地下水污染试验研究进展

本文在总结国内外众多学者有关污染物在含水层中的运移规律研究成果的基础上，介绍了地下水污染试验研究的最新进展；并据此认为，三维弥散，各向异性含水层介质中污染物的迁移，非饱和土层中的污染物多相迁移和吸力的关系等将成为今后地下水污染传播研究的主要问题。     

NUMERICAL STUDY OF NON-AQUEOUS PHASE LIQUID TRANSPORT IN A SINGLE FILLED FRACTURE BY LATTICE BOLTZMANN METHOD

In this article,the Non-Aqueous Phase Liquid(NAPL)transport in the single filled fracture was studied with the Shan-Chen multi-component multiphase Lattice Boltzmann Method(LBM)with special consideration of wettability effects.With the help of the model,the contact angle of the non-wetting phase and wetting phase interface at a solid wall could be adjusted.By considering a set of appropriate boundary conditions,the fractured conductivity was investigated in condition that the NAPL blocks the channels in the single filled fracture.In order to study the wettability effects on the NAPL transport,a constant driving force was introduced in the Shan-Chen multi-component multiphase LBM.Flow regimes with different wettabilities were discussed.Simulated results show that the LBM is a very instrumental method for simulating and studying the immiscible multiphase flow problems in single filled fracture.     

Equilibrium partitioning of a non-ionic surfactant and pentachlorophenol between water and a non-aqueous phase liquid

The partitioning of the non-ionic surfactant Tergitol NP-10 (TNP10) and pentachlorophenol (PCP) into a mineral oil light non-aqueous phase liquid (NAPL) were quantified in batch tests. Due to the ionizable nature of PCP, the effects of pH and ionic strength (micro) on the equilibrium partitioning were evaluated. NAPL:water partition coefficients (K(n:w)) of TNP10 ranged from 3 to 7 l(water)/l(NAPL). Enhanced PCP dissolution into water from the NAPL was achieved at aqueous TNP10 concentrations > or =200mg/l. Surfactant addition of 1200 mg/l TNP10 increased the aqueous PCP concentrations by 14-fold at pH 5 versus 2 to 3-fold at pH 7 as compared to PCP aqueous solubility. The more significant response at the lower pH is likely due to the greater hydrophobicity of PCP at the lower pH, which is approaching PCP's pK(a) of 4.7. Higher ionic strength (micro 0.11 versus 0.001 M) increased K(n:w) of PCP by 10-33% without surfactant, compared to a more than 150% increase with a dose of 4000 mg/l TNP10. This work contributes information relevant to the application of surfactants to remediate sites contaminated with NAPLs.     

ALTERED MOBILITY OF BENZ[a]ANTHRACENE IN THE PRESENCE OF p-XYLENE AND ITS IMPACT ON RISK IN THE SUBSURFACE

Concerns over soil and groundwater contamination by PAHs have been raised as they are often introduced into the subsurface as nonaqueous-phase liquid (NAPL) mixtures. However, characterizing the risk posed by a mixture of chemicals is a challenging task due to its uncertainty in quantifying the effects of the interaction between substances. This study focuses on the effects of phase-transforming interaction on the fate, transport, and risk assessment of a PAH in a PAH - NAPL mixture. The cell test was carried out using benz[a]anthracene (BaA) and p-xylene to verify the increased mobility of highly sorbed pollutants in the presence of less sorbed, mobile liquid pollutants. The experimental results showed that BaA had greater mobility in the presence of p-xylene than in its absence. The main transport mechanisms in the vadose zone were by dissolution into p-xylene or water. The developed model showed that transport of BaA was significantly faster in the presence of NAPL, but needs improvement. As well, risk assessment indicated that the oral carcinogenic risk of BaA calculated with the concentration in groundwater was 15～ 87 times larger when mixed with NAPL than when present as a single contaminant. This study demonstrated that consideration of phase-transforming interaction is necessary to analyze the risk of a PAH - NAPL mixture. The improvement of the transport model will be the topic of our continuing research.     

Influence of Initial Water Saturation on the Residual Saturation of an Organic Liquid in the Vadose Zone

The influence of initial water saturation on residual organic saturation was studied by determining pressure-saturation relationships with a pressure cell apparatus. Initial water saturations varying from 0.02 to 0.40 were investigated by allowing gasoline to imbibe into the water wet soil, displacing the mobile air. Subsequently, air was forced back into the soil in a series of incremental steps that resulted in gasoline drainage and a measurement of residual gasoline saturation. The data indicate that water saturation must be considered in order to understand retention and movement of liquid organics in the vadose zone. It is concluded that residual organic saturation decreases by the amount that initial water saturation increases, as Leverett's assumption implies, for initial water saturations less than a critical value. For initial water saturations greater than the critical values the residual oil saturation remained constant, the total liquid saturation increased and Leverett's assumption was no longer valid.     

Modeling water–NAPL–air three-phase capillary behavior in soils

Existing models relating the capillary pressures and the degrees of saturation of NAPL, water, air three-phase are briefly reviewed, and their limitations are discussed. Of particular interest is their inability to describe the intrusion and emission behavior of NAPL precisely, especially when the degree of saturation of NAPL is low. A new variable μ representing the relative magnitude of NAPL pressure to water pressure and air pressure is then defined as a state parameter, and a novel water–NAPL–air three-phase model is proposed, which considers the transition phenomena between water–NAPL–air three-phase and arbitrary two-phase system through the parameter μ. The validity of the proposed model is checked by comparing the calculation results with those obtained by an ordinary model and existing experimental results. The proposed model is shown capable of properly considering the interactions among the water–NAPL–air three-phase, and predicting the actual retention behavior of NAPL in unsaturated ground.     

Non-equilibrium partitioning tracer transport in porous media: 2-D physical modelling and imaging using a partitioning fluorescent dye

This paper describes an investigation into non-equilibrium partitioning tracer transport and interaction with non-aqueous-phase liquid (NAPL) contaminated water-saturated porous media using a two-dimensional (2-D) physical modelling methodology. A fluorescent partitioning tracer is employed within a transparent porous model which when imaged by a CCD digital camera can provide full spatial tracer concentrations and tracer breakthrough curves. Quasi one-dimensional (1-D) benchmarking tests in models packed with various combinations of clean quartz sand and NAPL are described. These modelled residual NAPL saturations, S_n, of 0–15%. Results demonstrated that the fluorescent partitioning tracer was able to detect and quantify the presence of NAPL at low flow rates. At larger flow rates and/or higher NAPL saturations, the tracer increasingly underpredicted the NAPL volume as expected and this is attributed primarily to non-equilibrium partitioning. Despite little change in permeability, change in NAPL saturations from 4% to 8% resulted in significant NAPL saturation underestimates at the same flow rates implying coalescence of NAPL into wider separated but larger ganglia. A 2-D investigation of an idealised heterogeneous residual NAPL contaminated flow field indicated little permeability change in the NAPL contaminated zone and thus little flow bypassing, leading to reduced underpredictions of NAPL saturations than for equivalent quasi 1-D cases. This was attributed to increased ‘sampling’ of the NAPL by the tracer. The process is clearly visually identifiable from the experimental images. This rapid and relatively inexpensive experimental method is of value in laboratory studies of partitioning tracer behaviour in porous media; in particular, the ability to observe full field concentrations makes it valuable for the study of complex heterogeneous systems.     

Microbial uptake of diesel oil sorbed on soil and oil spill clean‐up sorbents

Sorbent effects in the microbial uptake of diesel oil were determined for black cotton soil (BCS) and two oil spill clean‐up sorbents, ie peat sorb and spill sorb. Biodegradation studies were conducted in mass transfer limited batch slurry microcosms using microorganisms capable of direct interfacial uptake of diesel oil. Under identical loading conditions, the amounts of diesel oil initially loaded on the various sorbents were 178, 288 and 649 mg g^?1 for BCS, spill sorb and peat sorb, respectively. Total biodegradation of sorbed diesel was comparable for all the sorbents (45–52 mg), however, the biodegradation rates were significantly different. Peat sorb demonstrated a distinct initial lag phase, the biodegradation rate in spill sorb was initially slower, whereas biodegradation at a high rate commenced immediately for BCS. The maximum biodegradation rates observed for BCS, spill sorb and peat sorb microcosms were 7.9, 5, and 2.9 mg day^?1, respectively. Thus, the maximum biodegradation rate increased as the diesel oil loading decreased. Our results indicate that spill clean‐up sorbents have greater bioavailability limitations compared with soils and this is linked with their significantly higher loading capacity and internal porosity. Copyright © 2005 Society of Chemical Industry     

The photodegradation of trichloroethylene with or without the NAPL by UV irradiation in surfactant solutions

The photodegradation of trichloroethene (TCE) with or without nonaqueous phase liquids (NAPL) by ultraviolet irradiation in surfactant solutions was examined in this study. The photodecay of TCE was studied at monochromatic 254 nm UV lamps. The effects of the type of surfactants, initial surfactant concentrations, pH levels and NAPL concentrations were examined to explore the photodecay of TCE. All the photodegradation of TCE followed pseudo-first-order decay kinetics at various conditions. It was found that Brij 35 overdose and higher initial pH levels may retard or inhibit the photodecay of TCE, mainly due to protons, intermediate generation and change of surfactant structure in the processes. The optimal condition for TCE photodecay was suggested based on the analysis of kinetics data, from which the reaction mechanism of TCE in the presence of NAPL form was also studied. In general, the reactions of TCE in micellar solution and NAPL pool can be considered as independent pathways due to the low molecule diffusion between the two phases.     

Effect of temperature on cosolvent flooding for the enhanced solubilization and mobilization of NAPLs in porous media

This paper examines the potential for enhanced NAPL recovery from the subsurface through the combined application of hot water and cosolvent flushing. Batch experiments were conducted to determine the effect of temperature on fluid properties and the multiphase behavior of the ethanol-water-toluene system and to assess the impact of temperature on the capillary, Bond and total trapping numbers and on flooding stability. Column flooding experiments were also conducted to evaluate toluene NAPL recovery efficiency for different ethanol contents and flushing solution temperatures. The ethanol content considered ranged from 20 to 100% by mass, while the flushing solution temperatures were varied from 10 to 40°C. It is shown that small variations in the system temperature can strongly influence the solubilization, mobilization and stability of the multiphase system, but that the impact of temperature on the enhanced NAPL recovery is also dependent on the ethanol content of the flushing solution. The impact of hot water on NAPL recovery was most pronounced at intermediate ethanol contents (40-60% by mass) where the increase in system temperature led to enhanced NAPL solubilization as well as NAPL mobilization. This study demonstrates that coupling of hot water with in situ cosolvent flooding is a potentially effective remedial alternative that can optimize NAPL recovery while reducing the amount of chemicals injected into the subsurface.