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MAR 31 201koposed Location of the LCP Mid-Plume Extraction Well
<br /> Technical r�',/ p N��iR R Lodi,California
<br /> ENVIfi�®iV@i" EN ` 730392338
<br /> Memorandum ,,TQiTUnrP&plTgfjY ' Page 2of4
<br /> General Area Description,Area Geology and Hydrogeology
<br /> The LCP has historically been described as bounded by Pine Street to the north, Stockton
<br /> Street to the east, Pleasant Avenue to the west, and Park Street to the south. The area
<br /> contains a combination of residential, commercial, industrial, and municipal land uses.
<br /> Topography is generally flat, sloping to the north.
<br /> The LCP is underlain by at least 2,000 feet of Miocene to Holocene unconsolidated sediments,
<br /> including approximately 200 feet of Holocene and Pleistocene interbedded gravel, sand, silt and
<br /> clay, and 200 feet of Pliocene sands, silts, and clays. Soils are typically a heterogenous mix of
<br /> interbedded gravels, poorly to well-graded sands, silty sands, sandy clay, and clay. The soil
<br /> profile typically coarsens to the north, closer to the Mokelumne River.
<br /> The area geology and hydrogeology have been delineated into four groundwater zones —
<br /> shallow zone (up to 75 ft-bgs), the upper intermediate (between 75 to 100 ft-bgs) and the lower
<br /> intermediate zones (between 100 to 125 ft-bgs), the deep zone (between 125 to 175 ft-bgs),
<br /> and the deeper zone (>175 ft-bgs). Groundwater is encountered at approximately 50 feet ft-
<br /> bgs, and fluctuates throughout the year due to changes in precipitation, recharge, leakage from
<br /> the Mokelumne River, and pumpage by municipal and agricultural irrigation wells within the City
<br /> and the region. Groundwater flow directions are typically to the south-southeast in the shallow,
<br /> intermediate, and deep groundwater zones and to the southeast in the deeper groundwater
<br /> zone. The horizontal gradient is approximately 0.002 feet per foot (ft/ft) in the shallow
<br /> groundwater zone and 0.003 ft/ft in the intermediate, deep, and deeper groundwater zones.
<br /> Groundwater Contamination
<br /> Contaminants of concern (CoQ for the LCP are principally PCE, and to a lesser extent TCE.
<br /> Based on recent groundwater monitoring, the southern extent of the LCP extends to Mission
<br /> and Stockton Streets, where monitoring well MW-23B is located. PCE concentrations below
<br /> the California primary MCL have also been historically detected in City well 06R. This would
<br /> suggest that PCE has migrated from the shallower to the deeper zones. Figure 1 provides the
<br /> locations of the monitoring wells in the intermediate zones (except for wells G-13 and G-17A
<br /> which are screened in the shallow zone), the August 2014 PCE concentration data at these
<br /> wells, and the modeled extent of the Central plume in the intermediate zone.
<br /> As presented in Figure 1, the PCE concentration at MW-25B, which is an upper intermediate
<br /> zone well screened from 86 to 96 ft-bgs, is similar to those observed at the shallow zone
<br /> monitoring well G-13, which is screened from 63 to 68 ft-bgs. Shallow zone wells like G-17A,
<br /> which is south in a downgradient direction of G-13, have PCE concentrations an order of
<br /> magnitude below those shown in well G-13. This would indicate that the PCE plume appears
<br /> to be migrating from the shallow to the intermediate zone in the area between wells G-13 and
<br /> MW-25B. Additionally, the higher PCE concentration of the plume, as represented by the 500
<br /> micrograms per little (µg/I) contour (red shade in Figure 1), extends to south of Tokay Street.
<br /> Groundwater Flow Model
<br /> The Model was applied to approximate the migration of the LCP and provide data for the
<br /> location of possible extraction well/s for hydraulically containing and capturing the relatively
<br /> higher contaminant concentrations found in the mid-portion of the LCP. Groundwater data
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