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5157 WEST BUSINESS LOOP 205,TRACY <br /> PAGE 2 <br /> s <br /> defined. The decrease in the <br /> The lateral extent of the contamination in groundwater has not been def <br /> TPH-g concentration in MW-2 and the increase in MW-5 may s6ggest;6gration of the plume in a <br /> direction generally matching the northward flow of the groundwater. The well survey included in <br /> Quarterly Report-Second Quarter 2000 dated August 9, 2000 prepared by AGE identified the <br /> following domestic wells in the vicinity: it <br /> 5055 West Business Loop 205 (200 feet east) ;R <br /> 5157 West Business Loop 205 (230 feet west) <br /> 5225 West Business Loop 205 (250 feet north) <br /> i 5150 Lovely Road (400 feet south) <br /> { and an irrigation well at: ii <br /> 5225 West Business Loop 205 (600 feet north)at 734 feet total depth. <br /> The boring logs (MW-5, MW-6, B-7) included in the same report reported mostly clay to 30 feet <br /> below surface grade (bsg)with intermittent sand lenses between 10 feet to 20 feet bsg. <br /> PHS/EHD agrees with AGE's recommendation to install additional monitoring wells north and east <br /> of MW-6 to define the plume. PHS/EHD does not agree to delaying the installation for another <br /> quarter of sampling and observation, due to the close proximitykof existing domestic wells and the <br /> existence of a sand lens on top of clay. Therefore, you are required to define the plume and <br /> introduce measures to control the plume migration as soon as possible. Submit a work plan that <br /> will define the plume and control its migration to PHS/EHD by February 28, 2001. Also, sample <br /> i and analyze the domestic wells at 5055, 5157, and 5225 West Business Loop 205 in the next <br /> sampling event. ,w <br /> Direction of groundwater flow has been interpreted by AGE to range from approximately N5E to <br /> N35W, and the gradient to range from 0.003 to 0.01. However;application of the three-point <br /> problem graphical solution to MW-1, MW-3, and MW-4 yields afrelati�ely consistent direction of <br /> { groundwater flow at about N1 5W and gradient at around 0.01. <.'iThis would suggest that the <br /> I direction of plume migration should be toward MW-3 and awayfrom MW-6. Instead, the data <br /> indicate the plume is migrating in a northeast direction. Therefore,AGE needs to develop a site <br /> I conceptual model that includes an explanation as to why the plume does not appear to be <br /> r' <br /> migrating in the direction that would be expected based on calculated groundwater flow. <br /> The site conceptual model should include a narrative and graphical description of site <br /> characteristics that provides a foundation for understanding of the site. This should include: <br /> 1 sources of contamination, affected media (soil, air and water), routes of contaminant migration, <br /> 1 and contaminant receptors, as well as general physical conditions at the site that influence <br /> contaminant transport and receptor exposure. <br /> Development of a conceptual model is an iterative process and should not be left until the end of <br /> the project. The model should be developed early in the project and then refined as new data <br /> become available. The final conceptual model should provide enough information to make <br /> appropriate regulatory decisions and to justify the chosen preferred remedial alternative. <br /> PHS/EHD requests the site conceptual model be included in the next quarterly monitoring report. <br /> IJ <br /> If you have any questions, please contact Jeff Wong at (209)468-0335. <br /> Donna Heran, REHS, Director <br /> Environmental Health Division t€ <br /> C VL ,t <br /> Jeffrey�ngg EH� Dot Lofstorrr <br /> LOP/Site Mitigation Unit IV Registered Geologist, Unit IV <br /> c: Advanced GeoEnvironmental Inc.—Brien Millman <br /> c: RWQCB, Central Valley Region— Marty Hartzell <br /> ,i <br /> i <br />