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Mr. Philip S. Isore* <br />March 10, 1999 <br />Page 3 <br />detection of a chemical of concern (1.1 gg/L TCE). MW -5R exhibited rising <br />contaminant concentrations in the fall of 1998 with a maximum of 26 gg/L TCE in <br />November and a most recent concentration of 4.3 gg/L in January. Over the last 2.5 <br />years, the dominant gradient direction in this vicinity has been east-southeast. Therefore, <br />Parsons plans to drill four borings at offsite locations to the southeast of MW -5R as <br />shown on Figure 1. Groundwater will be sampled utilizing a HydropunchTM sampler <br />driven through a hollow stem auger. The target sampling depth in each boring will be <br />selected based on lithology and the first encountered groundwater. Analyses will be <br />performed on a rapid turnaround basis so that the investigation program can be modified <br />in the event of anomalous detections. Objectives of this sampling are to establish the <br />maximum extent of offsite chemical migration and develop data to calibrate a <br />groundwater fate and transport model that would simulate natural attenuation (dispersion <br />and adsorption) at the leading edge of the plume. <br />Sampling Offsite of Northeast Corner of AASC Facility. This offsite area was <br />investigated in early 1998 using a CPT rig and punch groundwater sampler (Parsons, <br />1998a). Four offsite wells (MW -11 through MW -14) were constructed in 1998. Three <br />of these wells have been sampled four times and exhibited generally stable concentrations <br />in the range of 10 to 20 µg/L total VOCs in the shallow groundwater. <br />The hypothesis Parsons will investigate is that chemicals were released from the site <br />sewer line and chemical concentrations should decrease with distance from this linear <br />source. Documentation from the City of Stockton indicates that the sewer line drains <br />northeastward from the AASC buildings, follows the shoulder of South Airport Way <br />northward, and joins laterals at Ralph Avenue. Parsons will drill six additional borings at <br />the locations indicated on Figure 2. Two borings will be drilled north of MW -11 in the <br />frontage road, two borings to the east of MW -12 and MW -11 in Michael Street, and two <br />borings in Ralph Avenue. Groundwater will be sampled utilizing a HydropunchTM <br />sampler driven through the hollow stem auger. <br />Contaminant Fate and Transport Modeling. The primary objective of this modeling task <br />will be to determine the site distances at which chemical concentrations are achieved at <br />levels that are protective of groundwater. <br />Conceptual Model - The saturated alluvial materials underlying the Former Hazardous <br />Materials/Waste Storage Area (source area) have been modeled as a four -layer system. A <br />groundwater flow simulation using MODFLOW was developed to evaluate the probable <br />groundwater capture zone for the newly installed extraction wells (Parsons, 1998c). This <br />calibrated flow model will be modified, i.e. the model domain will be expanded and used <br />as the basis for the transport modeling. <br />An evaluation of the fate and transport of 1,1-DCE, 1,1,1 -TCA, and TCE in groundwater <br />will then be performed using a three-dimensional groundwater flow model, MODFLOW <br />(McDonald and Harbaugh 1988), and a transport model (Zheng 1990). The Department <br />of Defense (DoD) Groundwater Modeling System (GMS), which contains these model <br />RESP_RWQ <br />