The Effect of Barometric Pumping on Field Determined Respiration Rate for Bioventing Process

Field measured respiration rates are necessary to size the appropriate oxygen supply equipment and to determine the proper injection/extraction flow rate and operating mode in the design of a bioventing system. Respiration rates are also used to estimate the amount of contaminant metabolized during the bioventing process. Almost all respiration tests are conducted in an open system subject to air diffusion and interaction with the lower atmosphere. Oxygen supply to shallow surface soil by diffusion can be significant, as indicated by field collected data. Temporal variations in barometric pressure due to weather patterns may induce air intrusion into the subsurface. However, the effect of fluctuations in barometric pressure on soil gas pressures and soil gas composition is generally ignored/neglected in in situ respiration tests. In this paper, the effect of barometric pumping on oxygen intrusion from the atmosphere to the shallow vadose zone is investigated using computer simulation. A model is also presented to correct the impact of barometric pumping on field-determined respiration rates. Bioventing is a process designed to provide indigenous organisms with adequate oxygen to aerobically degrade target contaminants. Bioventing has been used successfully to remediate soils contaminated with jet fuel, gasoline, and diesel fuel components. Respiration rate in a bioventing process is used to measure how fast a target contaminant in vadose zone soils is biologically degraded. The quantification of microbial respiration rates under field conditions is generally carried out using in situ respiration tests described by Hinchee et a1. (1992). During an in situ respiration test, narrowly screened soil gas monitoring points are placed into both contaminated and uncontaminated vadose zone soils. The soils with the monitoring points are vented with air for a given period of time, i.e., for 8 hours or longer. Then, soil vapor samples are collected and analyzed over time for oxygen, along with other soil gases. Almost all respiration tests are conducted in an open system subject to air diffusion and interaction with the lower atmosphere. Oxygen supply to shallow surface soil by diffusion can be significant, as indicated by data from FEW Air Force Base (Figure 1). Figure 2 illustrates the fluctuation of barometric pressure, also based on collected from FEW Air Force Base. Field data indicate that "fresh" air can migrate several meters into the subsurface with homogeneous soils in response to a typical barometric pressure cycle. This air intrusion can affect monitoring activities aimed at characterizing the composition and movement of gases in the vadose zone. It has been shown that relatively small gradients in total pressure can result in viscous gas fluxes larger than diffusive fluxes. However, the effect of air diffusion and fluctuations in barometric pressure on soil gas pressures and soil gas composition is generally ignored/neglected in in situ respiration tests. Therefore, the current method for field determination of respiration rate may