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4.0 . CONCLUSIONS, RECOMMENDATIONS AND PROJECT STATUS Based on the results of the investigation, most of the mass of petroleum hydrocarbons in sod and groundwater appears to be concentrated beneath the former UST system The petroleum hydrocarbons detected during this and previous investigations mostly consist of TPH-as- kerosene, TPH-as-gasoline, and BTEX compounds Concentrations of 1,2-DCA above water quality goals have also been detected in groundwater Concentrations of MTBE and other oxygenate compounds have been detected in groundwater, but below water quality goals Between December 11, 2003 and March 24, 2004, the groundwater surface rose an average of 1 09 feet, the groundwater gradient decreased slightly from 0 0238 to 0 0229 foottfoot, and the groundwater flow direction remained constant toward the east-southeast These groundwater surface characteristics are consistent with historical trends ' Between December 11, 2003 and March 24, 2004, the concentrations of petroleum hydrocarbons in groundwater beneath the former UST system remained relatively stable Since the monitoring wells were installed in July 2002, the concentrations of petroleum hydrocarbons in groundwater have generally declined, probably due to prior source removal efforts and on-going natural attenuation processes Based on the groundwater analytical results from the samples collected from the three on-site soil borings, the lateral extent of TPH-as-kerosene, TPH-as-gasoline and benzene appears to be defined to within the inside of the parking garage The concentrations of gasoline compounds detected in the groundwater samples collected from G-4 and G-5 were all below California Maximum Contaminant Levels (MCL) The lateral extent of impacted groundwater to the west of the former UST system remains undefined beneath the Hunter Street right-of-way The lateral distributions of TPH-as-kerosene, TPH-as-gasoline and benzene in groundwater are shown on isoconcentration maps (Figures 5, 6 and 7) The vertical extent of impacted soil and groundwater has not been investigated The results of the soil and groundwater samples collected from the three soil borings suggest that only concentrations of petroleum compounds below water quality goals have migrated from beneath the former UST system to the south and east walls of the parking garage The massive clay and silt soils observed beneath the site may be limiting the lateral migration of petroleum hydrocarbons in soil and groundwater r i 11