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GEOMATRIX <br /> Ms. Mary Meays <br /> San Joaquin County Public Health Services <br /> 29 September 1993 <br /> Page 5 <br /> phase hydrocarbons, has a low hydraulic conductivity and therefore does not contribute <br /> much water or hydrocarbon to the well when it is purged and sampled. The deeper <br /> sand unit, which is relatively clean, has a much higher hydraulic conductivity and <br /> therefore contributes most of the water collected during purging and sampling. The <br /> result is that only low concentrations of petroleum hydrocarbons are detected in <br /> samples from the well, even though separate-phase hydrocarbons are present in the <br /> soil. <br /> Further evidence of this phenomenon is the detection of benzene in most of the on-site <br /> monitoring wells. Benzene is the most mobile of the hydrocarbon constituents and the <br /> most easily removed from the separate-phase hydrocarbons held in the silt unit. <br /> The consequences of the distribution of hydrocarbons relative to the hydrogeology and <br /> well screen intervals at the site are that- <br /> - significant contamination, which is known to exist at the site, is not being detected <br /> in the monitoring wells; and <br /> - contamination is likely slowly desorbing from the upper silt unit into the well, <br /> moving down the well due to likely downward gradients during non-pumping <br /> conditions, moving outward from the well into the coarser, deeper sand unit, and <br /> migrating away from the site in this deeper unit. <br /> - The groundwater data from Boring B-7 (Alton, 5 July 1991) were not presented in this <br /> report. This is the only sample of groundwater collected at the water table that has <br /> ever been analyzed and the results (gasoline at 110,000 ppb, diesel at 60,000 ppb, and <br /> benzene at 560 ppb) demonstrate that separate-phase gasoline is present at the site. <br /> Section 1.2.6.2, Pages 1-9 and 1-10 <br /> - The results of hydraulic testing at the site are discussed on these pages. The analysis <br /> of the results and the conclusions based on the analysis are inconsistent in a number of <br /> ways. <br /> First, the pump test data were corrected for unconfined groundwater conditions; <br /> however, the most transmissive layer pumped during the pump tests, which is the <br /> lower sand unit that Alton states is 1 to 3 feet thick, at depths of 14 to 18 feet, is <br /> confined. The storativity value estimated from the test, 1.0 x 10', is indicative of <br />