desorption of certain organic compounds from natural sediments and soil minerals. Experiments were performed in which extractive solvents flowed through a liquid chromatographic column packed with dried estuarine and pond sediment samples and the effluent was analyzed for a test lipophilic compound, di(2-ethylhexyl) phthalate. The model predicts that the maximum desorption rate and the maximum extent of gel swelling should coincide and, conversely, that the desorption rate should be diffusion-limited if the polymer gel is only partially swollen. The conditions for maximum desorption and for diffusion-limited desorption were both observed experimentally.
Partition equilibria of nonionic organic compounds between soil organic matter and water
- Chiou
Sorption of halogenated organic solutes onto aquifer materials: comparisons between laboratory results and field observations Eng
- G P Curtis