Treatment of a Vadose Zone Plume Using Barometric Pumping: A Passive Soil Vapor Extraction Study at the Miscellaneous Chemicals Basin, Savannah River Site, S.C.

A passive soil vapor extraction (PSVE) treatability study at the Miscellaneous Chemical Basin (MCB) of the Savannah River Site (SRS) has been progressing since September 1996. During this time more than 243 pounds of chlorinated organic contaminants (183 lbs of TCE and 61 lbs of PCE) have been removed by natural barometric pumping of wells fitted with BaroBall ^TM valves (low-pressure check valves). In addition, contour maps of the vadose zone contaminant gas plume made before the study and after 4.5 years of operation show a significant decrease in the extent of the gas plume and identify the source location. To date, over 86% of all PCE and 89% of all TCE have been removed from the system. By projecting the exponentially declining concentration curves it is estimated that 99% of all PCE and TCE will be removed from the system by 2007 and 2009 respectively. In addition, soil samples were collected in July 2001 at various depths within the previously defined plume source area. Analysis of VOC content in these samples confirms that barometric pumping has significantly remediated the contamination in the MCB area. Barometric pumping harnesses natural atmospheric fluctuations to move air in and out of the vadose zone. Barometric pressure fluctuations are transmitted through the vadose zone, however, these pressure waves are damped and delayed in phase to degrees dependent on the effective vertical permeability of the formation. As a result, there is a pressure differential between the atmospheric pressure at the surface and the soil gas pressure in the subsurface. By connecting these two zones with a vadose zone well, the differential pressure will cause gas flow either into or out of the well. If the soil gas in the subsurface contains VOC's (gas phase), than flow out of the well will remove contaminants from the subsurface without mechanical pumping. The Baroball uses a lightweight ball in a conical seat to allow gas flow in only one direction with little pressure, but prevents gas flow in the opposite direction. Thus the Baroball can be used to allow contaminated soil gas out of the well, while preventing clean surface air from flowing into the well and diluting soil gas in the subsurface.