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Wm. J. Hunter & Associates 2220 Loma Vista Dr Registered Geologists Sacramento, CA 95825 �} Petroleum & Mineral Appraisers (916) 972-7941 irF (916) 972-1683 � April 4, 1997 APR 1 a 1997 Steven Sasson, Sr REHS E14V'11,1CI,,�;,�ENTAL HEALTH San Joaquin County Health Department PE-WiT SERVICES Environmental Health Division P . d Box 388 Stockton, CA 95201-0388 Ref Thirteenth Quarterly Groundwater Monitoring Report for 814 E . Charter Way, Stockton; Site Code 1061 We are enclosing a copy of the field report, laboratory analyses, and maps showing groundwater gradients and lines of equal concen- trations of benzene, toluene, ethylbenzene, xylene, & TPHg The field work was performed on January 24, 1997, by Del-Tech Geotech- nical Support Services as directed by our office The attached EXHIBIT A, shows the contours on the top of the water table on January 24, 1997 , the gradient continues to be to the east at about 0033 ' /ft ,a EXHIBIT B summarizes the Fourteen (14) hydraulic gradients measured to date on the property Except for the July 1995 sampling, all of the data indicate a rather stable gradient to the east/northeast The reason for the reversal in the potentio- metric surface in July, 1995 cannot be explained based on the data available at the time this report was prepared We have not received any data concerning activities at the Darpetro property since January, 1995 . EXHIBIT C shows the approximate contours based on lines of equal concentration of benzene from the January, 1997 sampling event MW-1 continues to below detection limits ; ,the only event that revealed any benzene was in February, 1994 Levels in MW-2 dropped significantly from 532 ug/L in July, 1996 , to 45 ug/L in January, 1997 . Levels of benzene measured during this latest event, 36 ug/L, are the highest detected since sampling began PLATES D, E, & F show lines of equal concentration of toluene, ethylbenzene, & xylene based on the 1/97 sampling All of the maps show an increase in levels of those components to the northwest . MW-3 , the farthest well from the removed UST' s, recorded the highest levels of those three substances since sampling began on the site PLATE G shows lines of equal concentration of THPG from the January, 1997 sampling Levels an MW-1 increased from 118 ug/L in July, 1996 to 199 ug/L in January, 1996 Levels in MW-2 1