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Jim Pinasco <br /> Page 4 <br /> August 1 , 1989 <br /> Concerns Regarding the One Hydrostratigraphic Unit Concept: <br /> Comment 5 <br /> On pages 3-17 and 3-18, paragraph 3 .6. 3 . 1, the Draft RI/FS <br /> refers to the Upper Tulare Formation as one hydrostratigraphic <br /> unit which overlies the Corcoran Clay. It is further stated <br /> that the Corcoran Clay acts as a regional barrier to <br /> groundwater flow. <br /> TSSU recommends that the hydrogeology of the upper <br /> (contaminated) aquifer(s) and underlying aquitard be <br /> characterized. <br /> Comment 6 <br /> Both of the constant discharge pumping tests performed at DDTC <br /> were performed using shallow pumping wells. The pumping well <br /> in the 1986 test was screened from 55 to 75 feet, and the <br /> observation wells were screened from approximately 55 to <br /> 65 feet BGS. The 1989 pumping test pumped from an extraction <br /> well screened' from 28 to 38 feet, and the observation wells <br /> were all screened above 35 feet BGS. <br /> TSSU questions that the Upper Tulare aquifer has been proven to <br /> be one hydrostratigraphic unit when pumping tests have not been <br /> performed deeper in the subsurface. Clarification is <br /> requested. <br /> Comment 7 <br /> On page 3-18, the Draft RI/FS states: <br /> "Although there is substantial spatial variability in <br /> concentrations on a small scale, as shown on the geologic <br /> cross-sections, the general trend is a pattern of <br /> decreasing concentrations with distance away from and <br /> downgradient from the potential on-site areas. <br /> TSSU suspects that the substantial spatial variability in <br /> contaminant concentrations is due to the presence of <br /> contaminant flow pathways (subsurface channels) which have not <br /> been, as of yet, properly characterized. Additional proposed <br /> studies should direct some effort to investigating pathways for <br /> contaminants. <br />