ARCHIVED REPORTS XR0001523 - Laserfiche WebLink

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EXECUTIVE SUMMARY This preliminary investigation and evaluation included the drilling of five on-site soil bonngs for soil sampling and analysis Three of the soil borings were converted to groundwater monitoring wells The newly installed wells were developed, surveyed, monitored and sampled Soils encountered beneath the site consist of a fining-upward sequence of fine-grained siliciclastic sediments The depth to static groundwater beneath the site ranges from 39 31 to 40 34 feet bgs Groundwater flow is to the south-southeast at a gradient of 0 05 foot/foot A metal detection survey conducted at the site discovered strong metallic signals in the vicinity of the former UST excavation The metal may be drums of waste not accounted for from earlier site investigation activities and other material used to backfill the former UST excavation Soil and groundwater beneath the site have been impacted by significant concentrations of gasoline hydrocarbons The five oxygenate compounds were not detected in any of the soil or groundwater samples analyzed during the investigation In soil, the highest concentration of gasoline hydrocarbons was detected in the sample collected at 30 feet below ground surface (bgs) from the soil boring for MW-2 This sample, collected directly beneath the former UST excavation, contained 2,400 milligrams per kilogram (mg/kg) of Total Petroleum Hydrocarbons (TPH)-as-gasoline and detectable concentrations of toluene, ethylbenzene and xylenes Laterally, the extent of impacted soil appears to be limited to the footprint of the former UST excavation Vertically, the extent of impacted soil beneath the former UST excavation is defined at a depth of approximately 55 feet bgs, on the basis of soil samples collected for analysis from the previously drilled soil boring B-1 In groundwater, the highest concentration of gasoline hydrocarbons detected was from the sample collected from MW-3, adjacent to the northwest corner of the former UST excavation This sample contained 120,000 micrograms per liter (pg/1) of TPH-as-gasoline, 7,800 µg/1 of benzene, 64,000 µg11 of toluene, 4,300 µg/I of ethylbenzene and 28,000 µg11 of xylenes These concentrations are above Title 22 Maximum Contaminant Levels for these compounds The lateral extent of impacted groundwater is undefined to the north (upgradient), west and south (downgradient) of the former UST excavation However, the lateral extent is defined to the northeast of the former USTs by MW-1 Vertically, the possible impact to deeper groundwater- bearing zones has not been investigated Based on the results of the results of this investigation, Ramage Environmental recommends the following course of environmental corrective action 1 Perform a geophysical survey to investigate for possible buried drums or other metal debris 2 Define the lateral extent of impacted groundwater by expanding the groundwater monitoring L well network 3 Monitor and sample groundwater from all monitoring wells on a quarterly basis for at least four consecutive quarters 4 Conduct a survey of water wells within a 2,000-foot radius of the site 5 Evaluate the risk posed by gasoline hydrocarbons on human health and the environment 6 Based on the findings of the risk assessment, request regulatory closure or proceed with further evaluation of possible remedial alternatives 3 �I �1