Technical_Memo_Woodlawn_Methane_Analysis_August_2025.pdf

SUBMITTED TO:

HDR

5551 10th Avenue NE, Suite 1200 Bellevue, WA 98004

BY:

Shannon & Wilson 400 N. 34th Street, Suite 100 Seattle, WA 98103

(206) 632-8020 www.shannonwilson.com

TECHNICAL MEMORANDUM

Woodlawn Methane Analysis

SEATTLE, WASHINGTON

August 7, 2025

Shannon & Wilson No: 114092-002

Technical Memorandum

114092-002 August 7, 2025 ii

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CONTENTS

1 Introduction

2 Applicable Regulations

3 Site Subsurface Conditions

3.1 Nearby Methane Monitoring Results

3.2 Woodlawn Avenue North General Subsurface Profile

4 Subsurface Construction and Anticipated Hazards

5 References

Figures Figure 1: Green Lake Methane 1,000-Foot Buffer Map

Appended Important Information

1 INTRODUCTION

Shannon & Wilson has prepared this methane analysis technical memorandum to assist Seattle Public Utilities (SPU) and HDR in the design and construction of the Woodlawn Avenue North Project, which is part of SPU’s North Seattle Water Main Rehab Package Six.

The memorandum will provide information detailing the potential risk of methane (CH4) intrusion, along with best management practices to be used during construction of the Project as per requirements of Title 10 of the King County Board of Health Solid Waste Regulations.

The Woodlawn Avenue North Project site is located in the Green Lake neighborhood of Seattle, Washington. The proposed Project includes improvements to the Woodlawn Avenue North right-of-way (ROW) between North 63rd Street and North 65th Street and in the Meridian Avenue North ROW between North 62nd Street and Woodlawn Avenue North.

The focus of the information in this memorandum will be associated with the north end of the Project site, located north of the intersection of North 65th Street. This portion of the Project site is situated within 1,000 feet of the former historic Green Lake Landfill site as identified in the City of Seattle’s 1984 abandoned landfill study. Figure 1 shows the approximate location of the Project site and the 1,000-foot CH4 buffer that surrounds the historic Green Lake Landfill.

The Woodlawn Avenue North Project site topography slopes down from the northeast to southwest, starting at the intersection of North 65th Street towards the intersection of North 63rd Street, with the overall topography surrounding the site sloping down to the west towards Green Lake.

Based on available information, the former Green Lake Landfill is not officially designated as a CH4-producing landfill; however, the presence of CH4 was documented by Herrera Environmental Consultants (Herrera) in 2021 as part of the proposed site redevelopment for the Green Lake Community Center. Landfill gas monitoring conducted by Herrera indicated that CH4 was present in the vicinity of the proposed Green Lake Community Center. CH4 readings collected by Herrara identified concentrations at levels above the lower explosive limit of 5% by volume (Herrera, 2021).

2 APPLICABLE REGULATIONS

Seattle Municipal Code 25.09.220 (Environmentally Critical Areas Code) indicates that development on abandoned landfills is subject to Seattle-King County Health Department requirements, per Title 22, Subtitle I, Building Code, and Seattle King County Health Department regulations. Title 22, Subtitle I, Building Code states that “areas within 1,000 feet of CH4-producing landfills may be susceptible to accumulations of hazardous levels of CH4 gas in enclosed spaces.”

Title 10 of King County Board of Health Solid Waste Regulation governs construction standards and CH4 controls on abandoned landfills. Authority is established under Revised Code of Washington (State) Chapter 70.05, Washington State Administrative Code (WAC) 173-304, Minimal Functional Standards for Solid Waste Handling, and WAC 173-351, Criteria for Municipal Solid Waste Landfills.

For construction on or within 1,000 feet of an abandoned landfill, Section 10.09.060 (A) states:

This construction restriction applies to all construction activities on or within one thousand feet (1,000’) of an active, closed or abandoned landfill that has been documented by the health officer to be generating levels of methane gas on-site at the lower explosive limit or greater levels. The distance shall be calculated from the location of the proposed structure to the nearest property line of the active or former landfill site.

For construction on or within one thousand feet of an abandoned landfill, Section 10.09.060

(B) states:

All enclosed structures to be built within the one-thousand-foot (1,000’) landfill zone must be protected from potential methane migration. The method for insuring a structure’s protection from methane shall be addressed in a report submitted by a licensed professional engineer to the local building department for approval. Such a report shall contain a description of the investigation and recommendation(s) for preventing the accumulation of explosive concentrations of methane gas within or under enclosed portions of the proposed building or structure. At the time of final inspection, the professional engineer shall furnish a signed statement attesting that the building or structure has been constructed in accordance with his/her recommendations for addressing methane gas migration.

http://seattle-wa.elaws.us/code/mc_title22

3 SITE SUBSURFACE CONDITIONS

3.1 Nearby Methane Monitoring Results

Based on the results of a Technical Memorandum prepared by Herrera in 2021, CH4 concentrations were detected, in some instances at explosive levels, in the vicinity of the existing Green Lake Community Center. CH4 concentrations were detected in both borings advanced and monitoring wells installed, as part of a field investigation completed to characterize potential landfill gas (LFG) conditions at the site. Additional landfill gases, including hydrogen sulfide (H2S) and carbon dioxide (CO2), were also monitored as part of this investigation. The information presented in the report was used to help make decisions regarding the design of a new community center planned for the site.

Elevated CH4 concentrations of up to 44%, exceeding the lower explosive limit of 5%, were detected during drilling within the hollow-stem augers. CH4 readings collected from monitoring wells installed at the site ranged from 0.1% by volume to a high of 62.4% by volume. No H2S was detected during drilling or from the finished monitoring wells installed as part of the project. CO2 was detected at concentrations that ranged from 0.1 to

13.6 parts per million by volume during drilling and in the monitoring wells.

3.2 Woodlawn Avenue North General Subsurface Profile

Review of available information, including Geologic Map of Seattle (Troost, et. al., 2005), the Final Geotechnical Report, Water Main Rehab Package 6 – North, prepared by SPU (2023), and geologic information presented at the Washington Geologic Information Portal website, the Project site appears to be generally underlain by Vashon glacial till. Historical explorations completed near the Project site presented in SPU’s Geotechnical Report indicated that the explorations completed generally encountered 6 feet of fill soils overlying medium dense to very dense glacial till deposits. Groundwater did not appear to be present in any of the historical explorations or soil cut observations (SPU, 2023).

4 SUBSURFACE CONSTRUCTION AND ANTICIPATED

HAZARDS

The Project will require subsurface excavation to complete some of the components associated with proposed site improvements. Site improvements that could be affected by the presence of landfill gases (which may include CH4 and CO2) include the following:

Installation of approximately 750 lineal feet of 8-inch restrained-joint, ductile iron pipe in Woodlawn Avenue North and intersections.

Installation of thirteen 8-inch gate values (three at North 63rd Avenue, five at North 64th Street, and five at North 65th Street).

Installation of two fire hydrants, one at North 64th Street and one at North 65th Street.

Based on the distance from the north portion of the Project site closest to the former historic Green Lake landfill, interpreted subsurface profile, and the general topography of the Project site, site construction activities, including excavation, are unlikely to encounter landfill gases or conditions that could impact the Project or worker health and safety.

However, since a portion of the Project site is located within 1,000 feet of the Green Lake Landfill, the proposed work must adhere to federal, state, and local regulations that are meant to protect the public from the risks of migrating landfill gases.

Specifically, construction restrictions as described in Sections A and B of 10.09.060 of King County Board of Health Solid Waste Regulation Title 10 for all construction activities within 1,000 feet of active, closed, or abandoned landfills that have been documented to generate levels of CH4 gas on-site at the lower explosive limit or greater levels apply to this Project.

Best management practices during construction of the Project in this area may include the following:

Monitoring subsurface soils for landfill gases (including CH4 and CO2) during the site excavation activities.

CH4 monitoring (Landfill Gas Monitoring) within portions of the project area that are located within the 1,000-foot CH4 buffer zone. Based upon available information, this works out to monitoring within the last 100 feet of the northern portion of the project area where excavation activities are being completed.

Monitoring should be completed using a calibrated four-gas meter. CH4 gas will be monitored relative to the lower explosive level (LEL).

The use of CH4 barriers or appropriate ventilation (positive pressure ventilation) in excavations completed within the 1,000-foot buffer.

If landfill gases are encountered during monitoring the use of appropriate ventilation (positive pressure ventilation) in excavations completed within the 1,000-foot buffer. For worker health and safety, CH4 barriers or trench dams should be considered where landfill gases are present. They will help prevent the lateral migration of landfill gases.

Positive pressure ventilation will help to ensure fresh air is moving within the area of excavation activities so that landfill gases do not accumulate to a point of concern to allow the work to be completed safely.

CH4 mitigation measures should be used if landfill gases are encountered at concentrations that exceed the following applicable action levels during monitoring:

Action levels for the landfill gases:

- Concentrations of CH4 at 10% of the LEL.

5 REFERENCES

Seattle, City of. (1984). Abandoned landfill study in the city of Seattle. Seattle-King County

Department of Public Health.

Troost, K. G., Booth, D. B., Wisher, A. P., & Shimel, S. A. (2005). The geologic map of Seattle (Open File Report 2005-1252) [map]. 1:24,000. U. S. Geological Survey.

Washington State Department of Ecology. (2024). Montlake landfill hazard assessment. Toxics Cleanup Program.

https://apps.ecology.wa.gov/cleanupsearch/reports/cleanup/sitedetails/1157 https://apps.ecology.wa.gov/cleanupsearch/reports/cleanup/sitedetails/1157

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PROJECT

LOCATION

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N 63RD ST

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N 65TH ST

N 59TH ST

N 60TH ST

N 61ST ST

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N 64TH ST

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PROJECT LOCATION

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(WOODLAWN AVE N)

STA: 38+27.92

(N 65TH ST)

STA: 38+47.46

(N 65TH ST)

STA: 15+00.00

(WOODLAWN AVE N)

COL

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12"TREE

12"TREE

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124.87'

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8"x6" TEE, FL 1-8" FLxRMJ ADAPTER 1-8" GV, FLxRMJ

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8" SL, RMJ W/(1-WRG)

STA 14+13.70, 10.00'LT

8" SL, RMJ W/(1-WRG)

STA 38+97.80, 10.00'LT

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8" 45° HB, RMJ

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TYPE 311 HYD

STA 37+84.50, 5.50'LT

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114092-002

FIG. 1

GREENLAKE METHANE

1,000-FOOT BUFFER MAP

Woodlawn Methane Analysis Seattle, Washington

August 2025

Approximate Location of Former Greenlake Landfill

LEGEND

1,000-Foot Methane Buffer of the Approximate Location of Former Greenlake Landfill

KEY MAP INSET

0 800 1,600

Scale in Feet

Green Lake

0 30 60

Scale in Feet

NOTES

Figure adapted from WATER MAIN REHAB PACKAGE 6

- NORTH, "Project Location Maps," Sheet G-002, 2 of 77 and "System Connection Details (2 of 5)," Sheet CU402, 40 of 77, dated 05/29/2025.

PROJECT AREA PLAN

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Important Information About Your Environmental Site Assessment/Evaluation Report

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ENVIRONMENTAL SITE ASSESSMENTS/EVALUATIONS ARE PERFORMED FOR

SPECIFIC PURPOSES AND FOR SPECIFIC CLIENTS.

This report was prepared to meet the needs you specified with respect to your specific site and your risk management preferences. Unless indicated otherwise, we prepared your report expressly for you and for the purposes you indicated. No one other than you should use this report for any purpose without first conferring with us. No one is authorized to use this report for any purpose other than that originally contemplated without our prior written consent.

The findings and conclusions documented in this site assessment/evaluation have been prepared for specific application to this project and have been developed in a manner consistent with that level of care and skill normally exercised by members of the environmental science profession currently practicing under similar conditions in this area.

The conclusions presented are based on interpretation of information currently available to us and are made within the operational scope, budget, and schedule constraints of this project. No warranty, express or implied, is made.

OUR REPORT IS BASED ON PROJECT-SPECIFIC FACTORS.

Our environmental site assessment is based on several factors and may include (but not be limited to) reviewing public documents to chronicle site ownership for the past 30, 40, or more years; investigating the site’s regulatory history to learn about permits granted or citations issued; determining prior uses of the site and those adjacent to it; reviewing available topographic and real estate maps, historical aerial photos, geologic information, and hydrologic data; reviewing readily available published information about surface and subsurface conditions; reviewing federal and state lists of known and potentially contaminated sites; evaluating the potential for naturally occurring hazards; and interviewing public officials, owners/operators, and/or adjacent owners with respect to local concerns and environmental conditions.

Except as noted within the text of the report, no sampling or quantitative laboratory testing was performed by us as part of this site assessment. Where such analyses were conducted by an outside laboratory, Shannon & Wilson relied upon the data provided and did not conduct an independent evaluation regarding the reliability of the data.

CONDITIONS CAN CHANGE.

Site conditions, both surface and subsurface, may be affected as a result of natural processes or human influence. An environmental site assessment/evaluation is based on conditions that existed at the time of the evaluation. Because so many aspects of a historical review rely on third-party information, most consultants will refuse to certify (warrant) that a site is free of contaminants, as it is impossible to know with absolute certainty if such a condition

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Unless your consultant indicates otherwise, your report should not be construed to represent geotechnical subsurface conditions at or adjacent to the site and does not provide sufficient information for construction-related activities. Your report also should not be used following floods, earthquakes, or other acts of nature; if the size or configuration of the site is altered; if the location of the site is modified; or if there is a change of ownership and/or use of the property.

INCIDENTAL DAMAGE MAY OCCUR DURING SAMPLING ACTIVITIES.

Incidental damage to a facility may occur during sampling activities. Asbestos and lead-based paint sampling often require destructive sampling of pipe insulation, floor tile, walls, doors, ceiling tile, roofing, and other building materials. Shannon & Wilson does not provide for paint repair. Limited repair of asbestos sample locations is provided. However, Shannon & Wilson neither warranties repairs made by our field personnel, nor are we held liable for injuries or damages as a result of those repairs. If you desire a specific form of repair, such as those provided by a licensed roofing contractor, you need to request the specific repair at the time of the proposal. The owner is responsible for repair methods that are not specified in the proposal.

READ RESPONSIBILITY CLAUSES CAREFULLY.

Environmental site assessments/evaluations are less exact than other design disciplines because they are based extensively on judgment and opinion and there may not have been any (or very limited) investigation of actual subsurface conditions. Wholly unwarranted claims have been lodged against consultants. To limit this exposure, consultants have developed a number of clauses for use in their contracts, reports, and other documents.

These responsibility clauses are not exculpatory clauses designed to transfer the consultant’s liabilities to other parties; rather, they are definitive clauses that identify where responsibilities begin and end. Their use helps all parties involved recognize their individual responsibilities and take appropriate action. Some of these definitive clauses may appear in this report, and you are encouraged to read them closely. Your consultant will be pleased to give full and frank answers to your questions.

Consultants cannot accept responsibility for problems that may develop if they are not consulted after factors considered in their reports have changed or conditions at the site have changed. Therefore, it is incumbent upon you to notify your consultant of any factors that may have changed prior to submission of the final assessment/evaluation.

An assessment/evaluation of a site helps reduce your risk but does not eliminate it. Even the most rigorous professional assessment may fail to identify all existing conditions.

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ONE OF THE OBLIGATIONS OF YOUR CONSULTANT IS TO PROTECT THE SAFETY,

HEALTH, PROPERTY, AND WELFARE OF THE PUBLIC.

If our environmental site assessment/evaluation discloses the existence of conditions that may endanger the safety, health, property, or welfare of the public, we may be obligated under rules of professional conduct, statutory law, or common law to notify you and others of these conditions.

The preceding paragraphs are based on information provided by the Geoprofessional Business Association (https://www.geoprofessional.org)