The soil and ground water at a General Motors plant site were contaminated with petroleum products from leaking underground storage tanks. Based on the initial assessment, the site was complex from the standpoint of geology (clay layers), hydrology (a recharge zone with a perched water table), and contaminant (approximately 4800 gallons of mixed gasoline and oil). After a thorough study of remedial alternatives, a synergistic remedial approach was adopted including pump and treat, product removal, vapor extraction, and bioventing. The system was designed and implemented at the site through 22 dual-extraction wells. Over a 21-month period, 4400 gallons of gasoline and oil were removed from the system, including 59 percent by vapor extraction, 28 percent by bioventing, and 13 percent by pump and treat. Synergism between the various remedial methods was demonstrated clearly. Ground water pump and treat lowered the water table, allowing air to flow for vapor extraction. The vacuum applied for vapor extraction increased the ground water removal rate and the efficiency of pump and treat. The vapor extraction system also added oxygen to the soil to stimulate aerobic biodegradation. 相似文献
This study looks at the influence of surface covers on the performance of a single pumping well system. Pilot tests were conducted on a sandy soil to determine the influence of surface confinement based upon both induced vacuum and pore gas velocity design criteria. The results demonstrate how covering the surface can significantly alter the associated air flow patterns and velocity distribution. Comparison of streamline iso‐contours obtained in covered scenarios reveals that the surface seal tended to prevent air from entering the subsurface near the extraction well and force air to be drawn from a greater distance. Calculated and measured pressure differentials, for open and semi‐confined scenarios, clearly show that adding a clay layer as a surface cover increased the vacuum induced within the soil. Pore gas velocity analysis showed that when the cover clay layer was used, the zone of capture of the soil vapor extraction system increased. The radius of influence of soil vapor extraction (SVE) systems, based on the attainment of a critical vacuum or pore gas velocity, can then be increased by including a surface seal in the design of such systems. The focus of this study is limited to air flow patterns contrasted between covered and uncovered conditions and not on the nuances of a full scale remediation implementation. 相似文献
Site closure for soil vacuum extraction (SVE) application typically requires attainment or specified soil concentration standards based on the premise that mass flux from the vadose zone to ground water not result in levels exceeding maximum contaminant levels (MCLs). Unfortunately, realization of MCLs in ground water may not be attainable at many sites. This results in soil remediation efforts that may be in excess of what is necessary for future protection of ground water and soil remediation goals which often cannot be achieved within a reasonable time period. Soil venting practitioners have attempted to circumvent these problems by basing closure on some predefined percent total mass removal, or an approach to a vapor concentration asymptote. These approaches, however, are subjective and influenced by venting design. We propose an alternative strategy based on evaluation of five components: (1) site characterization, (2) design. (3) performance monitoring, (4) rule-limited vapor transport, and (5) mass flux to and from ground water. Demonstration of closure is dependent on satisfactory assessment of all five components. The focus of this paper is to support mass flux evaluation. We present a plan based on monitoring of three subsurface zones and develop an analytical one-dimensional vertical flux model we term VFLUX. VFLUX is a significant improvement over the well-known numerical one-dimensional model. VLEACH, which is often used for estimation of mass flux to ground water, because it allows for the presence of nonaqueous phase liquids (NAPLs) in soil, degradation, and a lime-dependent boundary condition at the water table inter-face. The time-dependent boundary condition is the center-piece of our mass flux approach because it dynamically links performance of ground water remediation lo SVE closure. Progress or lack of progress in ground water remediation results in either increasingly or decreasingly stringent closure requirements, respectively. 相似文献
Increasingly, regulations by federal, state and local agencies are being developed that require the installation of vadose zone monitoring systems for hazardous chemical facilities in addition to, or in lieu of, conventional ground water monitoring wells. Compared to a ground water monitoring approach, vadose zone monitoring systems may permit earlier detection of chemical leakage and less costly cleanup of contamination. The effective use of vadose zone monitoring systems in detecting contamination depends on many factors. Without proper consideration of these factors, a vadose zone monitoring system may not give as high a level of reliability as a ground water monitoring system. Major factors to consider in installing a vadose zone monitoring system are: type of instrument to use, number of instruments, depth and location of instruments, and frequency of monitoring. Means to evaluate these factors in a comprehensive fashion have been lacking. Based on recent experience in installing and operating vadose zone monitoring systems, criteria and methods useful in resolving the preceding factors have been developed. Types of instruments can be classified as either direct (lysimeter, vapor probe) or indirect (tensiometer, conductivity probe). A combination of the two is needed for reliability. The depth, location and number of instruments depend on the geometry of the facility, the number and size of likely contaminant leakage points in engineered barriers, properties of the material being monitored, the effective radius of monitoring for each instrument, vadose zone properties, and types of remedial actions that are available. The freqency of monitoring largely depends on the rate of movement of the contaminant. Evaluating the preceding factors requires some level of modeling and preliminary field testing. 相似文献
Vapor extraction (soil venting) has been demonstrated to be a successful and cost-effective remediation technology for removing VOCs from the vadose (unsaturated) zone. However, in many cases, seasonal water table fluctuations, drawdown associated with pump-and-treat remediation techniques, and spills involving dense, non-aqueous phase liquids (DNAPLS) create contaminated soil below the water table. Vapor extraction alone is not considered to be an optimal remediation technology to address this type of contamination. An innovative approach to saturated zone remediation is the use of sparging (injection) wells to inject a hydrocarbon-free gaseous medium (typically air) into the saturated zone below the areas of contamination. The contaminants dissolved in the ground water and sorbed onto soil particles partition into the advective air phase, effectively simulating an in situ air-stripping system. The stripped contaminants are transported in the gas phase to the vadose zone, within the radius of influence of a vapor extraction and vapor treatment system. In situ air sparging is a complex multifluid phase process, which has been applied successfully in Europe since the mid-1980s. To date, site-specific pilot tests have been used to design air-sparging systems. Research is currently underway to develop better engineering design methodologies for the process. Major design parameters to be considered include contaminant type, gas injection pressures and flow rates, site geology, bubble size, injection interval (areal and vertical) and the equipment specifications. Correct design and operation of this technology has been demonstrated to achieve ground water cleanup of VOC contamination to low part-per-billion levels. 相似文献
In situ thermal desorption (ISTD) was used for the treatment of eight separate source zones containing chlorinated solvents in a tight loess (silt/clay) above the water table. The source areas were as much as 365 m (1200 feet) apart. A target volume of 38,200 m3 (49,950 cubic yards) of subsurface material to a depth of 9.1 m (30 feet) was treated in a period of 177 days. Energy was delivered through 367 thermal conduction heater borings, and vapors were extracted from 68 vertical vacuum wells. A vapor extraction and capture system, including a surface cover and vertical vacuum wells next to heater borings, provided for effective pneumatic control and capture of the chlorinated volatile organic compound (CVOC) vapors. A central treatment system, based on condensation and granular activated carbon filtration, was used to treat the vapors. Approximately 5675 kg (12,500 pounds) of contaminants was recovered in the extracted vapors. Forty-seven soil samples were used to document remedial performance. Based on these, the concentrations of the target contaminants were reduced to below the target remedial goals in all eight areas, typically with concentrations below 0.01 mg/kg in locations that had had CVOC concentrations higher than 1000 mg/kg. Turn-key costs for the thermal remediation were $3.9 million, and the unit treatment cost, including all utilities, was $103 per cubic meter treated ($79 per cubic yard). 相似文献
In situ air sparging (IAS) is becoming a widely used technology for remediating sites contaminated by volatile organic materials such as petroleum hydrocarbons. Published data indicate that the injection of air into subsurface water saturated areas coupled with soil vapor extraction (SVE) can increase removal rates in comparison to SVE alone for cases where hydrocarbons are distributed within the water saturated zone. However, the technology is still in its infancy and has not been subject to adequate research, nor have adequate monitoring methods been employed or even developed. Consequently, most IAS applications are designed, operated, and monitored based upon the experience of the individual practitioner. The use of in situ air sparging poses risks not generally associated with most practiced remedial technologies: air injection can enhance the undesirable off-site migration of vapors and ground water contamination plumes. Migration of previously immobile liquid hydrocarbons can also be induced. Thus, there is an added incentive to fully understand this technology prior to application. This overview of the current state of the practice of air sparging is a review of available published literature, consultation with practitioners, a range of unpublished data reports, as well as theoretical considerations. Potential strengths and weaknesses of the technology are discussed and recommendations for future investigations are given. 相似文献
This paper provides a new analytical model of airflow to inlet wells in soil vapor extraction systems. It is based on a recent analytical solution of airflow to a single vapor extraction well by Bahr and Joss (1995), which updated the previous model of Baehr and Hult (1991). Baehr and Joss (1995) treated the air leakage through the surface as an air flux boundary condition, whereas Baehr and Hult (1991) approximated the leakage as a distributed source imposed in the governing airflow equation. The new analytical model shows significant improvement on air-flow assessment over the previous model by Ge and Liao (1996), which could underestimate the efficiency of airflow to inlet wells by as much as 27% in a typical vapor extraction system. 相似文献
In recent years there has been increasing interest in the application of passive technologies to reduce or remove contaminants from the subsurface environment including soil and ground water. In most cases, the impetus for this interest lies in a perceived savings compared with more traditional remedial alternatives. In a few cases, the infrastructure at contaminated sites, such as buildings, paved areas, and utilities, makes the use of conventional remedial measures difficult and expensive. To demonstrate that natural processes are effective in reaching established goals, it is necessary to determine that transformation processes are taking place at a rate that is protective of human health and the environment and that these processes will continue for an acceptable period of time. The basic conditions that must be present to confirm natural attenuation processes arc taking place are discussed along with the behavior of contaminated plumes, monitoring requirements, data analysis, rates of degradation, and mathematical modeling. 相似文献
Tremendous resources have been and continue to be spent investigating and remediating petroleum hydrocarbon compounds (PHCs) in soil and ground water. Investigating and planning a remedial strategy for sites affected by PHCs is often a challenging task because of the complex chemical nature of the PHCs. the complex regulatory environment related to PHC cleanup, and the use of analytical methods that provide quantitation but not identification of PHCs. From a technical standpoint, the PHC impacting soil and/or ground water is frequently inadequately characterised, both in identification as well as in is general properties (solubility, toxicity). From a regulatory standpoint, promulgated or recommended total petroleum hydrocarbon (TPH) cleanup levels generally relate to assumed properties of specific unweathered products and are inconsistent among different agencies and regions. This produces a prime situation for unwillingly spending more resources on investigation or remediation than may be necessary, especially when the PHC in the subsurface has different properties from unweathered products such as gasoline or diesel. Accurately identifying the PHC and its nature, a process known as fingerprint characterization, is critical to the determination of appropriate regulatory goals and design of cost-effective remedial approaches. This paper presents several case studies in which fingerprint characterization made a significant difference in the project outcome. In each instance the nature of the organic material was better understood, the regulatory cleanup levels were negotiated based on the nature of the material, and a remedial approach was implemented that differed significantly from and was generally less costly than what would have been required without fingerprint characterization data. 相似文献
The results of an API-sponsored pilot-scale subsurface venting system study are presented. The purpose of this study was to evaluate the effectiveness of forced venting techniques in controlling and removing hydrocarbon vapors from a subsurface formation. Both qualitative and quantitative sampling and analytical procedures were developed to measure hydrocarbon vapors extracted from the soil. Vapor recovery and equivalent liquid product recovery rates were measured at each test cell evacuation rate. Two identical test cells were installed. Each cell contained 16 vapor monitoring probes spaced at distances from 4 to 44 feet from a vapor extraction (vacuum) well. Each cell was also configured with two air inlet wells to allow atmospheric air to enter the subsurface formation. The vapor monitoring probes were installed at three discrete elevations above the capillary zone. In situ vapor samples were obtained periodically from these probes to measure changes in vapor concentration and composition while extracting vapors from the vacuum well at three different flow rates (18.5 scfm, 22.5 scfm and 39.8 scfm). In situ vapor samples were analyzed using a portable gas chromatograph to quantify and speciate the vapors. Vacuum levels were also measured at each vapor sampling probe and at the vacuum well. The soil venting techniques evaluated during this study offer an alternative approach for controlling and eliminating spilled or leaked hydrocarbons from sand or gravel formations of high porosity and moderate permeability. These techniques may also be used to augment conventional liquid recovery methods. The data collected during this study will be useful in optimizing subsurface venting systems for removing and controlling hydrocarbon vapors in soil. Study results indicate pulsed venting techniques may offer a cost-effective means of controlling or eliminating hydrocarbon vapors in soil. 相似文献
Soil vapor extraction (SVE) is effective for removing volatile organic compound (VOC) mass from the vadose zone and reducing the potential for vapor intrusion (VI) into overlying and surrounding buildings. However, the relationship between residual mass in the subsurface and VI is complex. Through a series of alternating extraction (SVE on) and rebound (SVE off) periods, this field study explored the relationship and aspects of SVE applicable to VI mitigation in a commercial/light-industrial setting. The primary objective was to determine if SVE could provide VI mitigation over a wide area encompassing multiple buildings, city streets, and subsurface utilities and eliminate the need for individual subslab depressurization systems. We determined that SVE effectively mitigates offsite VI by intercepting or diluting contaminant vapors that would otherwise enter buildings through foundation slabs. Data indicate a measurable (5 Pa) influence of SVE on subslab/indoor pressure differential may occur but is not essential for effective VI mitigation. Indoor air quality improvements were evident in buildings 100 to 200 feet away from SVE including those without a measurable reversal of differential pressure across the slab or substantial reductions in subslab VOC concentration. These cases also demonstrated mitigation effects across a four-lane avenue with subsurface utilities. These findings suggest that SVE affects distant VI entry points with little observable impact on differential pressures and without relying on subslab VOC concentration reductions. 相似文献
Optimization of large‐scale injection‐based remedial systems requires engineering to intentionally capitalize on the biological, chemical, and physical mechanisms that occur within and between the zones of reagent application. These types of systems can be called hybrid designs as they employ multiple processes to achieve remediation endpoints ( Figure 1 ), resulting in optimized system performance and a reduction in the overall life‐cycle cost. While all remedial applications incorporate these mechanisms to some extent, the importance of each of these processes is magnified in large‐scale applications. This column discusses the dominant mechanisms responsible for mass reduction within both source and distal plume footprints, with a focus on the application of “Hybridized Design” for enhanced reductive dechlorination (ERD) systems. Figure 1 Open in figure viewer PowerPoint Diagram showing the hybrid design approach which encompasses physical (sorption, advection, diffusion), chemical (mass flux, abiotic degradation) and biological (metabolic and cometabolic degradation) processes. 相似文献
As a consequence of drilling and completion operations most wells have reduced permeability near the borehole (skin zone). This may result in a corresponding change of thermal conductivity of formations near the borehole. At the same time the mathematical models, which describe the process of thermal recovery of formations are based on the assumption that the thickness of the radius of heated zone (during circulation of the drilling mud) is significantly larger than the well radius (or skin zone). It is shown below that the radius of the heated zone (radius of investigation or radius of thermal influence) can be approximated by simple formulas. It is shown that for six field bottom-hole temperature surveys the radius of thermal influence (RTI) is too small, and for this reason, the duration of the drilling mud circulation period should increased. 相似文献
本文利用卫星重力反演与模拟软件ANGELS系统(ANalyst of Gravity Estimation with Low-orbit Satellites)对低低跟踪模式的重力卫星的关键载荷精度指标进行了深入分析.模拟结果表明:(1)对短弧长积分法而言,在低低跟踪模式的关键载荷精度指标中,重力场反演精度对星间距离变率精度最为敏感;(2)通过对目前在轨运行GRACE的载荷指标进行分析,发现轨道数据的误差主要影响重力场的低阶部分(约小于25阶),较高阶次部分(约大于26阶)主要受星间距离变率的误差限制;(3)如果下一代低低跟踪模式的重力卫星的目标之一是把重力异常反演精度较GRACE提高约10倍,则在保持轨道高度和GRACE相同的前提下,轨道、星间距离变率和星载加速度计等关键载荷指标需要达到的最低精度分别约为2cm、10nm·s-1和3.0×10-10 m·s-2;(4)轨道精度和混频误差将是影响下一代低低跟踪模式重力卫星重力场恢复能力进一步提高的主要制约因素,距离变率精度和加速度计精度存在盈余. 相似文献
This article describes a simplified method to calculate a building-specific subslab to indoor air attenuation factor using data collected during pressure-field extension testing similar to industry standards for radon mitigation. It also describes a simplified method to calculate the radius of influence for a conventional suction point using a mass flux-balance model. The analysis is based on three simple measurements: (1) the extraction flow rate, (2) cross-slab applied vacuum at a radial distance of 3 feet, and (3) cross-slab applied vacuum at a radial distance of 10 feet. The intent is to provide a practitioner with a rapid and useful screening-level assessment of whether the benefits of reduced mitigation system costs warrant an investment in a more detailed mathematical analysis of the flow and vacuum data. In addition, this may also help a practitioner to make real-time decisions regarding placement of communication test points during pressure-field extension testing. 相似文献
We present explicit analytic solutions describing the hydraulic head and discharge vector for two-dimensional, steady groundwater flow past an impermeable barrier embedded in a regional flow field. We use the solution to investigate the effects of open vertical barriers on the flow field; in particular, we examine the hydraulic containment of contaminant plumes or source zones by combination of a vertical barrier wall and extraction wells. We quantify the local reduction in discharge rates due to the barrier wall and the local increase in the size of the capture zone of an extraction well near an open, up-gradient barrier. We find that the combination of an open vertical barrier with down-gradient extraction wells can be very effective in decreasing the well discharge rate necessary to control a contaminant plume or source area. Design charts are presented for quantifying the effects of the barrier wall on the hydraulic control of the groundwater flow field and for estimating the jump in head across a barrier. The charts are appropriate for use in the preliminary design and cost estimating of remedial systems, and for the design of dewatering systems. 相似文献
Nongyrotropic (gyrophase bunched) ion distributions in a magnetoplasma are studied by analytical methods and by two-dimensional hybrid code simulations. Nongyrotropy may not occur in a plasma being simultaneously homogeneous, stationary, and solenoidal in phase space. A detailed study is performed for a homogeneous and stationary plasma with sources and sinks in phase space. The analytical investigation cast in the framework of linearized Maxwell-Vlasov theory yields a coupling of low-frequency left-handed, right-handed, and longitudinal modes. Nongyrotropic ion distributions are unstable; they excite left-handed waves. The growth rate is comparable to that of the ion ring instability. The hybrid code simulation study confirms the expected propagation direction parallel to the background magnetic field. Three diffusion processes are studied: arc lengthening, arc broadening, and arc radius decreasing corresponding to particle energy diffusion. The characteristic diffusion time-scales are found to be of the order of 101 wave cycles. 相似文献
Subslab soil gas sampling and analysis is a common line of evidence for assessing human health risks associated with subsurface vapor intrusion to indoor air for volatile organic compounds; however, conventional subslab sampling methods have generated data that show substantial spatial and temporal variability, which often makes the interpretation difficult. A new method of monitoring has been developed and tested that is based on a concept of integrating samples over a large volume of soil gas extracted from beneath the floor slab of a building to provide a spatially averaged subslab concentration. Regular field screening is also conducted to assess the trend of concentration as a function of the volume removed to provide insight into the spatial distribution of vapors at progressive distances away from the point of extraction. This approach minimizes the risk of failing to identify the areas of elevated soil vapor concentrations that may exist between discrete sample locations, and can provide information covering large buildings with fewer holes drilled through the floor. The new method also involves monitoring the extraction flow rate and transient vacuum response for mathematical analysis to help interpret the vapor concentration data and to support an optimal design for any subslab venting system that may be needed. 相似文献
