ARCHIVED REPORTS_REPORT SUMMARIZING INSTALL OF MONITORING WELLS MW4,MW5,MW7,MW8 & SOIL BORING RDM1 & 4TH QTR 2013 GROUNDWATER SAMPLING

Home Browse Search

r Ms.Angela Lee Huntbrook Holdings,LLC February 12,2014 Page 3 Summary of Previous Environmental Invesdeations The following site history is based solely on reports prepared by the previous consultants, Winzler & Kelly(W&K) and GHD Inc. of Sacramento, California. W&K merged with GHD Inc. in 2011. The USTs were removed in 1988. Soil samples collected from beneath the former USTs reported concentrations of total petroleum hydrocarbons as gasoline (TPHg) up to 4,700 milligrams per kilogram (mg/kg), and benzene up to 7.8 mg/kg in the UST basin located along the western property boundary. Approximately 80 cubic yards of soil were reportedly excavated from the UST basins • during removal activities. The UST basins were reportedly excavated to a depth of 14 feet below ground surface grade (bgs). The product lines and dispensers were not investigated during the UST removal activities. Groundwater was not encountered during the UST removal activities. No additional remediation has occurred since this initial excavation. • In 2011, W&K was awarded a contract by the State of California UST Cleanup Fund Emergency, Abandoned, and Recalcitrant(EAR)Account Program to continue the environmental investigation of the subject site. Work included a subsurface investigation and subsequent quarterly monitoring activities. During the September 2011 subsurface investigation, petroleum hydrocarbons were identified in soil borings SB-I through SB-4 (Figure 2), with the highest concentration reported in SB-4, located adjacent to the former fuel dispenser island (Figure 2). Soil concentrations for TPHg ranged from 3.9 to 11,000 mg/kg and total petroleum hydrocarbons as diesel (TPHd) ranged from 30 to 2,800 mg/kg. Benzene, toluene, ethylbenzene and xylenes were also detected at low concentrations, which suggest the release was relatively older and some natural attenuation may be occurring. A copy of the previous consultant's historical soil sample analytical data is included in Enclosure C. During the September 2011 subsurface investigation, petroleum hydrocarbon constituents were also detected in the groundwater samples collected from the newly installed monitoring wells (MW-1 through MW-3). During the initial groundwater sampling event on September 26, 2011, concentrations of TPHg ranged from 760 to 12,000 micrograms per liter(µg/L), TPHd ranged from 64 to 1,400 µg/L,and benzene ranged from 4.3 to 180 µg/L. Between September 26, 2011, and September 10, 2012, GHD (formerly W&K) conducted the first i five quarterly monitoring events at the site. Beginning in the fourth quarter 2012, the property's new owner, Huntbrook, contracted RDM to perform future groundwater sampling, assessment, and remediation activities associated with the site. Groundwater monitoring data collected at the site to date is summarized in Table 2. Regional Geology and Hydrogeology The site is situated within the Sacramento Valley, which comprises the northern portion of the Great (Central) Valley. The Great Valley is located between the California Coast Range to the west and the Sierra Nevada Range to the east. Quaternary sedimentary rocks and alluvial deposits of gravel, sand, silt, and clay underlie the Great Valley. Soil encountered beneath the subject property during the Phase I (September 2011) subsurface investigation included clayey silt, silty clay, fine to medium grained, poorly graded sand, and silty sand. Predominantly lean clay and silts with minor amounts of fine sand and gravel were present to approximately 55 to 60 togs. Groundwater was encountered in a 5- to 10-foot thick poorly graded silty sand lens present from approximately 50 to 60 fogs, which was underlain by a "hardpan" at approximately 60 to 65 fbgs.