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f, 28 October 1997 AGE-NC Project No 97-0330 Page 3 of 4 the site circumstances that should be weighed while considering site closure Closure critekalr soil and ground water impacted by fuel hydrocarbons in the diesel range has not been established In contrast to fuel hydrocarbons in the gasoline range, the diesel range fuel hydrocarbons contain fewer soluble components, are less mobile in the soil column, and are less volatile The subject site has been impacted by diesel-range fuel hydrocarbons, with TPH-d concentrations detected in soil samples as high as 12,000 parts per million (ppm reported as milligrams per kilogram) The lateral extent of the TPH-d impacted soil has not been determined, nor the source fully identified The probable source of the most highly impacted soil (samples from P2) is the former dispenser or former product line The concentrations and distribution of TPH-d detected in the former UST area may have come from either a minor release from the former UST or a larger release from the former product line/dispenser area The vertical extent of impacted soil encountered in boring P-2, slightly deeper than 50 feet bsg, probably closely approximates the total vertical extent of impacted soil on the site, although a release originating from the former dispenser may have penetrated slightly deeper The later al extent of impacted ground water has not been determined, but where encountered on the site, ground water has had minimal TPH-d impact (70 and 510 ppb) and has not been impacted by BTE&X or PNAs Ground water was encountered on the site at approximately 62 feet bsg The flow direction is generally toward the northeast in this portion of Stockton The closest well is located approximately 800 feet east of the site, in a cross-gradient to down- gradient relationship to the subject property The decline of TPH-d concentrations from 510 ppb in P3 to 70 ppb over a distance of approximately 24 feet suggests that significant concentrations of TPH-d will not impact the well The second closest known well is approximately 1,200 feet north of the site, in a cross-gradient to down-gradient direction The low mobility of diesel-range fuel hydrocarbons in ground water and the above noted decrease of TPH-d concentrations between P3 and P4 suggests that the well is not likely to be impacted by the release Natural attenuation of the TPH-d left on site is a possibility, although no data has been collected to demonstrate that the process is occurring or can occur on the site The concentrations of TPH-d detected are not so high as to be toxic to petroleum-degrading bacteria LIMITATIONS . Ourprofessional services were performed using that degree of care and skill ordinarily exercised by Advanced GeoEmironnzental,Inc