SITE INFORMATION AND CORRESPONDENCE_FILE 2

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Mr. Ken Gillies • • Gillies Trucking 3931 Newton Road, Stockton Page 3 of constantly (GT-16A) impacted since installation. The plume of impacted groundwater in the First Riverbank Aquifer (approx. 58 to 70 feet bsg) covers an area slightly exceeding an acre as drawn on Figure 17 of the QR. EHD interprets the overall data to demonstrate an undelineated plume of impacted groundwater in the First Riverbank Aquifer that is likely receiving TPH-d contaminants from the overlying Modesto Aquifer and Aqu itard(approximately 25 to 58 feet bsg). Dissolved TPH-d in the Second Riverbank sand (approx. 75 to 100 feet bsg) appears to be well delineated and limited in areal extent during the third quarter 2005. Dissolved TPH-d in the Third Riverbank sand, the deepest sand unit (approximately 110 to 130 feet bsg), while not tightly delineated, especially toward the southeast from the former tank pit, was present at low concentrations in the third quarter. Historical data for the interval indicates fluctuating low concentrations and undetectable concentrations. EHD believes this may be due to areally variable contaminant migration into the sand unit from the overlying and/or underlying aquitards. The interval in the third quarter had a low gradient of approximately 0.0006. As long as the contaminant concentration trends remain constant or decline further, and the evolving site conceptual model doesn't raise additional concerns, EHD will not require additional characterization of the Second and Third Riverbank sand units (approx. 75 to 100 feet and 1100 to 130 feet bsg, respectively) at this time. EHD's analysis of historical analytical data for the last year or two from wells screened in the First Riverbank aquifer (approx. 25 to 55 feet bsg) appears to show an elongated core (>220 feet in length) extending northeast and southwest from the former tank pit area; analytical data from wells GT-5 and GT-17A have fluctuating TPH-d concentrations of approximately 100 pg/I or that are less than the method detection limit, suggesting these well locations mark the margins of the plume toward the northwest and southeast from the former UST area, which would indicate a width of at least 100 feet for the dissolved plume. The data is consistent with plume migration along the eastern, north-trending sand channel axis and with the plume model shown on Figure 17 of the QR. At this time, EHD believes that the First Riverbank Sand unit is the most likely contaminant migration pathway with potential to impact nearby water supply wells and requires additional characterization. Submit a work plan to continue the lateral assessment of impacted groundwater in the First Riverbank Aquifer sand (approx. 58 to 70 feet bsg) to EHD by 28 February 2006. EHD is concerned that the detection limit for groundwater sample GT-17A was 385 micrograms per liter (µg/1) without explanation. Elevated detection limits may be caused by high concentrations of other contaminants. Please check with the laboratory on the suspected cause for the elevated detection limit and provide the information to EHD. As described or touched on above, the tasks and concerns for your site at this time include: