SITE INFORMATION AND CORRESPONDENCE FILE 1

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I 1.0 Introduction Enhanced vacuum recovery (EVR) is the practice of applying negative pressure to a pumping well to increase groundwater production. Research by Hackenberg et. al., (1994) specifies that it is theoretically possible to attain a two-fold increase in groundwater production when EVR is applied to a single pumping well in:a silty aquifer. This paper presents an empirical study in which a two-fold production increase was not only achieved; it was maintained for the duration of a 72 hour field test. The test site is located in the East Bay region of the San Francisco Bay Area in northern California. At the field test site, aquifer thickness is approximately 28 feet and depth to groundwater is approximately 23 feet below ground surface (bgs). Gasoline from a leaking underground storage tank (LUST) had impacted the soils and groundwater at the test site. As an interim measure, a soil vapor extraction (SVE) system consisting of a blower and activated carbon beds was placed into operation prior to the implementation of the EVR pilot test. The SVE remediation was very effective because the vadose soils are comprised of well graded gravels. Remediation of contaminated groundwater posed a significant challenge because the unconfined aquifer is predominantly comprised of highly plastic clay. The results of conventional pump tests conducted on groundwater wells at the site indicated that aquifer, production could not sustain pumping rates exceeding 0.5 gallons per minute (gpm). One well was pumped dry in 20 minutes at a discharge rate of 0.3 gpm. The results of the EVR pilot test reveal a sustained flow of 1 gpm. The EVR method has influenced the overall groundwater recovery and treatment system design at the test site. .The observed increase in sustainable flow has reduced the number of recovery wells needed to capture the contaminant plume, and shortened the estimated time period to complete aquifer... remediation. To date, little has been published about EVR. Rippberger (1989) briefly mentions that increased NAPL flow occurs in wells subjected to vacuum. Theoretical and practical work in this realm, like that by Hackenberg et. al., (1994), Mastroianni et. al., (1994), and Mastroianni and Hackenberg (1992) have not been widely applied. Groundwater recovery optimization methods, such as those discussed by Nolter and Kotziers (1991) and others; commonly employed LNAPL recovery methods, as presented by Kaluarachchi and Elliot (1995) and others; as well as SVE optimization modeling; as presented by Benson, et al. (1993), Sepehr and Samani (1993) and others do not include the use of EVR. In contrast, Parker (1995) and Schaal (1992) indicate that EVR can be used to remove LNAPL from wells without smearing well screens and the formation with LNAPL as do common dual pump water/product systems: Mastroianni et. al., (1994), Mastroianni and Hackenberg (1992), Millan (1993), and Parker (1995) indicate that LNAPL recovery can be increased through use of EVR. In these cases, EVR is used to overcome the interstitial pore pressures that retard LNAPL flow. 516