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• Although the water table was exceptionally high in 1998 and hydrocarbon concentrations did noticeably decline in many wells, concentrations in the Kwikee wells have not risen again since the water table lowered back to its 1996-97 elevation, as one would expect if the decline were clue merely to a high water table in 1998-99. Further, the TPH-g concentration in Chevron well MW-5 rose to a new historical peak (9,600 ppb) in 1998. This occurred when groundwater flow reversed and became southward toward this well (section 4.4). In 1999, a concentration of 10,500 ppb was detected in this well, when the static water level was still 4.5 feet above the screen. Thus, drowning of the well screens is an overly simplistic interpretation that does not adequately account for many of the important changes that are recorded by data from a number of wells. Figure 36 illustrates why drowning of the well screens has not been a dominant control on contaminant concentrations in well samples. Due to the much higher hydraulic conductivity of the aquifer sand body (500 to 1000 times greater than the underlying clay aquitard and between 150 and 300 times greater than the overlying silty-clay aquitard), the groundwater flow velocity is 2 to 3 orders of magnitude greater within the sand body than above or below it for any given hydraulic gradient.'Hence, gasoline dissolved in groundwater will migrate within the sand body much more readily and rapidly than above it, even when water-level data from monitoring wells imply that the piezometric surface is above the sand body ("High" water level in Figure 36). Contaminant migration will be further retarded"at high water levels because of the greater hydrocarbon affinity for clay-sized particles than for sand-sized' particles, and also because organic matter (such as root fragments or plant • debris) is more common within finer-grained sediment than within coarser sediment. Lightweight non-aqueous phase liquid hydrocarbons (LNAPLs) such as gasoline will therefore tend to migrate most rapidly either along the top of the sand body at the contact with the overlying aquitard (position A in Figure 36) or deeper within the sand body where pores spaces are larger and not clogged with fine-grained particles and the porosity and permeability (i.e. hydraulic conductivity) are therefore higher (position B in Figure 36). Regardless of the flow path at the intergranular scale, the result is that the contaminants arrive at the monitoring well within the screened interval, as long as the well is screened across the sand body. If the piezometric surface is above the screened interval during a particular sampling event, the water level will rise above the well screens, but LNAPLs will also rise to the static water level and be present in the sample. Purging of the well prior to sampling will further insure that contaminants will be drawn into the well, because the sand body is the only lithologic unit that provides significant quantities of water to the well during the purging process. o EHD commented that "Interestingly, the statement pre-interprets occasional contaminant increases during operation of the DPE unit as to not be inconsistent with the contention that the contaminants are from the Chevron site. Thus, whether the DPE unit is in operation or not, an increase of concentrations can be favorably interpreted!" Despite several readings, we are unable to make sense of this comment. In what sense is an increase in concentrations a favorable result? In the Second Quarter 2002 Report, we concluded that "absent • - 23