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1 ' Southland/Stockton 6 March 1991 1 WELL MONITORING AND SAMPLING RESULTS OVERVIEW Monitoring and sampling of monitoring wells and domestic wells has been ongoing at the site ' during both assessment and remediation activities A review of prior reports shows that the water elevation at the site has decreased from about: 20-feet below mean sea level (msl) in 1986 to about ' 35-feet below msl by November 1990 As a result, many of the monitoring wells have gone dry and have not been useable for groundwater monitoring and sampling throughout the duration of the project Monitoring wells which were not dry were sampled monthly until July 1990, after which time the monitoring well sampling frequency was reduced to quarterly Piezometric surface maps are provided showing the general groundwater gradient direction and magnitude on approximately a biannual basis from July 1986 through October 1990 (Appendix A) A review of the dissolved plume maps prepared from monitoring well sampling data shows that the lateral extent of the dissolved plume has reduced significantly since work was initiated at the site in �. 1986 Plume maps depicting TPH-as-gasoline concentrations are provided which show the extent of the plume on approximately a bi-yearly basis from July 1986 through October 1990 (Appendix B) The 1986 I results are shown In terms of parts per million (ppm), while results from 1987 to the present are in parts per billion (ppb) Additionally, a review of the total petroleum hydrocarbon analyses results for each Individual monitoring well shows that the magnitude of dissolved hydrocarbon concentrations in the water samples have also decreased significantly The concentrations have decreased to the point where the highest TPH-as-gasoline concentration reported for October 1990 was 50 ppb in the samples from MW-1 (MW-1 had previously been shown to contain separate-phase hydrocarbons) Analytical results for recent groundwater samples reported that benzene, toluene, ethylbenzene and xylenes are generally non detectable with the exception of samples from MW-17B which contained concentrations of 0 8 ppb of toluene (which is well below the State action level of 100 ppb) Summary tables of the TPH-as- gasoline concentrations for each well for the life of the project are provided in Appendix C All of the domestic wells shown on Figure 3 were sampled at least once between June and August 1986 Between August 1986 and January 1990, domestic well samples were taken at different times from all wells except 1701, 1741, 9155, 9147, 9177 and 9179 which were not sampled The Ifollowing seven domestic wells have been sampled monthly since February 1990 TMG, 1912, 1927, w GROUNDWATER ITECHNOLOGN, INC