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Y c Manna Pro Stockton Quarterly Groundwater Monitoring Report July 1992 2 4 046 be addressed by sampling it further and backfilling the pit with soil that meets certain agreed upon standards The remainder of the stockpiled soil not meeting those standards will then be treated via • on-site uncontrolled aeration The extent of petroleum hydrocarbons in the soil and groundwater has been evaluated in Soil and Groundwater lnvestigat�on at Manna Pr9, Inc Facility, da#ed December 1990, prepared by ERM- C-2 West, inc Additional groundwater monitoring wells may be required to establish plume perimeter y► monitoring points In our letter to PHS dated August 28, 1991, the overall remedial strategy for in place soil and groundwater was outlined Soil treatment will likely involve in-situ soil venting Groundwater remediation will involve groundwater extraction and treatment to maintain hydraulic control of the plume Groundwater Monitoring Data On April 23, 1992, the six existing groundwater monitoring wells were monitored for depth-to-water (DTW) The DTW measurements were taken from surveyed points on the top of each well casing, which are referenced to a common datum (designated as 100 feet above mean sea level) During this monitoring event the DTW in the six wells ranged from 37 35 to 39 56 feet This corresponds to static water level elevations ranging from 6161 to 63 58 feet above mean sea level (MSL) The results of groundwater monitoring are presented in Table 1 A potentiometric surface map as interpreted from the data in Table 1 is presented as Figure 1 As shown in Figure 1, the average groundwater gradient is approximately 0 01 ft/ft towards the west-southwest in the site vicinity The groundwater flow pattern suggests hydraulic mounding near, and potentially related to, the tank excavation° Groundwater Quality Data The six monitoring wells were sampled on April 23, 1992 following groundwater level monitoring Prior to sampling, the wells were purged by bailing to enable collection of samples representative of aquifer conditions The standard protocol for sampling is to purge 3 well casing volumes (approximately 50 gallons) prior to collecting samples However, because the wells bailed dry, they were allowed to recharge to approximately 80 percent of static water level prior to collecting samples' The well purging data are presented in Table 2 Samples were collected using a Teflon"'bailer and placed into sample containers supplied by the laboratory performing the analyses Sampling equipment was cleaned between wells using an AlconoxT'and water solution and rinsed with deionized water A field blank and rinsate sample were collected in the field for Quality Assurance/Quality Control (QA/QC) measures Samples were E1-1 GROUNDWATER TECHNOLOGY, INC