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4.3.1 Site Hydrology In order to compare water levels innumerous wells at a site,a known reference point,usually the top of ` each well casing,is surveyed into an known vertical datum. The water level within the well can then be measured from this known elevation and calculated relative to the vertical datum. Correlating water level information at a site enables geologists to map potentiometric contours,or lines of equal groundwater elevation, which provide vital information on groundwater flow direction and gradient. Initial top ofcasing well surveys at the site yield the groundwater elevations shown in Figure 2-2. Previous monitoring reports(WESTON,2006)have incorporated this information into potentiometric maps that showed a trough-shaped pattern in the groundwater flow direction. Groundwater flow direction was shown 4 as flowing towards the center ofthe trough;flow within the trough was towards the north-northeast. Under this regime,monitoring well MW-9 would be considered the background well,MW-2 and MW-2A would ` be the source wells,and MW-7 would be considered the downgradient well. This pattern does not seem to correlate with other similar sites in the area as accessed in the SWRCB's GeoTracker database (SWRCB, 2006). The monitoring well elevation that creates the groundwater trough at the site was surveyed at MW-6 in 1991. Removing this well elevation from the remaining on-site wells results in a groundwater flow direction towards the southeast, which seems to correlate with similar sites in the area (SWRCB, 2006). Additionally,groundwater flow in the Central Valley Groundwater Basin flows eastward from the San _ Joaquin Delta,located west of the site,toward a large groundwater depression approximately 8 miles east of the site (WESTON, 1992; San Joaquin County Department of Public Works, 2004). Due to the uncertainty ofthe groundwater flow direction at the site,WESTON recommends fillingthis data gap by resurveying the network ofmonitoring wells. This will support low risk site closure criteria#2 and #3 in the low risk site closure plan. r 4.3.2 Horizontal Extent of Groundwater Contamination The LLNL report found that petroleum plumes in the subsurface tend to stabilize due to natural biodegradation once the source is removed, as is the case at the site. Chemical concentrations of hydrocarbons in groundwater that decrease or do not change over time are the best indicators ofa stable plume. Comparison ofbackground and hydrocarbon plume concentrations of inorganic ions such as oxygen,iron,nitrate,and sulfate can also provide evidence ofbiodegradation and supplement other lines of evidence. In addition, a stable or decreasing plume can often display short term variability in groundwater concentrations due to natural variations in the environment(RWQCB, 1996). Historical groundwater data from the on-site monitoring wells shows dramatic initial drops in TPH-g and benzene, ` as discussed in Section 4.2.3.2,and has more recently begun to show asymptotic levels that fluctuate slightly. 14 r