首页 | 本学科首页   官方微博 | 高级检索  
相似文献
 共查询到20条相似文献，搜索用时 31 毫秒
1.
Organic contaminants present as nonaqueous phase liquids (NAPLs) in the subsurface often pose a long-term risk to human health and the environment. Investigating the distribution of NAPLs in porous media remains a major challenge in risk assessment and management of contaminated sites. Conventional soil coring and monitoring wells have been widely used over past decades as the primary means of subsurface investigation to determine NAPL extent. Known limitations of conventional approaches have led us to explore an alternative or a complementary technique to provide high-quality information of NAPL source zone architecture. This work advances an imaging tool for a variety of organic NAPL contaminants in unconsolidated soils through magnetic resonance imaging (MRI) of frozen cores. Using trichloroethylene (TCE) and o-xylene as model species, we illustrate that discriminatory freezing of water, while keeping the NAPL in a liquid state, enables high-resolution qualitative delineation of NAPL distribution within porous media. This novel approach may help improve site conceptual models and consequentially lead to highly tailored, more efficient remedial measures.  相似文献   

2.
Neat ethanol (75.7 L) was released into the upper capillary zone in a continuous-flow, sand-packed aquifer tank (8.2 m3) with an average seepage velocity of 0.75 m/day. This model aquifer system contained a residual nonaqueous phase liquid (NAPL) that extended from the capillary zone to 10 cm below the water table. Maximum aqueous concentrations of ethanol were 20% v/v in the capillary zone and 0.08% in the saturated zone at 25 and 30 cm downgradient from the emplaced NAPL source, respectively. A bench-scale release experiment was also conducted for a similar size spill (scaled to the plan area). The concentrations of ethanol in ground water for both the bench- and pilot-scale experiments were consistent with advective–dispersive limited mass transfer from the capillary to the saturated zone. Concentrations of monoaromatic hydrocarbons and isooctane increased in the pore water of the capillary zone as a result of both redistribution of residual NAPL (confirmed by visualization) and enhanced hydrocarbon dissolution due to the cosolvent effect exerted by ethanol. In the tank experiment, higher hydrocarbon concentrations in ground water were also attributed to decreased hydrocarbon biodegradation activity caused by preferential microbial utilization of ethanol and the resulting depletion of oxygen. These results infer that spills of highly concentrated ethanol will be largely confined to the capillary zone due to its buoyancy, and ethanol concentrations in near-source zone ground water will be controlled by mass transfer limitations and hydrologic conditions. Furthermore, highly concentrated ethanol releases onto pre-existing NAPL will likely exacerbate impacts to ground water, due to NAPL mobilization and dissolution, and decreased bioattenuation of hydrocarbons.  相似文献   

3.
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.  相似文献   

4.
In this work, we have applied a group-contribution activity-coefficient model, UNIFAC, and the solubility of alcohols in water to estimate partition coefficients for alcohol tracers between water and nonaqueous-phase liquids (NAPLs). The effects of temperature and mutual solubility between NAPL and aqueous phases on the estimation of partition coefficients were also investigated. By comparing the estimated results with experimental partition coefficients for 30 alcohol tracers between 10 NAPLs and water, we found that: i) the UNIFAC-solubility method, in which the UNIFAC model in its infinite-dilution form is applied to the NAPL phase and the solubility of tracers in water is used for estimation of the activity coefficient in the aqueous phase, works better than the UNIFAC model; ii) a linear relation between the logarithm of partition coefficients and the logarithm of tracer solubility in water is observed for those tracers having a similar chemical structure (i.e. the same number of branched methyl groups). This can serve as a useful tool for quick selection of the tracers that exhibit the desired partition coefficients; iii) the effect of mutual solubility between NAPL and aqueous phases can be neglected because such miscibility is very small, usually of the order of 10−3 mole/mole unit; and iv) temperature variation between 15° and 25°C does not significantly affect partition coefficients.  相似文献   

5.
DNAPL to LNAPL Transitions During Horizontal Cosolvent Flooding   总被引：1，自引：0，他引：1  
Cosolvent flooding is a technology with the potential to remove nonaqueous phase liquid (NAPL) sources from the subsurface. It can be used to initiate separate phase mobilization, which allows removal of NAPL within very few pore volumes. Mobilization may result in a sinking DNAPL bank during horizontal flooding of NAPLs denser than water. Reversal of phase density difference between aqueous and DNAPL phases could potentially avoid this downward migration of mobilized DNAPLs. We achieved phase density difference reversal and made DNAPLs float using two components in the cosolvent flooding solution. A low-density cosolvent partitions preferentially into the DNAPL and swells it, which causes a reduction in density of the DNAPL and reversal of the density difference between the NAPL and aqueous phases. A highdensity additive that remains in the aqueous phase allows the cosolvent flooding solution overall to have a density greater than that of water and permits control of the flooding instability. This study focused on tert-butanol as the swelling cosolvent and tetrachloroethylene as the contaminant. In batch tests with sucrose and glycerol as dense additives, phase density difference reversal occurred. To investigate the applicability of phase density difference reversal as a remediation technology, horizontal column and sandbox experiments were performed. These experiments demonstrated the occurrence of phase density difference reversal and effective remediation in horizontal cosolvent floods.  相似文献   

6.
A model is presented for estimating vapor concentrations in buildings because of volatilization from soil contaminated by non- aqueous phase liquids (NAPL) or from dissolved contaminants in ground water. The model considers source depletion, diffusive- dispersive transport of the contaminant of concern (COC) and of oxygen and oxygen-limited COC biodecay. Diffusive-advective transport through foundations and vapor losses caused by foundation cross-flow are considered. Competitive oxygen use by various species is assumed to be proportional to the product of the average dissolved-phase species concentration and a biopreference factor. Laboratory and field data indicate the biopreference factor to be proportional to the organic carbon partition coefficient for the fuel hydrocarbons studied. Predicted indoor air concentrations were sensitive to soil type and subbase permeability. Lower concentrations were predicted for buildings with shallow foundations caused by flushing of contaminants by cross-flow. NAPL source depletion had a large impact on average exposure concentration. Barometric pumping had a minor effect on indoor air emissions for the conditions studied. Risk-based soil cleanup levels were much lower when biodecay was considered because of the existence of a threshold source concentration below which no emissions occur. Computed cleanup levels at NAPL-contaminated sites were strongly dependent on total petroleum hydrocarbon (TPH) content and COC soil concentration. The model was applied to two field sites with gasoline-contaminated ground water. Confidence limits of predicted indoor air concentrations spanned approximately two orders of magnitude considering uncertainty in model parameters. Measured contaminant concentrations in indoor air were within model-predicted confidence limits.  相似文献   

7.
《Advances in water resources》2007,30(6-7):1618-1629
Residual dense non-aqueous liquids (NAPLs) in aquifers constitute a great challenge for groundwater cleanup. Active engineered treatment of regions that contain residual NAPLs is often required to shorten the long-term impact of NAPLs on groundwater quality. Enhanced residual NAPL cleanup can be achieved by promoting biodegradation of NAPL components in the aqueous phase, thereby increasing contaminant fluxes from the NAPL phase. Reaction-enhanced NAPL dissolution is often mathematically simulated under the assumption that lumped mass transfer coefficients, used to describe the dissolution behavior of the NAPL phase, are independent of the reactions. However, this assumption is not warranted because reactions occurring near the water–NAPL interface can reduce characteristic mass transfer lengths, which tend to enhance mass transfer over the no-reaction case.In this study, we mathematically investigated the connections between lumped mass transfer coefficients and reaction kinetics over an idealized residual NAPL domain. Since mass transfer is frequently a scale-dependent process, we also examined the influence of system extent on mass transfer coefficients. For our idealized domain with an assumed first-order decay reaction, the results show that lumped mass transfer coefficients depend on reaction kinetics and system scale. The mass transfer coefficient derived from the non-reactive case cannot properly represent the mass transfer process under the reactive conditions. When the advection time scale is long in comparison to the transverse dispersion time scale in the system, a fast reaction can increase significantly the lumped mass transfer coefficient. The mass transfer coefficient used for simulation was also found to be affected by the nature of the numerical scheme used.  相似文献   

8.
DyeLIF? is a new version of laser‐induced fluorescence (LIF) for high‐resolution three‐dimensional subsurface mapping of nonaqueous phase liquids (NAPLs) in the subsurface. DyeLIF eliminates the requirement that the NAPL contains native fluorophores (such as those that occur in compounds like polynuclear aromatic hydrocarbons [PAHs]) and can therefore be used to detect chlorinated solvents and other nonfluorescing NAPLs that had previously been undetectable with conventional LIF tools. With DyeLIF, an aqueous solution of water and nontoxic hydrophobic dye is continuously injected ahead of the sapphire detection window while the LIF probe is being advanced in the subsurface. If soil containing NAPL is penetrated, the injected dye solvates into the NAPL within a few milliseconds, creating strong fluorescence that is transmitted via fiber‐optic filaments to aboveground optical sensors. A detailed field evaluation of the novel DyeLIF technology was performed at a contaminated industrial site in Lowell, Massachusetts, USA where chlorinated solvent dense nonaqueous phase liquid (DNAPL) persists below the water table in sandy sediments. Continuously cored boreholes were drilled adjacent to 5 of 30 DyeLIF probes that were advanced at that site. The cores were subsampled in high resolution to generate discrete‐depth soil samples as splits at the same depths where DNAPL was detected in the colocated DyeLIF probes. The cores were analyzed above ground using (1) colorimetric screening using hydrophobic dye tests, (2) laboratory extraction and quantitative chemical analysis, (3) “Benchtop” DyeLIF, and (4) volumetric moisture content. Correlation between DyeLIF and aboveground analyses of the soil cores was excellent: 98% agreement with positive DNAPL detections in samples where DNAPL pore saturations were >0.7% (based on quantitative soil analyses) and the ex situ tests. DyeLIF produced the equivalent of one aboveground colorimetric dye test every 0.2 inch (0.5 cm) of probing. With average daily probing of 395 linear feet (120.4 m), this was the equivalent of 12,039 discrete‐depth colorimetric dye tests/day. Because DyeLIF is an in situ measurement, there are no issues with soil core recovery like there would be for conventional ex situ colorimetric dye tests and 100% characterization of the probed intervals is achieved. Tracking the injection rate and pressure of the dye solution provides simultaneous data regarding relative soil permeability, similar to other direct push (DP) hydraulic profiling tools. Conventional LIF is considered the premier DP tool to identify and map NAPL containing PAHs in the subsurface or confirm its absence. While chlorinated solvent DNAPLs at some field sites contain impurities (e.g., solvated greases or oils) that make them detectable with conventional LIF techniques, at other sites, the DNAPL cannot be detected with conventional LIF. At such sites, the injection of a hydrophobic dye ahead of the sapphire window with the DyeLIF system now makes the LIF technology applicable to the many types of NAPLs that were previously invisible using conventional LIF techniques.  相似文献   

9.
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.  相似文献   

10.
The objective of this study was to investigate whether 222Rn in groundwater can be used as a tracer for light non‐aqueous phase liquid (LNAPL) quantification at a field site treated by dual‐phase LNAPL removal. After the break of a pipeline, 5 ha of soil in the nature reserve Coussouls de Crau in southern France was contaminated by 5100 m3 of crude oil. Part of this oil seeped into the underlying gravel aquifer and formed a floating oil body of about 3.9 ha. The remediation consists of plume management by hydraulic groundwater barriers and LNAPL extraction in the source zone. 222Rn measurements were performed in 21 wells in and outside the source zone during 15 months. In uncontaminated groundwater, the radon activity was relatively constant and remained always >11 Bq/L. The variability of radon activity measurements in wells affected by the pump‐and‐skim system was consistent with the measurements in wells that were not impacted by the system. The mean activities in wells in the source zone were, in general, significantly lower than in wells upgradient of the source zone, owing to partitioning of 222Rn into the oil phase. The lowest activities were found in zones with high non‐aqueous phase liquid (NAPL) recovery. LNAPL saturations around each recovery well were furthermore calculated during a period of high groundwater level, using a laboratory‐determined crude oil–water partitioning coefficient of 38.5 ± 2.9. This yielded an estimated volume of residual crude oil of 309 ± 93 m3 below the capillary fringe. We find that 222Rn is a useful and cheap groundwater tracer for finding zones of good LNAPL recovery in an aquifer treated by dual‐phase LNAPL removal, but that quantification of NAPL saturation using Rn is highly uncertain.  相似文献   

11.
We investigated the dissolution of non-aqueous phase liquids (NAPLs) in a three-dimensional random sphere-pack medium using a pore-scale modeling approach to advance fundamental understanding and connect rigorously to microscale processes. Residual NAPL distributions were generated using a morphological approach and the entrapped non-wetting phase was quantitatively characterized by calculating volume, orientation, interfacial area, and shape of isolated NAPL regions. With a detailed aqueous-phase flow field obtained by a multiple-relaxation time lattice Boltzmann approach, we solved the advective–diffusive equation in the pore space using a high-resolution, adaptive-stencil finite-volume scheme and an operator-splitting algorithm. We show good agreement between the mass transfer rates predicted in the computational approach and previously published experimental observations. The pore-scale simulations presented in this work provide the first three-dimensional comparison to the considerable experimental work that has been performed to derive constitutive relations to quantify mass transfer from a residual NAPL to a flowing aqueous phase.  相似文献   

12.
This study presents a multiphase flow and multispecies reactive transport model for the simultaneous simulation of NAPL and groundwater flow, dissolution, and reactive transport with isotope fractionation, which can be used for better interpretation of NAPL-involved Compound Specific Isotope Analysis in 3D heterogeneous hydrogeologic systems. The model was verified for NAPL-aqueous phase equilibrium partitioning, aqueous phase multi-chain and multi-component reactive transport, and aqueous phase multi-component transport with isotope fractionation. Several illustrative examples are presented to investigate the effect of DNAPL spill rates, degradation rate constants, and enrichment factors on the temporal and spatial distribution of the isotope signatures of chlorinated aliphatic hydrocarbon groundwater plumes. The results clearly indicate that isotope signatures can be significantly different when considering multiphase flow within the source zone. A series of simulations indicate that degradation and isotope enrichment compete with dissolution to determine the isotope signatures in the source zone: isotope ratios remain the same as those of the source if dissolution dominates the reaction, while heavy isotopes are enriched in reactants along groundwater plume flow paths when degradation becomes dominant. It is also shown that NAPL composition can change from that of the injected source due to the partitioning of components between the aqueous and NAPL phases even when degradation is not allowed in NAPL phase. The three-dimensional simulation is presented to mechanistically illustrate the complexities in determining and interpreting the isotopic signatures with evolving DNAPL source architecture.  相似文献   

13.
Mass discharge across transect planes is increasingly used as a metric for performance assessment of in situ groundwater remediation systems. Mass discharge estimates using concentrations measured in multilevel transects are often made by assuming a uniform flow field, and uncertainty contributions from spatial concentration and flow field variability are often overlooked. We extend our recently developed geostatistical approach to estimate mass discharge using transect data of concentration and hydraulic conductivity, so accounting for the spatial variability of both datasets. The magnitude and uncertainty of mass discharge were quantified by conditional simulation. An important benefit of the approach is that uncertainty is quantified as an integral part of the mass discharge estimate. We use this approach for performance assessment of a bioremediation experiment of a trichloroethene (TCE) source zone. Analyses of dissolved parent and daughter compounds demonstrated that the engineered bioremediation has elevated the degradation rate of TCE, resulting in a two‐thirds reduction in the TCE mass discharge from the source zone. The biologically enhanced dissolution of TCE was not significant (～5%), and was less than expected. However, the discharges of the daughter products cis‐1,2, dichloroethene (cDCE) and vinyl chloride (VC) increased, probably because of the rapid transformation of TCE from the source zone to the measurement transect. This suggests that enhancing the biodegradation of cDCE and VC will be crucial to successful engineered bioremediation of TCE source zones.  相似文献   

14.
Nonaqueous phase liquid (NAPL)‐impacted lower permeability layers in heterogeneous media are difficult to fully remediate and can act as persistent sources of groundwater contamination through diffusive emissions to transmissive aquifer zones. This work investigated the benefits of partial remediation involving treatment focused near the high‐low permeability interface, with the performance metric being emissions reduction. A sequential base‐activated persulfate (S2O8 2?) delivery treatment strategy was studied in this work, involving 13–14 d deliveries of 10% w/w sodium persulfate (Na2S2O8) and 14–28 d deliveries of 19 g/L sodium hydroxide (NaOH). Treatment and control experiments were conducted in 1.2‐m wide × 1.2‐m tall × 5‐cm thick physical model tanks containing two soil layers differing in hydraulic conductivity by three orders of magnitude. The top 10 cm of the lower permeability layers contained 7400–7800 mg‐NAPL/kg‐soil; the NAPL was comprised of benzene, toluene, ethylbenzene, p‐xylene, o‐xylene, n‐propylbenzene, and 1,3,5‐trimethylbenzene (TMB) mixed in octane. Approximately 0.1 g‐Na2S2O8 was delivered per cm2‐interface area over 13–14 d. The S2O8 2? and SO4 2? concentration profiles suggest higher oxidant reaction rates when NaOH is delivered prior to, rather than after Na2S2O8. After 264 d and two treatments, hydrocarbon emissions from the NAPL source were reduced by 60% to 73% compared to a no‐treatment control tank. The incremental benefit of the second treatment was only about 10% of the effect of the first treatment.  相似文献   

15.
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.  相似文献   

16.
Electrical resistance heating (ERH) experiments were performed in a two‐dimensional water‐saturated porous medium comprising an electrically conductive, low‐permeability clay lens embedded within a less electrically conductive, higher permeability silica sand. These were compared to experiments performed in homogeneous silica sand. All experiments were performed in the absence of a non‐aqueous phase liquid (NAPL) or dissolved volatile organic compound (VOC). Temperature monitoring showed preferential heating in the clay lens and higher overall heating rates throughout the test cell compared to the homogeneous case. Gas production was localized around the sand–clay interface due to high temperature and low capillary displacement pressure. Above the clay lens, unexpected temperature plateaus were observed, similar to those observed in previous experiments during NAPL–water co‐boiling. A conceptual model based on the consumption of thermal energy as latent heat of vaporization in the highly localized heating and gas production region adjacent to the clay lens is proposed to explain the temperature plateaus. Supporting data is drawn from images of the gas phase and electric current measurements. These results show that the use of co‐boiling plateaus as an indicator of NAPL–water co‐boiling could be misleading during applications of ERH at sites containing electrically conductive, low‐permeability clay lenses embedded within less electrically conductive, higher‐permeability sands.  相似文献   

17.
Evaluation of volatilization as a natural attenuation pathway for MTBE   总被引：2，自引：0，他引：2  
Lahvis MA  Baehr AL  Baker RJ 《Ground water》2004,42(2):258-267
Volatilization and diffusion through the unsaturated zone can be an important pathway for natural attenuation remediation of methyl tert-butyl ether (MTBE) at gasoline spill sites. The significance of this pathway depends primarily on the distribution of immiscible product within the unsaturated zone and the relative magnitude of aqueous-phase advection (ground water recharge) to gaseous-phase diffusion. At a gasoline spill site in Laurel Bay, South Carolina, rates of MTBE volatilization from ground water downgradient from the source are estimated by analyzing the distribution of MTBE in the unsaturated zone above a solute plume. Volatilization rates of MTBE from ground water determined by transport modeling ranged from 0.0020 to 0.0042 g m(-2)/year, depending on the assumed rate of ground water recharge. Although diffusive conditions at the Laurel Bay site are favorable for volatilization, mass loss of MTBE is insignificant over the length (230 m) of the solute plume. Based on this analysis, significant volatilization of MTBE from ground water downgradient from source areas at other sites is not likely. In contrast, model results indicate that volatilization coupled with diffusion to the atmosphere could be a significant mass loss pathway for MTBE in source areas where residual product resides above the capillary zone. Although not documented, mass loss of MTBE at the Laurel Bay site due to volatilization and diffusion to the atmosphere are predicted to be two to three times greater than mass loading of MTBE to ground water due to dissolution and recharge. This result would imply that volatilization in the source zone may be the critical natural attenuation pathway for MTBE at gasoline spill sites, especially when considering capillary zone limitations on volatilization of MTBE from ground water and the relative recalcitrance of MTBE to biodegradation.  相似文献   

18.
Surface‐wetting properties are an important cause of changing the groundwater and two‐phase fluid flows. Various factors affecting the surface wettability were investigated in a parallel‐walled glass fracture with non‐aqueous phase liquid (NAPL) (gasoline, diesel, trichloroethylene, and creosote) wetted surfaces. First, the effect of the duration of NAPL exposure on wettability change was considered at pre‐wet fracture surfaces using the various NAPL species, and the result showed that the surface became hydrophobic after the exposure time of NAPL exceeded 2000 min. Second, the initial wetting state of the surface affected the timing when the wettability change begins as well as the extent of the wettability change in an NAPL‐wetted rock fractures. Under the dry condition, the wettability change was completed within a very short time of exposure to NAPL (~5 min), and then it finally reached the intermediate and weakly NAPL wetting (contact angle of 118°). Under the pre‐wet condition, a relatively long time of exposure (~5000 min) was needed to observe the obvious change of the surface wettability, which was changed up to strongly NAPL wetting (contact angle of 142°). Third, the wettability changed by NAPL exposure was stable and maintained for a long time, regardless of water flushing rate and temperature. Finally, the wettability change by the exposure of NAPL on parallel fracture surfaces was evaluated at various groundwater flow velocities. Result showed that groundwater flow velocity has an important impact upon measured contact angle. Although fracture surfaces were exposed to NAPL at the low groundwater flow velocity, the wettability was not changed from hydrophilic to hydrophobic when the contact time between NAPL and mineral surfaces was not sufficient owing to the pulse‐type movement of NAPL. This implies that the variation of exposure pattern due to groundwater flow on the wettability change can be an important factor affecting the wettability change of fracture surface and migration behaviour at natural fractured rock aquifers in case of NAPL spill. Copyright © 2015 John Wiley & Sons, Ltd.  相似文献   

19.
Approximately 190 kg of 2 μm‐diameter zero‐valent iron (ZVI) particles were injected into a test zone in the top 2 m of an unconfined aquifer within a trichloroethene (TCE) source area. A shear‐thinning fluid was used to enhance ZVI delivery in the subsurface to a radial distance of up to 4 m from a single injection well. The ZVI particles were mixed in‐line with the injection water, shear‐thinning fluid, and a low concentration of surfactant. ZVI was observed at each of the seven monitoring wells within the targeted radius of influence during injection. Additionally, all wells within the targeted zone showed low TCE concentrations and primarily dechlorination products present 44 d after injection. These results suggest that ZVI can be directly injected into an aquifer with shear‐thinning fluids to induce dechlorination and extends the applicability of ZVI to situations where other emplacement methods may not be viable.  相似文献   

20.
At three industrial sites in Ontario, New Hampshire, and Florida, tetrachloroethylene (PCE) and trichloroethylene (TCE), released decades ago as dense nonaqueous phase liquids (DNAPLs), now form persistent source zones for dissolved contaminant plumes. These zones are suspended below the water table and above the bottoms of their respective, moderately homogeneous, unconfined sandy aquifers. Exceptionally detailed, depth-discrete, ground water sampling was performed using a direct-push sampler along cross sections of the dissolved-phase plumes, immediately downgradient of these DNAPL source zones. The total plume PCE or TCE mass-discharge through each cross section ranged between 15 and 31 kg/year. Vertical ground water sample spacing as small as 15 cm and lateral spacing typically between 1 and 3 m revealed small zones where maximum concentrations were between 1% and 61% of solubility. These local maxima are surrounded by much lower concentration zones. A spacing no larger than 15 to 30 cm was needed at some locations to identify high concentration zones, and aqueous VOC concentrations varied as much as four orders of magnitude across 30 cm vertical intervals. High-resolution sampling at these sites showed that three-quarters of the mass-discharge occurs within 5% to 10% of the plume cross sectional areas. The extreme spatial variability of the mass-discharge occurs even though the sand aquifers are nearly hydraulically homogeneous. Depth-discrete field techniques such as those used in this study are essential for finding the small zones producing most of the mass-discharge, which is important for assessing natural attenuation and designing remedial options.  相似文献   

设为首页 | 免责声明 | 关于勤云 | 加入收藏

Copyright©北京勤云科技发展有限公司  京ICP备09084417号