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- - _,. _. -^ - - ^ - - - - - -- - (^ - - - ^ - - - - 1 510 685 91 GROUNDWRTER TECH 1kx +/92 :;15: 15 003 � II ii FI Mr. Steve Schneider SJCP HS , October S. 19W I Page 2 1 S. ProR2sW_.lindhrldual TACM Crtterla for STEX 1 You requested that individual target criteria be proposed for benzene,toluene, ethylbernzene, and total xylenes in the stockpiled soil. In addition, you asked that these criteria be based on the guidelines posited by Jon Marshack of the Califormia Regional Water Quality Control Board, Central Valley Region, i in his paper entitled: Wanated LWI yL odoloay for Waste Classification and ClWdup,Level I Determination. dated October 1986. Therefore, we have revised our proposed target criteria for BTEX. ,j The revised target criteria for individual constituents is shown in the following_table.JThese target criteria l r<are'considered quite conservative, and therefore mare than adequate to meet SJl'HS and RwOC�tank concerns, because no environmental attenuation has been assumed, and groundwater beneath th i excavation is already Impacted above the water quality goals 'j TARGET CRITERIA FOR SOIL TO BE RETURNED TO THE TANK EXCAVATION. CONSTITUENT WATER QUALITY LEACHABILITY TARdET CRITERIA GOAL FACTOR IN SOIL BENZENE 0.7 ug/1 10 0.007 mg/kg j TOLUENE 100 Ug/I 10 _1.0:mg/kg i i ETHYLBENZENE 680 ug/I 10 .6.13 mg/kg i TOTAL XYLENES 1.750 ug/I 10 17.5, mg/kg 4. Placement of a Btiffer Zone in the You requested that other soil stockpiled on the Manna Pf'o s—a-irom receht excavations on the northem:5 property boundary be used as a "buffer zone" between stockpiled soil returned to the tank excavation i and the highest groundwater observed at the site. We have found that the depth to groundwater } beneath the site typically is approximately 40 feet below grade. According to SJCPHS; during a recent, flood season, groundwater was observed as high as 30-35 feet below grade. To establish a reasonable buffer zone to prevent tank pit soils from.contacting high groundwater, we propose the following: i Before any stockpiled soil meeting target criteria Is placed in the excavatlonj a 7buffer =one' of clean fill from the excavation at the northern end of the site will be placed in the bottom of the excavation to a depth of 10 feet below grade. Notwithstanding the above, i stockpiled soil represented by composite samples showing Concentrations of petroleum hydrocarbons below laboratory detection 'levels may, in lieu of soil from the northern bour]dary(or alternative sources), be used to fill tteexcavation..to..10 feet.below rade. Manna Pro, however,will not lee-required-to abtaifaddctional soil from aftemative 0ff-si3e sources If there is not enough soil from the northern boundary or from the stockpile to' I'I fill the excavation up to 10 feet below grade (Manna Pro is informed that at least.,70Qj 1 cublc yards of soil from the northern boundary currently exlst.on the site)! P is Pl L GROUNDWATER . . . ...r ........_.._- .e