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FEE-18-99 THU 09;58 AM P, 14 <br /> FEB. -i V 99(THU) 15: 19 UNOCAL %%WT NORTH RE TEL:510 94ti/781 P. 413 <br /> DRAFT <br /> nitraie concentrations are"non dctwf- While sulfate concentrations see CWy 10 to 20 ppm. <br /> Thus, avail[Wc data provide evidence that the dmolved phase hydrocarbon pfulft in site <br /> grourldwaur is attcnuating and biodegrvAng via itanu, Sufte and ntrrm reductiam <br /> In considering possible knpacxs to oyer=and fucwro beneftial uses of <br /> groundwater,three factors are relevant: l)acconi ng to Department of WaW Re9DLtfCeS <br /> (DV.%)Bulletin 146 (1967) nerd petitioner's Myrrh 19911 sample analyses, shallow <br /> groundwater in the vidniry of the site is of'infariar quality,"2)the goM1undwaters <br /> monitored by site wells are nat in direct hydrz&c communication With deqm <br /> groundwa=aquifers or local supply wcU acid 3)as discussed 4xwe,the plume is stable <br /> and am1- 1atradons are decreasing as a consequence of natural aErnuation. A review of <br /> DWR Well Drillers' Reports for water supply wells located m the vicmxty of the sift show <br /> that these wells arc ronstmeted with saautary seals which rend to depths of 150 to 200 <br /> feet below ground surface. The purpose of this method of vm l consu ctian is to exdude <br /> Poor quality shallow grmmitwater(which generally,contains elevated concentrations of <br /> nitrate chloride, and TDS). The presence of this water in the V wuw Formation is a <br /> comegruence of the easterly mer mon of poor gena*water underlag the Deka ad the <br /> Stockton Deep Water Ship Channel aver'the past 5o years or so. An analysis of aver <br /> lewd elevation daM for site wdls also shows an absence of the seasonal moons one <br /> would erpect if the groimdwawr motdtored was m &ma hydraulic coamnmieation with <br /> local production wells_ Hance,the localized volume of affix=ed water is not=n=tly <br /> being used and, due to its InfmiEor quality,is not likely to be used in the>uoa-& <br /> Furthermore, histone gfixm water monitoring dats, in a clition to showing plurne xa6Flity <br /> and demising constituent concanrstions,shows that the poor quality of the shallow <br /> groundwater enbuces the on-going bioretnedbwon, consequently reducing the residual <br /> petroleum hydrocarbon mass in the subsurface_ Because constituent concean-aitions will <br /> continue to decline as the arse of resi4uW petroleum hydro=rbom deaease over fimS <br /> rordoued monitoring of site groundwater would serve no useful purpose. <br /> The only way ro ensure complete removal of lingering, residuak,detectable <br /> concentrations of petroicum c onsdzuents in sire groundwater r would be to excavate an <br /> additional several thousand cubic yards of zffeeted sail to depths in excess of 5o Rmi. This <br /> ' le <br />