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• s � l SECOR Work Plan for Additional Site Assessment and Interim Remediation Addendum 7-Eleven Store#32262 March 31, 2005 Page 2 { wells EX-1 and MW-5 prior to conducting the test to establish baseline groundwater levels and to monitor the amount of drawdown in each well during the test. Based on the analysis of the drawdown data for the step-test performed on August 11, 2004, an optimal pumping rate of 0.8 gpm was calculated and used for the original pump test. Maximum drawdown in EX-1 was 6.17 feet. SECOR recommends conducting an I additional step-drawdown test in order to determine a greater pumping rate which will I increase the drawdown in EX-1 as well as increase the radius of the associated cone of depression. The additional step-drawdown test will be conducted by incrementally increasing the pumping rate at selected time intervals (usually 60 to 90 minutes per step) and monitoring groundwater levels in EX-1 and MW-5. Hydraulic responses during the test ` will be monitored using a pressure transducer. Extracted groundwater will be discharged to the 6,900 gallon holding tank and flow rates will be monitored using a flowmeter. The step- drawdown test will be conducted to re-evaluate: '-1) the optimum discharge rate for the constant-discharge pumping test; 2) well losses; and 3) well efficiency. After conducting the step-drawdown test, the pump will be shut down and recovery water levels will be monitored until over 95% of the total drawdown has recovered. The recovery water levels can be analyzed with the weighted average of the discharge rate to estimate the hydraulic properties (transmissivity and hydraulic conductivity) of the tested interval. The constant-discharge pumping test will be performed for 80 hours or until the cone of depression reaches near steady-state conditions,/Aroundwater levels in the extraction well EX-1 and MW-5 will be monitored using the pressure transducers. After terminating the , pumping test, groundwater levels will be monitored in the extraction well EX-1 and - - monitoring well MW-5 until the levels have reached over 95% of pre-pumping levels. Thelie resulting drawdown and recovery data will be analyzed to estimate the aquifer parameters ¢-tO of transmissivity, hydraulic conductivity, and storativity. The resulting hydraulic parameters can then be used as part of the parameters input for the groundwater flow and transport ,. modeling. The data from the step-drawdown and constant-discharge pumping tests will be analyzed by methods described by Cooper and Jacob (1946), Theis (1935), and Moench (1997) that are contained in the computer program AQTESOLV. Groundwater samples will be collected from EX-1 before the step-draw down test and after completion of the test. In addition, a water sample will be collected from the holding tank.' The water samples will be submitted to a California-certified laboratory for the analysis of TPHg, BTEX, and fuel oxygenates by EPA Method 8260B. I Interim Remediation Using Groundwater Extraction The test results will be used to determine if periodic or continuous extraction of groundwater will be an effective remedial approach. The groundwater extraction rate and groundwater 1 concentrations observed during the test can be used to determine if groundwater treatment or offsite water disposal is more cost effective. 1 LW-ElevenV-11 Stores1322621Workplans132262 Addendum to Offsite well install and interim remediation workplan.doo