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. Mr. Kyle Christie Jun-- 12, 1990 Page 5 Benzene concentrations have varied in each well over time. The groundwater samples from wells E-6 and E-7 showed increases since they were last sampled (in February and August 1989, respectively). There are insufficient historical data from those wells to comment on the significance of the increases in benzene concentration. Decreases in benzene concentration were observed in the samples from the following wells: BHD-1,E-4,E-5,E-9,E-10,E-11,and E-12. (These wells were last sampled in either August or December 1989). A coniour map of the benzene concentrations in the March 1990 groundwater samples is shown on Figure: 1. The concentration of ethylbenzene is above the DHS MCI,of 680µg11 in wells E-3 and BfID-l. The concentration of toluene is above the DHS Drintdng Water Action Level of 100 µg11 in monitoring wells E-3, E-6, and E-10. The concentration of xylene is above the DHS MCI.of 1,750 µg11 in monitoring well E-3. Monitoring well E-3 has not been sampled for the previous 2 years because historically it has been dry. Groundwater Flow Groundwater levels at the site are being measured in each well on a biweekly basis using an electronic water level indicator. The levels are measured relative to top-of-casing elevations in order to determine groundwater elevations and monitor possible changes in flow direction. Beginning in late April 1988, the groundwater flow direction was influenc by the recovery system. Sometime in April 1989, the recovery puinp stopped operating. a malfunction was detected in June 1089. Repair of the recovery system is almost complete: owever,the recovery system is currently being hampered by unexplained fluctuations in the LEL meter, which is located at the recovery well head. Brown and Caldwell personnel are attempting to identify the cause of these fluctuations. As a result, the recovery pump is being operated on a manual basis for testing purposes. On April 15:I990, Brortm and Caldwell personnel measured water levels at the site. The recovery system was being treated at the time, and had been pumping at a rate of about 10 gallons per minute for 30 minutes,prior to the measurements being taken. Figure 2 is a water level contour map for April 15, 1990. The figure shows that water levels at that time generally were representative of static conditions, except that the early effects of the recovery system can be seen around the recovery well. During static conditions, groundwater flows to the northeast. Conclusions a About half of the March 1990 groundwater samples contained loser concentrations of the BETX constituents compared with past results. The decreased concentrations may be due !o flushing and dilution as the water table returned to static conditions. The direction of groundwater movement under static conditions is to the northeast 6. i it has been in that direction historically. The flow direction is readily apparent by the nose of high benzene concentrations pointing toward the northeast. / Brown and Caldwell