13900 Alton Parkway, Suite 122, Irvine, California 92618

Phone (949) 297-4444 • Fax (949) 297-3625

September 6, 2018

Mr. Jose Diaz Department of Toxic Substance Control 9211 Oakdale Avenue Chatsworth, CA 91311 RE: Supplemental Soil Vapor Sampling Workplan Former Napa-1 Manufactured Gas Plant Site 201 – 267 Riverside Drive Napa, California

Dear Mr. Diaz:

On behalf of Pacific Gas and Electric Company (PG&E), Terra Pacific Group (TPG) has prepared this Supplemental Soil Vapor Sampling Workplan (Workplan) describing additional soil gas sampling proposed for the former Napa-1 Manufactured Gas Plant (MGP) site (Site) (Figure 1).

Additional soil gas sampling is proposed at existing dual depth soil gas probe locations SG-5 through SG-12, B3-1, D3-1, E2-1 and H2-1 (Figure 2). The additional samples will be collected primarily along the western property line to provide additional data to support the use of non-default attenuation factors to evaluate vapor intrusion risks to neighboring residential properties under current conditions. The additional data to be collected will include fixed gases, notably, oxygen and methane, which will be used to evaluate potential bioattenuation conditions at the Site, as part of a revised, multiple lines of evidence approach to evaluating potential vapor intrusion concerns.

The soil gas sampling activities will be conducted in accordance with the attached procedures which incorporate the July 2015 Advisory - Active Soil Gas Investigations (ASGI) published jointly by the DTSC, Regional Water Quality Control Board, Los Angeles Region (RWQCB-LA), and Regional Water Quality Control Board, San Francisco Region (RWQCB-SF). The soil gas samples and field quality control samples will be analyzed for volatile organic compounds, including naphthalene, using EPA Method TO-15, as well as oxygen, methane and carbon dioxide using American Society of Testing and Materials (ASTM) Method D1946. In addition, 10 percent of the soil gas samples will be analyzed for helium also using ASTM Method D1946.

The results of this sampling event will be summarized and evaluated in revised versions of the Final Draft Feasibility Study and Remedial Action Plan (RAP) dated May 4, 2018, prepared by TPG and the Human Health Risk Assessment (HHRA) dated April 2018, prepared by RPS. These revised documents will be submitted to the DTSC within 90 days of receiving the laboratory analytical results.

If you have any questions or comments, please do not hesitate to call Murray Wunderly at (760) 213-5782.

Mr. Jose Diaz Supplemental Soil Vapor Sampling Workplan Former Napa-1 Manufactured Gas Plant Site September 6, 2018

Page 2

Respectfully submitted,

Terra Pacific Group Incorporated

Murray Wunderly, PG, CHG Principal Hydrogeologist/ Project Manager Attachments: Figure 1 – Site Location Map Figure 2 – Site Plan Showing Sampling Locations Attachment A – Field Sampling Procedures cc: Mr. Mike Lee, Pacific Gas and Electric Company Mr. Max Reyhani, Terra Pacific Group, Inc.

FIGURES

SITE LOCATION MAP

FORMER NAPA-1 MGP SITENAPA, CALIFORNIA

FIGURE 1

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Environmental Engineering, Consulting, and Construction

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FIGURE 2

NOTES:INFORMATION ABOUT MGP FEATURES WAS OBTAINED FROMAMEC'S 2015 SITE CHARACTERIZATION REPORT. PARCEL LINESOBTAINED FROM NAPA COUNTY GIS WEBSITE, MODIFIED BASEDON APN MAP AND FOULK CIVIL ENGINEERING SURVEY DATED10/30/2014. AERIAL BASE FROM ESRI.

PARCEL LINE

CURRENT PROPERTY BOUNDARY

FORMER MGP STRUCTURE

FORMER MGP 4-INCH BELOW GROUND OIL PIPELINE

FORMER MGP 6-INCH PIPE BELOW GROUND PIPELINE

SOIL GAS PROBE

ATTACHMENT A

Field Sampling Procedures

Attachment A – Field Sampling Procedures Former Napa-1 Manufactured Gas Plant

Page A-1

FIELD SAMPLING PROCEDURES

Sampling activities and procedures presented in this attachment will be followed during the implementation of soil gas sampling activities currently planned to be conducted in September 2018.

PERSONNEL AND PROCEDURES The environmental work described will be performed by qualified and experienced field staff working under the supervision of a California Professional Geologist or Civil Engineer. Supervised field activities will include the following: collecting soil gas samples; coordinating delivery of the samples to a certified analytical laboratory; and overseeing all phases of work including managing subcontractors, developing sampling forms and analyzing data. Field procedures will follow applicable local, county, and state guidelines at the time the work was performed.

HEALTH AND SAFETY PLAN A Health and Safety Plan (HASP) has been prepared in conformance with the U.S. Occupational Safety and Health Administration (OSHA) and California Division of Occupational Safety and Health (Cal/OSHA) guidelines set forth in “Hazardous Waste Operations and Emergency Response” (29 Code of Federal Regulations [CFR] 1910.120 and Title 8, CCR, Section 5192). The HASP provides information regarding anticipated site health and safety matters, and establishes policies and procedures adequate to protect site workers from predicted site hazards. The HASP includes specific information regarding chemicals of potential concern and the hazards associated with the fieldwork. The information includes action levels, responses to injuries that may be caused by exposure to potential site contaminants, directions to the nearest hospital, a list of emergency phone numbers, and material safety data sheets.

SOIL GAS SAMPLING PROCEDURES Soil gas samples will be collected in general accordance with the July 2015 “Advisory-Active Soil Gas Investigations (ASGI)” published jointly by the DTSC, the Regional Water Quality Control Board, Los Angeles Region and the Regional Water Quality Control Board, San Francisco Region. Soil gas samples will be collected from dual depth soil gas probes SG-5 through SG-12, B3-1, D3-1, E2-1, and H2-1 (Figure 2). Soil gas samples will be analyzed for volatile organic compounds (VOCs), including naphthalene, using EPA Method TO-15, as well as oxygen, methane and carbon dioxide by ASTM D-1946. Select samples will also be analyzed for helium by ASTM D-1946. Procedures which will be used to sample soil gas probes are described below.

Soil Gas Sampling Varying field conditions such as rainfall or fine-grained soils can impact soil gas sample collection. Soil gas sampling will not be conducted during or within a minimum of 5-days following a significant rain event (rainfall of ½ -inch or more over a 24-hour period).

Leak Test Leak testing will be conducted at each soil gas sampling probe to evaluate whether a good seal has been established in the sampling train, ground surface, and probe interface. Leak testing consisting of (1) a “vacuum shut-in test” to test the aboveground sampling equipment’s ability to hold vacuum in the tubing, valves, and connections located between the soil gas probe and passivated stainless-steel

Attachment A – Field Sampling Procedures Former Napa-1 Manufactured Gas Plant

Page A-2

canister and (2) a chemical tracer leak test using a leak-check compound (e.g., helium). Leak testing will be conducted prior to soil gas sample collection at each soil gas probe to test the integrity of the sampling system, probe, and borehole seal.

The “vacuum shut-in test” will be conducted at every soil gas probe prior to soil gas purging to verify there were no leaks in the aboveground sample train equipment and connection points, which includes the valves, tubing, and fittings between the soil gas probe and the passivated stainless-steel canister. This test will consist of applying a vacuum of at least 100” of water column (W.C.) to the tubing and valve system between the soil gas probe and passivated stainless-steel canister, closing the valves to seal the vacuum in the line, and verifying that the vacuum (100” W.C.) is maintained for at least 60 seconds in the line. If the vacuum is not maintained for 60 seconds, the tubing and valves will be changed and/or adjusted, and the test will be conducted again until the vacuum can be maintained for a minimum period of 60 seconds indicating a leak is not occurring in the aboveground sample train equipment and connection points.

Following a successful vacuum shut-in test, the chemical tracer leak test will be conducted during and after soil gas probe purging and during sampling at each soil gas sample probe using helium gas to check the ground surface seal and probe interface for ambient air leaks into the subsurface. A solid shroud, consisting of a plastic cap or box, will be placed to encompass only the soil gas probe at the surface. Once the shroud is in place, helium will be introduced into the shroud prior to purging the soil gas probe. The helium concentration in the shroud will be maintained at a minimum concentration of 30 percent by volume or at least two orders of magnitude higher than the reporting limit of the field helium detection meter used. During purging, the helium concentration within the shroud will be measured with a handheld helium detector to maintain the required minimum concentration of 30 percent by volume throughout the purge period.

During purging before sample collection, purged soil gas will be collected from the soil gas probe in 1-liter Tedlar bags (using a “lung box”). The helium concentration in soil gas collected in the Tedlar bag will be measured using the helium detector. If the helium concentration within the Tedlar bag is less than or equal to 5 percent of the minimum concentration maintained in the shroud during probe purging, then the soil gas sample will be considered to not have a significant leak, and the sample will be collected in a passivated stainless-steel canister. The leak check test helium concentrations measured in the Tedlar bag will be recorded on soil gas sampling forms for each soil gas sample. However, if the measured helium concentration within the Tedlar bag is more than 5 percent of the minimum helium concentration maintained in the shroud, then the chemical leak check test will be considered to have failed, and the shroud and sample train connections will be checked, readjusted, and replaced as needed. The vacuum shut-in test will be repeated followed by purging a minimum of 1 purge volume through the probe and sampling train to clear ambient air that may have entered the tubing during trouble-shooting. The chemical leak check test will then be repeated as described above. If the probe continues to fail the chemical leak check test, soil gas samples will be collected and the samples will be analyzed for helium by the offsite analytical laboratory.

The helium gas will continue to be applied to the shroud throughout the collection of the soil gas sample in the passivated stainless-steel canister. Helium will be analyzed in a minimum of 10 percent of the samples submitted to the off-site analytical laboratory.

Attachment A – Field Sampling Procedures Former Napa-1 Manufactured Gas Plant

Page A-3

Soil Gas Purging and Sample Collection Prior to purging, the purge volume (also referred to as the “dead space volume”) for each probe will be estimated by adding the following volumes: (1) the internal volume of tubing used over the length of probe from the tee-valve at the surface down-hole to the stainless-steel probe, (2) the estimated void space of the sand pack around the probe tip, and (3) the estimated void space of the dry bentonite in the annular space above the sand pack. Per ASGI, a default of three purge volumes will be used for all soil gas probes prior to sample collection. Purge volume calculations will be recorded on soil gas sampling forms for each soil gas sample.

Each soil gas probe will be purged at a consistent purge rate ranging between 100 and 200 mL/min using a pump, unless low-flow conditions were encountered. Vacuums less than 100” W.C. will be maintained during purging and sampling to minimize partitioning of vapors from pore water to soil gas, to prevent ambient air from diluting the soil gas samples, and to reduce the variability of purging rates. A flow meter and a vacuum gauge will be located in the sample train between the soil gas probe and the purging pump, but not in-line with the sample container. The sample container will be connected to the soil gas probe and purging line using a “tee” fitting or three-way manifold connection.

Soil gas samples will be collected using 1-liter passivated stainless steel canisters. The canisters will be appropriately decontaminated and batch-certified by the off-site stationary analytical laboratory prior to use. An evacuated canister will be attached to the probe tubing via a laboratory-provided manifold and other fittings assembled prior to the leak check tests. After the soil gas probe is appropriately purged and the leak check tests completed, the soil gas sample will be collected. A flow restrictor calibrated by the analytical laboratory to flow at a rate between 100 to 200 mL/min will be used to collect the soil gas sample in the canister. A vacuum gauge will also be used to ensure a vacuum of less than 100” W.C. was maintained during sampling.

Per ASGI, field duplicate samples were collected at a rate of one duplicate sample per 20 samples or per batch shipment to the laboratory, whichever was more often. Duplicate samples will be collected at the same time as the primary sample using a “tee” fitting and two separate passivated stainless-steel canisters, each with pre-set flow control regulators.

In addition to duplicate samples, field QC samples will also include material blanks and ambient air samples. The material blank will consist of sampling ambient air through an unused ¼-inch Teflon tubing section with the associated fittings and valves that will be used for collection the regular soil vapor samples. The material blank sample will be collected in a stainless-steel canister to verify and document the soil gas sampling equipment was not contributing to soil gas sample detections. A material blank will be collected at a rate of one per batch of new tubing used per ASGI. The ambient air sample will consist of collecting onsite ambient air over a 24-hour period, directly into a decontaminated passivated stainless-steel canister to assess ambient air background concentrations. The analytical laboratory will also analyze method blank samples as required by the analytical method to evaluate the analytical train in the laboratory. All field QC sample details will be recorded on a soil gas sampling form that identifies the location at which the field QC samples were collected. Material and ambient air samples will be collected simultaneously at locations where no recognizable sources of contaminants are being released to ambient air, including running vehicles or other operating engines. A minimum of one material blank and one ambient air sample will be collected per batch of tubing.

Attachment A – Field Sampling Procedures Former Napa-1 Manufactured Gas Plant

Page A-4

The soil gas samples will be appropriately labeled using securely attached labels and waterproof marker at the time of sample collection. Soil gas sample labels will include the sample ID, soil gas probe name and depth, and the date and time of sample collection. Chain-of-custody forms will be completed on-site following sample collection. The samples will be placed in a dry container for transportation to the laboratory.