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Work Plan for Additional Site�Cssessment • <br /> ARCO Facility No. 6100 <br /> Tracy,California <br /> Page 7 <br /> photoionization detector (PID). Delta will record PID readings, soil types, and other <br /> pertinent geologic data on the borehole log. Soil samples used for logging will be <br /> classified using the Unified Soil Classification System. A minimum of five soil samples <br /> from each boring will be submitted for chemical analysis, including the sample exhibiting <br /> the highest PID response, the sample from just above groundwater (if encountered), and <br /> the sample collected from the bottom of the boring. Additional samples may be collected <br /> at the discretion of the registered geologist overseeing the project. <br /> This method of drilling also allows the use of various geophysical logging instruments <br /> that measure several physical properties of the soil surrounding the borehole. Once the <br /> total depth of the soil boring has been reached, a wireline geophysical instrument will be <br /> lowered through the drilling mud to collect specific data, as outlined below. Following <br /> removal of the drilling mud, the hole will be pressure grouted using neat cement slurry <br /> containing approximately 5%bentonite. <br /> Wireline Geophysical LoQein> <br /> Wireline geophysical techniques will be used to evaluate soil lithology and identify <br /> perched water-bearing material and zones, if encountered. A sonde will be lowered by a <br /> mechanical winch within a borehole filled with drilling mud at a fixed rate of speed. The <br /> sonde will measure the physical properties of the soil type surrounding the borehole. The <br /> sonde will be lowered at the slowest practical speed to aid in identifying perched water <br /> zones that may be thin. <br /> A neutron logging tool will be used to identify soil lithology and zones of perched water, <br /> if encountered. A neutron logging tool emits neutrons from a radioactive source <br /> (typically plutonium-beryllium or americium-beryllium) and measures the attenuation <br /> rate of neutrons in the soil. Hydrogen is the most efficient element in attenuating <br /> neutrons because neutrons have a mass nearly identical to the mass of a hydrogen <br /> nucleus. Particles with a heavier nucleus (soil particles) do not attenuate neutrons <br /> because of the large differences in mass. Water should be the primary source of <br /> hydrogen in the subsurface because few rock/soil forming minerals contain hydrogen. <br /> Though petroleum hydrocarbons may be present in soil, water saturated soils should <br /> provide a distinctive response on the wireline log. A neutron tool should identify <br /> saturated soils because the tool measures the hydrogen content of the soil formation. The <br /> lowest neutron response on the wireline log should represent intervals of saturated soil. <br /> The neutron log should also identify the thickness of saturated soils within potential <br /> water bearing sand layers. The porosity of saturated soils can also be estimated. <br /> A gamma-gamma tool will be used to measure electron density of the soil around the <br /> borehole. Gamma photons are emitted from the sonde as the instrument is lowered <br /> through the drilling mud. The sonde measures the return rate of photons to the <br /> instrument, thus providing an attenuation rate of photons in the soil. Since electrons <br /> CAARCO\6 WREPORM6100 workplan 05-22-01.doc <br />