ARCHIVED REPORTS XR0005572 - Laserfiche WebLink

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. EXECUTIVE SUMMARY This investigation included the drilling of 3 soil borings for sod and groundwater-grab sampling and 3 additional soil borings for soil sampling and the installation, development, surveying and sampling of 3 monitoring wells Sods encountered consisted primarily of massive sand-based units with varying percentages of fine gravel, silt and clay During drilling, groundwater was first encountered between 13 and 19 feet below ground surface (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 0 00025 foot/foot in a northerly direction Analytical results indicate that soil and groundwater have been impacted by petroleum hydrocarbons The highest concentration of petroleum hydrocarbons detected in groundwater during this investigation (13,000 micrograms per liter [µg11] of Total Petroleum Hydrocarbons [TPH]-as-diesel, 2,800 µg11 of TPH-as-gasoline and 2 2 µg/l of benzene) was in the groundwater- grab sample collected from SB-7, near the former UST 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 µg11 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 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 The lateral extent is undefined to the east of MW-1 and to the north, Under State Highway 99 Soil impacted by greater than 100 milligrams per kilogram (mg/kg) of TPH-as-diesel extends up to approximately 40 feet away from the former UST excavation The lateral extent of impacted soil is not defined to the north of the site, under Highway 99 The vertical extent of impacted sod is undefined in the vicinity of the former UST Vertically, petroleum hydrocarbons are concentrated in soil within the zone of groundwater fluctuation (i e , capillary fringe) The greatest concentration of TPH-as-diesel (8,800 mg/kg) was detected in close proximity to the former UST excavation (SB-7) at a depth of 15 feet bgs The soil sample collected at the total depth of SB-7 (20 feet) contained 460 mg/kg of TPH-as-diesel Based on the results of this investigation, Wright Environmental recommends the following course of corrective action 1 Further define the lateral extent of impacted soil and groundwater within the Highway 99 right-of-way, the later extent of impacted groundwater east of MW-1 and the vertical extent of impacted sod in the vicinity of the former UST 2 Excavate the heavily impacted soil under and in proximity to the former UST 3 Monitor the concentration and stability of the groundwater contaminant plume by monitoring and sampling the existing wells on a quarterly basis for a period of at least one year 4 Model the future fate and transport of residual petroleum hydrocarbons, assess the risk to human health and the environment and submit a request for regulatory closure or a plan for additional environmental corrective action 1