[en] Petroleum released to the subsurface may be held in capillary tension above the water table for years, serving as a source of groundwater and soil gas contamination. Soil venting can be used to attack this ongoing source, sometimes in conjunction with biodegradation to permanently destroy the released hydrocarbons vapors. These processes were explored using intact soil cores from the site of an aviation gasoline release. Hydrocarbon vapor concentration profiles were analyzed by gas chromatography and interpreted using mathematical models. In the venting experiment, an intact core was subjected to a horizontal sweep flow of nitrogen. Residual petroleum in the soil volatilized and hydrocarbon vapors diffused upward. Soil venting significantly increased the rate of contaminant removal relative to ambient field conditions. No correlation between hydrocarbon vapor exit flux and sweep flow rate was observed, indicating that flow rates in excess of a minimum value were no more effective. A steady state model balancing volatilization and diffusion successfully predicted the shape of the hydrocarbon concentration profiles. The volatilization source was construed as an LNAPL droplet surrounded by an air water aggregate surrounded by a free air pore, with the aggregate reducing the mass transfer of hydrocarbons from LNAPL to air. Source strength, estimated from a diffusive flux model, decreased with time as LNAPL droplets became smaller. The biodegradation experiment employed an intact core from mid-depth in the unsaturated zone which was subjected to a upward flow of nitrogen, oxygen, water vapor, and hydrocarbon vapors. Significant biodegradation was indicated by reductions in hydrocarbon concentration with elevation in the core. First order biodegradation rate constants were estimated by calibrating the experimental data to a simple model balancing advection and biodegradation

[en] Soil gas samples from intact soil cores were collected on adsorbents at a field site, then thermally desorbed and analyzed by laboratory gas chromatography (GC). Vertical concentration profiles of predominant vapor phase petroleum hydrocarbons under ambient conditions were obtained for the zone directly above the capillary fringe. Water and residual phase weathered aviation gasoline were present in this region of the profile. The sampling, trapping, and GC methodology was effective in most respects. Reproducibility, trapping, and desorption efficiency were generally satisfactory, and different sorbent tubes gave similar results. A minor shortcoming of the method occurred with the most volatile compound, 2,3-dimethylbutane, which was poorly retained during several weeks of storage time and was also poorly desorbed. Vapor phase concentrations of predominant hydrocarbon compounds all increased with depth at one sampling location. At a more highly contaminated location, concentrations of highly volatile compounds increased with depth while concentrations of less volatile compounds remained constant or decreased, possibly indicating distillation effects. Scatter in the data was attributed to heterogeneities in water and residual phase distribution

[en] Aerobic biodegradation of vapor-phase petroleum hydrocarbons was evaluated in an intact soil core from the site of an aviation gasoline release. An unsaturated zone soil core was subjected to a flow of nitrogen gas, oxygen, water vapor, and vapor-phase hydrocarbons in a configuration analogous to a biofilter or an in situ bioventing or sparging situation. The vertical profiles of vapor-phase hydrocarbon concentration in the soil core were determined by gas chromatography of vapor samples. Biodegradation reduced low influent hydrocarbon concentrations by 45 to 92% over a 0.6-m interval of an intact soil core. The estimated total hydrocarbon concentration was reduced by 75% from 26 to 7 parts per million. Steady-state concentrations were input to a simple analytical model balancing advection and first-order biodegradation of hydrocarbons. First-order rate constants for the major hydrocarbon compounds were used to calibrate the model to the concentration profiles. Rate constants for the seven individual hydrocarbon compounds varied by a factor of 4. Compounds with lower molecular weights, fewer methyl groups, and no quaternary carbons tended to have higher rate constants. The first-order rate constants were consistent with kinetic parameters determined from both microcosm and tubing cluster studies at the field site

[en] The diffusion of 2,2,4-trimethylpentan (TMP) and 2,2,5-trimethylhexane (TMH) vapors out of residually contaminated sandy soil from the US Environmental Protection Agency (EPA) field research site at Traverse City, Michigan, was measured and modeled. The headspace of an intact core sleeve sample was swept with nitrogen gas to simulate the diffusive release of hydrocarbon vapors from residual aviation gasoline in and immediately above the capillary fringe to a soil-venting air flow in the unsaturated zone. The resulting steady-state profile was modeled using existing diffusivity and air porosity estimates in a balance of diffusive flux and a first order source term. The source strength, which was calibrated with the observed flux of 2,2,4-TMP leaving the sleeve, varied with the residual gasoline remaining in the core, but was independent of the headspace sweep flow rate. The finding suggested that lower soil-venting air flow rates were in principle as effective as higher air flow rates in venting LNAPL vapors from contaminated soils

[en] The diffusion of 2,2,4-trimethylpentane (TMP) and 2,2,5-trimethylhexane (TMH) vapors out of residually contaminated sandy soil from the US Environmental Protection Agency (EPA) field research site at Traverse City, Michigan, was measured and modeled. The headspace of an intact core sleeve sample was swept with nitrogen gas to simulate the diffusive release of hydrocarbon vapors from residual aviation gasoline in and immediately above the capillary fringe to a soil-venting air flow in the unsaturated zone. The resulting steady-state profile was modeled using existing diffusivity and air porosity estimates in a balance of diffusive flux and a first order source term. The source strength, which was calibrated with the observed flux of 2,2,4-TMP leaving the sleeve, varied with the residual gasoline remaining in the core, but was independent of the headspace sweep flow rate. This finding suggested that lower soil-venting air flow rates were in principle as effective as higher air flow rates in venting LNAPL vapors from contaminated soils. The saturated vapor concentration ratio of 2,2,4-TMP to 2,2,5-TMH decreased from 6.6 to 3.5 over the duration of the experiments in an expression of distillation effects. The vertical profile model was tested against sample port data in four separate experiments for both species, yielding mean errors ranging from 0 to -24% in magnitude

[en] The authors measured and modeled hydrocarbons vapor constituent sparging out of residually contaminated soil in two intact core sleeves from the US Coast Guard Air Station in northern Michigan. The LNAPL source was a 0.3 m thick zone of residual aviation gasoline in the contaminated capillary fringe about 5 m below the ground surface in uniform sandy soil. The sampling location was far downgradient of the origin of the fuel spill, so that the configuration of separate phase contamination was due to surface tension in the capillary fringe and trapping by a historically fluctuating water table. The gas sparging efficiency was assessed with a simple transport model balancing gaseous advection and a zero order source term. The predicted vertical profiles accurately matched the data for all constitutents, exhibiting source strengths that compared favorably with values from a companion study of diffusive soil venting. The sparging efficiency was found to be independent of gas flow rate, so that low flows generate relatively high exit concentrations. This finding suggests that local mass transport constrains the removal efficiency for LNAPL sources trapped by fluctuating water tables

[en] Aerobic biodegradation of vapor-phase petroleum hydrocarbons was evaluated in an intact soil core from the site of an aviation gasoline release. A mid-depth unsaturated zone soil core was subjected to a flow of nitrogen gas, oxygen, water vapor, and vapor-phase hydrocarbons in a configuration analogous to a biofilter or an in situ bioventing or sparging situation. The vertical profiles of vapor-phase hydrocarbon concentration in the soil core were determined by gas chromatography of vapor samples. Steady-state concentrations were input to a simple analytical model balancing advection and first-order biodegradation of hydrocarbons. First-order rate constants for each major hydrocarbon compound were used to calibrate the model to the concentration profiles. Compounds with lower molecular weights, fewer methyl groups, and no quaternary carbons tended to have higher rate constants. The first-order rate constants were consistent with kinetic parameters determined from microcosm studies at the same field site, suggesting that both estimation methods were effective