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I09 August 2000 AGE-NC Project No. 95-0106 ' Page 3of10 2.4.' MONITORING WELL INSTALLATION Between July 1996 and April 1998 a total of six wells,were installed on the site or on surrounding .properties in order to determine the lateral and vertical limits of the dissolved-hydrocarbon plume. In July:.1996, four soil borings were drilled and completed as 2-inch diameter ground water monitoring wells with slotted screen from approximately 5 to 25 feet bsg. In April 1997, soil borings B5 (MW-5)',and B6 (MW-6)were.drilled and completed as ground water Imonitoring wells. Monitoring well MW-5 was completed using 2-inch diameter PVC casing with a slotted screen from approximately 3 to 10 feet bsg. Ground water monitoring well MW-6 was dual- completed with screen intervals installed from a depth of 20.to 23 feet bsg and 3 to 13 feet bsg.-The installation of the first four monitoring wells was summarized in the PIER prepared by AGE, dated 10 September 1996: IOn 07 May 1998, soil borings B 13 through B 15-were advanced manually and one boring was completed as a ground water monitoring well. Borings B13, B 14'(MW-7) and B15 were established I • on the property at 52 West Beverly Place. The installation of the borings was performed using a . six-inch diameter hand auger. A total .of 15 soil borings have been advanced on and off the site, seven of which have been I completed as ground water monitoring wells,-providing data concerning Hydrocarbon impact on soil and ground water, soil composition, and ground water flow direction. Laboratory analysis of soil samples collected from monitoring wells and soil probe borings constrain the plume of impacted soil . ' to the immediate vicinity of the former UST excavation. The lateral extent of impacted ground water appears to be delineated northeast,.east, south, west and northwest of the former UST excavation. Laboratory analysis of ground water samples collected north of the former UST excavation, from. well.MW4, have consistently detected total petroleum hydrocarbons quantified as gasoline (TPH--g), benzene, toluene, ethylbenzene. and total xylenes (BTEX) and have generally detected the-sample analytes in samples from MW-1. -During the initial ground water.monitoring event of 09 July 1996, THP-g was detected in the ground water samples from MW-1 and MW-4 at concentrations of 15,000 ug/1 'and 18,000 ,ug/l, . respectively. Benzene was detecied'in the same samples at concentrations of 190 µg/l and 590 ktg/l. The.concentrations iri both wells declined roughly an order of magnitude by the next monitoring event.(05 November 1996). Over the.last ten monitoring events (not counting the current event), TPH-g and benzene concentrations in ground water samples collected from MW4 have averaged 2,330 ,ug/l and 100.9µg/l, respectively. THP-g and benzene concentrations in samples from MW-I over the sable period averaged 533 4g11 and 139.4 E.cg/l, respectively. Laboratory results of monitoring well-samples are.in Table 3. The benzene concentrations exceed the"California Maximum " Advanced GeoEnvironmental,Inc.