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ACE07-106-H Effective: April 00 interconnected. The two aquifers are normally 5 to 12 feet thick and are often not entirely saturated. The A- and B -zones consist mostly of sand -silt mixtures as described in Section 2.2.1 above. The C- and D -zones are most likely contained within the upper Laguna Formation. These zones are comprised of medium to coarse-grained sand, gravel, clayey sand, and silty -gravelly sand deposits usually no more than 10 feet thick. The C- and D -zone aquifers normally consist of more saturated deposits than the A- and B -zone aquifers. Groundwater flow direction in the vicinity of Building 199 is primarily north to northwest. Seasonal fluctuation in groundwater levels is observed in the A-, B-, and C -zones, especially during the summer months when large quantities of groundwater are withdrawn for local agricultural use. The average maximum groundwater fluctuation for the three zones ranges from approximately four to six feet annually. 2.2.3 Previous Investigations Activities The project site is located north of Building 199 in an area known to formerly contain maintenance operations. A 750 -gallon diesel tank (Tank #27) located in the vicinity of Building 180 was removed in 1990 (see Figure 2-3). Previous investigations conducted in the area showed concentrations of diesel, gasoline, and volatile organic compounds (VOCs) in the soil to 20 feet bgs in the vicinity of Tank #27. Based on the conclusions presented in Remediallnvestigation/Feasibility Study at Sharpe Site: Problem Assessment Report, for Underground Storage Tanks (ESE 1992), additional investigation work was warranted to defined soil contamination in the area of Tank #27. A 6,000 -gallon oil/water separator (Tank #26) was formally located on the east side of Building 199 (see Figure 2-3). The oil/water separator and adjacent contaminated soil were removed in 1991. Soil borings in the area of Building 199 completed prior to the removal of the oil/water separator showed elevated ` concentrations of diesel and benzene, ethylbenzene, toluene, and xylene (BTEX) compounds. The area of these detections was over -excavated as part of the Tank #26 removal. Two monitoring wells were installed in the area of Building 199 as a part of previous investigations. Existing monitoring well 439-A is located approximately 200 feet northeast of Building 199 and is used to monitor groundwater quality (A -zone) in the vicinity of a VOC release. The maximum concentration of BTEX constituents detected in groundwater samples collected from the well was 400 micro -grams per liter (µg/L). Monitoring well 472-A was formerly located north of Building 199 (and the recent soil excavation area) to monitor groundwater quality (A -zone) in the vicinity of former Tank #27. A groundwater sample collected from the well in November 1991 showed BTEX at 42 µg/L and trichloroethylene at 140 µg/L. The North Balloon Area Groundwater Treatment System is located northeast of Building 199 (see Figure 2-2). Based on existing documentation, the area north of Building 199 is within the capture zone of this groundwater collection and treatment system (ESE, 1992). In August 1999, apparent contaminated soil was encountered north of Building 199 during RCP storm drain installation and paving activities being performed by others. It was believed that the contamination may have been attributed to former site operations (including vehicle maintenance) and spillage or leakage from former fuel tanks in the area. On August 17, 1999, three soil samples (DPI 60, DPI 61 and DPI 62) were collected by IT to assess the type of contaminants present at the site (see Figure 2-3). Samples were collected at approximately one C ft\Apr00\PCRBLDG199.doc 2-2