Analyzing Barometric Pumping to Characterize Subsurface Permeability

Abstract

The prevalence and persistence of subsurface contamination by volatile organic compounds (VOCs) and the shortage of funds to remediate this contamination below regulatory action levels has led to a determined search for low cost clean up strategies. As an alternative to long term active soil vapor extraction (ASVE), passive soil vapor extraction and injection using atmospheric pressure changes as a driving force is being used at an increasing number of sites. Employing barometric pumping, either for passive injection of nutrients to enhance bioremediation or passive extraction of contaminated soil gas, is an easily verified low cost strategy for remediation. For mass-transfer limited, or remote sites, and also as an interim measure while more aggressive strategies are being selected, barometric pumping may be an appropriate cleanup choice. To verify the viability of these passive techniques, subsurface flow characteristics are vital yet an inexpensive characterization method for acquiring these parameters must be employed to match the low cost of the passive remediation systems. Figure 16.1 illustrates the scenario for barometric pumping through a vadose zone well. In this figure, the well is screened below a substantial clayey zone. The atmospheric pressure signal will penetrate the subsurface but is delayed and damped by this low permeability material and the other materials between the screen and the ground surface. As a result, there will be a pressure difference between the subsurface at the screen depth (Pz) and the surface (Patm), hence, flow through the well. Primarily the effective permeability of the sandy materials in the screen stratum combined with the magnitude of the pressure difference will control the magnitude of the flow. As the VOCs in the vicinity of the screened zone slowly diffuse out of the fine grain materials into the sandy zone, they are removed during regular periods of flow out of the well. As a result, the magnitude of VOC concentration in the gas phase changes