|
'i:=' f"..."JJ-'::::• • '-- -... .. -.s-_ sssvGLL4....�..�.s-.+� i - -_s.... . ."--..-__-- - - __- --______-
<br /> i
<br /> .f/.+%!//.rr ri} /'-:'.'':i"rlrfl./r'r:rf. - '•:S .�:_-_f Yf+Fi--s::..._.._si! i L. ....
<br /> y
<br /> f..
<br /> ASSCvvATC
<br /> O C I ATE S INC ,
<br /> On July 18, 1995, Smith Technology supervised the installation of three groundwater monitoring
<br /> wells, MW-1, MW-2, and MW-3 (Figure 2). During installation of the monitoring wells, soil f,
<br /> samples were collected from the borings. On July 21, 1995, Smith Technology developed and
<br /> sampled monitoring wells MW-1, MW-2 and MW-3. Analytical results of the soil and
<br /> groundwater samples collected indicated elevated concentrations of petroleum hydrocarbons
<br /> were present in the soil and groundwater downgradient of the former gasoline UST locations.
<br /> On September 24 and October 2, 1996, Smith Technology conducted an additional subsurface E
<br /> investigation at the site. A total of 9 soil borings were advanced to depths up to 15 feet below
<br /> ground surface. Soil, soil vapor, and groundwater samples were collected from the borings.
<br /> Analytical results of the soil and groundwater samples indicated the general extent of total
<br /> petroleum hydrocarbons as gasoline (TPHg), benzene, toluene, ethylbenzene, and xylenes
<br /> (BTEC, and methyl tertiary butyl ether (MTBE) in soil and groundwater under the site.
<br /> Analytical results for geochemical parameters in soil and groundwater indicated bioremediation
<br /> is occurring in the plume of petroleum hydrocarbons in groundwater beneath the site. Analytical
<br /> results of soil vapor samples indicated the availability of oxygen, carbon dioxide, nitrogen and
<br /> methane in the vadose zone. Also,results of laboratory geophysical testing on soil samples from
<br /> the site indicated that the potential receptor on-site, the water supply well, would not be affected
<br /> by the plume of petroleum hydrocarbons in groundwater within the next 20 years, in the worst
<br /> case.
<br /> On March 9, 1999, ATC supervised the installation of three groundwater monitoring wells, MW-
<br /> 4, MW-5, and MW-6 (Figure 2). During installation of the monitoring wells, soil samples were
<br /> collected from the borings. On March 11, 1999, ATC developed and sampled monitoring wells
<br /> MW-4, MW-5, and MW-6. Analytical results of the soil and groundwater samples collected
<br /> indicated are contained in ATC's Summary Report Additional Subsurface Investigation, 7675
<br /> West Eleventh Street, Tracy, California, dated May 12, 1999.
<br /> GROUNDWATER SAMPLING AND GRADIENT EVALUATION
<br /> ATC personnel performed the latest quarterly groundwater monitoring of monitoring wells on
<br /> June 24, 1999. Field work consisted of measuring depth-to-water levels in all wells, collecting
<br /> groundwater samples from all wells, and submitting groundwater samples for laboratory
<br /> analysis.
<br /> Water level measurements were collected from groundwater monitoring wells MW-1 through
<br /> MW-6. The depth to water ranged from 6.58 to 8.22 feet below top of well casing elevation.
<br /> Groundwater elevations have increased an average of 0.31 feet since March 11, 1999. The water
<br /> level data were used to develop the groundwater elevation contour map (Figure 2). Assuming
<br /> that horizontal isotropic conditions prevail, groundwater in the uppermost aquifer beneath the
<br /> site flowed in a north-northwest direction. The average hydraulic gradient on June 24, 1999 was
<br /> := calculated to be 0.003 ft/ft or about 17 feet/mile. The data indicates a groundwater gradient that
<br /> _ 2
<br /> w:117 8611reportsl4gmr1998.doc
<br />
|