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July 6, 2017 Monitoring Well Installation Workplan
<br />1.3. Local Hydrogeologic Conditions
<br />The Lathrop area is located in the basin-trough geologic zone of the San Joaquin Valley,
<br />which consists of continental Pleistocene and Holocene stream-laid and still-water
<br />deposits2. These deposits are derived from a mixture of Sierra Nevada and Coast Range
<br />rocks 3. The near surface deposits are mapped primarily as Quaternary alluvial flood plain
<br />deposits 4. The local depositional environment is a sedimentary river basin, and the shallow
<br />sediments are primarily flood plain and over bank deposits of sand, silt and clay. The near
<br />surface deposits are characterized by discontinuous inter-bedded silty sand, sandy silt, silty
<br />clay, clay, and sand depositss. Soils are derived from mixed Sierran and Coast Range source
<br />materials, and are generally poorly drained.6
<br />The Lathrop area groundwater basin is comprised of an approximately 150-feet thick upper
<br />water-bearing zone overlying a 75 to 1,000 feet thick lower water-bearing zone. The lower
<br />water-bearing zone provides drinking water to the City of Lathrop. The hydraulic
<br />connection between the upper and lower zones appears limited due to extensive low
<br />permeability strata.7
<br />Depth to groundwater varies depending on location, season, and local influences. The
<br />shallowest groundwater levels are generally observed adjacent to the San Joaquin River,
<br />and groundwater depths tend to increase with distance from the river. In the alluvial flood
<br />plain deposits, groundwater is typically within 12 feet of land surface.
<br />1.4. Proposed Replacement Monitoring Well Installation
<br />Figure 2 shows the existing monitoring well locations in the CLSP area, proposed
<br />replacement well locations, and planned future disposal areas. There are no planned Use
<br />Areas near the former CLSP-10 location, therefore, the proposed replacement well for
<br />CLSP-10 (CLSP-11) is located to the south where it will be downgradient and adjacent to
<br />planned Land Application Areas (LAA). The well will be placed east of the existing sidewalk
<br />on Golden Valley Parkway and near the edge of the LAA. The proposed replacement location
<br />for CLSP-2 (CLSP-12) is set back from De Lima Road approximately 15 feet into the parcel to
<br />preserve the integrity of the well during planned pond construction and ensure future
<br />sampling access.
<br />2 Jennings CW and Strand RG, 1958, "Geologic map of California, Santa Cruz Sheet," California
<br />Division of Mines and Geology, Sacramento, California.
<br />3 Miller RE, Green JH, Davis GH, 1971, "Geology of the compacting deposits in the Los Banos-
<br />Kettleman City subsidence area, California," U.S. Geological Survey Professional Paper 497-E.
<br />4 Atwater BF, 1982, "Geologic maps of the Sacramento-San Joaquin Delta, California," US Geological
<br />Survey Miscellaneous Field Studies Maps, MF-1401.
<br />5 Kleinfelder, "Hydrogeology of the Mossdale and Reiter Property, Lathrop, California," November 24,
<br />2004.
<br />6 Soil Conservation Service, 1992, Soil Survey of San Joaquin County.
<br />7 Page RW, 1985, Geology of the fresh groundwater basin of the Central Valley, California, with
<br />texture maps and sections. U.S. Geological Survey Professional Paper 1401-C.
<br />HydroFocus, Inc. 2
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