包气带土壤对石油烃的截留作用研究

包气带土壤对地表污染物的截留作用是地下水免受污染的一道天然屏障.土壤截留污染物的能力对地下水石油烃污染的治理起着关键作用,在对实际污染场地进行调查研究的基础上,采用土柱淋滤、吸附/解吸、石油挥发等室内模拟实验,研究包气带土壤对石油烃的截留作用及其影响因素.结果表明,细砂、中砂和粗砂3种土壤对石油烃的截留率分别为81.0...     

Investigation of the vadose zone using barometric pressure cycles

This paper documents a technique for investigating one-dimensional airflow in the vadose zone. Variations in pore gas pressures resulting from barometric cycles were measured at depths as great as 180 m in several gas monitoring wells. The data were transformed to the frequency domain, enabling comparison with closed-form analytic expressions of one-dimensional pressure transport in layered porous media. The data reveal evidence for vertical fracture flow that was not apparent from in situ measurements of permeability. The data also reveal that the basalt underlying the site at depths greater than 100 m has permeability exceeding 1000 darcies, and is vented to the atmosphere at an estimated distance of a few kilometers from the site.     

Plant aided bioremediation in the vadose zone: model development and applications

Phytoremediation has the potential to enhance clean up of land contaminated by various pollutants. A mathematical model that includes a two-fluid phase flow model of water flow as well as a two-region soil model of contaminant reactions was developed and applied to various bioremediation scenarios in the unsaturated zone, especially to plant-aided bioremediation. To investigate model behavior and determine the main parameters and mechanisms that affect bioremediation in unplanted and planted soils, numerical simulations of theoretical scenarios were conducted before applying the model to field data. It is observed from the results that parameters affecting the contaminant concentration in the water phase, such as aqueous solubility, the octanol-water partition coefficient, and organic carbon content of the soil controlled the contaminant fate in the vadose zone. Simulation using the developed model also characterized the fate and transport of the contaminants both in planted and unplanted soils satisfactorily for field applications. Although phytoremediation has the potential for remediation of contaminated soils, results from both modeling and field studies suggested that plants may not always enhance the remediation efficiency when the soil already has a high microbial concentration, when the contaminant bioavailability is low, or when the overall reaction is mass transfer-limited. Therefore, other steps to increase contaminant bioavailability are needed in phytoremediation applications; natural purification mechanisms such as aging, volatilization, and natural bioremediation should be considered to maximize the plant effect and minimize the cost.     

Two-dimensional laboratory simulation of LNAPL infiltration and redistribution in the vadose zone

A quantitative two-dimensional laboratory experiment was conducted to investigate the immiscible flow of a light non-aqueous phase liquid (LNAPL) in the vadose zone. An image analysis technique was used to determine the two-dimensional saturation distribution of LNAPL, water and air during LNAPL infiltration and redistribution. Vertical water saturation variations were also continuously monitored with miniature resistivity probes. LNAPL and water pressures were measured using hydrophobic and hydrophilic tensiometers. This study is limited to homogeneous geological conditions, but the unique experimental methods developed will be used to examine more complex systems. The pressure measurements and the quantification of the saturation distribution of all the fluids in the entire flow domain under transient conditions provide quantitative data essential for testing the predictive capability of numerical models. The data are used to examine the adequacy of the constitutive pressure-saturation relations that are used in multiphase flow models. The results indicate that refinement of these commonly used hydraulic relations is needed for accurate model prediction. It is noted in particular that, in three-fluid phase systems, models should account for the existence of a residual NAPL saturation occurring after NAPL drainage. This is of notable importance because residual NAPL can act as a non negligible persistent source of contamination.     

有机污染物在包气带中迁移转化试验研究

采集一定浓度的有机物废水 ,在试验室内进行了静态吸附、静态降解和动态土柱试验 ,对COD在包气带中迁移转化规律进行研究 ,提出了描述COD在包气带中迁移转化规律的数学模型。结果表明 :包气带对COD的吸附过程是线性的 ,可用亨利吸附模式s =Kdc +s0 表示 ,吸附系数Kd=0 .0 6 93;包气带对COD的降解曲线基本符合一级动力学方程c =c0 e-k1 t,降解系数k1=0 .0 4 99d-1;弥散试验测得弥散系数D =0 .0 0 2 4 2m2 /d。COD在包气带中的迁移转化过程是弥散、吸附、降解等多种作用共同作用的结果。     

氨氮在滦河三角洲典型包气带介质上的吸附性能研究

包气带是地下水的天然屏障,也是氮素污染地下水的主要通道。以滦河三角洲包气带4种典型土壤粉砂、砂粉土、粉土以及粉质粘土为研究对象,通过静态吸附实验查明了4种不同土壤对氨氮的吸附性能。结果表明,4种土壤对氨氮的吸附主要发生在0~2 h,其吸附均符合二级吸附动力学方程;其等温吸附曲线均符合Langmuir模式,且最大吸附量分别为粉砂0.138 mg/g ,砂粉土0.382 mg/g,粉土0.428 mg/g,粉质粘土0.534 mg/g。不同土壤对氨氮的吸附能力随着粒径的不同差异较大,表现为：粉质粘土>粉土>砂粉土>粉砂。说明粉质粘土防污能力最强,粉土次之,粉砂最弱。结果为查明氨氮在包气带的迁移转化规律提供理论依据,同时对水土氮污染防治有着重要的意义。     

渗滤液污染包气带中铁的形态变化

从渗滤液场龄和包气带岩性两方面出发,研究了新、老渗滤液对亚粘土和细砂包气带环境中Fe的含量及存在形态的影响。结果表明：新、老渗滤液分别能使细砂包气带介质中除残渣态以外Fe的含量增加16.68%或降低13.82%。亚粘土比细砂作为包气带介质更能缓冲渗滤液对介质中Fe的影响程度,其受影响范围在包气带0～20 cm深度处。当亚粘土为介质的包气带被新渗滤液污染后,其表层介质中碳酸盐结合态Fe的含量会增加15倍之多,为缓冲渗滤液Fe的污染做出了巨大贡献,但这部分Fe的存在也是潜在的二次污染源,在环境pH急剧变化的情况下,它可能会引起地下水高铁污染。     

An analytical solution for vertical transport of volatile chemicals in the vadose zone

An analytical solution is presented for one-dimensional vertical transport of volatile chemicals through the vadose zone to groundwater. The solution accounts for the important transport mechanisms of the steady advection of water and gas, diffusion and dispersion in water and gas, as well as adsorption, and first-order degradation. By assuming a linear, equilibrium partitioning between water, gas and the adsorbed chemical phases, the dependent variable in the mathematical model becomes the total resident concentration. The general solution was derived for cases having a constant initial total concentration over a discrete depth interval and a zero initial total concentration elsewhere. A zero concentration gradient is assumed at the groundwater table. Examples are given to demonstrate the application of the new solution for calculating the case of a non-uniform initial source concentration, and estimating the transport of chemicals to the groundwater and the atmosphere. The solution was also used to verify a numerical code called VLEACH. We discovered an error in VLEACH, and found that the new solution agreed very well with the numerical results by corrected VLEACH. A simplified solution to predict the migration of volatile organic chemical due to the gas density effect has shown that a high source concentration may lead to significant downward advective gas-phase transport in a soil with a high air-permeability.     

粉砂介质包气带中甲苯蒸气挥发的迁移转化

挥发性有机物通过包气带向上迁移到地表或室内引发的蒸气入侵(vapor intrusion,VI)问题越发受到关注。以甲苯作为研究对象,采用土柱实验,对粉砂介质包气带中甲苯蒸气发生的扩散、生物降解的规律进行研究。结果表明,粉砂介质中,甲苯蒸气扩散穿透70 cm土柱时间为24 h,甲苯蒸气在土柱内扩散10 d时,土柱达到短暂的稳态平衡;12～30 d时,处于动态平衡;30～60 d时,土柱内甲苯蒸气浓度下降速率发生变化,土柱内O2体积分数下降了8%,CO2体积分数增加了6%;60 d时,甲苯蒸气基本被降解。说明覆盖一定厚度的粉砂对甲苯蒸气的迁移有比较好的阻滞和生物降解作用。     

基于DRASTIC的包气带阻滞污染物能力研究

对包气带阻滞污染物能力的研究,对保护地下水脆弱性具有重要意义.将传统DRASTIC方法结合层次分析法,对各评价指标赋以新的权重,并依托地理信息系统(GIS)平台,对河南省土壤包气带阻滞污染物的能力进行评价,得到了河南省包气带阻滞能力分区图.结果表明,针对现状所进行的包气带阻滞污染物能力的评价对实际情况具有一定的代表性,...     

Transport and degradation of propylene glycol in the vadose zone: model development and sensitivity analysis

Transport and degradation of de-icing chemical (containing propylene glycol, PG) in the vadose zone were studied with a lysimeter experiment and a model, in which transient water flow, kinetic degradation of PG and soil chemistry were combined. The lysimeter experiment indicated that aerobic as well as anaerobic degradation occurs in the vadose zone. Therefore, the model included both types of degradation, which was made possible by assuming advection-controlled (mobile) and diffusion-controlled (immobile) zones. In the mobile zone, oxygen can be transported by diffusion in the gas phase. The immobile zone is always water-saturated, and oxygen only diffuses slowly in the water phase. Therefore, the model is designed in a way that the redox potential can decrease when PG is degraded, and thus, anaerobic degradation can occur. In our model, manganese oxide (MnO₂, which is present in the soil) and NO \(_{3}^{-}\) (applied to enhance biodegradation) can be used as electron acceptors for anaerobic degradation. The application of NO \(_{3}^{-}\) does not result in a lower leaching of PG nor in a slower depletion of MnO₂. The thickness of the snowcover influences the leached fraction of PG, as with a high infiltration rate, transport is fast, there is less time for degradation and thus more PG will leach. The model showed that, in this soil, the effect of the water flow dominates over the effect of the degradation parameters on the leaching at a 1-m depth.     

Assessing tungsten transport in the vadose zone: from dissolution studies to soil columns

This study investigates the dissolution, sorption, leachability, and plant uptake of tungsten and alloying metals from canister round munitions in the presence of model, well characterized soils. The source of tungsten was canister round munitions, composed mainly of tungsten (95%) with iron and nickel making up the remaining fraction. Three soils were chosen for the lysimeter studies while four model soils were selected for the adsorption studies. Lysimeter soils were representatives of the typical range of soils across the continental USA; muck-peat, clay-loamy and sandy-quartzose soil. Adsorption equilibrium data on the four model soils were modeled with Langmuir and linear isotherms and the model parameters were obtained. The adsorption affinity of soils for tungsten follows the order: Pahokee peat > kaolinite > montmorillonite > illite. A canister round munition dissolution study was also performed. After 24 d, the measured dissolved concentrations were: 61.97, 3.56, 15.83 mg L⁻¹ for tungsten, iron and nickel, respectively. Lysimeter transport studies show muck peat and sandy quartzose soils having higher tungsten concentration, up to 150 mg kg⁻¹ in the upper layers of the lysimeters and a sharp decline with depth suggesting strong retardation processes along the soil profile. The concentrations of tungsten, iron and nickel in soil lysimeter effluents were very low in terms of posing any environmental concern; although no regulatory limits have been established for tungsten in natural waters. The substantial uptake of tungsten and nickel by ryegrass after 120 d of exposure to soils containing canister round munition suggests the possibility of tungsten and nickel entering the food chain.     

Natural attenuation/phytoremediation in the vadose zone of a former industrial sludge basin

The natural attenuation of polyaromatic hydrocarbons (PAHs) in the vadose zone of a naturally revegetated former industrial sludge basin (0.45 ha) was examined. This was accomplished by comparing the concentration of 16 PAH contaminants present in sludge collected below the root zone of plants with contaminants present at 3 shallower depths within the root zone. Chemical analysis of 240 samples from 60 cores showed the average concentration of total and individual PAHs in the 0-30 cm, 30-60 cm, and bottom of the root zone strata were approximately 10, 20, and 50%, respectively, of the 16, 800 ppm average total PAH concentration in deep non-rooted sludge. Statistically significant differences in average PAH concentrations were observed between each strata studied and the non-rooted sludge except for the concentrations of acenaphthene and chrysene present at the bottom of the root zone in comparison to sludge values. The rooting depth of the vegetation growing in the basin was dependent on both vegetation type and plant age. Average rooting depths for trees, forbs (herbaceous non-grasses), and grasses were 90, 60, and 50 cm, respectively. The deepest root systems observed (100-120 cm) were associated with the oldest (12-14 year-old) mulberry trees. Examination of root systems and PAH concentrations at numerous locations and depths within the basin indicated that plant roots and their microbially active rhizospheres fostered PAH disappearance; including water insoluble, low volatility compounds, i.e. benzo(a)pyrene and benzo(ghi)perylene. The reduced concentration of PAHs in the upper strata of this revegetated former sludge basin indicated that natural attenuation had occurred. This observation supports the concept that through appropriate planting and management practices (phytoremediation) it will be possible to accelerate, maximize, and sustain natural processes, whereby even the most recalcitrant PAH contaminants (i.e. benzo(a)pyrene) can be remediated over time.     

An analytical solution for estimating percolation rate by fitting temperature profiles in the vadose zone

We present a simple analytical solution for one-dimensional steady heat transfer with convection and conduction through a multilayer system such as a vadose zone. We assume that each layer is homogeneous and has a constant thermal diffusivity. The mass/heat flow direction is perpendicular to the layers, and the mass flow rate is a constant. The analytical solution presented in this study also assumes constant known temperatures at the two boundaries of the system. Although the analytical solution gives the temperature as a function of a few parameters, we focus on the inverse application to estimate the percolation rate in a vadose zone. Example applications have shown that with reliable field observation data, the solution can be used to determine the percolation rate to high degree of accuracy (e.g., to mm/year). In some other cases, the solution may also be helpful in characterizing potential lateral flow along layer divides.     

Estimation of primary drainage three-phase relative permeability for organic liquid transport in the vadose zone

The modeling of transport of organic liquid contaminants through the vadose zone often requires three-phase relative permeabilities. Since these are difficult to measure, predictive models are usually used. The objective of this study is to assess the ability of eight common models to predict the drainage relative permeability to oil in a three-phase system (water-oil-air). A comparison of the models' estimates using data set from Oak [Oak, M.J., 1990. Three-phase relative permeability of water-wet Berea. In: Seventh Symposium on Enhanced Oil Recovery, Paper SPE/Doe 20183. Tulsa, OK, April 22-25] showed that they provide very different predictions for the same system. The goodness of the models does not increase with the amount of data or computation that the models require. Also, the calculations showed how different interpretations of the models and of the terminology associated with them can significantly impact the predictions. Thus, considerable error may be introduced into the simulations of organic liquid transport in the vadose zone depending on the selection and interpretation of the three-phase relative permeability model.     

Effects of soil moisture and physical-chemical properties of organic pollutants on vapor-phase transport in the vadose zone

Vapor-phase transport of organic pollutants is one of the important pathways in the distribution and attenuation of volatile organic compounds in the vadose zone. In this study, the impact of vapor-phase partitioning and of the physical-chemical properties of organic pollutants on vapor-phase transport was assessed. An experimentally derived relationship to predict vapor sorption for a variety of soil types under varying soil moisture conditions was incorporated into the two-dimensional finite-element model, Vocwaste. The revised model was then used to simulate the transport of volatile organics. Vapor-phase partitioning in the model accounted for vapor uptake by sorption onto moist mineral surfaces as well as sorption at the liquid-solid interface and dissolution into soil water. Under dry conditions, vapor-phase sorption of volatile organic pollutants was shown to have a retarding effect on transport of organic vapors. However, for shallow, contaminated soils, volatilization was controlled by vapor diffusion, even under dry conditions where vapor-phase sorption was high. The influence of Henry's law constant and of the aqueous-phase (solid-liquid) partition coefficient for volatile organic pollutants was considered in the simulations. Volatilization of organic vapors was shown to be favored for contaminants with high Henry's law constants and low aqueous-phase partitioning coefficients. Because of the interdependence of these two physical-chemical properties, individual properties of the contaminant should not be considered in isolation in the evaluation of vapor transport.     

不同龄渗滤液及其在包气带中的迁移转化研究

通过实验室土柱模拟方法,研究了不同场龄渗滤液中有机污染物、氮以及 Fe、Mn、Zn、Cd 在包气带中的迁移转化规律.结果表明,不同场龄渗滤液理化性质差别很大,随着场龄的增加,COD 从40 194 mg/L降低到 1 778 mg/L,NH4 浓度从1 758 mg/L升高到2 166 mg/L,金属浓度则减小.经过以细砂为介质的包气带后,新渗滤液易对地下水造成高浓度有机物污染,而老渗滤液更容易造成地下水的高浓度氮污染.Fe、Mn 和 Cd 在包气带中比较稳定,而 Zn 的迁移能力很强,易对地下水构成威胁.     

The application of ground penetrating radar attenuation tomography in a vadose zone infiltration experiment

Cross-borehole ground penetrating radar (XBGPR) is used in monitoring a long-term vadose zone infiltration experiment at a test site in Socorro, NM in order to examine contaminant transport in the vadose zone. XBGPR attenuation tomography is conducted in order to test the ability of using images of electromagnetic attenuation for hydrogeologic investigations. The results of four pre-infiltration attenuation inversions shows standard deviations below 0.1 Np/m, and demonstrate the consistency of the XBGPR tomography technique for making time-lapse observations. Correlation to the core records indicates that XBGPR attenuation tomograms provide high-resolution images of clay distribution in the vadose zone. Water infiltration at the ground surface was initiated in February 1999 at a constant rate of 2.7 cm/day, and continued at this rate throughout the data collection experiment. Time-lapse attenuation tomograms show that attenuation increases by approximately 0.3 Np/m during the water infiltration, and indicate a snowplow effect may be occurring where salts are dissolved by the water and concentrated at the front of the plume. Seasonal temperature changes may also cause changes in electromagnetic attenuation images, and masking the evidence of water infiltration. Thus caution must be taken when using time-lapse attenuation images to interpret the movement of a water plume during a long-term experiment as temperature changes.     

Coupled effects of solution chemistry and hydrodynamics on the mobility and transport of quantum dot nanomaterials in the vadose zone

To investigate the coupled effects of solution chemistry and hydrodynamics on the mobility of quantum dot (QD) nanoparticles in the vadose zone, laboratory scale transport experiments involving single and/or sequential infiltrations of QDs in unsaturated and saturated porous media, and computations of total interaction and capillary potential energies were performed. As ionic strength increased, QD retention in the unsaturated porous media increased; however, this retention was significantly suppressed in the presence of a non-ionic surfactant in the infiltration suspensions as indicated by surfactant enhanced transport of QDs. In the vadose zone, the non-ionic surfactant limited the formation of QD aggregates, enhanced QD mobility and transport, and lowered the solution surface tension, which resulted in a decrease in capillary forces that not only led to a reduction in the removal of QDs, but also impacted the vadose zone flow processes. When chemical transport conditions were favorable (ionic strength of 5 × 10(-4)M and 5 × 10(-3)M, or ionic strengths of 5 × 10(-2)M and 0.5M with surfactant), the dominating phenomena controlling the mobility and transport of QDs in the vadose zone were meso-scale processes, where infiltration by preferential flow results in the rapid transport of QDs. When chemical transport conditions were unfavorable (ionic strength of 5 × 10(-2)M and 0.5M) the dominating phenomena controlling the mobility and transport of QDs in the vadose zone were pore-scale processes governed by gas-water interfaces (GWI) that impact the mobility of QDs. The addition of surfactant enhanced the transport of QDs both in favorable and unfavorable chemical transport conditions. The mobility and retention of QDs was controlled by interaction and capillary forces, with the latter being the most influential. GWI were found to be the dominant mechanism and site for QD removal compared with solid-water interfaces (SWI) and pore straining. Additionally, ripening phenomena were demonstrated to enhance QDs removal or retention in porous media and to be attenuated by the presence of surfactant.     

定流量条件下壤土夹层对苯和Br-穿透包气带的影响

为研究壤土夹层对VOCs穿透包气带的影响,以苯为特征污染物,NaBr为示踪剂开展室内的渗流模拟土柱实验。模拟包气带对苯的吸附实验,确定吸附平衡时间、最大吸附量,分析砂土和粉质壤土对苯的吸附特性;设计渗流土柱实验,模拟定流量降雨条件下苯在不同构型的土柱中的迁移过程,分析壤土夹层在时间和空间上对苯穿透包气带到达含水层的影响,并得出弥散系数和阻滞系数。结果表明,相对黏粒含量高的壤土夹层显著改变了溴离子的穿透曲线,对污染水有明显的阻水减渗作用,不仅延迟了苯向含水层的迁移时间,而且在一定程度上起到了吸附、降解的阻滞作用,有效降低了特征污染物穿透包气带的含量。