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' v . A S S O C I A T E S I N C steadily to below maximum contaminant levels (MCLs) as specified in Title 22 of the California Code of Regulations (Calitorma's drinking water standards) ATC is currently continuing with quarterly monitoring and sampling of wells at the request of Mr Harlin Knoll of PHS/EHD During ATC's most recent communication with PHS/EHD in December 2000, Mr Knoll indicated that the previously proposed workplan to install a vapor extraction/air sparge system at the site is not warranted However, Mr Knoll also indicated that either long term monitoring or another alternative would be required prior to the site being considered for closure ATC discussed the possibility of performing a long-term groundwater extraction test at the site, to which Mr Knoll agreed The workplan for performing a groundwater extraction pump test at the site is included in this report QUARTERLY MONITORING ACTIVITIES Groundwater Monitoring and Sampling ATC performed quarterly groundwater monitoring and sampling on December 20, 2000 Field work included measuring the depth-to-water in wells MW1 through MW6 and purging and sampling the water from wells MW3 and MW4 in addition to the nearby Thornton No 2 • Municipal Supply Well (Supply Well) The samples were analyzed for the presence of dissolved petroleum hydrocarbons Depth to water measurements were collected on December 20, 2000 from monitoring wells MW 1 through MW6 According to these measurements, groundwater beneath the site generally flows in a southwesterly direction at an average gradient of 0 001 ft/ft, or approximately 5 feet/mile, with apparent mounding in the vicinity of MW3 Flow direction and average gradient are consistent with historical data for the subject site A groundwater gradient map for the December 20, 2000 monitoring episode is shown on Figure 2 Groundwater samples were collected from wells MW3 and MW4 subsequent to purging and stabilization of physical parameters (pH, electrical conductivity, and temperature) The monitoring wells were purged and sampled according to ATC's Field Procedures on file with PHSIEHD Monitoring well purge logs are included in Attachment 1 Furthermore, ATC coordinated with San Joaquin County Utility Maintenance Division personnel and collected a water sample from the sampling port of the Supply Well Groundwater samples were placed on ice and transported under chain-of-custody to State- certified Argon Laboratories (ELAP #2359) Samples were analyzed for TPHg and BTEX by EPA method 8015/8020 and for methyl tertiary butyl ether (MTBE) by EPA method 8020 w 1625821quart1yl4gr-2000 doe 4