Laserfiche WebLink
02 December 1998 <br /> AGE-NC Project No. 95-0106 <br /> Page 2 of 4 <br /> A total of 15 soil borings have been advanced on and off the site, 7 of which have been completed <br /> as ground water monitoring well,providing data concerning hydrocarbon impact on soil and ground <br /> water, soil composition, and ground water flow direction. Laboratory analysis of soil samples <br /> collected from monitoring wells and soil probe borings constrain the plume of impacted soil to the <br /> immediate vicinity of the former UST excavation. The lateral extent of impacted ground water <br /> appears to be delineated northeast, east, south, west and northwest of the former UST excavation, <br /> but ground water samples collected north of the former UST excavation, from well MW4, have <br /> consistently contained total petroleum hydrocarbons quantified as gasoline (TPH-g), benzene, <br /> toluene, ethylbenzene and total xylenes(BTEX). Over the last five monitoring events, benzene and <br /> TPH-g concentrations in ground water samples collected from MW4 have averaged 131.8 <br /> micrograms per liter (µg/1) and 3,040 ug/l, respectively. Benzene concentrations exceeding the <br /> California maximum contaminant level (MCL) for drinking water of 1 4g/1 represents a potential <br /> threat to human health and ground water in a residential area. Lower concentrations of petroleum <br /> hydrocarbons have been detected in samples collected from MW1, located approximately 25 feet <br /> north-northeast of the former UST excavation, but were not detected during the last monitoring <br /> event. <br /> The occurrence of TPH-g and BTEX in ground water encountered in an off-site well (MW4) <br /> indicates that a plume of dissolved hydrocarbons has migrated north of the site. The lateral extent <br /> of the impacted ground water has not been delineated <br /> Two potential migration routes for the hydrocarbon-impacted ground water are suggested by the <br /> impacted ground water encountered in well MW4: <br /> • Petroleum hydrocarbons may be migrating as dissolved constituents at shallow depth, <br /> approximately 5 to 10 feet bsg through clay-rich soil, in the shallow saturated zone and <br /> capillary fringe zone, or <br /> • As dissolved constituents migrating through sandy soil layers lying 10 to 25 feet bsg. <br /> In the former case, greater risk to human health and safety may be anticipated, but in the latter case, <br /> greater risk to ground water may exist due to the higher degree of mobility of dissolved <br /> hydrocarbons in sandy soil. Figures 3 and 4 show available soil and selected analytical data in cross <br /> sectional view. <br /> Advanced GeoEnviron mental,Inc. <br />