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s� Manna Pro Stockton 2 Quarterly Groundwater Monitoring Report April 1992 Groundwater Monitoring_Data On January 10, 1992, the six existing groundwater monitoring welts were monitored for depth-to-water (D—,W). The DTVV 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 39.50 to 41.77 feet. This corresponds to static water level elevations ranging from 59.37 to 61.43 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 0.03 ft/ft towards the west-southwest in the site vicinity. The groundwater flow pattern suggestshydraulic mounding near,and potentially related to,the tank excavation. Groundwater Qualms Data The six monitoring wells were sampled on January 10, 1992 following groundwater level monitoring. Prior to sampling, the wells were purged by balling to enable collection of samples representative of aquifer conditions. The standard protocol for sampling is to purge 3 to 5 well casing volumes prior to collecting samples. However,when a well is bailed dry before 3 to 5 casing volumes are removed,the water level in the well is allowed to recharge to approximately 80 percent of static water level prior to collecting samples. One well (MW-1) bailed dry after bailing 10 gallons of water. However, 15 to 25 gallons of water(3 to 5 casing volumes)were bailed from the other wells(MVV-2 through MW-6)as shown 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 Alconox"'and water solution and rinsed with delonized water. A field blank and rinsate sample were collected in the field for Quality Assurance/Quality Control(QA/QC)measurer. Samples were labeled and placed in an Ice-chilled cooler for transport to GTEL Environmental laboratories,Inc.In Concord,California,and were accompanied at all times by a chain-of-custody record. Samples from the January 10, 1992 sampling event were analyzed for the following parameters: o pH using Standard Method 4500 H • Specific Conductance using Standard Method 2510 s Aromatic Volatile Organics and Total Petroleum Hydrocarbons as Gasoline using EPA Methods 5030,8020 and Modified 8015. • Total Petroleum Hydrocarbons as Diesel Fuel using Modified EPA Methods 3510/8015 i • Total Lead using EPA Method 7421 The results of laboratory analyses are presented in Table 3. The laboratory analytical reports are also included as an attachment to this letter. As shown in Table 3, pH ranged from 7.2 to 7.8. Specific conductance ranged from 920 to 2,100 micro-mhos per centimeter(umho/cm) at 25 degrees Centigrade. Because the results of pH and specific conductance in the samples collected were generally similar, this suggests that the water is from the same hydrologic zone. GROUNDWATER ^_ -)TECHNOLOGY,INC.