SITE INFORMATION AND CORRESPONDENCE

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- ENVIRONMENTAL HEALTH�EPARTMEN ENVIRO��VI u�N SAN JOAQUIN COUNTY Unit Supervisors G n Donna K. Hera ,R.E.H.S. Third Floor Carl Borgman,R.E.H.S. z ? 304 East Weber Avenue, Director Mike Huggins,R.E.H.S., R.D.I. Laurie A. Catulia, R.E.H.S. Stockton,California 95202-270 Douglas W.Wilson,R.E.H.S. Program Manager Telephone: (209)468-3420 Margaret Lagorio,R.E.H.S. c4�fi ou`'. P Fax: (209) 464-0138 Robert McCtellon, R.E.H.S. Website:www.sjgov.orglehdl 3eff Carruesco,R.E.H.S. BOB YAMADA MAR 3 1 20D6 15406 SOUTH TRACY BLVD STOCKTON CA 95205 SITE CODE: 4067 RE: YAMADA BROTHERS 15406 S TRACY BLVD STOCKTON CA 95206 San Joaquin County Environmental Health Department (EHD) has reviewed the Fourth Quarter Monitoring Report 2005 dated March 1,2006,prepared by Advanced GeoEnvironmental Inc. (AGE). To define the lateral extent of the groundwater contamination at the site and to provide a sentinel monitoring points for the existing down-gradient domestic drinking water well DW-1, four additional monitoring wells MW-4 through MW-7 ,were installed in 2004. MW-4 and MW-7 were installed southwest of the former underground storage tanks (UST)in the down-gtadient direction of groundwater flow,and MW-5 was installed toward the west in a cross-gradient direction. With the exception of MW-4,analytical results of the other 6 monitoring wells were non-detect (ND) for all constituents the last two quarters(September and November) of 2005. The MTBE concentration in MW-4 was ND during the first 3 quarters of 2005,but rose to 55 ug/l during the 4th quarter in November. In contrast, the concentration of Total Petroleum Hydrocarbons as gasoline (ITH-g) dropped to ND in November,but had been 600 ug/l of TPH-g in September 2005. Depth to groundwater ranged from about 4 feet below surface grade (bsg) to 6 feet bsg,and the general groundwater flow direction is southwestward. The dissolved MTBE plume at MW-4 appears to be migrating toward DW-1,possibly due to MW- 4's pumping influence,though at concentrations significantly lower than previously reported at MW- 2 (a cross-gradient well from the former UST)when it had concentrations as high as 1,200 ug/l Dune 2002). MTBE has also been found between November 1999 and June 2005 at the up-gradient well MW-3,which is located approximately 60 feet north of the former UST. MTBE at NfW-1,which is the closest well to the former UST area,had been mostly ND and never detected at a concentration above 3 ug/l. The MTBE concentration in the grab water sample from B1 at the former UST was 9.9 ug/l as reported in Preliminary Site Asren-ment Deport,dated February 12, 1999,prepared by AGE. Based on the distribution of the dissolved MTBE, the current MTBE plume does not appear to be solely the result of a release from the former UST system. EHD concludes that the multiple aboveground storage tanks (AST) at the site are potential source(s) of some or all of the dissolved MTBE observed in groundwater,possibly the result of long period low-level release such as dripping at the valve. Low level release(s) over the years may have allowed MTBE to infiltrate through the surface and into the normally high groundwater table,but migration of gasoline components such as aromatic hydrocarbons may have been retarded since they are more likely to adsorb to the soil at the surface, subjected to degradation or volatilization. Also, the presence of TPH-g at MW-4,but not at the other wells,indicates that the ITH-g at MW-4 may be a release separate from the former UST system. The lack of aromatic hydrocarbons, except at very low concentrations in MW-4,possibly due to degradation,also suggests the above scenario of a release in the distant past.