ARCHIVED REPORTS XR0005572 - Laserfiche WebLink

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4.0 CONCLUSIONS AND RECOMMENDATIONS This environmental site investigation included the drilling of three soil borings for soil and groundwater-grab sampling and the drilling of three additional soil borings for sod sampling and the installation, development, surveying and sampling of three monitoring wells Field observation and analytical laboratory results indicate that diesel and gasoline hydrocarbons have impacted soil and groundwater beneath the site Soils beneath the site consist primarily of massive sand-based units with varying percentages of fine gravel, silt and clay The deepest soil encountered during drilling (at a depth of approximately 20 to 25 feet bgs) consisted of sandy silt with up to 25% clay During drilling, groundwater was first encountered between 13 and 19 feet bgs Prior to groundwater purging and sampling, the static depth-to-groundwater ranged from 12 37 feet bgs in MW-2 to 13 25 feet bgs in MW-3 Following a wellhead elevation survey, the groundwater gradient was calculated at approximately 0 00025 footlfoot in a northerly direction The greatest concentration of petroleum hydrocarbons detected in groundwater during this investigation was in the groundwater-grab sample collected from SB-7 This sample contained 13,000 µg11 of TPH-as-diesel, 2,800 µgll of TPH-as-gasoline and 2 2 µg/l of benzene The groundwater sample collected from MW-1, located approximately 40 feet east of the former UST contained 2,000 µg11 of TPH-as-diesel and 420 µgfl of TPH-as-gasoline MTBE was detected at 1 1 µg11 in MW-1 and at 0 65 µg11 in MW-3 No other alcohol- or ether-based oxygenate compound was detected in any of the groundwater samples analyzed Given the proximity of the State Highway 99 right-of-way immediately north of the former UST, the lateral extent of impacted groundwater is not defined to the north of the site in addition, given the concentrations of petroleum hydrocarbons detected in the groundwater sample collected from MW-1, the lateral extent of impacted groundwater to the east of the former UST is also undefined The lateral extent of impacted groundwater is defined to the east of the former UST by MW-2 and to the south of the former UST by MW-3 As expected, the soil samples with the highest concentrations of petroleum hydrocarbons were collected inclose proximity to the former UST Sod impacted by greater than 100 mg/kg of TPH- as-diesel extends up to approximately 40 feet away from the former UST excavation Again, due to the proximity of the former UST to the State Highway 99 right-of-way, the lateral extent of impacted soil is not defined to the north of the site • 10