13    FINITE ELEMENT MODELING OF AIR FLOW IN
SOIL VENTING
Syed Asim Husain, Graduate Assistant
Bruce A. DeVantier, Associate Professor
Department of Civil Engineering and Mechanics
Southern Illinois University at Carbondale
Carbondale, Illinois 62901
INTRODUCTION
Innovative technologies for soil remediation are constantly being developed and applied. The in situ
vacuum extraction process, also known as soil venting has been demonstrated to be a viable technology for remediating volatile organic compounds (VOCs) in the unsaturated zone. It has been successfully field-demonstrated on trichloroethylene (TCE) contaminated soil, under the EPA Superfund
Innovative Technology Evaluation (SITE) program.1 This process and its variations have been used,
and will continue to be used for the cleanup of numerous sites contaminated with gasoline, solvents or
other volatile organic carbons due to spills or leaking underground storage tanks.
The basic technology of soil venting is relatively simple. It consists of removal of VOCs from
subsurface soils by mechanically drawing or venting air through the soil matrix. The basic components of the system include extraction wells, high vacuum pumps, treatment units to control air
emissions and monitoring systems. More complex systems incorporate trenches, air injection wells,
passive wells and surface seals. Further variations to this basic technology include air reinjection
systems (closed loop systems), simultaneous groundwater treatment, steam or hot air enhancement,
and reuse of recovered hydrocarbons.
The basic underlying phenomena that govern the performance of soil venting systems are quite well
understood. Application of vacuum and removal of vapors from the extraction wells induces vapor
flow through the subsurface causing the contaminants to volatilize from the soil matrix. These vapors
are swept by the carrier gas flow (primarily air) to the extraction wells. Many complex processes occur
on the microscale however. The three main factors that control the performance of a venting operation are the chemical composition of the contaminant, vapor flow rate through the unsaturated zone,
and the flow path of carrier vapors relative to the location of the contaminants.
Although the process in itself is well understood, the actual design is mostly based on trial and error
and previous experience of the design engineer. Design criteria involving size and number of wells,
their spacing and location are decided more by experience than by rigorous logic. Since economy of
the remediation operation is one of the key factors in deciding the most applicable technology for a
site, a comprehensive model of the whole air venting process for preliminary studies can clearly reduce
project design and operation costs.
The available published information on vacuum extraction technology is limited to brief process
descriptions2,3 controlled experimental studies,4 and field experience.5"8 More recently a field application study by Johnson et al.9 presented a case study of a gasoline spill and demonstrated use of
simplified air flow and mass transfer equations in conjunction with field test data.
The analytical screening models of Johnson et al.10 are the first attempt to quantify the vapor flow
rates and mass removal rates in an air venting operation. Although very practical, these simplistic
models are constrained by the assumptions inherent to the two dimensional equations upon which
these are based. The application of a perfectly horizontal flow assumption no doubt simplifies
matters, but there are very few field situations where perfectly horizontal flow will be encountered.
Since most of the sites to be remediated by this method are quite small in area, homogenous soil
conditions can be reasonably assumed in general, but assumption of isotropic air permeability in soil
neglects the typical added resistance to vertical air flow due to natural compaction of soils. The
assumption of purely horizontal flow negates the need for vertical permeabilty data, but vertical flow
components are seldom negligible.
46th Purdue Industrial Waste Conference Proceedings, 1992 Lewis Publishers, Inc., Chelsea, Michigan 48118.
Printed in U.S.A.
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