Performance Of Soil Vapor Extraction Research Articles

Orthogonal Experimental Study on Remediation of Ethylbenzene Contaminated Soil by SVE

Soil vapor extraction (SVE) technology has strong potential value in the decontamination of soils dominated by volatile contaminants. In this paper, in order to evaluate in detail the influence of the main factors on the efficiency of SVE, L9(34) orthogonal tests and response surface analysis were carried out using a self-developed one-dimensional SVE system model. A first-order kinetic reaction model was also employed to analyze the relationship between pollutant concentration and time. The thermal reaction unit of SVE technology with a scale consistent with the soil column of the indoor test was simulated using COMSOL simulation software. The obtained results indicate that the most important factors affecting the performance of SVE are time, temperature, and contaminant concentration, while the influence of the extraction flow rate is not significant. A first-order kinetic reaction model can be used to predict the half-life of contaminant concentrations. Combined with the desirability function, the optimal conditions for the removal of ethylbenzene from soil were: time 180 min, temperature 20 °C, extraction flow 6000 mL/min, and contaminant concentration 2%. The developed numerical model, 3D-SVE, nicely simulates laboratory findings. These results can provide ideas to improve the efficiency of SVE.

A Rapid Decision Support Tool for Estimating Impacts of a Vadose Zone Volatile Organic Compound Source on Groundwater and Soil Gas

AbstractDiminishing rates of subsurface volatile contaminant removal by soil vapor extraction (SVE) oftentimes warrants an in‐depth performance assessment to guide remedy decision‐making processes. Such a performance assessment must include quantitative approaches to better understand the impact of remaining vadose zone contamination on soil gas and groundwater concentrations. The spreadsheet‐based Soil Vapor Extraction Endstate Tool (SVEET) software functionality has recently been expanded to facilitate quantitative performance assessments. The updated version, referred to as SVEET2, includes expansion of the input parameter ranges for describing a site (site geometry, source characteristics, etc.), an expanded list of contaminants, and incorporation of elements of the Vapor Intrusion Estimation Tool for Unsaturated‐zone Sources software to provide soil gas concentration estimates for use in vapor intrusion evaluation. As part of the update, SVEET2 was used to estimate the impact of a tetrachloroethene (PCE) vadose zone source on groundwater concentrations, comparing SVEET2 results to field‐observed values at an undisclosed site where SVE was recently terminated. PCE concentrations from three separate monitoring wells were estimated by SVEET2 to be within the range of 6.0–6.7 μg/L, as compared to actual field concentrations that ranged from 3 to 11 μg/L PCE. These data demonstrate that SVEET2 can rapidly provide representative quantitative estimates of impacts from a vadose zone contaminant source at field sites. In the context of the SVE performance assessment, such quantitative estimates provide a basis to support remedial and/or regulatory decisions regarding the continued need for vadose zone volatile organic compound remediation or technical justification for SVE termination, which can significantly reduce the cost to complete for a site.

Removal of Aliphatic Hydrocarbons from Gas Oil Contaminated Clay Soil via Soil Vapor Extraction

In this research, the performance of soil vapor extraction (SVE) for the removal of aliphatic hydrocarbons from gas oil contaminated soil and the effect of soil type on this method is studied. To explore the effect of soil type in the removal of hydrocarbons, SVE tests were conducted on 3 types of soil: 1) fine sand, 2) fine sand with 20% of kaolinite clay and 3) fine sand with 40% of kaolinite clay. Three extraction periods of 8, 16 and 24 hours were used. The results have shown that the efficiency of this method exceeds 78% in the removal of all hydrocarbons from fine sand, while an increase in clay significantly decreases the efficiency. Accordingly, the efficiency of this method decreases by 53.1% and 54.6% in fine sand with 20% of clay and fine sand with 40% of clay respectively. Furthermore, the results show that the higher concentration of some heavy and medium hydrocarbon leads to the more increase in early hours of SVE process, which indicates the alteration of hydrocarbons into each other.

Characterization and Remediation of Chlorinated Volatile Organic Contaminants in the Vadose Zone: An Overview of Issues and Approaches.

Contamination of vadose-zone systems by chlorinated solvents is widespread, and poses significant potential risk to human health through impacts on groundwater quality and vapor intrusion. Soil vapor extraction (SVE) is the presumptive remedy for such contamination, and has been used successfully for innumerable sites. However, SVE operations typically exhibit reduced mass-removal effectiveness at some point due to the impact of poorly accessible contaminant mass and associated mass-transfer limitations. Assessment of SVE performance and closure is currently based on characterizing contaminant mass discharge associated with the vadose-zone source, and its impact on groundwater or vapor intrusion. These issues are addressed in this overview, with a focus on summarizing recent advances in our understanding of the transport, characterization, and remediation of chlorinated solvents in the vadose zone. The evolution of contaminant distribution over time and the associated impacts on remediation efficiency will be discussed, as will the potential impact of persistent sources on groundwater quality and vapor intrusion. In addition, alternative methods for site characterization and remediation will be addressed.

Assessing performance and closure for soil vapor extraction: Integrating vapor discharge and impact to groundwater quality

Soil vapor extraction (SVE) is typically effective for removal of volatile contaminants from higher-permeability portions of the vadose zone. However, contamination in lower-permeability zones can persist due to mass transfer processes that limit the removal effectiveness. After SVE has been operated for a period of time and the remaining contamination is primarily located in lower-permeability zones, the remedy performance needs to be evaluated to determine whether the SVE system should be optimized, terminated, or transitioned to another technology to replace or augment SVE. Numerical modeling of vapor-phase contaminant transport was used to investigate the correlation between measured vapor-phase mass discharge, MF r , from a persistent, vadose-zone contaminant source and the resulting groundwater contaminant concentrations. This relationship was shown to be linear, and was used to directly assess SVE remediation progress over time and to determine the level of remediation in the vadose zone necessary to protect groundwater. Although site properties and source characteristics must be specified to establish a unique relation between MF r and the groundwater contaminant concentration, this correlation provides insight into SVE performance and support for decisions to optimize or terminate the SVE operation or to transition to another type of treatment.

Soil Vapor Extraction Performance in Layered Vadose Zone Materials

A pilot soil vapor extraction (SVE) system was installed at a small landfill within the Savannah River Site to address trichloroethylene (TCE) contamination present in the vadose zone. The SVE system has been operating since September 1999 and numerous tests have been performed on the system. A model was developed to simulate SVE at this site, incorporating the effects of contaminant behavior in a layered subsurface as well as the effects of contaminant diffusion into and out of soil aggregates. The objectives of this study were to: (i) compare the field data from the site with predictions from this mechanistic model; and (ii) establish the case for closure based on field observations and model predictions. A dense non‐aqueous‐phase liquid TCE source was discovered at the site during the course of operation. Location of this source compares well with the predicted residual source from the application of the diffusion components of the SVE model to soil gas TCE concentration rebound observations. Collectively, the field observations and the model predictions strongly support the observations that a substantial portion of the source contamination at the site has been removed by the SVE system and that the criteria for site closure have been met.

Effects of skin and hydraulic fractures on SVE wells

Soil vapor extraction (SVE) systems are intended to cause substantial volumes of air to flow through the subsurface with the purpose of removing volatile contaminants. The effectiveness of SVE can be influenced by any effect that changes the specific gas capacity (discharge as a function of vacuum) of a well. Skins of low permeability material enveloping a well bore are widely recognized to affect the performance of wells used to recover water, natural gas, or petroleum, and skin can also significantly diminish the performance of an SVE well. Skins a few mm thick consisting of material whose gas phase permeability is 0.01 of the formation can reduce the specific gas capacity of an SVE well by factors of 2 to 10 or more. Hydraulic fractures created in the vicinities of shallow wells commonly resemble sand-filled layers shaped like flat-lying disks or gently dipping saucers. The contrast between the gas-phase permeability of the sand in the fracture and that of the formation is particularly important, with significant effects requiring the ratio to be greater than approximately 50. Shallow hydraulic fractures filled with several tenths of m 3 of sand in formations that are several orders of magnitude less permeable than that of the enveloping formation should increase specific gas capacity by factors of 10 or more. Field tests of the effects of hydraulic fractures on the performance of SVE were conducted by creating four wells intersecting fractures and a suite of control wells created using conventional methods in silty saprolite. Specific gas capacities ranged over more than an order of magnitude for 10 wells completed within a small area (2 m 2) and at the same depth. Specific capacities correlate to the drilling method that was used to create the bore for the well: lowest values occurred in wells drilled with a machine auger, slightly better results were obtained using a Shelby tube, and the best results were obtained from conventional wells bored with a hand auger. Skin factors determined for wells created with a machine auger could be explained by a layer 1 cm thick that has 0.007 times the permeability of the enveloping material, which could readily have been created during the drilling procedure. Specific capacities of wells intersecting hydraulic fractures were 5 to 100 times more than those of conventional wells. The large difference in performance appears to be due in part to the beneficial effects of the fracture, and in part to the detrimental effects of well skin.

Use of Rebound Testing for Evaluation of Soil Vapor Extraction Performance at the Savannah River Site

AbstractIn 1999, a pilot soil vapor extraction (SVE) system was installed at a waste area within the Department of Energy's Savannah River Site located near Aiken, South Carolina, to remediate trichloroethylene (TCE) contamination and to evaluate monitoring and operational strategies for SVE application in layered heterogeneous materials. The specific objectives of the results reported here were (1) to evaluate the use of rebound analysis of soil gas concentrations as the basis for operational strategies, and (2) establish the endpoint criteria for active remedial action. Three soil gas TCE concentration rebound tests were conducted over a period of 18 months to assess system performance and progress. For each rebound test, the system was shut down and allowed to equilibrate for two to four weeks. Soil gas TCE concentrations were measured several times during this equilibration period. Comparison of these rebound test results has been used for evaluating SVE system performance. A transient two‐dimensional diffusion model has been used to convert soil gas TCE rebound concentrations to estimates from distance to source, and the model predictions correspond with observed dense nonaqueous phase liquid at the site. Also, these rebound tests can provide sufficient information about contaminant distribution and SVE mass transfer limitations to select a reasonable and appropriate endpoint for active remedial operations.

Axi-symmetric simulation of soil vapor extraction influenced by soil fracturing

Fracturing, either pneumatic or hydraulic, is a method to improve the performance of soil vapor extraction (SVE) in relatively low permeability soils (<10 −5 cm/s). A two-dimensional model is presented to simulate trichloroethylene (TCE) soil vapor extraction modified by fracturing. Flow and transport is modeled using mobile macropore and micropore networks, which also have been identified in the literature as dual porosity, dual permeability, or heterogeneous flow models. In this model, fluids can flow in both the macropore and micropore networks. This represents a more general model compared to immobile micropore, mobile macropore models presented thus far in the literature for vapor flow and transport in two dimensions. The model considers pressure- and concentration-driven exchange between the macropore and micropore networks, concentration-driven exchange between the gas and sorbed phases within each network, and equilibrium exchange between the gas and water and a sorbed phase within each network. The parameters employed in an example simulation are based on field measurements made at a fractured site. Considered in the simulations were the influence of the volume percentage of fractures, the length of fractures, the relative location of the water table, and the influence of pulsed pumping. For these simulations, inter-network concentration-driven exchange most significantly affected mass removal. The volume percentage of fractures more significantly influence flow and mass removal than the length of fractures. The depth of the water table below the contamination plume only significantly influenced flow and mass removal when the water table was within 60 cm of the bottom of the contaminated soil in the vadose zone for the parameters considered in this study. Pulsed pumping was not found to increase the amount of mass removed in this study.

Practical Modeling of SVE Performance at a Jet-Fuel Spill Site

A coupled flow and transport model (VENT3D) was used to simulate the performance of soil vapor extraction at a jet-fuel (JP-4) contaminated site. The contaminant mixture was approximated with 10 surrogate components categorized by carbon number class. The contaminant was distributed heterogeneously throughout the soil. The initial total contaminant mass was the only parameter that was adjusted, and it was fit to the observed removal during the pilot tests. The predicted total mass removal using the most general approach matched the observed hydrocarbon mass removal within ±4%. The effects of the following selected simplifying assumptions were examined with the model: (1) uniform contaminant distribution; (2) 2D horizontal flow; and (3) single equivalent contaminant. The uniform soil contamination distribution and the horizontal flow assumptions yielded similar results—both estimated removal rates that were initially higher by 30% and eventually overpredicted the total removal by 10%. The equivalent single-component approximation underpredicted the removal by as much as 50% during the first half of the operation and then overpredicted the removal, ultimately overpredicting the total removal by <10%.

Augmenting in‐situ remediation by soil vapor extraction with six‐phase soil heating

AbstractThe use and performance of soil vapor extraction (SVE) as an in‐situ remedial technology has been limited at numerous sites because of both geologic and chemical factors. SVE systems are not well suited to sites containing low permeability soils or sites contaminated with recalcitrant compounds. Six‐phase soil heating (SPSH) has been developed by the Battelle Pacific Northwest Laboratories (Battelle) to enhance SVE systems. The technology utilizes resistive soil heating to increase the vapor pressure of subsurface contaminants and to generate an in‐situ source of steam. The steam strips contaminants sorbed onto soil surfaces and acts as a carrier gas, providing an enhanced mechanism by which the contaminants can reach an extraction well.Full‐scale applications of SPSH have been performed at the U.S. Department of Energy's Savannah River Site in Aiken, South Carolina; at a former fire training site in Niagara Falls, New York; and at Fort Richardson near Anchorage, Alaska. At each site, chlorinated solvents were present in low permeability soils and SPSH was applied in conjunction with SVE. The results of the three applications showed that SPSH is a cost‐effective technology that can reduce the time required to remediate a site using only conventional SVE.

Radio Frequency Heating for Soil Remediation

ABSTRACT Radio frequency heating (RFH) is a technology that increases the cost-effectiveness of a variety of site remediation technologies by accelerating the rate of contaminant removal. Heating makes the physical, chemical, and biological properties of materials such as contaminants, soil, and groundwater more amenable to remediation. RFH brings controlled heating to the subsurface, enhancing the removal of contaminants by soil vapor extraction (SVE), groundwater aeration (air sparging), bioremediation, and product recovery. The results presented are from a bench-scale study and a field demonstration that both used RFH to enhance the performance of SVE. The bench-scale study performed on PCE-contaminated soil revealed an increase, by a factor of 8, in the removal rate when RFH was used to heat soil to 90 °C. The application of RFH for a three-week period at a former gasoline station near St. Paul, MN, resulted in raising the ambient soil temperature from 8 °C to 100 °C in the immediate vicinity of the RFH applicator and to 40 °C 1.5 m (5 ft) away. Most significantly, the use of an integrated RFH/SVE system achieved an overall 50% reduction in gasoline range organics (GRO) in soil over a two- to three-month period. The discussion includes applications of RFH for enhancing bioremediation and product recovery.

Soil Cleanup by In-Situ Aeration. XXI. Effects of Desorption Rates and Equilibria on Remediation Rates

Abstract A model for soil vapor extraction (SVE) is developed which permits the use of nonlinear adsorption isotherms and takes desorption kinetics into account. The configuration modeled is a single vertical well. The model is used to exhibit the dependence of cleanup rate on the parameters of the nonlinear adsorption isotherm and on a rate constant k f associated with adsorption. Cleanup curves similar to those found in modeling diffusion-limited SVE by the lumped diffusion parameter method are observed for small values of k f. Slow cleanup rates can also result from isotherm parameter values. In particular, severe tailing in the terminal phase of remediation may result from an equilibrium isotherm which approaches the form C gas=K(C sorbed) B , where B > 1 as C sorbed approaches zero. As with diffusion-limited SVE, short-term pilot-scale experiments may well not identify conditions which cause tailing during terminal phase cleanup. One can distinguish between poor SVE performance due to small rate cons...