ARCHIVED REPORTS_XR0010580 - Laserfiche WebLink

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EXECUTIVE SUMMARY • On behalf of the San Joaquin County Facilities Management Department, Ramage Environmental installed three groundwater monitoring wells (MW-1, MW-2 and MW-3) in the vicinity of three former underground storage tanks (UST) at the site Sods encountered beneath ' the site consist of a fining-upward sequence of fine-grained sediments The depth to static groundwater beneath the site ranges from 7 74 to 8 16 feet bgs Based on surveyed elevations of the three wells, groundwater flow is to the north at a relatively low gradient of 0 004 foot/foot Groundwater samples collected from the newly-installed wells were analyzed for Total Petroleum Hydrocarbons (TPH)-as-diesel, TPH-as-gasoline, benzene, toluene, ethylbenzene, xylenes (BTEX), methyl tertiary butyl ether (MTBE), and four other alcohol-and ether-based oxygenate compounds The three groundwater samples were also analyzed for phystochemical and microbial parameters Only TPH-as-diesel was detected in groundwater from one of the three wells (MW-2) at a relatively low concentration of 140 micrograms per liter (µg11) As such, the plume of impacted groundwater appears to have migrated from the area of the former USTs at least 30 feet to the northeast Based on the presence of free product previously observed in close proximity to the former USTs and the relatively low concentration of diesel in groundwater at MW-2, contaminant migration appears to be slow and probably stable This is supported by the relatively flat groundwater gradient Nitrate concentrations and both general and selective microbial populations in all three groundwater samples are relatively low, however, the percentage of selective degraders to the total number of heterotrophs is high This suggests that the existing microbes can degrade the contaminant, but that nutrient addition (nitrate and oxygen) would enhance blo activity No water wells or other sensitive receptors appear to be threatened by the plume of impacted groundwater During a walk of the site vicinity, the two wells nearest to the subject site could not be located The site and site vicinity is connected to city water service ' Based on the results of the well installation and sampling activities, Ramage Environmental recommends the following course of environmental corrective action 1 Monitor and sample groundwater from the three monitoring wells on a quarterly basis for at least three more quarters Analyze samples by EPA Methods 8015/8020 only 2 Based on the results of four quarters of groundwater monitoring and sampling and on the ' results of previous soil and groundwater grab samples collected at the site, evaluate the risk posed by residual petroleum hydrocarbons on human health and the environment 3 Based on the findings of the risk assessment, request regulatory closure or proceed with ' further evaluation of possible remedial alternatives 1 3