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��-RZNMA • 4th Quarter 1992 Monitonng Report Laidlaw Corp , 1444 Tillie Lewis Dr,Stockton,CA January 13, 1993 Page 2 In a two phased approach beginning on March 27, 1991, RESNA personnel advanced six soil borings (MW1-MW6) in the vicinity of the former underground storage tank and subsequently converted them to groundwater monitoring;wells at depths of approximately 30 feet below grade The purpose of these borings was to investigate and define hydrocarbon impaction in soil and groundwater. The bonng/well locations are plotted on Figure 2 Field activities performed at the site are summarized in separate Problem Assessment Reports submitted by RESNA on May 20 and May 30, 1991 QUARTERLY MONITORING AND SAMPLING Groundwater monitoring and sampling was conducted on November 23, 1992, by RESNA personnel Water level measurements were collected from each site monitoring well prior to purging and sampling (Table 1) Groundwater elevations across the site have decreased an average of 0 73 feet since August 1992 A groundwater gradient map was calculated using the November 23, 1992 groundwater elevation data A variant groundwater gradient exists beneath the site with an average magnitude of approximately 0 01 ft/ft or 53 ft/mile in a southeastern direction (Figure 2) Prior to groundwater sample collection, a minimum of 3 casing volumes of water was purged from each well All purged water was contained onsite in DOT approved 55-gallon drums pending analytical results Groundwater samples were collected from each of the six wells after stabilization of groundwater parameters (pH, electrical conductivity, temperature) Water samples were collected using a dedicated disposable bailer Field purge logs are contained in Attachment 1 Groundwater samples were placed on ice and transported under chain-of-custody protocol to state-certified RESNA Environmental Laboratories in Fremont, California All samples were analyzed for benzene, toluene, ethylbenzene, xylenes (BTEX) using EPA method 8020, and total petroleum hydrocarbons as gasoline (TPHG) using modified EPA method 8015 Dissolved petroleum hydrocarbons and BTEX constituents were not detected in monitoring wells MW1, MW4, MW5 and MW6 Petroleum hydrocarbon constituents were detected at various levels in wells MW2 and MW3 Laboratory results are summarized in Table 2 Laboratory reports and chain-of-custody documentation are presented in Attachment 2 Laidlaw Corporation will continue with groundwater monitoring and sampling The next round • of sampling will be performed in February 1993 QUARTLY1QR120992 FNL