SITE INFORMATION AND CORRESPONDENCE

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FE&-1$-99 THU 09:56 AM P. 13 FEB. -I1' 99(THU) 15: 19 UNOCAL sow,T NORTH R£ TEL:510 94` 781 P. 012 DRAFT respectively,as dernonstrated by the monitoring rte dts of the past ye=r. Reported concentrarlons ofTPH g and ane in groundwazer flus: Lim iuvieity; well (NNW-4) immedinely down-gradient of MW-2,have ranged from 54 to 460 ppb and"non-dciece to 2.0 ppb. re WoWvaW (32 sampling eve=over nixie years) and eoncenuz6ons in wells Poviphwnf to the faster pump island have been"host detect" for at least tate past six quarterly mosritormg events. Similarly, =mceatrativns of toluene, ethylbeazcme, mid xylene in groundwwar samples from well MW-2 have historically ranged from 25,000 to 92,000 ppb, 2,700-to 9.900 ppb, and 14,000 to 37,400 ppb, resile dvd,y. Haweves, 1Ai1MLtinsfions of mese consfitmts are less tban MQ,s(generally"non detect" to less than 10 ppb)in growkiwattir f om all other site monitor wells. Tims the data fly show the plume is stable, concemratioes of residual peanlaum lrydror rbaas is groundwater are decreasing,and that residua,amceatra#ions is clews of Dam Piaa objectiim ase Ifinited to less tium 60 feet off-sate with respect to toluew, othylbenzene,and xylem.about 70 feet with respect to banmmw, and an eati maW 150 Scat with regard to TPH-S Nas:iral attetasatioa includes physiwl, ctxcal and 6ialogia processes. Biologic processes are the most import=for reducing sbe naris of residual petroleum hydrocarbons and occ%=under bath aerobic and waerobic conditivas. The primary evidence showing that natural attetwation is o=ming at the site is the demon dmonstrating that the p,saumio is stable and conC=ntratlons of&ssolved phase hydrocwbons are decreasing. Secondary euidem deasonstratiag that biormediafion is occurring are gmehemicW mdicaralrs. Analyses of gornidwater splen w lecred ftvm site wells in March 1998 show that concenumdons of.reduced iron(ferrous)are greater,and ditrate and sulfate couc entmfions arc sigttfcantly less, in groundwater samplers from wells widim the plume when compared to samples from."backgrcwd" wells. For example, groundwater samples from up-gradlem and crass-gradient wells have ferrous iron concentrations of less that,0.1 ppm while groundwater within the plume hive reported cor=tratiora ranging fr ern 0.5 to 3.4 ppm. Up-gm&cm nitrate and sulfate COUCeIM atiow are about 70 ppm and 170 Ppm respectively whsle concentrations of these constituents at Lhe core of the plume are"cost dared"and in down-gradient groundwater• =One:=MPW-colli imt Aug= 1992,had a itperied TPH-g amee im..ion of Lim ppb.