SITE INFORMATION AND CORRESPONDENCE

Home Browse Search

ST Services Stockton Ter - 2 - 15 May 2007 Monitoring Reports and C erization Report during the fourth quarter 2006 and first quarter 2007. The low groundwater conditions observed in January 2007 were similar to historical levels despite January 2007 being one of the driest on record for the Stockton area. Precipitation amounts in Stockton in January generally average 2.71 inches. However, Stockton received only 0.5 inches of rain in January 2007 . Depth to groundwater ranged from about one to 11 feet below ground surface (bgs) over the three quarters. Ash Creek calculated that horizontal gradients ranged from 0.001 to 0.0006 feet per foot in a east-southeasterly direction in the four water bearing units. Downward vertical gradients were dominant between the water bearing zones during the three quarters and ranged from - 0.001 to - 0.031 feet per foot. Historically, downward vertical gradients have persisted between the water bearing units. The monitoring well closest to the source area, PS/MW-14, has historically contained free product and therefore is not usually sampled. COC concentrations displayed dramatic fluctuations and showed no discernable trends between quarters. Over the three quarters, maximum dissolved COC concentrations were mostly detected in samples obtained from monitoring well ST/MW-1 and PS/P-12. These observations are consistent with historical trends. During the three quarters, maximum TPHg and TPHd concentrations included 220,000 micrograms per liter (Ng/1) and 13,000 pg/I, respectively, in the first quarter 2007 and fourth quarter 2006. Maximum BTEX concentrations were 17,000 pg/I, 9,500 pg/l, 3,200 pg/I, and 19,000 pg/I, respectively, and occurred over the three quarters. Two oxygenates detected in the highest concentrations included MTBE and TAME at 250,000 pg/I and 1 ,100 pg/l, respectively. ST Services did not sample monitoring wells OW-6D, OW-7A, OW-713, OW-7C, OW-7D, OW- 8A, OW-813, or OW-8C during the third quarter 2006. Monitoring and Reporting Program (MRP) No. R5-2004-0822 required ST Services to sample these wells semi-annually for two years beginning in the third quarter of 2004. The MRP states, "If concentrations remain stable or decrease, the Discharger may eliminate sampling of these wells." As shown in Appendix F of the Annual Report, OW-8A, OW-813, and OW-8C did not meet the requirement for two years of semi-annual monitoring because they were not sampled in the first quarter of 2005. In addition, in a 2 October 2006 letter, Regional Water Board staff stated that OW-6D should be sampled annually, and OW-8C needs to be sampled for two more semi-annual sampling events to evaluate the increasing concentrations of TPHd and benzene. Based on these changes, the new wells installed since 2004, and our review of site documents, we have prepared the attached draft, revised MRP to reflect current site conditions. By 15 June 2007, please provide your comments on this document and a site map showing all the wells, which will be used as Figure 1 for the MRP. The Report describes well installation and groundwater monitoring conducted in September and October 2006. A total of four monitoring wells were installed: two in the A zone (ACA-1A and ACA-2A) and two in the B-zone (ACA-1 B and ACA-213). ST Services installed the wells to assist in evaluating data gaps at the presumed edge of the COC plume in the A and B zones. Laboratory analytical results show that in the closer well cluster, TPHg and TPHd concentrations were higher in the A-zone (ACA-1 A) compared to the B-zone (ACA-1 B). The http://www.weatherunderground.com/history/airport/KSCK/2007/l/31/DailyHistory.html?req_city=NA&req_state=NA& req_statename=NA