Development of recommendations and methods to support assessment of soil venting performance and closure.

摘要

Soil venting, which includes gas injection as well as gas extraction in subsurface media, has become the primary method used in the United States to remove volatile organic compounds (VOCs) from unsaturated subsurface media. The popularity and widespread use of venting is due to its simplicity of operation and proven ability to remove contaminant mass inexpensively compared to competing technologies. Despite the common use of venting in the Superfund program, there is little consistency in approach to assessment of performance and closure. Assessment of the technology’s performance and eventual decisions on closure are based primarily on negotiations between responsible parties and regulators. In this process there is widespread use and reliance on empirical methods as opposed to an emphasis on understanding fundamental physical, chemical, and biological processes controlling mass removal during the venting operation. This results in the technology not being used to its fullest potential, nor its limitations being well understood.;The overall purpose of the work described in this dissertation was to improve the “state of the art” and “state of the science” of soil venting application. This purpose was accomplished by attainment of three specific objectives. The first objective was to develop an overall regulatory approach to assess venting performance and closure including measures to ensure consistency in ground-water and vadose zone remediation. The second objective was to provide comprehensive and detailed literature reviews on gas flow and vapor transport. These reviews formed the basis of recommendations and methods to improve venting design and monitoring. The third objective was to perform research to improve various aspects of venting application. This research consisted of: (1) analysis of linearization of the gas flow equation, (2) one-dimensional steady-state analysis of gas slippage, (3) two-dimensional steady-state analysis of gas flow and permeability estimation in a domain open to the atmosphere, (4) two-dimensional steady-state analysis of gas flow and permeability estimation in a semi-confined domain, (5) two-dimensional transient gas flow analysis and permeability estimation, (6) analysis and comparison of radius of influence versus critical pore-gas velocity based venting design, (7) modification of a gas extraction well to minimize water-table upwelling, (8) simulation of rate-limited vapor transport with diffusion modeling, (9) assessment of respiration testing, (10) development of a one-dimensional, analytical, vadose zone transport code to simulate mass flux to and from the capillary fringe, and (11) analysis of water-table mounding during sparging.