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<br /> _. 3-2
<br /> Site Geology
<br /> Geology underlying ARCO Products Company (ARCO) Station 434 and general vicinity
<br /> was determined from the boring logs of the five monitoring wells,one extraction well, and eight
<br /> vapor wells installed at the site during this investigation, and the boring logs from the ten
<br /> monitoring wells previously installed.
<br /> E , The location of cross sections A-A' and B-B' are shown on Figure 3-1. The boring logs
<br /> of wells V-6, V-5, V-7, W-10, W-1, W-5, and W-12 were used to construct cross section A-A'
<br /> shown on Figure 3-2. The boring logs of wells W-8, W-7,W-3, V-2, V-3, V-6,and W-11 were
<br /> used to eonsttuct cross section B-B' shown on Figure 3-3. Underlying ARCO Station 434 and
<br /> vicinity is a series of sand, silt, and clay that can vary widely over short distances. These
<br /> sediments are probably stream channel and flood basin deposits associated with the Ivlokelutnne
<br /> r j River and its tributaries. In general, finer-grained sediments underlie ARCO Station 434.
<br /> Borehole logs from wells located at the station show mainly clayey silt,silt,clay,sandy silt,silty
<br /> sand, and minor amounts of sand. Large amounts of clay underlie the station and range from
<br /> 2 feet to as much as 22 feet in thickness. These sediments were most likely deposited by slow-
<br /> moving flood waters. The sediments appear to be predominantly silt from ground surface to
<br /> approximately 12 feet below ground surface under the ARCO station, Borehole logs from wells
<br /> located off the station site show that in general,coarser-grained sediments are located in the areas
<br /> north,east, south,and southwest of the station. These areas mainly consist of silts, sandy silts,
<br /> and sands. The coarsest grained sediments exist to the north, east,and southeast of the ARCO
<br /> station as indicated by the borehole logs from wells W-10,W--11,and W-12. These areas contain
<br /> mainly silty sands,sands, and silts with depth and were most likely deposited by slightly faster
<br /> moving waters. The sediments appear to be predominantly silt from
<br /> ground to
<br /> "^ approximately 35 feet below ground surface under the American Savings facility andethe
<br /> Mervyn's parking lot. In all areas, the sediments do not exist as continuous sheets, but rather
<br /> as interlacing channels, indicating that these sediments are probably braided stream channel
<br /> deposits associated with the Mokelumne River and associated drainages.
<br /> Groundwater Gradient
<br /> r-
<br /> Groundwater elevations obtained by Brown and Caldwell on May 8, 1990,are summarized
<br /> on Figure 3-4. These absolute elevations were used to construct the direction of groundwater
<br /> flow and the hydraulic Y gradient using methods outlined in U.S.Geological Survey Water-Supply
<br /> • Paper 2224. The general groundwater flow direction for the ARCO station and vicinity is to the
<br /> southwest,although at the station,theundwater flow
<br /> direction south to sonthea;t The large amount of clay directly underlying heeARCO station may be
<br /> west to
<br /> acting as a barrier to theflow_of.groundwater-beneath-the-ARCO station.
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