SITE INFORMATION AND CORRESPONDENCE

Home Browse Search

Ms. Joachim - 3- 22 June 2012 gradient of extraction well EU-3. As of 23 February 2012, the TCE concentration in M-49A had not returned to pretest levels. Although it is clear that M-49A is outside of the well capture zone, it is unclear why the TCE concentration in M-49A increased when Nestle decreased the pumping rate of the extraction system. This observation seems to indicate that there is significant mass of TCE in the Upper A-Zone located up-gradient of monitoring well M-49A. Directive: Future investigations must characterize the TCE mass in groundwater west of the facility and up-gradient of monitoring well M-49A; and future remedy selection must prevent the continued migration of the TCE mass towards the northwest, through and beyond the location of M-49A. Below, the "Additional Directives" section of this letter requires Nestle to install monitoring wells to characterize this TCE mass. 4. Discussion: Monitoring well M-49A is part of the M-6 nest of monitoring wells; these wells contain elevated concentrations of TCE and varying concentrations of its breakdown products including cis-1,2-dichloroethene (cis-1,2-DCE), and vinyl chloride (VC). Monitoring well M-66 is screened within the B-Zone which is the most shallow water bearing zone beneath Ripon; monitoring well M-49A is screened within the Upper A-Zone which is beneath the B-Zone. These two wells appear to be hydraulically connected because the TCE and cis-1,2-DCE concentrations are similar. Figure 1 shows the TCE concentration trends over the last ten years in well M-613 and well M-49A. Since Nestle installed M-49A in 2008, TCE concentrations in both wells have been in the 100 pg/L range and cis-1,2-DCE concentrations very low or not detected. Well M-6A, screened within the Lower A-Zone, is below the Upper A-Zone and separated by discontinuous fine-grained layers. Well M-6C1, screened within the shallow part of the C-Zone, appears connected hydraulically to the Lower A-Zone because the TCE and cis-1,2-DCE concentrations are similar. Well M-6C2, screened within the deep part of the C-Zone, appears separated from the shallow C-Zone because the relative TCE to cis-1,2-DCE concentration in M-6C-2 is less than in shallow C-Zone well M-6C1. Central Valley Water Board staff used the TCE and cis-1,2-DCE concentration ratio to differentiate the chemical signatures of samples collected from the M-6 nest of six monitoring wells. Staff calculated the ratio in the groundwater samples collected from all six wells over the last ten years. Figure 2 presents the trends of these ratios. It immediately becomes apparent from Figure 2 that the ratios of shallow wells M-613 and M-49A are very similar, as are their TCE concentrations shown in Figure 1. The ratios are between 50 and 150, whereas the wells screened in all of the deeper zones have ratios that are less than approximately 10. Figure 3 shows the similarity of ratios in Lower A-Zone well M-6A and shallow C-Zone well M-6C1. As indicated above, these two wells appear connected hydraulically. Figure 4 shows the similarity between these hydraulically connected wells and D-Zone well M-6D which is hydraulically separated from them by the Corcoran Clay. It appears that an idle, nearby production well may have provided a conduit for TCE contaminated groundwater to migrate immediately from the Upper A- or shallow C-Zone, across both the deeper C-Zone and the Corcoran Clay, and into the D-Zone. Figure 5 shows a different chemical signature for deeper C-Zone well M-6C2. The TCE/cis- 1,2-DCE ratios of the deeper C-Zone well have been less than one over the last ten years.