SITE INFORMATION AND CORRESPONDENCE

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Fuller Mobile Home Park 365 Louise Ave. - 2 - i' ' " 28 August 1998 Lathrop, San Joaquin County F �, On 28 April 1998, one grab groundwater sample was collected from boring B-12, which was drilled 'as a slant boring toward the eastern property boundary, from'al'location"near monitoring well MW-3. The ! analytical results from the B-12 water sample showed low, concentrations of ethylbenzene and xylenes, TPHg.was detected at 3000 µg/L, and benzene, toluene and the oxygenateswerenot detected. A Hydrocarbon Degrading Bacteria Enumeration Assay was conducted on three soil samples collected from the contaminated zone at 12.5 feet bgs. The Assay identified;both long and short chained ` ' hydrocarbon degraders in the soil samples. Dissolved oxygen levels have also been detected in nearby wells MW-3-and MW-6, at concentrations of 2.2 mg/L and 4.0 mg/L, respectively. The presence of Hydrocarbon Degrading Bacteria, in conjunction with the�indicated concentrations of dissolved oxygen does suggest that the environment is appropriate for biodegradation of contaminants, according to Dri Sean P. Bushart, Ph.D, with Cyto Culture International, Inc , (see the 28 May 1998, "Phase 11 Site Characterization "Continued"report-b)i- VHF,Inc In the location of the former UST excavation, a Storm Drain Pond was installed in approximately 1993. Based on a phone conversation with William H. Fox, of'WHF, Inc, soil was excavated from the storm drain pond area and used to fill low spots on the property.�Some of the excavated soil may have contain r. residual concentrations of petroleum constituents. However, the total depth of the pond is approximately 6 feet bgs, so it is unlikely that the excavated material will pose a health risk, or adversely impact water quality. The location of the pond may actually increase the infiltration of dissolved oxygen into the ! contaminated zone. I� The groundwater gradient has been fairly flat,but variable within 1 order of magnitude. The flow direction has also been variable, with historic flows in nearly all compass directions. The groundwater flow and gradient appear to be controlled by precipitation+'rstorm drain runoff, and infiltration. Based on the mass balance calculations provided by WHF Inc.,the remaining volume of petroleum hydrocarbon contamination at the site is 37.6 gallons. This remaining contamination is-presented as a� approximately 36.07 gallons of diesel, and 1.57 gallons of gasoline constituents. The contaminated subsurface volume is estimated to be 1,150 cubic yards, in the vicinity of the former UST location. ` - Summary::". � _ --'�- ". F'."-x-= " -�� --s_=a`'• '.c"` _, -'-'6 ;- �....., _ - The contaminant concentrations in soil and groundwater:b low th�!site have declined significantly over the past.5 years. There is evidence of biological activity and likely Pio of li. petroleum 11 , hydrocarbons at the site. The lateral and vertical extent of soil and groundwater contamination appears to be adequately defined, and an estimate of the volume of contamination remaining, and the contaminant distribution has been provided. 1 After a review the information provided, and based on tlJlow concentrations of contamination' remaining in groundwater, and the apparent presence of biodegradation, the likelihood of continued, significant impact to groundwater resources appears to be low. It doles appear that this site will pose little threat to water quality. I recommend that we concur with the SJCEHD and the SWRCB/UST-CUF determination that no further action related to the undergound storage tank activities be required. a;