Olivia R. West

Preferential flow path development and its influence on long-term PRB performance: column study

The operating life of an Fe(0)-based permeable reactive barrier (PRB) is limited due to chemical reactions of Fe(0) in groundwater. The relative contributions from mineral precipitation, gas production, and microbial activity to the degradation of PRB performance have been uncertain. In this controlled field study, nitrate-rich, site groundwater was treated by Fe(0) in large-volume, flow-through columns to monitor the changes in chemical and hydraulic parameters over time. Tracer tests showed a close relationship between hydraulic residence time and pH measurements. The ionic profiles suggest that mineral precipitation and accumulation is the primary mechanism for pore clogging around the inlet of the column. Accumulated N(2) gas generated by biotic processes also affected the hydraulics although the effects were secondary to that of mineral precipitation. Quantitative estimates indicate a porosity reduction of up to 45.3% near the column inlet over 72 days of operation under accelerated flow conditions. According to this study, preferential flow through a PRB at a site with similar groundwater chemistry should be detected over approximately 1 year of operation. During the early operation of a PRB, pH is a key indicator for monitoring the change in hydraulic residence time resulting from heterogeneity development. If the surrounding native material is more conductive than the clogged Fe-media, groundwater bypass may render the PRB ineffective for treating contaminated groundwater.

Sediment surface effects on methane hydrate formation and dissociation

Abstract The effects of sediment surfaces on methane hydrate formation and dissociation were investigated using colloidal suspensions and new experimental methods developed for a large volume (72 liters), temperature-controlled pressure vessel. Hydrates were formed by bubbling methane gas through test solutions at temperatures and pressures within the hydrate stability field. Hydrate formation was visually detected by the accumulation of hydrate-encrusted gas bubbles. To measure hydrate dissociation conditions, the pressure vessel was warmed while temperature was monitored within a zone of previously formed hydrate-encrusted gas bubbles. Hydrate dissociation was indicated by a distinct plateau in the hydrate zone temperature, while temperatures of the gas and liquid phases within the vessel continued to rise. The ‘dissociation plateau’ appears to be a phenomenon that is unique to the large volume of the pressure vessel used for the experiments. In experiments where hydrates were formed in pure water, temperature and corresponding pressure conditions measured during the temperature plateau matched model-predicted values for hydrate stability in water, thus confirming the validity of this new method for measuring hydrate dissociation conditions. Formation and dissociation conditions were measured for methane hydrates in colloidal suspensions containing bentonite. Hydrate formation experiments indicated that the presence of bentonite in water at 200 mg/l significantly decreased pressures required for hydrate formation relative to formation in pure water alone. On the other hand, hydrate dissociation conditions measured in bentonite and silica suspensions with solids concentrations of 34 g/l did not differ significantly from that of water. These results are relevant to the origin and stability of natural gas hydrate deposits known to exist in deep permafrost and marine sediments, where the effects of sediment surfaces are largely unknown.

Predicting the precipitation of mineral phases in permeable reactive barriers

The working lifetime of permeable reactive barriers (PRBs) using Fe0 as the reactive media is limited by precipitation of secondary minerals, due to reaction of groundwater with Fe0. Since PRBs are emplaced at sites with widely differing groundwater chemistry, the suite of minerals that precipitate, as well as the rate of their formation, can vary widely. Using plausible phases obtained from field PRBs, the study shows that chemical equilibrium modeling can correctly predict the amounts of precipitates formed, based on the thermodynamic properties of Fe0 and groundwater constituents. These predictions were compared to the results from the solid phase analysis from a field column experiment and from a field-installed PRB at Y-12 Plant, Oak Ridge, TN. Using the column chemical data molar distributions of the precipitates along the flow path were modeled. The maximum precipitation at the Fe0-sand interface at the influent end was predicted, where pore water showed high saturation index (SI) with respect to c...

Measurement error and spatial variability effects on characterization of volatile organics in the subsurface.

The effects of measurement error and spatial variability on establishing subsurface contaminant distributions were demonstrated in a study described herein, where soil underlying a former land treatment facility was intensively sampled and analyzed for volatile organic compounds (VOCs). Concentrations of VOCs measured on-site using a heated headspace/gas chromatography method were typically 10 times higher than concentrations measured at an off-site laboratory using a purge-and-trap/gas chromatography/mass spectrometry method. This was attributed either to VOC losses during storage and preparation of off-site samples and/or inefficient VOC extraction in the off-site method. Three contaminant distribution models developed from the on-site VOC measurements were evaluated through cross-validation and subregion sampling methods. Order of magnitude discrepancies existed between predicted and measured concentrations. These results show that increasing sampling density with cost-effective field analyses can be more effective than using complex spatial models to overcome the lack of spatial information in sparse data sets comprised of off-site laboratory analyses. 25 refs., 8 figs., 2 tabs.

A new experimental facility for investigating the formation and properties of gas hydrates under simulated seafloor conditions

A seafloor process simulator (SPS) has been developed for experimental investigations of the physical, geochemical, and microbiological processes affecting the formation and stability of methane and carbon dioxide hydrates at temperatures and pressures corresponding to ocean depths of 2 km. The SPS is a corrosion-resistant pressure vessel whose salient characteristics are: (i) an operating range suitable for study of methane and carbon dioxide hydrates; (ii) numerous access and observation ports, and (iii) a large (0.0722 m3) internal volume. Initial experiments have shown that the SPS can be used to produce large amounts of high-purity methane hydrate over a wide range of experimental conditions.

Investigation of jet breakup and droplet size distribution of liquid CO2 and water systems—implications for CO2 hydrate formation for ocean carbon sequestration

Abstract An experimental investigation has been conducted into the effect of fluid velocity and orifice size on the breakup patterns of liquid CO2 in water, as well as those for water in CO2. Under high-pressure and low-temperature conditions, the jet breakup patterns follow distinct Rayleigh, transitional, and spray modes. Droplet size distribution was determined in the different modes, with the spray mode producing the smallest droplets and the most uniform size distribution. The system appears to progress from transitional to spray mode when the Ohnesorge number is approximately 18 Re-1. Using this relationship, it is possible to predict the minimum injection rate necessary for spray mode at any injector diameter. Under hydrate-forming conditions, the jet breakup did not appear to be affected because breakup occurred faster than hydrate formation. However, injection into a confined space could promote droplet coalescence, resulting in a larger average drop size. These results can be used to control hydrate conversion in an ocean CO2 injection system and to ensure a large dispersion of injected CO2 during its sequestration in the ocean.

In situ mixed region vapor stripping in low-permeability media. 2. Full-scale field experiments.

This paper is the second in a three-part series that describes mixed region vapor stripping (MRVS) for in situ treatment of fine-grained soils contaminated by volatile organic compounds (VOCs) including trichloroethene (TCE), 1,1,1-trichloroethane (TCA), and related halocarbons. As described in this paper, MRVS processes were studied during full-scale field experiments wherein ambient or heated air was injected at high volumetric flow rates during in situ soil mixing, and VOCs were volatilized and advectively removed from the subsurface, captured in a shroud covering the mixed region, and then treated on-site. The field test was conducted at an inactive land disposal site in southern Ohio where dense silty clay soils were contaminated by VOCs at concentrations in the 10-500 mg kg -1 range. During the field studies, seven columns, each 3.0 m diameter and 4.6 or 6.7 m deep, were treated with ambient air (∼15-25°C) or heated air (∼120-130°C) injected at flow rates of 28-40 m 3 min -1 . Intensive monitoring and measurement activities defined contaminant behavior and key MRVS operation and performance parameters. The field testing revealed that MRVS could rapidly reduce the concentrations of VOCs (i.e., TCE, TCA,...) in dense silty clay soil by 88-98%. The rate and extent of reduction was somewhat higher with the injection of heated air as compared to ambient air. Regardless of injection air temperature, as treatment progressed, the rate of VOC removal became increasingly mass transfer limited.

Using 14C-Labeled Radiochemicals Can Cause Experimental Error in Studies of the Behavior of Volatile Organic Compounds.

This study demonstrates that using {sup 14}C-labeled radiochemicals can cause potential experimental artifacts and error in studies of the behavior of volatile organic compounds (VOCs), such as trichloroethylene (TCE) and tetrachloroethylene (PCE). Results indicated that the volatilization rates of {sup 14}C-labeled TCE and PCE (analyzed by {sup 14}C radioactivity) were much lower than those of unlabeled TCE and PCE (analyzed by GC). These observations are attributed to (1) the presence of impurities that may be nonvolatile or less volatile than the target organic compound and (2) a possible isotopic discrimination between {sup 14}C-labeled and unlabeled VOCs. It is therefore suggested that caution be used in using radiochemicals in studies of VOC behavior in environmental media and in interpreting experimental data. 33 refs., 3 figs.

In situ mixed region vapor stripping in low-permeability media. 3. Modeling of field tests.

A model was developed to simulate in situ mixed region vapor stripping process for removal of volatile organic compounds from fine-textured soils. The model consists of a thermal response submodel for simulating temperature, moisture content, and exhaust gas flow rate changes in the zone being treated and a mass transfer submodel for predicting the removal of a linearly sorbing, dissolved contaminant. The treatment zone was assumed to be completely mixed. Model parameters were obtained independently from measurements and literature correlations where possible. Calibration of first-order mass transfer rates and energy input (via soil mixing and high-pressure gas injection) was completed using the results of a full-scale field test. The calibrated thermal submodel predicted temperature changes observed in three other field tests. The model could simulate contaminant mass removal rates for the three independent tests. The mass transfer rates, ranging between 0.0009 and 0.0015 s -1 , were independent of temperature up to at least 30°C but appeared to be dependent on the frequency of contact between the mixing blades and various soil depths.

In situ mixed region vapor stripping in low-permeability media. 1. Process features and laboratory experiments.

This is the first paper in a three-part series on mixed region vapor stripping (MRVS) for in situ treatment of fine-grained soils contaminated by volatile organic compounds (VOCs). In this paper, the MRVS process is described followed by the results of laboratory experiments in which the efficiency of MRVS for removing trichloroethene (TCE) was measured. These experiments revealed that MRVS could rapidly remove >90%. of the initial TCE from fine-grained, silty clay soils (>80% silt and clay particles, and hydraulic conductivity <10 -6 cm s -1 ). A spherical diffusion model for TCE mass removal during MRVS was developed and used to simulate the measured reductions in soil TCE level as a function of treatment time. The conceptual model for VOC transport was subsequently used to simulate VOC removals measured in the MRVS field tests. The field-scale evaluation of MRVS is the subject of the second paper, while model simulations of field VOC removals are described in the third paper in this series.