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3.3.2 Groundwater Gradient and Flow Direction The depth to groundwater has been measured on more than two dozen occasions at this site, and the gradient and flow direction have been calculated from the depth data (Table 4) hutially, it appeared that the flow direction fluctuated periodically from northwest to northeast, but as more wells were installed to provide more information, it has become apparent that the flow direction vanes from place to place because the gradient is not planar The December 1998 map (Figure 20) is typical in the central part of the site the gradient lines indicate that there is a very slight groundwater "high" which trends to the northwest through the former UST cavity Groundwater flows off this ridge to the northeast (toward MW-4), to the northwest (toward MW-5), and toward the southwest (toward SW-2 and VW-2) West of the site, the flow appears to be dominantly to the northeast beneath Yosemite Street Convergence of flow off the "lugh" with flow beneath the street produces a combined flow direction to the north There are at least two possible explanations for this non-planar gradient One possibility is that the groundwater trough beneath Yosemite Street is caused by reduced infiltration of surface water through the pavement This possibility was particularly attractive in early 1998, when the trough trended northward, parallel to the street In late 1998, the trough trended northwestward and extended southeastward as far as SW-1, making this explanation less satisfactory The second possibility is that the subtle variations in groundwater gradient are controlled by the geometry of hydrogeolopc unit 2 and the gram size m the lower part of unit 1 The groundwater "high" through the center of the site is roughly coincident with the axial "thick"of unit 2 (Figure 19),and the cross sections (Figures 17 and 18) show that the top of unit 2 rises to the southeast and falls to the northeast (away from the axial thick) In addition, the sandy lower part of unit 1 is present between 22 5 and 26 5 feet in MW-4, but is as shallow as 18-20 feet in MW-2 and other borings along the axis of unit 2 The slight rise in the water table that causes the groundwater "high" may be due to greater hydraulic conductivity in both units along the axis of unit 2 The top of unit 2 is also high in the area of B-3 and VW-1 (Figure 17), where the upper part of the unit thickens abruptly 3.3.3 Depth to Groundwater The depth to groundwater has changed during the course of the investigation, consistently decreasing over time (Figure 21) In late 1993, the water table was at 29 feet in MW-2, but by Apnl of 1998 it had risen to 10 5 feet The static water level has been above the screened interval in MW-1, MW-2, and MW-3 since early 1995, and was as much as 17 feet above the screened interval in early 1998 The water level has been above the screen in the other monitor wells since early 1997 At the present depth of approximately 14 feet, the water level is 15 feet above the screened interval in the two spargung wells Despite this, groundwater samples from SW-1 have been among the most contaminated of any collected at the site 6