Appendix D3 Preliminary Subsurface and Investigation

Appendix D3 Preliminary Subsurface Methane Gas and Soil Investigation

1.0 Introduction: GeoKinetics has completed a limited subsurface methane gas investigation and soil sampling investigation for the property located at 21141 Strathmoor Lane in the city of Huntington Beach, California. The general site location is shown in Figure 1 while a recent aerial photograph of the property is provided as Figure 2. We understand that eighty-seven (87) homes are proposed to be constructed on the property. The property is located within the Newport West Oil Field as it has been mapped by the California Geologic Energy Management Division (CalGEM). No historical oil wells are shown to exist at the property based upon our review of the CalGEM oil field map. A "Plugged" historical oil well (Gisler, "Tad Travers" #2) is located approximately 115 feet west of the subject property based upon our review of the CalGEM oil field map. The mapped location of this oil well is shown on Figure 2.

The purpose of the current investigation was to screen for the presence of elevated levels of subsurface methane gas that may be present at the site due to the oil field setting. In addition, pesticide screening is being performed in order to classify former potential fill soils. This report presents the results of the limited methane gas investigation and soil sampling investigation as well as recommendations regarding the need for methane or soil mitigation measures with respect to construction of the proposed residences in general accordance with HBFD guidelines.

2.0 Methane Occurrence and Characteristics: The presence of methane gas in the subsurface is common within former oil production areas and other locations where organic material - such as grass, leaves, wood, manure, etc. are present in the soil. Methane is generated by the bacteriological digestion, or biodegradation, of organic matter in the absence of oxygen. Methane generated by this process is referred to as "biogenic gas". Methane can also be generated by petrogenic processes at depth and subsequently migrate to the ground surface from relatively deep oil and gas bearing zones. Our experience indicates that biogenic methane gas is common and can be found in the soil beneath a relatively high percentage of properties. Methane is not toxic, however it is combustible and potentially explosive at concentrations above 55,000 ppm in the presence of oxygen. This concentration is referred to as its Lower Explosive Level or LEL. Methane is lighter than air and therefore has a natural tendency to rise to the ground surface where it typically dissipates into the atmosphere. The presence of subsurface methane associated with the biodegradation of low levels of organic material in the soil is normally not problematic. The rates at which the organic material is decomposed, and methane is generated, are slow enough such that the gas dissipates naturally under normal circumstances. However, as methane migrates to the ground surface, the potential exists for its accumulation beneath slab-on-grade foundation systems. If the gas accumulates to high concentrations, and becomes pressurized, and a crack or other penetration is present in the floor slab of the home, detectable levels of methane may enter the interior of the home. Improvements - such as sub-slab vent lines or gas membranes - are often installed as an additional precaution when elevated subsurface gas levels are detected.

Report on Limited Methane Gas and Soil Sampling Investigation 21141 Strathmoor Lane ? Huntington Beach, California

October 19, 2020

3.0 Methane Soil Gas Field Investigation: The field work associated with the methane gas investigation included the installation and monitoring of five (5) subsurface gas probes at the subject property. The probes were installed on September 1st, 2020 and they were subsequently monitored on September 2nd, 2020 during periods of falling barometric pressure. The locations of the gas probes, which were installed using hand augering equipment, are shown in Figure 2. A schematic illustrating the "as-installed" configuration of the gas probes is provided as Figure 3. As shown in this figure, each gas probe consists of ?-inch diameter polyethylene tubing with an attached porous polypropylene tip. A single gas sampling tip was installed at 5 below grade at each location. Each sampling tip was embedded within a 12-inch interval of washed Monterey #3 sand. Bentonite clay seals were placed above and below each sand interval in order to isolate the gas sampling tip. Gas tight quick connect fittings were installed on the ends of the polyethylene tubing at the ground surface in order to seal the probes between monitoring events.

As discussed previously, monitoring of the gas probes was performed on September 2nd, 2020. The subsurface gas pressure relative to atmospheric, and the concentrations of methane, oxygen, and carbon dioxide were measured in the subsurface probes during each monitoring event. The monitoring equipment that was utilized and the associated detection limits, or resolutions, are summarized in Table 1. As indicated, subsurface gas pressures were measured to the nearest 0.05 inches of water prior to each sampling event using a Magnahelic gauge while the barometric pressure was measured and recorded to the nearest 0.05 inches of mercury using a digital barometer.

As indicated in Table 1, methane, oxygen and carbon dioxide concentrations were measured in the field using a portable, methane specific, LandTec GEM2000-Plus infra-red gas analyzer. A volume of gas equivalent to ten times that of the ?-inch diameter polyethylene gas probe tube was extracted through the LandTec GEM2000-Plus during the monitoring process. It should be noted that steady state readings for methane and the other recorded gases (oxygen and carbon dioxide) are typically obtained after approximately three tubing volumes of gas have been extracted.

The LandTec GEM2000-Plus was calibrated at the beginning and the end of each day of monitoring using a certified mixture of 15% methane, 15% carbon dioxide, and 70% nitrogen calibration gas (Note: all gas concentrations referred to in this report are on a volumetric basis). A Photovac Flame Ionization Detector (FID) was used to confirm combustible gas levels where

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Report on Limited Methane Gas and Soil Sampling Investigation 21141 Strathmoor Lane ? Huntington Beach, California

October 19, 2020

sufficient oxygen was present in the probe to operate the FID. The gas levels measured with both detectors were found to be consistent.

Ambient gas levels in the air four feet above the ground surface were recorded at the site periodically during the monitoring. In each instance, the measured gas level fell within the following range:

Gas

Methane Carbon Dioxide

Oxygen

Measured Range ................
................

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