MENU ▼
Read by QxMD icon Read
search
OPEN IN READ APP
JOURNAL ARTICLE

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

Jee-Won Choi, Fred D Tillman, James A Smith
Environmental Science & Technology 2002 July 15, 36 (14): 3157-64
12141498
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.

Comments

You need to log in or sign up for an account to be able to comment.

No comments yet, be the first to post one!

Trending on Read

Available on the App Store

Available on the Play Store
Remove bar
Read by QxMD icon Read
12141498
×

Search Tips

Use Boolean operators: AND/OR

diabetic AND foot
diabetes OR diabetic

Exclude a word using the 'minus' sign

Virchow -triad

Use Parentheses

water AND (cup OR glass)

Add an asterisk (*) at end of a word to include word stems

Neuro* will search for Neurology, Neuroscientist, Neurological, and so on

Use quotes to search for an exact phrase

"primary prevention of cancer"
(heart or cardiac or cardio*) AND arrest -"American Heart Association"