COMPLIANCE INFO_CASE 2 - Laserfiche WebLink

Home Browse Search

M. Scott Mansholt - 4 - 11 March 2008 TAOC Tracy Gravel Pits Based on the recent investigative trenching results, it appears that the lateral extent of affected soil has not been adequately defined in the southeast and northwest ends of the gravel excavation pit between the recent trenches and the property boundary. It is not known if disposal has occurred in those areas. As noted on historical and current aerial photographs the gravel excavation extended offsite laterally from these areas and additional definition is needed. In addition, according to the findings, the lateral extent of affected soil was not defined further north of the property boundary. However, review of historical aerial photographs combined with the various field investigations, does not indicate that disposal to the former ponds/sumps extended beyond the northern property boundary. Geomatrix concluded that based on the current and previous investigations, affected soil appears to occur at several locations primarily in the north part of the site, and does not occur in the south-southwest portion of the property. GROUNDWATER Groundwater samples were obtained from nine CPT borings, three onsite and six offsite/downgradient locations at approximate depths of 48 to 58-feet using temporary grab- groundwater sampling devices. Two onsite samples were submitted for general minerals analysis only; the other seven for TPH, PAHs, and BTEX compounds. With the exception of low concentrations of TPHd in three of seven samples analyzed (65, 96, and 160 pg/L), the results were ND; also, TPH as gasoline (TPHg) was ND. A TPHd detection of 360 pg/L in the QA/QC equipment rinse sample appears to be responsible for the TPHd detections listed. Review of the lab data results supports this observation by Geomatrix. The offsite sampling locations were chosen to evaluate the downgradient extent of affected groundwater previously identified in two onsite borings reporting elevated TPHd present in the northeast and northwest corners of the site. The offsite borings/groundwater sampling was conducted in streets within the adjacent residential neighborhood north of the site. Geomatrix stated that based on the ND analytical results for the TPH, BTEX and PAH compounds the lateral extent of affected groundwater has been delineated, is limited to the area along the northern portion of the site. The well search for water wells within 2,000-feet of the site reported that 199 of the 226 wells present ranged from 72 to 240 feet deep; eight irrigation or industrial wells ranged from 75 to 860-feet deep; 20 were monitoring wells located on an adjacent property to the west, and two were community/municipal wells at 705 and 1,080-feet deep. The majority of the wells had a minimum 50-foot bentonite sanitary surface seal. None of the wells identified were downgradient (to the north within the adjacent residential development) and none identified were south of the site, upgradient. Previously, we requested that an effort should be made to obtain permission to sample the well observed northwest of the site. A representative for the business on the adjacent company stated his company uses water provided by the City and they had no knowledge of the well observed. According to the earlier well search information, one industrial well,