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2015 Annual Correctie Action Evaluation Report ® Austin Road Unit, Forward Landfill — Manteca California Paee 3 thickness and porosity. (For example, the average TVOC concentration between the 25 ug/L and 50 ug/L contours would be 37.5 ug/L,.) Consistent with the Evaluation Procedures Report, the aquifer thickness was assumed to be 40 feet and the aquifer porosity was assumed to be 0.25. n ® The TVOC masses corresponding to all contour pairs on a given quarter's isoconcentra.tion map were then summed to estimate the total, plume -wide TVOC mass for that quarter. Tables 6 through 9 summarize the TVOC mass estimation calculations for the past four quarters. The historical TVOC masses for the facility through second quarter 2015 are summarized on Table 10. A historical database summary and time series plot for TVOC mass are presented in Appendix A. The TVOC mass for each quarter was compared to the TVOC mass TL of 789.5 kg. The calculated TVOC mass did not exceed the TL during the third quarter 2014, fourth quarter 2014, first quarter 2015, or second quarter 2015. 2.0 TVOC Mass Trend The estimated TVOC masses for the past four quarters were reviewed for increasing trends. TVOC mass has not increased for four consecutive quarters at any time in the past year. 3.0 Individual Well TVOC Concentrations The TVOC concentrations observed at individual wells for the past four quarters are presented on Tables 2 through 5 and summarized on Table 11. A historical database summary and time series plots for TVOC concentrations are presented in Appendix B. The TVOC concentrations for each quarter were compared to the established TVOC concentration TLs. There was only one exceedance of an individual well TL during the third quarter 2014, fourth quarter 2014, first quarter 2015, or second quarter 2015. The third quarter (August) 2014 TVOC concentration at deep domestic supply well 8106-A (76.4 ug/L) exhibited an exceedance of its TL of 22.94 ug/L. This exceedance was due primarily to detections of 1,2-dichloropropane (26 ug/L) and diisopropyl ether (44 ug/L), which were not confirmed in the subsequent fourth quarter (November) 2014 monitoring event. 4.0 Condition of Landfill Cover Title 27 of the California Code of Regulations (CCR) § 20950(a)(2)(AX 1) specifies that "[fJor landfills and for waste piles and surface impoundments closed as landfills, the goal of closure, including but not limited to the installation of a final cover, is to minimize the infiltration of water into the waste, thereby minimizing the production of leachate and gas." The goal is to minimize infiltration of water rather than eliminate the infiltration of water. The Austin Road Landfill reported that the facility's cover performed as designed from the third quarter 2014 through second quarter 2015 in that it minimized the infiltration of water into the waste. There were no significant problems with landfill cover that resulted in exposed waste from the third quarter 2014 through second quarter 2015. Minor erosion issues were addressed by placement and compaction of soil material. There were reportedly no leachate seeps identified from the third quarter 2014 through )�HRST & ASSOCIATES, INC:`