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Former Chevron Service Station 9-0557 Page 2 <br /> 139 South Center Street April 29, 2009 <br /> Stockton, California <br /> • Cross sections showing subsurface geological features, depth to groundwater, man-made <br /> conduits, monitoring well construction and the interpreted vertical extent of soil and <br /> groundwater contamination; <br /> • Cross-plots of key chemical concentrations verses time for representative wells that <br /> demonstrate significant contaminant concentration trends; <br /> • Summary tables of contaminant concentrations in the different media; <br /> • Rose diagram showing directions of dominant groundwater flow; <br /> • Well and boring logs (including construction/screening); and <br /> • Narrative description of the SCM that describes controls on contaminant distribution; <br /> contaminant migration mechanisms, pathways and rates; plume disposition over time; <br /> and sorbed and dissolved contaminant masses. <br /> While not necessarily showing all data in diagrams and drawings, an adequate SCM can be <br /> utilized to account for most of the known data, and to evaluate the fate of impacted groundwater <br /> with .an estimate of when background conditions will be restored. All analytical data should be <br /> included in tables. Residual contaminant concentrations should be compared to Environmental <br /> Screening Levels (ESL) and California Human Health Screening Levels (CHHSLs) established ` <br /> by California Environmental Protection Agency San Francisco Bay Regional Water Quality <br /> Control Board and an evaluation of the potential for vapor intrusion inside future onsite structures <br /> should be included. <br /> The SCM,should be updated as new data are acquired that cause a change of interpretation or <br /> expands the model until model modifications are not needed to explain subsequently acquired <br /> data. As the SCM becomes complete, it .can be utilized to reasonably predict additional j <br /> investigation results and plume response to various remedial technologies or to changes in <br /> groundwater flow regime. The model should adequately demonstrate all interpretations and <br /> opinions. <br /> Submit the SCM to the EHD by July 30, 2009. Please contact Vicki McCartney at (209) 468- j <br /> 9852, or by email at vmccartney@sjcehd.com if you have any questions or concerns regarding <br /> this letter. <br /> Sincerely, <br /> Victoria L. McCartney,REHS Nuel C. Henderson, Jr.,PG <br /> Senior Registered Environmental Health Specialist Engineering Geologist <br /> c: Mr. David M. Evans, ARCADIS, 950 Glen Drive, Suite 125,Folsom, California 95630 <br /> Mr. James L.L. Barton, PG, California Regional Water Quality Control Board Central <br /> Valley Region, 11020 Sun Center Drive, Suite 200, Rancho Cordova, California 95670 <br /> Mr. Mike Wallace, Senior Real Property yAgent, Ci hof Stockton Office of Economic <br /> Development - Redevelopment Department, Real Property Section, 425 North El Dorado <br /> Street,Third Floor, Stockton, California, 95202 <br /> Greyhound Bus Lines;Post Office Box 660362,Dallas, Texas 75266-0362 <br />