ARCHIVED REPORTS XR0010092 - Laserfiche WebLink

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4 north Groundwater motion and plume configuration and length is, in our opinion, typical for petroleum fuel dissolved plumes in shallow alluvial deposits Data from soil bonngs shows that contaminants were not detected on an assumed northwesterly flow direction from the tank pit Soil samples collected outside of the tank pit vicinity of the former tank location show that contaminants were not detected in the vadose zone soil or near the capillary fringe (at a depth of about 8-feet) Previous excavation in the tank area apparently removed vadose zone contaminated soil Wright interprets this data to indicate that the leakage under the tanks probably arrived very near the capillary fringe and then moved away as dissolved components This agrees with the EVAX (1992) data and observations of the excavation adjacent to the building during tank removal and contaminants were not detected in groundwater upgradient of the former tank pit area at that time Plume Movement and Microbial Activity The aquifer is of relatively low permeability with relatively low groundwater velocities, causing the plume spread slowly to the north (see Wnght report dated July 29, 1996) Comparing this data to the previous study, it suggests that contaminants have migrated slowly along the capillary fringe under the parking lot and possibly along the agricultural pipeline backfill to a limited extent Excavation in the tank area has removed the contaminated soil, and excavation wall samples laboratory data indicate that only residual concentrations remain Previous subsurface investigations have shown that there are TPHG contaminants have extended slightly onto the Tracy High School property, but the BTEX components were very low However, groundwater sampled from well MW-4 showed that contaminants were not detected, indicating that dissolved contaminants are not mobile The microbial counts show a decrease in the plume area compared to upgradient well MW-6 This could indicate that the biologic degradation of the more volatile components has been occurring for some time The ferrous iron is also low, suggesting that anaerobic reactions may have operated upon the plume, degrading the TPH component This indicates that volatiles are highly depleted hydrocarbon plume (as compared to a "fresh" or recent fuel spill) and that biodegradation processes have degraded the volatiles A comparison of TPHG values to Benzene show a depleted Benzene component in all wells except MW- 9 Data at MW-9 suggests that more fuel appears to have been absorbed in this local vicinity and when water levels rise, contact with the absorbed residual causes an increased in observed contaminants The volatiles are relatively low and appear to fluctuate with rising and declining water levels (see Tables 1 and 2) That is, the concentrations appear to increase with rising water levels, which contact the absorbed contaminants As the absorbed contaminants desorb into the water, the concentrations tend to increase As water levels decline, the concentrations also tend decline indicating that the contaminants are not especially mobile The down gradient wells do not display wide changes, and contaminants are not detected in MW-4, the extreme downgradient well A rough contaminant mass calculation was made using the available observed data (Lyman, Reidy and Levy, 1992) The quantity of fuel leakage at Mr Ed's is unknown The values for TPHG and Benzene were summed from Table 2 and used in the calculation to estimate approximate gallons of fuel in the interpreted contaminant area (about 66,250 cubic feet) shown on Figure 1 The estimates are about 2,773 gallons TPHG and 0 0043 gallons Benzene The lower Benzene number is a reflection of the degradation processes operating on the plume These numbers further support a degraded fuel spill in which the TPH is depleted in volatiles (see previous chemical and microbial data discussion) Off-site Plume The occurrence of contaminants in MW-6 is attributed to an offsite source (presumably from south across East Eleventh Street) since MW-6 is upgradient of the known on-site contaminant source This well data confirms a separate source across the street south from 595 East Eleventh Street The data Page 7