Experimental Investigation of Surfactant-Enhanced Dissolution of Residual NAPL in Saturated Soil

Nonaqueous phase liquid (NAPL) is a long-term source of ground water contamination as the pollutant slowly partitions into the air and water phases. The objective of this work was to study the efficacy of aqueous surfactant solution to enhance the dissolution of a residual NAPL below the capillary fringe, hence reducing the time needed for aquifer restoration. An analytical technique was developed to measure the concentration of NAPL in a nonionic surfactant. Soil column experiments simulated conditions in the saturated soil where a NAPL may become trapped as a discontinuous immobile phase. Experimental results indicate that dissolution was a rate-limited process, approaching equilibrium concentrations after 24 hours. The relative permeability of the aqueous phase initially decreased as surfactant was injected, but increased over time as the saturation of residual NAPL was reduced through mass transfer into the surfactant-enhanced aqueous phase. These findings suggest that enhancing the aqueous phase with a nonionic surfactant may significantly enhance the in situ recovery or residual NAPL.     

Biogeochemical Coring and Preservation Method for Unconsolidated Soil Samples

Developing an accurate conceptual site model (CSM) is an important process before a decision can be made regarding effective remedial actions. A critical aspect of an accurate CSM is thoroughly understanding the biogeochemistry occurring at the site in the area of concern. To collect media samples that accurately preserve the in situ biogeochemistry, a new Rotosonic core barrel and core preservation protocol was developed. The new biogeochemical core barrel (BCB) successfully isolated and preserved the in situ biogeochemical conditions of the soil core and minimized the soil core's exposure pathways to air. The BCB's success was achieved by a modified Rotosonic core barrel, a specialized drive shoe, an internal BCB core barrel piston, hydraulic extrusion of the soil core into a stainless core tube with an internal piston, and specialized core tube sealing, handling, and subsampling methods. Detailed subsampling of 65-foot (nominally 20 m) soil core in 2-inch (nominally 51 mm) increments within a specialized anaerobic glovebox confirmed the presence of five biogeochemical redox transition zones within the soil core. The BCB also allowed for split soil core samples for detailed mineralogical and live microbiological studies. Success of the BCB method is further evidenced by the presence of the highly redox-sensitive surface bound iron sulfide mineral mackinawite. The BCB allowed detailed analysis of the soil core including Fe and S concentration gradients, oxidation–reduction potential gradients, volatile organic compound analysis, and live microbiological assessments.     

A Comprehensive Method for Fractionating Soil Organic Matter Not Protected and Protected from Decomposition by Physical and Chemical Mechanisms

The objective of this work was to describe a method for isolating meaningful and measurable soil organic matter (SOM) pools that differ in the mechanisms by which they are protected from decomposition. The proposed method is appropriate for soil C stabilization and sequestration studies. Unlike previous fractionation schemes, this procedure allows free SOM located between aggregates (unprotected C pool) and SOM occluded within both macroaggregates and microaggregates (C weakly and strongly protected by physical mechanisms, respectively) to be recovered separately, freed from the soil mineral matrix and the mineral‐associated SOM pool (C pool protected by chemical mechanisms) and thus well suited to advanced chemical characterization by ¹³C‐NMR. Briefly, free SOM is isolated by an initial density separation. Stable macroaggregates are broken up into stable microaggregates and intra‐macroaggregate SOM, which is then separated by density. Finally, intra‐microaggregate SOM is isolated from mineral‐associated SOM by a third density separation after ultrasonic disruption. The SOM dissolved during the fractionation procedure is also recovered. Results obtained on soil samples with contrasting textures suggested that clay content induces a decrease of the proportion of free organic C and an increase of mineral‐associated organic C content. Free SOM is characterized by a marked presence of undecayed organic material and biologically labile substances, such as carbohydrates and proteins. In contrast, SOM occluded within aggregates, especially within microaggregates, represents a more decomposed fraction, relatively enriched in unsubstituted‐aliphatic material, most probably lipid biopolymers.     

Effect of NAPL film stability on the dissolution of residual wetting NAPL in porous media: A pore-scale modeling study

Wettability profoundly affects not only the initial distribution of residual NAPL contaminants in natural soils, but also their subsequent dissolution in a flowing aqueous phase. Under conditions of preferential NAPL wettability, the residual NAPL phase is found within the smaller pores and in the form of continuous corner filaments and thick films on pore walls. Such films expose a much greater interfacial area for mass transfer than would be exposed by the same amount of non-wetting NAPL. Importantly, capillary and hydraulic continuity of NAPL filaments and thick films is essential for sustaining NAPL–water counterflow during the course of NAPL dissolution in flowing groundwater—a mechanism which maintains and even increases the interfacial area for mass transfer. Continued dissolution results in gradual thinning of the NAPL films, which may become unstable and rupture causing disconnection of the residual NAPL in the form of clusters. Using a pore network simulator, we demonstrate that NAPL film instability drastically modifies the microscopic configuration of residual NAPL, and hence the local hydrodynamic conditions and interfacial area for mass transfer, with concomitant effects on macroscopically observable quantities, such as the aqueous effluent concentration and the fractional NAPL recovery with time. These results strongly suggest that the disjoining pressure of NAPL films may exert an important, and hitherto unaccounted, control on the dissolution behaviour of a residual NAPL phase in oil wet systems.     

Effect of Different Sampling Methodologies on Measured Methane Concentrations in Groundwater Samples

Analysis of dissolved light hydrocarbon gas concentrations (primarily methane and ethane) in water supply wells is commonly used to establish conditions before and after drilling in areas of shale gas and oil extraction. Several methods are currently used to collect samples for dissolved gas analysis from water supply wells; however, the reliability of results obtained from these methods has not been quantified. This study compares dissolved methane and ethane concentrations measured in groundwater samples collected using three sampling methods employed in pre‐ and post‐drill sampling programs in the Appalachian Basin. These include an open‐system collection method where 40 mL volatile organic analysis (VOA) vials are filled directly while in contact with the atmosphere (Direct‐Fill VOA) and two alternative methods: (1) a semi‐closed system method whereby 40 mL VOA vials are filled while inverted under a head of water (Inverted VOA) and (2) a relatively new (2013) closed system method in which the sample is collected without direct contact with purge water or the atmosphere (IsoFlask^®). This study reveals that, in the absence of effervescence, the difference in methane concentrations between the three sampling methods was relatively small. However, when methane concentrations equaled or exceeded 20 mg/L (the approximate concentration at which effervescence occurs in the study area), IsoFlask^® (closed system) samples yielded significantly higher methane concentrations than Direct‐Fill VOA (open system) samples, and Inverted VOA (semi‐closed system) samples yielded lower concentrations. These results suggest that open and semi‐closed system sample collection methods are adequate for non‐effervescing samples. However, the use of a closed system collection method provides the most accurate means for the measurement of dissolved hydrocarbon gases under all conditions.     

Influence of Shallow Geology on Volatile Organic Chemical Attenuation from Groundwater to Deep Soil Gas

Vapor intrusion pathway evaluations commonly begin with a comparison of volatile organic chemical (VOC) concentrations in groundwater to generic, or Tier 1, screening levels. These screening levels are typically quite low reflecting both a desired level of conservatism in a generic risk screening process as well as limitations in understanding of physical and chemical processes that impact vapor migration in the subsurface. To study the latter issue, we have collected detailed soil gas and groundwater vertical concentration profiles and evaluated soil characteristics at seven different sites overlying chlorinated solvent contaminant plumes. The goal of the study was to evaluate soil characteristics and their impacts on VOC attenuation from groundwater to deep soil gas (i.e., soil gas in the unsaturated zone within 2 feet of the water table). The study results suggest that generic screening levels can be adjusted by a factor of 100× at sites with fine‐grained soils above the water table, as identified by visual observations or soil air permeability measurements. For these fine‐grained soil sites, the upward‐adjusted screening levels maintain a level of conservatism while potentially eliminating the need for vapor intrusion investigations at sites that may not meet generic screening criteria.     

A Model-Averaging Method for Assessing Groundwater Conceptual Model Uncertainty

This study evaluates alternative groundwater models with different recharge and geologic components at the northern Yucca Flat area of the Death Valley Regional Flow System (DVRFS), USA. Recharge over the DVRFS has been estimated using five methods, and five geological interpretations are available at the northern Yucca Flat area. Combining the recharge and geological components together with additional modeling components that represent other hydrogeological conditions yields a total of 25 groundwater flow models. As all the models are plausible given available data and information, evaluating model uncertainty becomes inevitable. On the other hand, hydraulic parameters (e.g., hydraulic conductivity) are uncertain in each model, giving rise to parametric uncertainty. Propagation of the uncertainty in the models and model parameters through groundwater modeling causes predictive uncertainty in model predictions (e.g., hydraulic head and flow). Parametric uncertainty within each model is assessed using Monte Carlo simulation, and model uncertainty is evaluated using the model averaging method. Two model-averaging techniques (on the basis of information criteria and GLUE) are discussed. This study shows that contribution of model uncertainty to predictive uncertainty is significantly larger than that of parametric uncertainty. For the recharge and geological components, uncertainty in the geological interpretations has more significant effect on model predictions than uncertainty in the recharge estimates. In addition, weighted residuals vary more for the different geological models than for different recharge models. Most of the calibrated observations are not important for discriminating between the alternative models, because their weighted residuals vary only slightly from one model to another.     

A New Method for Collecting Core Samples Without a Drilling Rig

A new piston sampler allows the collection of high-quality core samples from sand, silt or clay, up to depths of 18 meters. The sampler is operated by a one- or two-person crew without a drilling rig. The sampler and ancillary equipment fit easily into a half-ton truck, making this a highly portable sampling system. Other advantages include minimal mechanical disturbance and precisely known sample depth. Casing is not required to maintain an open corehole below the water table and drilling fluid is not used in the corehole, so the solids and pore water of the sample should not be contaminated by foreign fluids. High-quality samples for physical, geochemical, and microbiological characterization of the subsurface are easily obtained with this new device.     

干旱地区单层及多层介质土壤中降低接地电阻的方法

以甘肃省酒泉市金塔地区某变电站工程为实例,对戈壁、沙漠等高土壤电阻率地区如何降低接地电阻进行了组合分析。由于干旱地区年降雨量低,日照时间长,蒸发量大,用常规的降阻剂或局部更换土壤等方法难以长期稳定的解决高土壤电阻率地区接地电阻过大的问题。本文采用了一些特殊方法进行处理,这对促进此类地区电力建设项目及其他项目建设具有重要意义。     

A Surface-Flux Measurement Method for Screening Contamination from Volatile Organic Compounds

Measurement of the vapor flux from volatile organic compounds (VOCs) provides a rapid means for screening large areas of potential contamination. The vapor flux is determined from the rate of VOC concentration buildup inside a 3.1L accumulator device that is sealed to the surface of the contaminated soil. After the VOC concentrations are allowed to increase for a few minutes, they are analyzed with a portable gas chromatograph or a total organic vapor analyzer.
The measurement approach was evaluated at a field site in an area where the ground water and soil had been impacted with Jet Fuel No. 4 (JP-4). An indication of the areal extent of impact was determined by mapping the surface VOC vapor flux. The pattern revealed by the flux measurements was found to coincide, in rough outline, with the known extent of toluene concentrations in the ground water and with conventional soil-gas survey results. In addition, a mathematical model describing VOC diffusion into the accumulator device was verified by performing laboratory measurements of the surface VOC vapor flux on a sandbox designed to simulate a hazardous waste site.     

A practical model for mobile, residual, and entrapped NAPL in water-wet porous media

Flow of nonvolatile nonaqueous phase liquid (NAPL) and aqueous phases that account for mobile, entrapped, and residual NAPL in variably saturated water-wet porous media is modeled and compared against results from detailed laboratory experiments. Residual saturation formation in the vadose zone is a process that is often ignored in multifluid flow simulators, which might cause an overestimation of the volume of NAPL that reaches the ground water. Mobile NAPL is defined as being continuous in the pore space and flows under a pressure gradient or gravitational body force. Entrapped NAPL is defined as being occluded by the aqueous phase, occurring as immobile ganglia surrounded by aqueous phase in the pore space and formed when NAPL is replaced by the aqueous phase. Residual NAPL is defined as immobile, nonwater entrapped NAPL that does not drain from the pore spaces and is conceptualized as being either continuous or discontinuous. Free NAPL comprises mobile and residual NAPL. The numerical model is formulated on mass conservation equations for oil and water, transported via NAPL and aqueous phases through variably saturated porous media. To account for phase transitions, a primary variable switching scheme is implemented for the oil-mass conservation equation over three phase conditions: (1) aqueous or aqueous-gas with dissolved oil, (2) aqueous or aqueous-gas with entrapped NAPL, and (3) aqueous or aqueous gas with free NAPL. Two laboratory-scale column experiments are modeled to verify the numerical model. Comparisons between the numerical simulations and experiments demonstrate the necessity to include the residual NAPL formation process in multifluid flow simulators.     

基于三轴试验的软土震陷简化计算方法研究

本文通过对天津滨海新区典型软土的静、动力学试验,分析了软土残余应变的变化规律,提出了结合地区震陷经验系数的软土震陷简化计算方法,并利用1976年唐山7.8级地震中天津塘沽地区软土震陷观测资料对计算方法进行了验证。结果表明,本方法用于估算自由地表震陷量是可行的,对中小工程中软土震陷量的估算具有一定的参考价值。     

A Simple Field Method for Assessing Near‐Surface Saturated Hydraulic Conductivity

This technical note describes an effective and inexpensive field technique for measuring the saturated hydraulic conductivity of both undisturbed cores and repacked soil samples. The method requires no specialized equipment; everything that is required can be obtained in a hardware store. The method is a straightforward field implementation of the widely used falling‐head laboratory analysis directly derived from Darcy's law. As such, it sidesteps the need for empirical assumptions about soil texture and the relationship between saturated and unsaturated flow components which many permeameter‐based methods rely upon. The method is shown to produce results that are consistent with K values obtained elsewhere in the same homogeneous sand formation. Furthermore, the proposed method is useful for measuring hydraulic conductivity in drill cuttings obtained from direct push or auguring drill techniques, which cannot be done with any other field method. The range of hydraulic conductivity values that this test is appropriate for is on the order of 1E ? 7 m/s to 1E ? 3 m/s.     

A Supplementary Method for Assessing the Reliability of Fluids Sampled from Deep Aquifers

In deep aquifers, temperatures of formation fluids recovered during drill-stem tests were usually higher than regular bottom-hole temperatures recorded at the same borehole and interval. Wherever the opposite was observed, the sampled formation fluids were diluted with mud filtrate or cushion water. Temperature measurements are a useful criterion for differentiating between representative and contaminated formation-fluid samples.     

基于二分法的CT投影快速仿真

投影仿真是CT仿真研究的基础。本文基于二分法原理,研究了一种CT投影快速仿真方法,并对仿真过程进行了优化。针对由凸物体组成的待扫描体,可实现任意轨迹的扫描,投影仿真精度可无限逼近于解析投影精度。数值仿真实验验证了本文方法的有效性。     

一种基于二元回归的时深转换新方法

南堡凹陷C油田沙河街组发育一套披覆在潜山之上的生屑云岩储层,为该油田的主力含油层系,生屑云岩分布不稳定,横向变化较大,先期钻探的5口探井有2口落空。基于周边油田分析,古地貌是生屑云岩储层分布的主要因素。受上覆火成岩的影响,该油藏速度横向变化剧烈,古地貌恢复困难,使预测储层分布范围困难较大。考虑到速度横向变化的影响,利用传统的时深转换方法难以准确地计算构造深度进而恢复古地貌。本文从多口已钻井的时深关系精细对比分析出发,明确了该区的速度影响因素主要有压实效应和火成岩厚度,同时基于对传统时深转换方法的适用性分析,提出了一种基于二元回归的时深转换新方法。实际资料应用表明,利用该方法计算的构造深度与实钻结果吻合较好,能够有效恢复古地貌和预测生屑云岩的分布,该时深转换方法对于速度横向变化剧烈的地区有较好的推广应用价值。