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ll 11 �} December 20, 1994 Harding Lawson Associates 256661 San Joaquin County Mr Harlin Knoll Page 3 � Site A and Site B Invests aFP To evaluate potential impacts to groundwater quality from the USTs, HLA conducted a ' groundwater/soil gas survey at each site on June 30, 1994 The groundwater/soil gas survey was performed by Tracer Research Corporation (TRC) of Tucson, Arizona The groundwater/soil gas survey are qualitative methods that can be used to assess in the delineation of volatile organic compounds in soils and groundwater The soil gas survey is performed by sampling and analyzing soil gas from the pore spaces of the unsaturated zone above the water table Because many common organic compounds exhibit significant vapor pressures, their introduction into the subsurface results in vapor phase transport Should these chemicals reach the water table and travel with the groundwater, organic vapors can emanate from the water into the overlying soil Groundwater survey sampling is conducted using the same techniques as soil gas sampling, except that a sample of groundwater is collected instead of soil gas Both of the sampling methods are reconnaissance tools only and do not provide quantitative data on chemical concentrations in soil or groundwater However for surveying large areas where minimal disruption to operations and minimal damage to property is required these methods provide preliminary data for the evaluation of soil �( and/or groundwater contamination At Site A, three groundwater samples designated WS-1, WS-2, WS-3 and WS-5 and two soil gas samples designated SG-1, and SGA were collected to evaluate the potential groundwater plume n associated with the former UST (Plate 2) At Site B, one groundwater sample designated WS-4 and four soil gas samples designated SG-2, SG-3, SG-5, and SG-6 were collected to assess the potential _ groundwater plume at this location (Plate 3) The groundwater/soil gas sampling equipment was mounted directly on a street legal van, equipped with a hydraulic ram mechanism to push the sampling probes to the desired depth The sampling probe consisted of 1-uich-diameter steel pipe with a detachable tip The pipe was driven to the desired sampling depth using the hydraulics on the truck Groundwater samples were collected by attaching one end of teflon tubing to the probe, and the other end to a vacuum pump Once the tubing had filled with water, the water within the tubing was poured into a 40-milliliter VOA vial The tubing was discarded after each sampling event and J replaced Soil gas samples were collected by attaching the tubing to the probe and pump, purging the probe, and collecting the sample in an air tight syringe from the top of the probe Typically, soil gas samples were collected at locations where low permeability soils prolubited the collection of groundwater samples Soil gas samples were collected at depths between 12 and 12 5 feet bgs Groundwater samples were collected at depths of 11 to 17 feet bgs The groundwater and soil gas samples were analyzed for total volatile hydrocarbons (TVHC) and benzene, toluene, ethylbenzene, and xylenes (BTEX) Samples of groundwater were analyzed using the headspace method as described in the attached TRC Report The data collected from these surveys was used to qualitatively assess the groundwater quality at each site prior to the installation of groundwater monitoring wells A groundwater monitoring well was drilled and installed at each site Based on the results of the groundwater/soil gas survey, a monitoring well was drilled and installed at Site A approximately 40 feet downgradient of the UST excavation While RWQCB Tri-Regional Guidelines require a morutormg well within ten feet of the UST excavation, the SJCEHS recommended that this well be installed downgradient of the UST because a groundwater sample from the UST excavation had been obtained previously and the data provided information on groundwater quality in the vicinity of the this UST �y r l