REMEDIAL ACTION WORK PLAN OPERABLE UNIT 2, LANDFILL ...

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2.7.3 Gas Probes
Housing is currently within 100 feet of Area A. After relocation of refuse in Area A, the nearest
housing will be approximately 350 feet from a landfill area (east of Area F). Subsurface soil
between the landfill and the housing reportedly consists of sands and silty sands which are
conducive to both vertical and lateral LFG migration.
Because the distance to the housing is significantly greater than the depth of the landfill (the
distance LFG must travel to reach the ground surface), and because the area between the landfill
and the housing areas is undeveloped and its soil is conducive to LFG venting, it is probable that
even after the landfill is covered, migrating LFG will vent to the atmosphere before it reaches the
housing areas. To monitor potential LFG migration, LFG monitoring probes will be installed on
the perimeter of the landfill between the landfill and the housing areas. The perimeter probes and
the passive vents will be monitored for methane.
In addition, LFG monitoring probes will be installed around the closed landfills. The probes will
be spaced a maximum of 1,000 feet apart outside the landfill boundary at the edge of the cover.
The probe locations are shown as “SGP-XX” on HLA drawing sheets C22 to C27. The probes
will be sampled and analyzed quarterly for the first year in accordance with California Code of
Regulations (CCR) Title 14 and the Final Closure and Postclosure Maintenance Plan (HLA,
1995 b). The need for continuing monitoring will be evaluated after one year.
2.7.4 Settlement Plates
The additional weight of the final cover and general fill is expected to induce settlement over the
landfill trenches, with less settlement over the native material, possibly causing some differential
settlement over the entire cover. Settlement will be monitored by settlement plates. Because of
the elimination of the winter shutdown, settlement plates will be installed in the vegetative cover
layer afier the completion of the earthworks. Surveying of the settlement plates for changes in
elevation will occur on installation and at completion of the project. The settlement plates will
be used to monitor the settlement of the landfill cover during the postclosure maintenance period.
2.8 Vegetative Cover
The vegetative cover that will be installed at Areas B through F will be two feet thick. This layer
is intended to protect the FML and support the growth of native vegetation. The vegetative layer
will consist of dune sand placed directly over the FML barrier and planted with native plants. It
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