Relating soil solution Zn concentration to diffusive gradients in thin films measurements in contaminated soils

The technique of diffusive gradients in thin films (DGT) has been suggested to sample an available fraction of metals in soil. The objectives of this study were to compare DGT measurements with commonly measured fractions of Zn in soil, viz, the soil solution concentration and the total Zn concentration. The DGT technique was used to measure fluxes and interfacial concentrations of Zn in three series of field-contaminated soils collected in transects toward galvanized electricity pylons and in 15 soils amended with ZnCl2 at six rates. The ratio of DGT-measured concentration to pore water concentration of Zn, R, varied between 0.02 and 1.52 (mean 0.29). This ratio decreased with decreasing distribution coefficient, Kd, of Zn in the soil, which is in agreement with the predictions of the DGT-induced fluxes in soils (DIFS) model. The R values predicted with the DIFS model were generally larger than the observed values in the ZnCl2-amended soils at the higher Zn rates. A modification of the DIFS model indicated that saturation of the resin gel was approached in these soils, despite the short deployment times used (2 h). The saturation of the resin with Zn did not occur in the control soils (no Zn salt added) or the field-contaminated soils. Pore water concentration of Zn in these soils was predicted from the DGT-measured concentration and the total Zn content. Predicted values and observations were generally in good agreement. The pore water concentration was more than 5 times underpredicted for the most acid soil (pH = 3) and for six other soils, for which the underprediction was attributed to the presence of colloidal Zn in the soil solution.     

Kinetics of Zn release in soils and prediction of Zn concentration in plants using diffusive gradients in thin films

Effective concentrations (CE) of Zn measured by the technique of DGT (diffusive gradients in thin films) were compared, along with total concentrations of Zn and the concentrations of Zn in soil solutions, to Zn concentrations in plants. Soils variously contaminated with Zn were collected in the vicinity of two galvanized electrical transmission towers (pylons) and two motorway crash barriers. Lepidium sativum was grown in each soil and in corresponding control soils amended with ZnCl2 to similar total Zn concentrations. CE, concentrations in soil solution, and total Zn were measured in all soils, and total Zn was measured in the plant shoots. The CE values, soil solution Zn, and shoot Zn concentrations were all larger in ZnCl2 amended soils than in field contaminated soils at corresponding total Zn. Correlations between the concentration of Zn in the plants and the measured soil parameter followed the order CE > soil solution > total Zn. The low scatter in the plot of log plant concentration versus log CE revealed a relationship with two distinct features. Plant Zn was between 100 and 300 mg/kg up to an effective Zn concentration of about 2 mg/L, above which plant Zn increased steadily with increasing CE. Use of a dynamic model to interpret the DGT measurement suggests that the intrinsic rate of release of Zn from solid phase to solution, expressed as a rate constant, is much higher for soils that receive fresh supplies of Zn. This finding provides a mechanistic basis for reconciling laboratory experiments, where metal is freshly amended, to data obtained in the field. The potential of DGT as a surrogate for metal availability to plants is further confirmed by this work.     

Evaluating the performance of diffusive gradients in thin films for predicting ni sediment toxicity

Diffusive gradients in thin films (DGTs) rapidly measure labile fractions of metal and are promoted as an assessment tool for bioavailability. Using macroinvertebrate community composition as a response, this study compared the predictive ability of DGT-measured Ni with acid volatile sulfide (AVS) and organic carbon (OC) corrected Ni [(SEM(Ni)-AVS)/f(OC)] and total Ni concentrations. In two experiments, sediments were amended with Ni and placed within either a streamside mesocosm or deployed in situ. DGT-measured Ni concentrations (C(DGT)) increased with increasing total Ni, were greater at depth, and decreased over time. Relationships between Ni C(DGT) and sediment geochemistry indicated a shift in Ni partitioning from AVS-bound to Fe- and Mn-associated Ni. In both experiments, DGT-measured Ni poorly predicted the invertebrate response to metal, whereas models that included total Ni or (SEM(Ni)-AVS)/f(OC) effectively predicted the invertebrate response for the streamside mesocosm and in situ experiments, respectively. C(DGT) overestimated the available Ni fraction, possibly due to sampling either nonbioavailable solid-phase Ni or Ni irrespective of cations competing at the biotic ligand. We suggest that C(DGT) cannot replace (SEM(Ni)-AVS)/f(OC) for predicting invertebrate response to sediment Ni, and greater understanding of metal species lability to DGTs is needed before assuming equivalence between bioavailable and DGT-labile metals in sediments.     

Use of diffusive gradients in thin films to measure cadmium speciation in solutions with synthetic and natural ligands: comparison with model predictions

The performance of the technique of diffusive gradients in thin films (DGT) was characterized in well-defined systems containing cadmium with chloride and nitrate ions, simple organic ligands (nitrilotriacetic acid and diglycolic acid), and Suwannee river fulvic acid for the pH range 5-8. Cd was fully labile in all Cd, Cl-, and NO3- solutions tested (I= 0.1 and 0.01 M), even atvery low Cd concentrations (10 nM), consistent with there being no binding of Cd to the diffusive gel. Diffusion coefficients of Cd-nitritotriacetic acid (NTA) and Cd-diglycolic acid (DGA) species were measured and found to be ca. 25-30% lower than the equivalent coefficient for free metal ions. These values were used to calculate concentrations of labile Cd from DGT measurements in solutions of Cd with NTA or DGA. Cd-NTA and Cd-DGA species were found to be fully DGT-labile. DGT devices that used a diffusive gel with a reduced pore size, which retarded the passage of fulvic acid species through the gel, were used to estimate the proportion of Cd complexed by fulvic acid. These results were compared with predictions of the solution speciation from models with default parameter values. ECOSAT, incorporating the NICA-Donnan model, correctly predicted the magnitude of the binding and its pH dependence, while predictions from WHAM V (with humic ion binding model V) and WHAM 6 (with humic ion binding model VI) were less satisfactory at predicting the pH dependence. Reasonable fits to the data could be obtained from WHAM 6 when the effective binding constant log K(MA) was changed from 1.6 to 1.5, the value of deltaLK1 from 2.8 to 1.0 to minimize the dependence on pH, and the value of deltaLK2 from 1.48 to 1.0 to decrease the strength of the strong bidentate and tridentate binding sites.     

Trace metal speciation in brackish water using diffusive gradients in thin films and ultrafiltration: comparison of techniques

Diffusive gradients in thin films (DGT) and ultrafiltration were used to measure trace metal concentrations in the Baltic Sea. The results provide the first comparison of these two fundamentally different speciation methods for trace metals. Cd, Cu, Mn, Ni, and Zn were measured at two sites with different total trace metal concentrations. DGT units prepared with APA-gel as diffusive layer and Chelex 100 resin as binding agent were used throughout the study. The ultrafiltration was performed with Millipore Prep/Scale modules with cutoffs of 1 and 10 kDa. Concentration levels of Mn, Zn, and Cd measured by DGT agreed with the concentrations measured in 1 kDa ultrafiltered samples. For Cu and Ni the ultrafiltered concentrations exceeded the DGT-labile concentrations. The ability of DGT to preconcentrate metals was found to be an analytical advantage compared with ultrafiltration. DGT appears to be a good alternative to 1 kDa ultrafiltration for measurement of truly dissolved Mn, Cd, and Zn in the Baltic Sea.     

Measurement of dissolved reactive phosphorus using the diffusive gradients in thin films technique with a high-capacity binding phase

Measurement of dissolved reactive phosphorus (DRP) by the diffusive gradients in thin films (DGT) technique was investigated using a new binding phase. Half-dried amorphous zirconium oxide (with 50 ± 5% of water content) was mixed with acrylamide solution for the preparation of the new binding phase. The resulting binding gel had a high binding capacity (223 μg P cm(-2)) for phosphate. The solution of NaOH (1 M) was used for elution of phosphate from the gel, and an elution efficiency of 0.95 was obtained. A test of DGT uptake with this gel showed its dependence on temperature, and there was no influence of pH (3 to 10) and ionic strength (10 nM to 0.1 M). Its capacity for DGT response exceeded 100 μg P cm(-2), corresponding to a DRP concentration of more than 20 mg L(-1) for a 24 h deployment with a standard DGT device at 25 °C, which was at least 50 times of the Fe-oxide gel commonly used in the present DGT technique. Measurements with this high-capacity DGT technique in a laboratory microcosm of homogeneously mixed sediments gave smooth and reproducible mass-depth profiles. This technique was well demonstrated by in situ measurements in algal- and macrophyte-dominated regions of Lake Taihu. The DGT-measured concentrations of DRP were on average 20% and 40% of the DRP concentrations in pore waters, respectively, indicating a partial resupply of the sediments to the pore waters with DRP.     

High-resolution, two-dimensional measurement of dissolved reactive phosphorus in sediments using the diffusive gradients in thin films technique in combination with a routine procedure

Dissolved reactive phosphorus (DRP) is the most available P form in sediments and often directly controls phytoplankton blooms in aquatic systems. In this study, a novel procedure was developed for two-dimensional (2D) measurement of DRP in sediments at a spatial resolution of 0.45 mm using the diffusive gradients in thin films (DGT) technique with a revised high-capacity binding phase (Zr oxide gel). This procedure involves DGT uptake of P in sediments, 2D slicing of the binding gel on a 0.45 × 0.45-mm grid system, elution of P from each gel square with 1 M NaOH, and microcolorimetric determination of DRP in each eluted solution using 384-microwell plates. Measurements of DRP via this procedure were tested in homogeneous solutions and sediments and produced an acceptable error (<20% relative standard deviation) for the analysis once the accumulated mass of P in each gel square reached 1.2 μg cm(-2). This method was successfully applied to produce 2D images of the DRP distribution in sediments with and without the influence of tubificid worm bioturbation, revealing a much more pronounced and localized impact from tubificid worms than that found using a one-dimensional measurement of pore water DRP concentrations at 1-cm resolution.     

聚天冬氨酸为结合相的薄膜梯度扩散技术富集测量自来水中痕量铬(Ⅲ)

建立了以0.05mol/L聚天冬氨酸(PASP)溶液为结合相的薄膜梯度扩散(DGT)装置(PASP DGT)富集测量自来水中痕量CrIII的分析方法。DGT法测得配制水中CrIII的回收率为94.4%~105.6%,相对标准偏差(RSD)为2.78%~5.03%;测得自来水中CrIII的浓度为2.99~3.47μg/L,加标回收率为93.2%~107.7%;DGT方法对水中CrIII的检出限为0.081μg/L(采样48h),可应用于自来水中痕量CrIII的定量检测与早期预警。      

Biological reduction of nanoengineered iron(III) oxide sculptured thin films

Sculptured thin films (STFs) are assemblies of nominally identical, parallel nanowires with tailored shapes such as chevrons and spirals. A series of iron(lll) STFs were produced with varied crystallinity (from hematite toferrihydrite) and nanowire shapes (slanted columnar, clockwise helical, and counterclockwise helical). When the dissimilatory metal-reducing bacterium Shewanella putrefaciens CN32 was used to measure their bioreducibility, it was found that bioreduction was controlled primarily by oxide crystallinity. STFs were characterized by scanning electron microscopy, atomic force microscopy, and grazing incidence small-angle X-ray scattering. Postbioreduction characterizations determined that mineralogy of the film materials did not change, but surface roughness generally increased. Changes caused by bioreduction were assessed in terms of both transmittance and reflectance of light incident normal to the STFs. The greatest optical changes were obtained with crystalline hematite films. These results underscore the feasibility of an STF-based fiber optic iron(lll) reduction sensor for in situ subsurface deployment.     

Diffusive gradients in thin films sampler predicts stress in brown trout (Salmo trutta L.) exposed to aluminum in acid fresh waters

Increased levels of aluminum ions released from nutrient-poor soils affected by acid rain have been the primary cause of fish deaths in the acidified watersheds of southern Norway. The complex aluminum chemistry in water requires speciation methods to measure the gill-reactive species imposing toxic effects toward fish. Previously, aluminum speciation has mainly followed the fractionation principles outlined by Barnes/Driscoll, and several analogues of these fractionation principles have been used both in situ and in the laboratory. Due to rapid transformation processes, aluminum speciation in water samples may change even during short storage times. Thus, results obtained by laboratory fractionation methods might be misleading for the assessment of potentially toxic aluminum species in the water. Until now, all in situ field fractionation methods have been time and labor consuming. The DGT technique (diffusive gradients in thin films) is a new in situ sampler collecting a fraction of dissolved metal weighted according to the rate of diffusion and dissociation kinetics. In a field experiment with acid surface water we studied the DGT sampler as a new prediction tool for the gill accumulation of aluminum in trout (Salmo trutta L.) and the induced physiological stress responses measured as changes in blood glucose and plasma chloride. Aluminum determined with DGT (DGT-AI) was higher than labile monomeric aluminum (Ali) determined with a laboratory aluminum fractionation procedure (PCV--a pyrocatechol violet analogue of Barnes/Driscoll), a difference due to collection of a fraction of organically complexed aluminum by DGT and a reduction of the Ali fraction during sample storage. DGT-AI predicted the gill uptake and the aluminum-induced physiological stress responses (increased blood glucose and decreased plasma chloride, r2 from 0.6 to 0.9). The results indicate that DGT-AI is a better predictor for the stress response than laboratory-determined Ali, because the DGT sampler collects a more correct fraction of the gill-reactive aluminum species that induces the stress.     

Measurement and dynamic modeling of trace metal mobilization in soils using DGT and DIFS

The technique of diffusive gradients in thin-films (DGT) accumulates metals on a Chelex resin after their diffusive transport through a hydrogel. It lowers metal concentrations in soil solution adjacent to the device and induces resupply of metal associated with the solid phase. DGT devices were deployed in an alluvial gley soil for 21 different time periods between 4 h and 19.5 d. The accumulated masses of Cu, Cd, Ni, and Zn were used to calculate the distribution coefficient for labile metal, Kdl, and adsorption and desorption rate constants. Calculations were performed using a dynamic numerical model of DGT-induced fluxes in soils (DIFS). It assumes first-order exchange between solid phase and solution and diffusional transport in both the soil solution and the hydrogel. The DIFS model fitted changes in accumulated mass with time very well. Values of Kdl calculated from DIFS of 100 (Cd), 250 (Cu), 150 (Ni), and 150 (Zn) were larger than values of distribution coefficients estimated by exchange with Ca(NO3)2 but similar to those estimated by isotopic exchange (Cd and Zn only). These results suggest that the solid-phase pool of metal affected by the removal of labile metal by DGT, which operates on a time scale of minutes, is similar to the solid-phase pool of metal that can isotopically exchange with solution on a time scale of 2 d. Response times of minutes were consistent with interaction rates with surfaces, and desorption rate constants agreed with other reported values. An appraisal of the DIFS model demonstrated the importance of the labile pool size in the solid phase for controlling supply to a sink, such as DGT or a plant. As values of Kdl and kinetic parameters are obtained using DGT with minimal soil disturbance and by a similar mechanism to that involved in plant uptake, they may be pertinent to bioavailability studies.     

Desorption kinetics of Cd, Zn, and Ni measured in soils by DGT

DGT (diffusive gradients in thin films) was used to measure the distribution and rates of exchange of Zn, Cd, and Ni between solid phase and solution in five different soils. Soil texture ranged from sandy loam to clay, pH ranged from 4.9 to 7.1, and organic carbon content ranged from 0.8% to 5.8%. DGT devices continuously remove metal to a Chelex gel layer after passage through a well-defined diffusion layer. The magnitude of the induced remobilization flux from the solid phase is related to the pool size of labile metal and the exchange kinetics between dissolved and sorbed metal. DGT devices were deployed over a series of times (4 h to 3 weeks), and the DIFS model (DGT induced fluxes in soils) was used to derive distribution coefficients for labile metal (Kdl) and the rate at which the soil system can supply metal from solid phase to solution, expressed as a response time. Response times for Zn and Cd were short generally (<8 min). They were so short in some soils (<1 min) that no distinction could be made between supply of metal being controlled by diffusion or the rate of release. Generally longer response times for Ni (5-20 min) were consistent with its slow desorption. The major factor influencing Kdl for Zn and Cd was pH, but association with humic substances in the solid phase also appeared to be important. The systematic decline, with increasing pH, in both the pool size of Ni available to the DGT device and the rate constant for its release is consistent with a part of the soil Ni pool being unavailable within a time scale of 1-20 min. This kinetic limitation is likely to limit the availability of Ni to plants.     

Rhizosphere gradients of polycyclic aromatic hydrocarbon (PAH) dissipation in two industrial soils and the impact of arbuscular mycorrhiza

Phytoremediation of organic pollutants depends on plant-microbe interactions in the rhizosphere, but the extent and intensity of such rhizosphere effects are likely to decrease with increasing distance from the root surface. We conducted a time-course pot experiment to measure dissipation of polycyclic aromatic hydrocarbons (PAHs) in the rhizosphere of clover and ryegrass grown together on two industrially polluted soils (containing 0.4 and 2 g kg(-1) of 12 PAHs). The impact of the fungal root symbiosis arbuscular mycorrhiza (AM) on PAH degradation was also assessed, as these fungi have previously improved plant establishment on PAH-polluted soils and enhanced PAH degradation in spiked soil. The two soils behaved differently with respect to the time-course of PAH dissipation. The less polluted and more highly organic soil showed low initial PAH dissipation rates, with small positive effects of plants after 13 weeks. At the final harvest (26 weeks), the amounts of PAHs extracted from nonplanted pots were higher than the initial concentrations. In parallel planted pots, PAH concentrations decreased as a function of proximity to roots. The most polluted soil showed higher initial PAH dissipation (25% during 13 weeks), but at the final harvest PAH concentrations had increased to values between the initial concentration and those at 13 weeks. An effect of root proximity was observed for the last harvest only. The presence of mycorrhiza generally enhanced plant growth and favored growth of clover at the expense of ryegrass. Mycorrhiza enhanced PAH dissipation when plant effects were observed.     

Evaluation of the diffusive gradient in a thin film technique for monitoring trace metal concentrations in estuarine waters

Monitoring trace metal concentrations in dynamic estuarine waters is not straightforward. This study demonstrated that important information could be obtained from intensive sampling of physicochemical parameters and trace metal concentrations, in the Gold Coast Broadwater, Australia. A regular pattern of variation in Cu and Ni concentrations was related to the movement of water passed point sources with tidal flows, rather than due to conventional estuarine mixing of end-member waters. However, this approach was logistically demanding and expensive. The diffusive gradients in a thin film (DGT) technique was used as an alternative method due to its continual time-integrated response to changes in trace metal concentrations. Significant correlations were found between 24 h DGT-labile measurements and 0.45-microm filterable measurements, on time-averaged composite samples (grab samples combined every 4 h for 24 h), for Cu (n = 24, r = 0.965, p < 0.001), Pb (n = 24, r = 0.799, p < 0.001), Zn (n = 17, r = 0.909, p < 0.001), and Ni (n = 23, r = 0.916, p < 0.001). DGT-labile measurements as a fraction of 0.45 microm-filterable concentrations were 21 +/- 2% for Cu, 29 +/- 11% for Pb, 28 +/- 5% for Zn, and 27 +/- 12% for Ni, demonstrating the speciation capabilities of DGT. Although DGT measurements were confirmed as being highly operationally defined, DGT was still found to be very promising as a monitoring approach, particularly for dynamic estuarine waters.     

涤纶基表面溅射纳米铜膜导电性能

采用正交试验方法优化涤纶基表面溅射纳米铜膜的工艺参数,其结果为:镀膜时间30 min,溅射功率120 W,气体压强0.2 Pa,基材表面等离子体预处理时间为3 min。测定了不同环境温湿度和不同水洗次数下薄膜的方块电阻,以及样品在室温下放置90 d后的形貌变化和电阻变化,以评价薄膜材料导电时效性。     

A rapid method for estimating nitrate-nitrogen concentration in field soils

A rapid method is described to enable farmers to measure the NO₃-N levels in a field simply and cheaply. It involves taking soil cores from the field, kneading them with water in polythene bags, adding the resulting suspensions to test strips of reagent impregnated paper and estimating the intensity of colour that develops. A formula to relate the errors incurred at each step to the overall error of the method was derived. These components of error were estimated and their substitution into the formula enabled the overall errors in using the method to assess nitrate concentrations in different fields to be calculated. The largest component of error was the variation in NO₃-N from core to core within a field. The median value over a range of sites was 57%. For a soil with this value the overall standard error in estimating NO₃-N concentration was 22% which is similar to that found with the methods of soil analysis currently used for advisory purposes. The increased number of determinations in the rapid method compensated for larger errors in each individual measurement so that the overall error in estimation by rapid and standard methods were both dominated by core to core variability. Measurements of NO₃-N concentration in each of 18 different fields by the rapid method were in reasonably good agreement with those made entirely independently by a laboratory procedure.     

涤纶基表面磁控溅射纳米铜膜的电磁屏蔽性能

通过射频磁控溅射的方法,将金属铜溅射到涤纶机织布表面;讨论了基材编织密度、溅射时间、低温等离子体预处理对镀膜织物电磁屏蔽效能的影响。结果表明,随基材编织密度的增加,镀铜织物导电能力有所增强,当电磁波频率相同时,增加基材编织密度,样品的电磁屏蔽性能略微提高;延长溅射时间,导电涤纶织物的电磁屏蔽效能明显增强;等离子体预处理后,样品屏蔽效能提高。     

Electrophoretic assembly of naturally occurring humic substances as thin films

Polydisperse humic acid thin films on optically transparent electrodes (OTEs) have been prepared by electrophoretic deposition from a solution of Suwanee River humic acid (SHA) in ethanol/acetonitrile. The thickness of the film and the rate of deposition of SHA are dependent on the applied voltage and the concentration of the solution. Tapping-mode atomic force microscopy (TM-AFM) confirms the assembly of SHA aggregates on the electrode surface. The ability of these thin films to incorporate redox-active species such as ferrocene from solution is demonstrated by cyclic voltammetry experiments. A linear dependence of the peak current for the oxidation of ferrocene as a function of scan rate indicates that the ferrocene is incorporated into the humic membrane.     

Use of hyperspectral imaging for evaluation of the shelf-life of fresh white button mushrooms (Agaricus bisporus) stored in different packaging films

The shelf life of mushrooms packaged using different polymer top-films (PVC, PET with different levels of perforations) was investigated using hyperspectral imaging (HSI). Packaged mushrooms were stored at 4 ± 0.2 °C for 14 days and weight loss, Hunter L, a, b values, maturity index and in-pack gas composition (% CO₂ and O₂) were also measured. The results obtained showed that the PET film perforated with small holes (1 mm in diameter) was generally superior in terms of maintaining overall mushroom quality. Regression models were built to correlate HSI data with measured quality parameters. Prediction maps were generated from hyperspectral data to show the model performance at pixel level. Results presented in this work show hyperspectral imaging can be used to evaluate the effect of different packaging systems on mushroom quality and that perforated PET packaging film is a viable alternative to the conventional PVC packaging, facilitating an increase in shelf life from 10 to 14 days.Industrial relevanceThe present study demonstrates HSI can be used for rapid evaluation of mushroom quality facilitating non-destructive evaluation of the effect of packaging systems on mushroom shelf-life. In addition, this work suggests an effective packaging solution to extend shelf life of mushrooms during storage. The proposed solution potentially improves the packaging recyclability as the same polymer material (PET) is used for the tray and top film, compared to conventional mushroom packaging where PVC is used for the top film and polypropylene (PP) for the tray.     

Relative importance of gas-phase diffusive and advective tichloroethene (TCE) fluxes in the unsaturated zone under natural conditions

It was hypothesized that atmospheric pressure changes can induce gas flow in the unsaturated zone to such an extent that the advective flux of organic vapors in unsaturated-zone soil gas can be significant relative to the gas-phase diffusion flux of these organic vapors. To test this hypothesis, a series of field measurements and computer simulations were conducted to simulate and compare diffusion and advection fluxes at a trichloroethene-contaminated field site at Picatinny Arsenal in north-central New Jersey. Moisture content temperature, and soil-gas pressure were measured at multiple depths (including at land surface) and times for three distinct sampling events in August 1996, October 1996, and August 1998. Gas pressures in the unsaturated zone changed significantly over time and followed changes measured in the atmosphere. Gas permeability of the unsaturated zone was estimated using data from a variety of sources, including laboratory gas permeability measurements made on intact soil cores from the site, a field air pump test, and calibration of a gas-flow model to the transient, one-dimensional gas pressure data. The final gas-flow model reproduced small pressure gradients as observed in the field during the three distinct sampling events. The velocities calculated from the gas-flow model were used in transient, one-dimensional transport simulations to quantify advective and diffusive fluxes of TCE vapor from the subsurface to the atmosphere as a function of time for each sampling event. Effective diffusion coefficients used for these simulations were determined from independent laboratory measurements made on intact soil cores collected from the field site. For two of the three sampling events (August 1996 and August 1998), the TCE gas-phase diffusion flux at land surface was significantly greater than the advection flux over the entire sampling period. For the second sampling event (October 1996), the advection flux was frequently larger than the diffusion flux. When averaged over the second sampling event, the advection and diffusion fluxes were comparable in magnitude. Sensitivity analyses indicate that diffusion fluxes increase significantly with increases in air-filled porosity near land surface, whereas advection fluxes do not. For October 1996, the comparable advection and diffusion fluxes were caused by high moisture content near land surface and a subsequent reduction in the diffusion flux relative to the advection flux. These results indicate that under certain environmental conditions, the organic vapor advection flux from the unsaturated zone to the atmosphere may be equal to or greater than the diffusion flux.