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I 2255 Airport,Stockton - ZE Auto August 2004,Page 2 look into technologies(such as cone penetrometer testing)-that will help characterize the geology and have the ability to advance to deeper depths in order to define the vertical extent of the contamination. Due to the high concentrations of contamination in groundwater and in the soil,interim remediation to protect possible impact to local well receptors and or prevent further degradation of groundwater by source removal in the vadose zone should be considered. To this end,EHD requests the next quarterly report'or investigation report to include a site conceptual model. EHD has listed below the components generally used in an SCM. Simply stated,the purposes for an SCM are to demonstrate where the contaminants came from,where they are at the present time,how they move through the subsurface,how they will respond to changes in the ground water flow characteristics or to potential remediation efforts,what the contaminants'ultimate environmental fate will be,and to help evaluate the risk posed by the contaminants to ground water supplies and sensitive receptors.The components of an effective SCM commonly include: • Local and regional plan view maps with sources,boring and monitoring well locations,lines of cross section,extent of contaminants in each media,direction and rate of ground water flow,and receptor locations,including water supply wells within 2,000 feet of the site. • Cross sections showing subsurface geological features,depth to ground water,man-made conduits, monitoring well construction and an interpretative drawing of the vertical extent of soil and ground water contamination. • Cross-plots of key chemical concentrations vs. time for representative wells that demonstrate significant contaminant concentration trends. • Summary tables of contaminant concentrations in the different media. • Well and boring logs. • A narrative description of the SCNI that describes controls on contaminant distribution;contaminant migration mechanisms pathways and rates;plume disposition over time;and sorbed and dissolved $r' , 5 P P P . contaminant masses. While not necessarily showing all data in diagrams and drawings,an adequate SCINI can be utilized to account for most of the known data and to illustrate additional assessment needs.All analytical data should be included in tables.Methods and calculations of contaminant masses should be included in the SC\I„and may be submitted as an appendix.The site conceptual model should be updated each time new data is acquired that causes a change of interpretation or expands the model until model modifications are not needed to explain subsequently acquired data.As the SCM becomes complete,it can be utilized to reasonably predict additional investigation results and plume response to various remedial technologies or to changes in ground water flow regime.The model should adequately demonstrate all interpretations and opinions. If you have any questions contact Jeffrey Wong at(209)468-0335. 1 p, Donna Heran,REHS,Director Environmental Health Division Jeffrey Wong,Senior REHS s ;\Margaret agorio,REHS V LOP/Site Mitigation Unit Supervisor c: CRWQCB,Central Valley Region—James L.L.Barton,RG c: .Horizon Environmental,Inc.--Rexford K.Smith,RG&Karen P.Liptak