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Legend <br /> Fault;arrows show <br /> relative sense of — <br /> vertical offset <br /> �- Water Table Eastern GSA <br /> w <br /> i f T 1 1 7 Sewage treatment �" ✓ --- <br /> 1 tlntfaf�CiS r, Pond Water <br /> `+ supply <br /> ®� � -+i '°"Shallowagmfer i 'a`�'� <br /> /►`��e Regional <br /> _ a ow .00" atfuifer ; <br /> a uif <br /> er / <br /> Low permeability <br /> sedimentary rock <br /> Not to scale <br /> M-S3R.92-0925j <br /> Figure 4. Conceptual hydrogeologic model of the General Services Area. <br /> and regional aquifer beneath the GSA, creating soil and ground water in relatively high concen- <br /> ground water VOC plumes. As shown in Figure trations(up to 360,000 parts per billion [ppb]in <br /> 5,a shallow ground water VOC plume originates soil and 240,000 ppb in ground water). Thir- <br /> near the buildings in central GSA and extends teen other VOCs have been less frequently de- <br /> about 200 ft to the east-southeast from the dry tected at significantly lower concentrations in soil <br /> well source area. TCE has also been detected in and ground water. <br /> ground water in the regional aquifer west of the <br /> Site 300 sewage treatment pond (Fig. 5). TCE Ongoing Remedial Actions <br /> concentrations have generally been <br /> decreasing in the regional aquifer in the central LLNL has implemented CERCLA Removal <br /> GSA since 1990. Actions to remediate VOCs in soil and ground <br /> In the eastern GSA, TCE in ground water Fater in the central GSA and in ground water in <br /> extends eastward from the debris burial trench the eastern GSA. To date, approximately 25 <br /> area and has migrated northward in the Corral pounds of VOCs have been extracted and treated. <br /> Hollow shallow water-bearing zone. The plume Since April 1993, a ground water extraction <br /> extends approximately 2,300 feet from the and treatment system has been in operation in <br /> debris burial trench release site in this shallow the central GSA at the former Building 875 dry <br /> aquifer(Fig. 6). TCE has also been detected in well area. Over 270,000 gallons of ground wa- <br /> ter have been extracted and treated. Since ground <br /> ground water in the regional aquifer in the vi- <br /> cinity of the debris burial trenches. For the most water remediation activities were initiated,TCE <br /> part, TCE in the regional aquifer is limited to concentrations in ground water samples from <br /> portions of the regional aquifer which underlie monitor wells in the dry well area have decreased <br /> the shallow water bearing zone that contains from a historical maximum concentration of <br /> VOCs. 240,000 ppb to a maximum concentration of only <br /> Dissolved and undissolved TICE has been 10,000 ppb by third quarter 1994. <br /> identified as the primary chemical of concern Following dewatering of the shallow water- <br /> bearing zone at the dry well area through ground <br /> because it has been most frequently detected in <br /> 5 <br />