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{y • DISCUSSION The results of this monitoring event indicated that the concentration levels of dissolved gasoline hydrocarbons are consistent or were slightly lower than levels detected during previous sampling events The groundwater gradient and direction of groundwater flow were also generally consistent with previous data Groundwater elevation increased approximately 5 06 feet from the previous sampling event (November 1996) The SVE system is operating efficiently to reduce petroleum hydrocarbon impact to sod and groundwater A total of 623 pounds of TPHG was removed during the first quarter of 1997 A cumulative amount of 22,894 pounds of TPHG has been removed since commencing SVE activities (see Table 4) Figures 4 and 5 present the rate of TPHG concentration over time and the cumulative mass and extraction rate of TPHG removed over time, respectively A small increase in the concentration of TPHG and benzene was observed during this sampling event probably due to air sparging Air sparging was initiated on September 5, 1996, and continued through first quarter 1997 Air sparge flow rates and dissolved oxygen (DO) concentrations are presented in Table 5 EMCON will continue to monitor the effects of air sparging during the second quarter 1997 Air sparging has resulted in an increase in influent TPHG and benzene concentrations The DO readings were measured in the groundwater monitoring wells DO isoconcentration map is depicted on Figure 6 Based on the data, DO concentrations have increased near the sparge points With declining influent vapor concentrations, the mass of hydrocarbons removed has declined below 10 pounds per day Concurrently, the air sparge system has been increasing DO concentrations in the groundwater and petroleum hydrocarbons in groundwater have declined Therefore, EMCON • recommends shutting the system off to evaluate natural biodegradation of the site and evaluate possible risk-based closure ATTACHMENTS • Table 1 Groundwater Monitoring Data • Table 2 Operational Uptime Information • Table 3 Flow Rates and Results of Air Sample Analyses • Table 4 Extraction and Emission Rates • Table 5 Air Sparge Flow Rates and Dissolved Oxygen Levels • Figure 1 Site Location Map • Figure 2 Generalized Site Plan • Figure 3 Groundwater Contour Map (March 24, 1997) • Figure 4 TPHG Concentration vs Time • Figure 5 Cumulative Mass and Extraction Rate of TPHG Removed vs Time • Figure 6 Dissolved Oxygen Isoconcentration Map • Appendix A Sampling and Analysis Procedures • Appendix B Cham-of-Custody Records and Laboratory Analysis Records • Appendix C Feld Data Sheets cc Elizabeth Thayer, California Regional Water Quality Control Board Ben Singh, San Joaquin Unified Air Pollution Control District SAC\N\PJO 080SA8051140 8AS 97prc 1