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<br /> Mr. Kyle Chri;tie
<br /> June 29, 1989
<br /> Page 3
<br /> Benzene concentrations in wells E-2, E-5, E-8, E-9, i~-10, and E-12 have increased since the
<br /> fall quarter 1988 (November 28 and 29, 1988). Tire benzene concentration in well PRA has
<br /> gone from 6.4 ug/I to nondetectable, since tine fall quarter 1988. Benzene concentrations in
<br /> well E-I I have deer�.;:.icd since Lire fall quarter 1988. A contour map of February 27, 28, and
<br /> March 2, 1989, benzene concentrations in groundwater is shown on Figur L T;tble 2
<br /> summarizes the benzene concentrations for spring, summer, fall 1988, and winter 1989
<br /> quarterly sampling events. This table shows than benzene concentrations have varied in each
<br /> well over time.
<br /> The concentration of cthylbenzene is above the DHS MCL in monitoring well E-10. The
<br /> concentration of toluene is above the DHS Drinking 'Yater Action Level in monitoring rtnb7
<br /> well
<br /> E-10. The cortecrnrations of xylene are above DHS MCLS in monitoring wells E-5, E-6, E-
<br /> 9, E-10, and E-IL
<br /> Groundwater Flow
<br /> Groundwater levels a-.t the site are being measured in each well on it weekly basis. Tire levels
<br /> are measured with an electronic water level indicator to determine groundwater elevations
<br /> relative to trip of casing elevations, Since late April 1988, the groundwater flow direction has
<br /> been influenced by the recovery system, which has been functioning continuously since early
<br /> April. Figure 2 is a typical groundwater contour map for recovery system pumping conditions
<br /> with the pump operating at a rate of 20 gallons per minute (gpm). The map is based on
<br /> water level measurements taken February 22, 1989.
<br /> On February 26, 1989, a malfunction of the recovery system caused a shut down of the
<br /> recovery pump. The pump was inoperative for approximately 3 weeks. Water level
<br /> measurements taken on February 27 and ?.8, 1989, were taken during static groundwater
<br /> conditions. A groundwater contour map fir March 8, 1989, is shown on Figure 3. The
<br /> figure shows groundwater returning to true',static conditions. During true static conditions,
<br /> groundwater flows to the northwest. if the reovery pump were shut down for a longer time
<br /> period, the water level contort, would return to'true static conditions,
<br /> i
<br /> .� On March 17, 1989, the recovery pump was repaired and the pumping rate was adjusted
<br /> from 20 gpm to 5 gpm. Water level measurements taken on April 12, I989, approximately
<br /> I month after the pump was restarted, indicate that groundwater flow is being influenced by
<br /> the recovery system. Figure 4 is a groundwater contour map produced from the April 12,
<br /> 1989, water level measurements.
<br /> Figures 2, 3, and 4 are representative of the groundwater elevations relative to mean sett
<br /> level. In early April 1989, all of the monitoring wells ^ssociated with this project were
<br /> surveyed to a Stockton city benchmark with a known elevation of 2.312 feet below mean
<br /> sea level. The wells had previously been surveyed to an arbitrary benchmark of 100 feet
<br /> above mean sea level.
<br /> BROWN AND CALDWELL
<br /> 723 S STREET SACF"ENTO.CALIFORNIA 95614,7092.19165 444'0123 -
<br />
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