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.may 10`0 Manna Pro Stockton 2 Cluarterly Groundwater Monitoring Report January 1992 GroundmMter MonitorIng Data On October 2,1991,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 sic wells ranged from 41.25 to 43.4 feet. This corresponds to static water level elevations ranging from 57.77 to 59.68 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 preoented 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 suggests hydraulic mounding near,and potentially related to,the tank excavation. GroundwWor Qualkv Data The six monitoring wells were sampled on October 2, 1991 following groundwater level monitoring. Prior to sampling, the welis 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. All six wells balled dry after bailing between 10 and 15 gallons of water from each well as shown in Table 2. + Samples were collected using a Teflon TM bailer and placed Into sample containers supplied by the laboratory performing the analyses.pe alyse . Sam lin p g equipment was cleaned between wells using an AlconoxT""and water wash and delonized water rinse. 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 October 2, 1991 sampling event were analyzed for the following parameters: • pH using Standard Method 4500 H • Specific Conductance using Standard Method 2510 • 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 • 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 7.1 to 7.9. Specific --- conductance ranged from 980 to 2,_4.00 micro-mhos per eentlmeterat 25 degrees Centigrade(umno/cm,25)..--- ------ —.---- — —.— -= --- ------ Because the results of pH and specific conduct=ance in the samples collected were generally similar, this suggests that the water Is from the same hydrologic zone. �.J ` =GROUNDWATER TECHNOLOcY,INC,