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I I3.0 GROUNDWATER DEPTH AND GRADIENT GeoAudit measured the depth-to-groundwater at the site in January, February and May 1994 The depth to groundwater ranged from approximately 72 to 73 feet The gradient ranged from 0 003 ft/ft to 0 0075 ft/ft Groundwater flow direction was directed southwest The gradients calculated by GeoAudit are more shallow than the previous gradient of 0 05 Ift/ft directed westward (November 1992) as calculated by NGA 4.0 SCOPE OF WORK I 4.1 Vapor test Low concentrations of TPH were detected in soil samples collected from beneath the USTs at the time of removal 1990, additional soil was removed from the UST excavation at this time However, the PHS/EHD has no record of confirmatory sampling Slotted 4-inch PVC casing was installed at the bottom of the excavation Approximately 5 inches of pea gravel were placed across the bottom of the excavation and covered with plastic sheeting The new UST's were then installed on top of the plastic sheeting To determine if petroleum hydrocarbons still remain beneath the USTs, we propose to Iperform a limited vapor extraction test A regenerative blower will be used to apply a vacuum to the vapor piping Air samples will then be collected and analyzed 42 Extended Purge ' Low concentrations of TPH as gasoline continue to be detected in water samples from MW-2 No petroleum hydrocarbons were detected in any other samples from the May 1994 monitoring event, indicating a limited extent of petroleum hydrocarbons in groundwater A submersible pump will be used to purge approximately 500 to 1,000 gallons of water from MW-2 A regenerative blower will be used to apply a vacuum to the well head during pumping A negative pressure applied to the well head has been shown to increase well recharge and pumping rates 5 0 PROCEDURES 51 Vapor Test The following procedures will be followed to perform the limited vapor extraction test beneath the UST's 1 Notifythe EHD 48 hours nor to commencement of any work P 0-1111-1 Alda S—1*1 5 laeW-1i Boon.�p119 9060 5 I