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® SG-120, located at the southwestern limit of Area 2 and adjacent to groundwater monitoring well MW-5. These wells allow the sampling of gas in the 60 to 90-ft depth range and are a more ideal alternative to the existing single-probe well locations. Description of Existing Wells: The existing SG-2 to SG-11 wells were installed with the approval of RWQCB staff, allowing sampling from a pea-gravel-filled boring that extends to a depth of 65 feet below ground surface. A single probe extends into the top of the pea gravel, allowing the collection of a sample representative of the gas in the soil adjacent to the pea- gravel-filled boring. Although the pea-gravel-filled boring does allow sampling of the gasses that may exist in the strata penetrated by the boring itself, it is highly unlikely that it would allow penetration of shallow level soil gases deeper into the soil beneath the boring, because the gravel-fill zone of these boreholes penetrates mostly stiff, dense, or hard clayey soils. This is also the case with the deeper perimeter migration monitoring wells. The wells function to monitor the gasses within the strata penetrated by the boring, but it is understood that the boring does not force gasses from the ' penetrated strata into the strata beneath the boring. A geological observation of soil gas penetration through the strata at the site is observed at the SG-1 and SG-9 locations. SG-1 is 255 ft deep with the borehole sealed with cement to a depth 247 ft. However, soil gas containing VOCs are consistently encountered at the 255 ft. depth. The migration of soil gas to that depth did not pass through the borehole, but through the strata. tAnother geological observation in regards to VOC migration to groundwater is that MW-3 is generally free of VOCs, even though VOCs are present in the pore space of the strata above groundwater. The likely explanation is that although pore space gases contain VOCs, the pore water does not; or that the pressure of the gases above groundwater are not high enough to impact groundwater. The only times that VOCs are present in groundwater, even though they are present in a gaseous phase above groundwater, is when there is an explainable greater LFG driving force in the waste mass; such as ponding which occurred in the late 1990's or the recent episode of trapped gases by the HDPE liner over refuse. iAnother geologic observation of soil gas penetration of VOC migration to groundwater is that groundwater at well MW-2R, which is adjacent to SG-9; has always been free of VOCs even though VOCs are detected in the pore space to a potential depth of 67 ft., the borehole depth of SG-9. This supports the conclusion that the gravel-fill borehole below the probe does not provide a potential for VOC migration to groundwater. Based on the above geologic observations of soil gas penetration, the County has collected evidence to support the following facts on groundwater impact from LFG: • In the case of unlined Area"I", impending VOC groundwater impact is foretold by preventing LFG buildup in the waste mass by limiting moisture or entrapment. ® There is an approximate 4 to 5 year reaction time between surface actions and groundwater response. The length of time between a surface action and a groundwater response is based on the thickness and composition of the soil strata at the site. Foothill Sanitary Landfill 6 Department of Public Works/Solid Waste Engineering Feasibility Study County of San Joaquin—August 2014