SITE INFORMATION AND CORRESPONDENCE

Home Browse Search

. arta UaKer Stockton Terminals Technical C*ittee 3 25 April 2008 cumulative system operation is about 50%, and the system frequently delivers oxygen instead of ozone. The STTC conducts an OS system monitoring program in six A and B zone remedial observation wells (PS/MW-18, PS/WC-1S, AR/MW-21, AR/MW-18B, AR/MW-21B, PS/WC-1M). A contingency plan is triggered if COC concentrations exceed historical concentrations by one order of magnitude for three consecutive quarters in any of these six wells. During the fourth quarter of 2007, historical concentration exceedences of BTEX in monitoring well AR/MW-18B varied from one to two orders of magnitude. TPHg and MTBE were detected at 480 and 2.0 pg/L, respectively. These COC concentrations were the highest ever observed in this well. Based on the groundwater monitoring results described in the Annual Report, the STTC proposes to (1) expand the scope of the ozone injection monitoring program to closely monitor concentration trends exhibited in selected C and D zone wells to ensure that off-site migration of COCs does not become a problem, (2) install an off-site D zone monitoring well to address a gap in the monitoring well network between wells OW-8D and OW-51), (3) revise MRP No. R5-2004- 0823 to conduct quarterly monitoring in wells PS/P-26 and PS/MW-1M, (4) conduct annual sampling of PS/P-26, TS-1, and TS/M-1 for ethanol and methanol, and (5) continue to work with Regional Water Board staff to obtain all necessary permits to implement the 2 May 2007 Work Plan for Enhanced Bioremediation Study(Pilot Study) on the BP facility. The Pilot Study proposes injecting a nutrient solution to mitigate the fuel hydrocarbons in groundwater. If proven effective, STTC intends to expand the enhanced bioremediation program treatment to a full-scale program. Our comments are presented below. 1 Regional Water Board staff do not concur that expanding the scope of the C and D zone monitoring program sufficiently addresses our previously stated concern regarding contaminant migration in the C and D zones beyond the OS system. Some of the highest C- zone detections were observed in off-site monitoring well OW-9C, which is about 800 feet downgradient of TS/MW-21). As shown in the above table, TS/MW-2D typically contains the highest on-site concentrations of CDCs in the D-zone. The time-concentration graphs provided in the Response show that the average benzene concentration in TS/MW-2D has increased by about 1,500 pg/L over the last two years. These data show that the increasing oxygenate concentrations observed in monitoring wells OW-9C and OW-8D are caused by migration of CDCs from TS/MW-2D through a gap in the west OS system. The increasing benzene concentrations observed in OW-3C, coupled with the recent high concentration observed in downgradient monitoring wells AR/MW-18C and OW-4C offer additional evidence that the east and west OS barriers are ineffective at preventing the downgradient, off-site migration of CDCs in the C and D zones. Breaches in the OS system offer a more appropriate explanation for the observed increasing concentrations than steeper pre-1997 horizontal gradients and a 1998 spill. These events occurred more than 10 years ago and likely would now manifest as decreasing concentration trends. The additional monitoring proposed by the STTC will not mitigate the spread of contaminants through the OS barrier. The STTC must focus its efforts on supplementing the east and west OS systems, installing a different remediation system, and/or reducing the source area CDCs, as proposed in the Pilot Study.