Soil column evaluation of factors controlling biodegradation of DNT in the vadose zone

High concentrations of 2,4-dinitrotoluene (2,4-DNT) and 2,6-dinitrotoluene (2,6-DNT) are present in vadose zone soils at many facilities where explosives manufacturing has taken place. Both DNT isomers can be biodegraded under aerobic conditions, but rates of intrinsic biodegradation observed in vadose zone soils are not appreciable. Studies presented herein demonstrate that nutrient limitations control the onset of rapid 2,4-DNT biodegradation in such soils. In column studies conducted at field capacity, high levels of 2,4-DNT biodegradation were rapidly stimulated by the addition of a complete mineral medium but not by bicarbonate-buffered distilled deionized water or by phosphate-amended tap water. Biodegradation of 2,6-DNT was not observed under any conditions. Microcosm studies using a DNT-degrading culture from column effluent suggest that, after the onset of 2,4-DNT degradation, nitrite evolution will eventually control the extent of degradation achieved by two mechanisms. First, high levels of nitrite (40 mM) were found to strongly inhibit 2,4-DNT degradation. Second, nitrite production reduces the solution pH, and at pH levels below 6.0, 2,4-DNT degradation slows rapidly. Under conditions evaluated in laboratory-scale studies, 2,4-DNT biodegradation enhanced the rate of contaminant loss from the vadose zone by a factor of 10 when compared to the washout due to leaching.     

Colloid stability in vadose zone Hanford sediments

We experimentally determined colloid stability of natural colloids extracted from vadose zone sediments from the U.S. Department of Energy's Hanford Reservation. We also used reference minerals, kaolinite, montmorillonite, and silica,for comparative purposes. Colloid stability was assessed with two different methods: the batch turbidity method and dynamic light scattering. Critical coagulation concentrations (CCCs) were determined for pure Na and pure Ca electrolyte solutions, as well for mimicked Hanford vadose zone pore waters with varying sodium adsorption ratios (SARs). Critical coagulation concentrations obtained from the batch turbidity method were sensitive to initial colloid mass concentrations, settling time, and CCC criteria. The lower the initial colloid concentration and the shorter the settling times were, the larger was the CCC. The CCCs determined from the dynamic light scattering, where diluted colloidal suspensions are used, were not dependent on settling time and arbitrary CCC criteria, so dynamic light scattering is therefore the preferred method to determine colloid stability. The CCC values determined from dynamic light scattering ranged from 90 to 200 mmol/L for Na systems and 1.7 to 3.8 mmol/L for Ca systems. The stability of natural colloids was intermediate between that of pure kaolinite and montmorillonite. The results indicate that colloids in the Hanford vadose zone form stable suspensions, i.e., are in the slow aggregation regime. Nonetheless, due to the long travel times in the vadose zone, nearly all colloids will aggregate and be removed from the water column before reaching groundwater levels.     

Direct volatilization of naphthalene to the atmosphere at a phytoremediation site

Phytoremediation systems are known to reduce groundwater contamination by at least three major mechanisms: plant uptake, phytovolatilization, and enhanced rhizosphere bioremediation. The potential for such systems to enhance a fourth remediation pathway--direct surface volatilization of contaminants through the subsurface and into the atmosphere-has not yet been investigated in the field. A vertical flux chamber was used to measure direct surface volatilization of naphthalene over nine months at a creosote-contaminated site in Oneida, Tennessee, where a phytoremediation system of poplar trees was installed in 1997. A maximum flux of 23 microg m(-2) h(-1) was measured in August 2004, and naphthalene removal by the direct volatilization pathway is estimated to be 50 g yr(-1) at this site. Results suggest that direct volatilization fluxes are most strongly affected by the groundwater level (thickness of the saturated zone), soil moisture, and changes in atmospheric pressure. At this site, transpiration and canopy interception resulting from the phytoremediation system significantly reduce the saturated thickness, increasing the vertical concentration gradient of naphthalene in the groundwater and thus increasing the upward diffusive flux of naphthalene through the subsurface. The presence of the trees, therefore, promotes direct volatilization into the atmosphere. This research represents the first known measurement of naphthalene attenuation by the direct volatilization pathway.     

Mechanisms affecting the infiltration and distribution of ethanol-blended gasoline in the vadose zone

One- and two-dimensional experiments were conducted to examine differences in the behavior of gasoline and gasohol (10% ethanol by volume) as they infiltrate through the unsaturated zone and spread at the capillary fringe. Ethanol in the spilled gasohol quickly partitions into the residual water in the vadose zone and is retained there as the gasoline continues to infiltrate. Under the conditions tested, over 99% of the ethanol was initially retained in the vadose zone. Depending on the volume of gasoline spilled and the depth to the water table, this causes an increase in the aqueous-phase saturation and relative permeability, thus allowing the ethanol-laden water to drain into the gasoline pool. Under the conditions tested, the presence of ethanol does not have a significant impact on the overall size or shape of the resulting gasoline pool at the capillary fringe. Residual gasoline saturations in the vadose zone were significantly reduced however because of reduced surface and interfacial tensions associated with high ethanol concentrations. The flux of ethanol in the effluent of the column ranged from 1.4 x 10(-4) to 4.5 x 10(-7) g/(cm2 min) with the LNAPL and from 6 x 10(-3) to 3.0 x 10(-4) g/(cm2 min) after water was introduced to simulate rain infiltration. The experimental results presented here illustrate that the dynamic effects of ethanol partitioning into the aqueous phase in the vadose zone create an initial condition that is significantly different than previously understood.     

Influence of sources on plutonium mobility and oxidation state transformations in vadose zone sediments

Well-defined solid sources of Pu(III) (PuCl3), Pu(IV) (Pu (NO3)4 and Pu (C2O4)2), and Pu(VI) (Pu02(NO3)2) were placed in lysimeters containing vadose zone sediments and exposed to natural weather conditions for 2 or 11 years. The objective of this study was to measure the release rate of Pu and the changes in the Pu oxidation states from these Pu sources with the intent to develop a reactive transport model source-term. Pu(III) and Pu(IV) sources had identical Pu concentration depth profiles and similar Pu release rates. Source release data indicate that PuIV(C2O4)2 was the least mobile, whereas Pu(VI)O2(NO3)2 was the most mobile. Synchrotron X-ray fluorescence (SXRF) revealed that Pu was very unevenly distributed on the sediment and Mn concentrations were too low (630 mg kg(-1)) and perhaps of the wrong mineralogy to influence Pu distribution. The high stability of sorbed Pu(IV) is proposed to be due to the formation of a stable hydrolyzed Pu(IV) surface species. Plutonium X-ray absorption near-edge spectroscopy (XANES) analysis conducted on sediment recovered at the end of the studyfrom the Pu(IV)(NO3)4- and Pu(III)(III)Cl3-amended lysimeters contained essentially identical Pu distributions: approximately 37% Pu(III), 67% Pu(IV), 0% Pu(V), and 0% Pu(VI). These results were similar to those using a wet chemistry Pu oxidation state assay, except the latter method did not detect any Pu(III) present on the sediment but instead indicated that 93-98% of the Pu existed as Pu(IV). This discrepancy was likely attributable to incomplete extraction of sediment Pu(III) by the wet chemistry method. Although Pu has been known to exist in the +3 oxidation state under microbially induced reducing conditions for decades, to our knowledge, this is the first observation of steady-state Pu(III) in association with natural sediments. On the basis of thermodynamic considerations, Pu(III) has a wide potential distribution, especially in acidic environments, and as such may warrant further investigation.     

Review: Technical and policy challenges in deep vadose zone remediation of metals and radionuclides

Contamination in deep vadose zone environments is isolated from exposure so direct contact is not a factor in its risk to human health and the environment. Instead, movement of contamination to the groundwater creates the potential for exposure and risk to receptors. Limiting flux from contaminated vadose zone is key for protection of groundwater resources, thus the deep vadose zone is not necessarily considered a resource requiring restoration. Contaminant discharge to the groundwater must be maintained low enough by natural attenuation (e.g., adsorption processes or radioactive decay) or through remedial actions (e.g., contaminant mass reduction or mobility reduction) to meet the groundwater concentration goals. This paper reviews the major processes for deep vadose zone metal and radionuclide remediation that form the practical constraints on remedial actions. Remediation of metal and radionuclide contamination in the deep vadose zone is complicated by heterogeneous contaminant distribution and the saturation-dependent preferential flow in heterogeneous sediments. Thus, efforts to remove contaminants have generally been unsuccessful although partial removal may reduce downward flux. Contaminant mobility may be reduced through abiotic and biotic reactions or through physical encapsulation. Hydraulic controls may limit aqueous transport. Delivering amendments to the contaminated zone and verifying performance are challenges for remediation.     

Equilibrium partitioning of chlorinated solvents in the vadose zone: low f(oc) geomedia

A series of gas (vapor)-advecting water-unsaturated column experiments using a low organic content (f(oc)) silica sand was conducted to determine mass distributions of chlorinated-volatile hydrophobic organic compounds (C-VHOCs) in a natural sorbent system. C-VHOCs used were trichloroethene (TCE), tetrachloroethene (PCE), chlorobenzene (CB), and 1,3-dichlorobenzene (DCB). Four volumetric water contents (theta(w) = 0.07, 0.12, 0.17, 0.20) and several influent gas-phase C-VHOC (solute) concentrations were considered. The method of temporal first moments was applied to complete breakthrough curve data to determine total C-VHOC gas-phase retardation and associated gas-phase C-VHOC mass fraction. Results were compared to an equilibrium partitioning advective-dispersive formulation of total gas-phase retardation. Literature-derived values of Henry's law constants and independent measurements of gas/water interface areal extent and interface phase adsorption allowed quantification of C-VHOC mass fractions in the aqueous and gas/water interface phases. Unaccounted C-VHOC mass, derived from comparison of measured C-VHOC retardation to independent phase prediction, was attributed to solid-phase sorption. Results indicate that for all conditions tested, gas/water interfacial adsorption exhibited only a small effect on C-VHOC vapor retardation (accounting for < or = 10% of the total C-VHOC distributions). Solid-phase association was the dominant uptake mechanism, accounting for 46-91% of the total C-VHOC mass in the porous system. Evaluation of the solid-phase C-VHOC uptake results in terms of a modified form of the Dubinin-Radushkevich (DR) isotherm equation provided strong evidence supporting the mechanism of pore-filling in this natural, low f(oc) sorbent.     

Small-volume releases of gasoline in the vadose zone: impact of the additives MTBE and ethanol on groundwater quality

A controlled gasoline spill experiment was performed under outdoor conditions typical for winter in temperate regions to study the fate of methyl tert-butyl ether (MTBE), ethanol, benzene, and selected other petroleum hydrocarbons. Artificial gasoline containing MTBE and ethanol (5% w/w of each) was placed at a defined depth into a 2.3 m thick unsaturated zone of alluvial sand overlying a gravel aquifer in a lysimeter. During an initial period of 41 days without recharge, MTBE and hydrocarbon vapors migrated by vapor-phase diffusion to groundwater, while ethanol vapors were naturally attenuated. In a subsequent period of 30 days with 5-mm daily recharge, all soluble compounds including ethanol were transported to the groundwater. Ethanol disappeared concomitantly with benzene and all other petroleum hydrocarbons except isooctane from the aerobic groundwater due to biodegradation. MTBE persisted for longer than 6 months at concentrations larger than 125000 microg L(-1). No evidence for MTBE biodegradation was found, whereas > 99.6% of ethanol removal from the lysimeter was due to biodegradation. It is concluded that MTBE-free gasoline would be less harmful for groundwater resources and that ethanol is an acceptable substitute.     

Distribution of atmospheric SF6 near a large urban area as recorded in the vadose zone

Local emissions of SF6 are of interest for studying their impact on the use of SF6 as a groundwater-dating tool near source regions as well as for investigating the spatial distributions of (inert) gaseous compounds spreading from urban or industrial centers. A precondition for the use of SF6 in such studies is the capability to document the temporal and spatial evolution of SF6 in and around source regions with sufficient resolution. Here we present a time series of SF6 measurements in soil air at a site (Sparkill, NY) about 25 km north of New York City carried out between May 2000 and January 2002. The data show that, below about 2 m depth, the vadose zone integrates atmospheric SF6 mixing ratios over time scales greater than 1 month. SF6 mixing ratios in soil air at these depths match averaged high-resolution atmospheric measurements performed at Lamont-Doherty Earth Observatory in Palisades, NY, located about 3 km south of Sparkill. To a first-order approximation, a simple one-dimensional diffusion model reproduces the measured SF6 profiles in the vadose zone, suggesting that the soil indeed acts as a low-pass filter for inert atmospheric gases. These findings indicate that measurements of soil air can be used to determine the spatial pattern of SF6 excess relative to the remote atmosphere for a given region. A transect of soil profiles from Manhattan to the tip of Long Island indicates that emissions from sites close to New York City lead to significant SF6 excesses (ca. 25% or more) above the clean air mixing ratios over distances of the order of 80 km.     

Diffusive partitioning tracer test for nonaqueous phase liquid (NAPL) detection in the vadose zone

This paper proposes the theory and practical application of a new partitioning tracer test for nonaqueous phase liquid (NAPL) detection in the vadose zone, which is based on diffusion. A mixture of chlorofluorocarbons as gaseous tracers is injected into the vadose zone to form a point source at the injection point. While the tracers diffuse away, small volumes of gas are withdrawn from the injection point. The quantitative determination of the NAPL saturation is based on a comparison of the concentration decline of tracers with different air-NAPL partitioning coefficients. The test has been evaluated in laboratory sand columns contaminated with dodecane. NAPL in saturations of 0.8-4% of the total porosity have been quantified in a wide range of different water contents. Actual and measured NAPL saturations calculated as an average from four different tracer pairs agreed within +/-30%. The new method was successfully used for repeated NAPL quantification in a large-scale field lysimeter contaminated with artificial kerosene. This rapid and inexpensive test is potentially of value for site investigations especially in combination with soil gas measurements, because it requires similar equipment. Possible applications are source delineation and repeated NAPL quantification in situ during a remediation.     

Spectroscopic evidence for uranium bearing precipitates in vadose zone sediments at the Hanford 300-area site

Uranium (U) solid-state speciation in vadose zone sediments collected beneath the former North Process Pond (NPP) in the 300 Area of the Hanford site (Washington) was investigated using multi-scale techniques. In 30 day batch experiments, only a small fraction of total U (approximately 7.4%) was released to artificial groundwater solutions equilibrated with 1% pCO2. Synchrotron-based micro-X-rayfluorescence spectroscopy analyses showed that U was distributed among at least two types of species: (i) U discrete grains associated with Cu and (ii) areas with intermediate U concentrations on grains and grain coatings. Metatorbernite (Cu[UO2]2[PO4]2 x 8H2O) and uranophane (Ca[UO2]2[SiO3(OH)]2 x 5H2O) at some U discrete grains, and muscovite at U intermediate concentration areas, were identified in synchrotron-based micro-X-ray diffraction. Scanning electron microscopy/energy dispersive X-ray analyses revealed 8-10 microm size metatorbernite particles that were embedded in C-, Al-, and Si-rich coatings on quartz and albite grains. In mu- and bulk-X-ray absorption structure (mu-XAS and XAS) spectroscopy analyses, the structure of metatorbernite with additional U-C and U-U coordination environments was consistently observed at U discrete grains with high U concentrations. The consistency of the mu- and bulk-XAS analyses suggests that metatorbernite may comprise a significant fraction of the total U in the sample. The entrapped, micrometer-sized metatorbernite particles in C-, Al-, and Si-rich coatings, along with the more soluble precipitated uranyl carbonates and uranophane, likely control the long-term release of U to water associated with the vadose zone sediments.     

Spectroscopic and diffraction study of uranium speciation in contaminated vadose zone sediments from the Hanford site, Washington state

Contamination of vadose zone sediments under tank BX-102 at the Hanford site, Washington, resulted from the accidental release of 7-8 metric tons of uranium dissolved in caustic aqueous sludge in 1951. We have applied synchrotron-based X-ray spectroscopic and diffraction techniques to characterize the speciation of uranium in samples of these contaminated sediments. UIII-edge X-ray absorption fine structure (XAFS) spectroscopic studies demonstrate that uranium occurs predominantly as a uranium(VI) silicate from the uranophane group of minerals. XAFS cannot distinguish between the members of this mineral group due to the near identical local coordination environments of uranium in these phases. However, these phases differ crystallographically, and can be distinguished using X-ray diffraction (XRD) methods. As the concentration of uranium was too low for conventional XRD to detect these phases, X-ray microdiffraction (microXRD) was used to collect diffraction patterns on approximately 20 microm diameter areas of localized high uranium concentration found using microscanning X-ray fluorescence (microSXRF). Only sodium boltwoodite, Na(UO2)(SiO3OH) x 1.5H20, was observed; no other uranophane group minerals were present. Sodium boltwoodite formation has effectively sequestered uranium in these sediments under the current geochemical and hydrologic conditions. Attempts to remediate the uranium contamination will likely face significant difficulties because of the speciation and distribution of uranium in the sediments.     

New field method: gas push-pull test for the in-situ quantification of microbial activities in the vadose zone

Quantitative information on microbial processes in the field is important. Here we propose a new field method, the "gas push-pull test" (GPPT) for the in-situ quantification of microbial activities in the vadose zone. To evaluate the new method, we studied microbial methane oxidation above an anaerobic, petroleum-contaminated aquifer. A GPPT consists of the injection of a gas mixture of reactants (e.g., methane, oxygen) and nonreactive tracer gases (e.g., neon, argon) into the vadose zone and the subsequent extraction of the injection gas mixture together with soil air from the same location. Rate constants of gas conversion are calculated from breakthrough curves of extracted reactants and tracers. In agreement with expectations from previously measured gas profiles, we determined first-order rate constants of 0.68 h(-1) at 1.1 m below soil surface and 2.19 h(-1) at 2.7 m, close to the groundwater table. Co-injection of a specific inhibitor (acetylene) for methanotrophs showed that the observed methane consumption was microbially mediated. This was confirmed by increases of stable carbon isotope ratios in methane by up to 42.6 %. In the future, GPPTs should provide useful quantitative information on a range of microbial processes in the vadose zone.     

Advective removal of intraparticle uranium from contaminated vadose zone sediments, Hanford, U.S

A column study on U(VI)-contaminated vadose zone sediments from the Hanford Site, WA, was performed to investigate U(VI) release kinetics with water advection and variable geochemical conditions. The sediments were collected from an area adjacent to and below tank BX-102 that was contaminated as a result of a radioactive tank waste overfill event. The primary reservoir for U(VI) in the sediments are micrometer-size precipitates composed of nanocrystallite aggregates of a Na-U-Silicate phase, most likely Na-boltwoodite, that nucleated and grew within microfractures of the plagioclase component of sand-sized granitic clasts. Two sediment samples, with different U(VI) concentrations and intraparticle mass transfer properties, were leached with advective flows of three different solutions. The influent solutions were all calcite-saturated and in equilibrium with atmospheric CO2. One solution was prepared from DI water, the second was a synthetic groundwater (SGW) with elevated Na that mimicked groundwater at the Hanford site, and the third was the same SGW but with both elevated Na and Si. The latter two solutions were employed, in part, to test the effect of saturation state on U(VI) release. For both sediments, and all three electrolytes, there was an initial rapid release of U(VI) to the advecting solution followed by slower near steady-state release. U(VI)aq concentrations increased during subsequent stop-flow events. The electrolytes with elevated Na and Si depressed U(VL)aq concentrations in effluent solutions. Effluent U(VI)aq concentrations for both sediments and all three electrolytes were simulated reasonably well by a three domain model (the advecting fluid, fractures, and matrix) that coupled U(VI) dissolution, intraparticle U(VI)aq diffusion, and interparticle advection, where diffusion and dissolution properties were parameterized in a previous batch study.     

Volatilization of parathion and chlorothalonil from a potato crop simulated by the PEARL model

The volatilization of pesticides from crop canopies in the field should be modeled within the context of evaluating environmental exposure. A model concept based on diffusion through a laminar air-boundary layer was incorporated into the PEARL model (pesticide emission assessment at regional and local scales) and used to simulate volatilization of the pesticides parathion and chlorothalonil from a potato crop in a field experiment. Rate coefficients for the competing processes of plant penetration, wash off, and phototransformation in the canopy had to be derived from a diversity of literature data. Cumulative volatilization of the moderately volatile parathion (31% of the dosage in 7.6 days) could be simulated after calibrating two input data derived for the related compound parathion-methyl. The less volatile and more slowly transformed chlorothalonil showed 5% volatilization in 7.6 days, which could be explained by the simulation. Simulated behavior of the pesticides in the crop canopy roughly corresponded to published data.     

TCE diffusion to the atmosphere in phytoremediation applications

The phytoremediation of trichloroethylene (TCE) and other chlorinated compounds has been studied over the past decade, and full-scale systems are in place. The results regarding TCE fates and removal pathways are inconclusive and conflicting, particularly the results regarding volatilization to the atmosphere. Research presented here demonstrates that TCE is taken up by trees and volatilized to the atmosphere. TCE diffusion along the transpiration pathway is shown to be the primary process for TCE volatilization, although volatilization can occur from both stems and leaves. Two concurrent processes influence the eventual fate: transport with transpiration stream through xylem tissues and diffusion from transpiration stream to atmosphere. TCE diffusion flux invariably decreased with height for trees planted in soil or grown hydroponically. In both laboratory experiments and field sampling, TCE concentrations in the transpiration stream (e.g., xylem tissues) decreased with elevation. In field samples, TCE concentrations also decreased in the radial direction, providing fundamental evidence for diffusion. The TCE concentrations in tissues responded linearly to the exposure concentrations at the roots, while TCE diffusion from tree stems was influenced by concentration and transpiration rates.     

Volatilization of the pesticides chlorpyrifos and fenpropimorph from a potato crop

Volatilization of pesticides from crops in the field can be an important emission pathway. In a field experiment with characterization of meteorological conditions, the pesticides chlorpyrifos and fenpropimorph were sprayed onto a potato crop, after which concentrations in the air and on/in the plants were measured. Rates of volatilization were estimated with the aerodynamic profile (ADP), energy balance (EB), relaxed eddy accumulation (REA), and plume dispersion (PD) methods. The volatilization rates obtained with the ADP and EB methods were similar, while some rates obtained with the REA and PD methods in the initial period were lower. Cumulative volatilization of chlorpyrifos during daylight hours (ADP and EB methods) was estimated to be about 65% of the dosage. By far the majority of this volatilization occurred in the first few days. Competing processes at the plant surface had a considerable effect on the dissipation of fenpropimorph, so cumulative volatilization during daylight hours was estimated to be only 7% of the dosage. Plant surface residues were higher than would correspond with the volatilization rate, indicating that penetration into the leaves had occurred.     

Seasonal and interannual mobility of arsenic in a lake impacted by metal mining

Detailed examination of the water column, sediments, and interstitial waters was conducted in Balmer Lake, Ontario, Canada, in 1993-1994 and 1999 in order to assess the seasonal and interannual controls governing the behavior of As. High-resolution profiles of dissolved (<0.45 microm) Fe, Mn, SO4(2-), and sigmaH2S across the sediment-water interface indicate the presence of reducing conditions in close proximity to the benthic boundary during ice-free periods, which are characterized by fully oxygenated bottom waters. Dissolved As is remobilized as As(III) in suboxic sediment horizons via the redox-controlled dissolution of Fe (and perhaps Mn) oxide phases. During 1993-1994, As fluxes to the water column were relatively low (2-15 microg cm(-2) year(-1)) and contributed between 2 and 18% of the water column inventory. Dissolved As in the lake waters was derived primarily from external mining-related loadings during this period. Between 1993 and 1999, external loadings of As to Balmer Lake decreased while [As]aq within the lake increased, suggesting an increase in the proportion of sediment-derived As. Indeed, benthic dissolved As fluxes in 1999 ranged from 179 to 380 microg cm(-2) year(-1), representing approximately 33-60% of the water column burden. The relatively recent importance of sedimentary arsenic sources is suggested to reflect changes to sediment redox conditions associated with a postulated increase in lake primary productivity. Ironically, the increased contribution of dissolved arsenic to the water column appears to have resulted from an otherwise improvement in water quality. Reduced loadings of Cu, Zn, and Ni to the lake since 1994 appear to have allowed increased phytoplankton production that has stimulated arsenic release.