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Groundwater Quality Monitoring - 152 East 11 th Street, Tracy Page 6 above the water table during the dry season Such seasonal fluctuations are commonly is observed at sites affected by releases of hydrocarbon fuels Examination of the data compilation presented in Table 4 shows that, while there have been seasonal variations in the concentrations of components of hydrocarbon fuels in the groundwater, there has been a significant net reduction in the concentration of contaminants in the groundwater over the period since contamination of the subsurface at this site was first discovered This reduction is attributed to natural remediation by bacterial digestion of hydrocarbons (bioremediation) that converts these materials into carbon dioxide and water Natural bioremediation of components of fuel hydrocarbons in soil and groundwater at sites where leakage of fuels into the subsurface has been eliminated (as is the case at the subject site following the removal of the underground storage tanks in April 1984) is commonly observed RECOMMENDATIONS FOR FUTURE GROUNDWATER QUALITY MONITORING The round of sampling performed on February 25th 1994 completed a full cycle of groundwater quality monitoring that has included sampling of all wells installed for both the 152 and 153 East I lth street sites to establish a comprehensive data base, observation of seasonal variations in groundwater elevations, flow directions and gradients and, observation of seasonal fluctuations in the concentrations of hydrocarbon components in the groundwater as well as the long term trends in these data The results of the program have satisfied each of the objectives established by the Groundwater Sampling Plan - Pr 152 EasLljth. Tracy, California . As noted previously, analysis of the substantial body of groundwater-quality data now available for the subject site 1 ) confirms the interpretation shown on Figure 2 that indicates the extent to which the subsurface has been affected by hydrocarbons released at the 152 East Street site, 2 ) shows that seasonal variations in groundwater table elevations are typically less than 1 ft and seasonal variations in the groundwater flow direction and gradient are small, 3 ) indicates that, while there are minor seasonal fluctuations in the concentrations of fuel hydrocarbon components in groundwater beneath the affected areas of the site, over time, these concentrations have decreased significantly Together with other geotechnical data obtained for the site, the groundwater-duality database that has been established is sufficient to permit development of a corrective action plan for the site, including selection of cost-effective technologies and evaluation of principal design parameters However, as a conservative measure to check for unexpected changes in the distribution or concentration of contaminants, it is recommended that groundwater quality continue to be periodically monitored until the site can be released from regulatory oversight Because the concentrations of hydrocarbon fuels in groundwater have been observed to be slightly elevated during the wet season compared to the dry season, an annual round of sampling should be conducted in February To permit monitoring of any changing conditions within the interior of the 152 East Street