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r groundwater depths at various dates and for approximating the direction of the dip of the water table (hydraulic gradient) The water table dip is and has been approximately east from the project site , perhaps with a northerly component . This is shown on Figure 3 Figure 5 shows that the water table in the Spring , 1991 was at an elevation of about -50 ' below MSL Because the ground elevation is about +27 ' , the depth to groundwater would be about 77 ' from the surface This is shown on Figure 6 , a depth to groundwater ' map The highest water levels in recent years occurred in the Spring of 1986 , Figure 7 This shows the depth to ' groundwater to be -20 ' below MSL, or -47 ' below ground level The excavation and sampling by JTO in 1988 was at depths of -28 ' and -29 ' below ground level (bgl ) , only about ' 18 ' above the high water level in the vadose zone that j occurred in 1986 This means that any migrating plume of hydrocarbons would not be found above about -47 ' because ' lateral migration is curtailed as a general rule to a 30 to 40 degree angle from the vertical That is , as an example , at a depth of 50 ' bgl , lateral migration might be in the order of 40 ' or so and would not reach test wells located at ' much greater distances , unless carried by groundwater flow. Therefore , sampling and testing in the vadose zone would not be worthwhile above the highest groundwater levels Any contaminants found at shallow depths would have had other sources The sampling program proposed herein is based on this concept ' Figures 8 and 9 are from SOILS OF SAN JOAQUIN COUNTY CALIFORNIA, By Walter Weir Soils in the vicinity of the project are identified as Stockton Adobe Clay, although the ' spoil excavated from the tank area was called "sand' WORK PLAN ' GENERAL It is proposed that three monitor wells be drilled located as shown on Figure 3 MW q# 1 is located within 10 ' of the leak source and in a probable down gradient location It is proposed that this well be sampled at -25 ' bgl , -35 ' bgl and ' -47 'bgl (highest water level ) and then at -57 ' bgl , -67 ' bgl and -77 ' bgl or at the top of the water table if found higher The excavation was sampled at -29 ' bgl in 1988 It is ' recommended that the first core sample in MW 2 and 3 to be at --47 ' bgl One and one half to two feet long sampling devices will be used (detailed below) This depth will give samples at the highest level reached by groundwater in the vadose zone The next sample would be 5 ' lower at -52 ' and then at 10 ' intervals , --62 ' and -72 ' The water table is expected to be found at about -77 ' bgl This program should ' give ample coverage through the vadose zone while keeping 1 3