SITE INFORMATION AND CORRESPONDENCE

Home Browse Search

• 05,07,077,",712.,,07,07 ation ember of The IT Group Ms. Devra Lewis Project 812717 April 9, 2001 Page 2 2. The Report states that the BIC Aquii and is continuously saturated through the stratigraphic interval. An explanation is needed as to why a zone considered to be an aquitard is saturated. Dynamic pore water pressure measixements from the cone penetrometer indicates that the B/C Aquitard is saturated but has a low permeability which appears to restrict the vertical migration of contaminants. Analyti-at data collected from the various zones indicates that although petroleum hydrocarbons have migrated below the B/C Aquitard in several areas, there is a significant concentrationadient across this zone. The B/C aquitard is the only fine-grained unit(predominantly silt�nd clay) that is continuous across the site. By definition, an aquitard refers to a less-permeable bed in a stratigraphic sequence. These beds may be permeable enough to transmit water but the difference in permeability between the aquitard and the surrounding zones is typically high. The term "aquiclude" is often applied to low permeability zones that are not saturated and do not transmit water under ordinary hydraulic gradients. In reality, the term aquiclude refers to a saturated low permeability geologic unit that is incapable of transmitting significant volumes of water under ordinary hydraulic gradients. 3. The monitoring wells proposed based on CPT and HydroPunch data near CPT/H00-08, -09, and-14 illustrated on Figures 24 and 25 should be further to the east to be outside of the contoured plume area. These monitoring wells will be moved further to the east and beyond the inferred plume boundary. The new locations will have to be approved by the Port of Stockton. The Port requested that downgradient wells be installed at locations that will not interfere with future property development. 4. Monitoring wells need to be installed in the area of CPTIH00-16 to delineate the WE detected in that area, north of the site, and northwest of monitoring well TO/WC-IS, TO/WC- IM, and PSIP-29 to fully delineate groundwater contamination and to provide consistent data about plume migration. Benzene was detected in the B-and C-Zone groundwater samples collected at CPT/H00-16 at concentrations of 1.0 µg/L and 1.5 µg/L, respectively. MtBE was detected in the A- B- and C-Zone groundwater samples from CPT/H00-16 at concentrations of 929, 7,650, and 4,360 µg/L analyzed by EPA Method 8020. The results of the confirmatory analysis of the A-Zone groundwater samples shows that MtBE was below detection limits. Unfortunately, the analytical laboratory failed to perform confirmatory analysis on the B- and C-Zone samples. The groundwater analytical data collected during the groundwater investigation indicates that benzene and MtBE (if present) occur as an isolated mass downgradient of the leading edge of the benzene and MtBE plume. These data suggest there may be a separate source for these compounds originating from a source other than the STTC facilities. II /T CORPORATION SAC:\C:1My Dmuments:Projccts\Pon of Stockton\CPT HP 6tudy\Response to CommenmAm-98M 1 812717