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z 1 k' r 'Mo-W- :s J.N11moi/1 AlIF l'14'r 'fir 3D MarctLjg�4._-'1St?i.��11>14x�..��Lti An investicjaLion was conducted by WA between 12 and 15 March to determine 3 whether the 13-inch thick concrete slab in the bottom of the excavation could be left in place. Eleven soil samples were collected from 0.5 fL to 1 ft below the slab. Four samples were analyzed for total petroleum t1 centration of hydrocarbons by Canonic Environmental (CC). The highest con TPII was detected at 55 ppm in sample SL-D collected at 0.5 ft below the trio slab in the southwestern corner of the excavation. Analytic results are presented in Table ,, and sample locations are presented in Figure 4. 14,- A groundwater elevation contour map for depth to-„pater measurements on 2 March 1988 indicated that the estimated direction of groundwater flow was to the north, Which was contradictory to the northwestward direction 1�xi,s determined on 15 October 1987 (Figure 5). The variation was attributed pz� to seasonal changes in groundwater recharge conditions. S An additional downgradient well MW-4, to be located on the north side of the excavation, was recommended for installation. Un ut Locs1� 1.11i1�i�tLi� Jg May 19Q�_liardio_fhlil IRK A request was made by WA to the San Joaquin Local Health District (SJLHi)) to aerate 1000 cubic yards of pea gravel and soil removed from the tank excavation on site. Samples of the pea gravel PG-East and PG..We:t had been collected on 25 August 1987, and samples TP-B-2A and TP-B-3A hl� --aen collected on 23 January 1988, prior to the removal of the pea gravel from r the excavation pit. One composite of the stockpiled native soil had also been collected on 14 March 1988. The analytic results of these previously r collected samples indicated that an average of between 100 ppm and 500 ppm of hydrocarbons remained in the stockpiled material . 10 June 19P - QQrdQD Boggs, Cal j[grnia Eggional Water Qual itY COOL 1 ' - CenLrjal �99jgn zT A request was made by WA to the California Regional Water Qu lity Control Board - Central Valley Region (RWQCB-CVR) to leave the 13-inch thick concrete slab in the excavation based on the data obtained from past (U% sampling of soil beneath the slab, soil and groundwater sampling related to installation of the three monitoring wel :,, and soil sampling during excavation of the tank pit. The basic argument presented by WA suggester that hydrocarbons, which leaked from the tanks, had migrated laterally and �_ downward away from the edges of the slab and are contained in soil adjacent to the southwestern corner of the excavation. 07611 nta9 l —;Iiibi 11100! mei!