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Closure Report The Southland C2Mrefion,9110 Thornton Road Stockton,CA December 5 19955 that approximately 0 003 gallon of gasoline remains dissolved in groundwater Based on the estimated initial and residual volumes, approximately 99 99 percent of dissolved petroleum hydrocarbons have been removed (Table 4) Dissolved benzene and TPH-G concentrations plotted on hydrographs for monitoring wells MW-1 and MW-7 (Figures 21 and 22) document the decline in dissolved concentrations so RATIONALE FOR CLOSURE Results of site remediation activities, both soil and groundwater, have documented the decrease of petroleum hydrocarbons in the subsurface at the site The presence of separate-phase hydrocarbons in soil or groundwater is no longer detected The detection of low levels of BTEX and TPH-G concentrations in groundwater samples has been reported at random intervals since 1990 Results of sampling indicated levels of hydrocarbon constituents have been at or below California action levels for drinking water since 1989 California action levels for hydrocarbon constituents in drinking water are I part per billion (ppb) benzene, 100 ppb toluene, 680 ppb ethylbenzene, and 1,750 ppb xylenes Groundwater Technology recommends site closure for the referenced site based on u It is estimated that at least 99 percent of the initial hydrocarbon mass in the subsurface has been removed, and that less than 10 gallons of gasoline remain All liquid phase hydrocarbons have been removed, approximately 98 percent of adsorbed phase hydrocarbons have been removed, and virtually 100 percent (99 99) of dissolved phase hydrocarbons have been removed ■ Evidence of only a limited vertical and lateral extent for low concentrations of residual petroleum hydrocarbons impacting subsurface sods suggests a low driving mass and low potential for migration to groundwater Areas of sod which still contain residual TPH-G concentrations do not contain detectable benzene concentrations The lack of detection of lighter petroleum hydrocarbon compounds when TPH-G is detected indicates that residual hydrocarbons are highly degraded This degradation indicates that petroleum compounds which pose the greatest risk to human health, the environment, and beneficial uses are no longer present in detectable quantities ■ Analytical results from groundwater sampling indicate levels of hydrocarbon constituents have been at or below California action levels for drinking water since 1989 Approximately 120 sampling events for monitoring wells and 135 sampling events for domestic wells have been conducted to document the decline and disappearance of dissolved petroleum hydrocarbons ■ It is not economically feasible to remediate residual petroleum hydrocarbons at the site considering the low concentrations, highly degraded nature of residual hydrocarbons, limited horizontal and vertical extent, and the limited potential risk to human health,the environment, and beneficial uses Benzene, toluene, and ethylbenzene were not detected in any air samples collected during the SVE test G4osuie2 rpt[SfFIL-B 1) E,DC Glt()UNI)1'1'XI I.It ❑E17 TCCHNOI.OG)