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.i' t,d TECHNICAL APPROACH - Cleanup Technology Review Cleanup actions of further soil excavation, and soil and groundwater extraction and treatment do not appear to be cost effective for this site's contaminant composition, n state guidance. Active extraction/injection technology is. distribution and pending g J not feasible given the difficulty of construction'in the street. Since contaminants are present only on the capillary fringe near the former tank pit, potential contaminant mass removal is not justified by costly clean-up effort In addition, the City of Tracy has indicated that excavation under the street is impractical at any time in the near future. In our opinion, the contaminants under the street are confined to the capillary fringe near the former tank pit and dissolved contaminants as revealed by the previous studies. An excavation (into the street) of this magnitude is, in our opinion, not warranted by the observed data. Given the degraded Benzene, Toluene, Ethylbenzene, and total Xylenes (BTEX) volatile component chemistry of the dissolved plume, Wright feels that natural biologic action will continue to degrade the : gasoline; however, continued groundwater ` monitoring is warranted and the existing groundwater monitoring array should be expanded for this purpose. Discussions with the PHS-EHD representative indicate that the groundwater plume has not been defined by the work presented in the PAR. PHS-EHD requires a monitoring well at the downgradient plume boundary to consider the plume boundary complete. NATURAL ATTENUATION- Monitoring Well and Monitoring Approach Based on telephone conversations with PHS-EHD, and a subsequent letter date stamped February 3, 1997, a minimum of one additional monitoring well will be installed at a downgradient location. The new and existing wells will be monitored at quarterly intervals to meet the requirements of the PHS-EHD for underground tank petroleum releases. The data collected in this periodic monitoring program can be used to track hydrocarbons in groundwater. This well will be added to the monitoring array and used for bio-attenuation monitoring of containments. The PHS-EHD indicated that methods for data collection to estimate the rate of contaminant plume stabilization or bio-attenuation should be proposed for this workplan. Wright recommends biologic parameters be added to the monitoring program to estimate biologic activity [i.e., dissolved oxygen, oxidation reduction potential, pH, conductivity, temperature, alkalinity, nitrate, ferrous iron, sulfate and biologic screening (for assumed psuedomonas genera); Wiedemeier and others, 19951. This will,establish baseline site conditions regarding biologic activity, and gathers data for determining indigenous biodegradation conditions for natural attenuation. Wright also recommends permeability testing of soil core samples from within each of the two prominent aquifer lithofacies identified in previous work. Along with continued quarterly groundwater monitoring data, the results can be used to estimate groundwater flow rates and the rate of containment plume stabilization.