WORK PLANS - Laserfiche WebLink

Home Browse Search

Soil Vapor Monitoring Wells Installation and Sampling Work Plan San Joaquin County Public Works 1810 E. Hazelton Avenue, Stockton,CA Page 2 In a letter dated May 31, 2022, the Central Valley Water Board staff requested that zone injection remediation cease due to ineffective remediation and to submit a remedial action plan that outlines a new remedial approach. In a subsequent letter dated September 9, 2022 (Appendix D), the Central Valley Water Board staff indicated that they had evaluated current Site conditions against State Water Resources Control Board's Low-Threat Underground Storage Tank Case Closure Policy(LTCP)and identified remaining impediments to case closure. As outlined in the letter, Site conditions meet all eight of the LTCP General Criteria. Additionally, Site conditions meet three of the four criteria (a, b, and d) of LTCP Groundwater Media-Specific Class: (a) the maximum extent of petroleum hydrocarbon impact to groundwater above water quality objectives is less than 250 feet in length, (b)there is no free product, and (d) maximum dissolved benzene and methyl tertiary butyl ether (MTBE) concentrations are less than 3,000 micrograms per liter (ug/L) and 1,000 ug/L, respectively. There are two public water supply wells located within 1,000 feet of the defined extent of the Site plume, and thus criterion c is not met. California Water Service Company(CalWater)wells are located approximately 580 feet south and 990 feet northwest of the Site. However, these wells are listed as inactive and are located tip and cross gradient, respectively. As such,risk to human health and safety and the environment from the Site release is low and Site conditions meet LTCP Groundwater Media-Specific Class 5 criteria. As a result of the LTCP criteria, a proposal for a new remediation approach was no longer necessary. Central Valley Water Board staff found that since dissolved benzene greater than 1,000 ugtL, extends to or under the office building north and soil collected from the well OW7 borehole located on the north side of the maintenance building contained elevated levels of total petroleum hydrocarbons (TPH) at 5 and 10 feet below ground surface (bgs), a vapor intrusion health risk may exist, and thus the active fueling facility exception is not met. Central Valley Water Board staffrequested a Work Plan for the installation and sampling of permanent soil gas wells to evaluate vapor intrusion risk to the office and maintenance buildings immediately north and south of the source area. This Work Plan is in response to that request. 3.0 SUBSURFACE CONDITIONS The Site surface is completely paved with either asphalt black top or concrete. Based upon previous investigations and groundwater monitoring well installation soil boring logs, soil beneath the Site consists of silt and clay from beneath the pavement to approximately 30 feet bgs, silty sand from approximately 30 feet to approximately 35-45 feet bgs, and interbedded sands and silts from 35-45 feet bgs to approximately 55 feet bgs, the maximum depth explored. The depth to groundwater, as measured in September 2022, was between 30 and 33 feet bgs. The direction of groundwater flow beneath the Site is towards the northeast. Appendix A contains recent groundwater surface map (Figure 3), TPH-g Isoconcentration Map (Figure 4), Benzene in Groundwater Map(Figure 5), and MTBE in Groundwater Map(Figure 6). Appendix B contains table summarizing historical soil (Table B-1) and groundwater data collected from the Site (Table B-2). 4.0 PROPOSED SOIL VAPOR PROBE LOCATIONS Three soil vapor monitoring wells are proposed as shown on Figure 7, Appendix A. One soil vapor well (SVP-1) will be installed north of the maintenance building and east of the dispensers near where shallow soil contamination was encountered during the installation of ozone injection wells; a second soil vapor well (SVP-2) will be located south of the office building and east of the dispensers; and a third soil vapor well (SVP-3) will be located near the southeastern corner of the office building, at the approximate downgradient extent of the 100 ug/L benzene Isoconcentration contour. A i** CONDOR