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Treatment of Petroleum-Contaminated Soils
Barometric pumping is similar to soil vapor extraction but relies on natural variations in barometric pressure rather than blowers to create the flow of soil gas through the subsurface (Looney and Falta 2000). On a daily basis, barometric pressure will vary slightly because of the cycle of the sun. Wider variations in barometric pressure will be seen over a span of days as weather fronts move in and out of the area being monitored. As the barometric pressure changes in the atmosphere, a pressure gradient is established between the atmosphere and air contained in subsurface soil. An airflow is created into or out of the subsurface soil in response to this gradient. This airflow can be used to remove volatile compounds from contaminated soil. Placing screened wells in the region of contaminated soil and adding a check valve to restrict the flow to extraction rather than injection focuses the pressure sink created by the change in barometric pressure, causing air to flow through the contaminated soil to the screened portion of the well or wells. The screened portion of the extraction wells should be placed deep enough to take advantage of the greatest pressure differential. Contaminants are removed as described in the discussion on soil vapor extraction.
The section on SVE described the complexities of using in situ venting processes in cold, wet regions. Acknowledging the reduction in the removal rate of volatile compounds because of cold temperatures, there is an advantage to a decrease in the soil temperatures at the ground surface. Ice will begin to form in the pore space in the top few centimeters of soil as temperatures drop below freezing. In soils with moderate to high moisture content, permeability will decrease at the ground surface, establishing a partial barrier to airflow. This partial barrier will increase the time required for the two pressures (atmospheric and subsurface) to reach equilibrium. Once the two temperatures are at equilibrium, subsurface airflow is stopped. Freezing at the surface increases the time required for these two pressures to establish equilibrium, increasing the rate at which contaminants are removed.
This technique has an obvious advantage in remote regions. After installation, the system can be left alone, except for regular monitoring.
Table 16 shows the items to be considered when developing a cost estimate for barometric pumping. The assumptions used to construct this table are similar to those used in the section on SVE.
| Cost estimating factors | |
|---|---|
| • | Mobilization and demobilization. |
| • | Piping. |
| • | Slotted pipe or well screen. |
| • | Gravel pack for area around screens. |
| • | Liner (if required). |
| • | Exhaust gas treatment system. |
| • | Sampling of off gas to monitor treatment progress. |
| • | Confirmation sampling—The number of samples depends on the size of the contaminated site and on the regulatory agency. |
| • | Accommodations at the site during system insallation. |
| • | Operation and maintenance visits. |
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