San Francisco Bay Regional Water Quality Control Board
February 3, 2026
Cost Recovery ID: 2020994
GeoTracker ID: T10000023282
TC III SPP North, LLC
Attn: Kyle Winkler
351 California Street, Suite 1110
San Francisco, CA 94101
[email protected]
SUBJECT: SMP Approval - 510, 920, 930, 950 De Guigne Drive and 935, 945 Stewart Drive, Sunnyvale, Santa Clara County
Dear Kyle Winkler:
This letter approves the December 23, 2025, Site Management Plan (Plan) and requests TC III SPP North, LLC (TC) to submit a completion report for the additional soil vapor and subslab sampling event, and a vapor intrusion mitigation system (VIMS) design plan, and an operation, maintenance, and monitoring (OMM) plan for the subject Property (Property). This letter will be maintained in GeoTracker for the regulatory record.
DoFuPent ,nIorPationௗௗ
x Date and Title: December 23, 2025, Site Management Plan x Prepared by: AEI Consultants
x Prepared on Behalf of: TC III SPP North, LLC
Regional Water Board Reviewௗ
x Reviewer’s Name: Emma Hoffman-Davies
x Result of Review: The Plan is accepted
BaFNgroundௗ
The Property is approximately 20 acres and consists of two parcels with five commercial office buildings. No documented discharges have been reported at the Property. Soil vapor quality at the Property is impacted by volatile organic compounds (VOCs) including trichloroethene (TCE), benzene, tetrachloroethene (PCE), from off property sources. Elevated concentrations of lead were detected in soil along the southern portions of the Property. The Property is proposed to undergo residential redevelopment, with the demolition of the existing buildings and construction of several townhomes. VIMS will likely be necessary at the townhomes that are proposed to be constructed in areas where elevated concentrations of TCE and PCE have been Attachment 16
2 detected in soil vapor. A Soil Vapor Investigation Work Plan proposing the collection and analysis of soil vapor samples to assess soil vapor quality at the Property prior to redevelopment activities was approved in our July 30, 2025, letter. The investigation identified elevated concentrations of VOCs in soil vapor primarily at the western property boundary. The Plan was submitted to address the elevated concentrations of VOCs in soil vapor prior to redevelopment and is summarized below.
Plan Summary
The Plan also proposes the collection of additional soil vapor sampling:
x Collect soil vapor samp les at 13 locations for the analysis of VOCs. x Collect subslab soil vapor samples beneath the existing buildings for the analysis of VOCs.
x The existing soil vapor data and the results for these samples will be used to determine the locations of the VIMS and to address data gaps.
The Plan provides the following information regarding potential risks to be encountered during redevelopment activities and VIMS for the residential units:
x Areas with lead impacts to soil and VOC impacts to soil vapor and groundwater will require mitigation/risk management.
x Risk management measures identified in Section 5.0 include soil management information, procedures when contamination is encountered, and construction impact mitigation measures.
x Buildings proposed to be located within an approximate 100-foot radius from a soil vapor sample containing 70 μg/m3 PCE and/or 55 μg/m3 TCE will include a passive VIMS and a low-permeability barrier.
x Prior to construction, a VIMS design and an OMM plan will be submitted for Regional Water Board review.
Requested Reports
TC is requested to submit the following reports by the following dates:
1. A VIMS Design and OMM Plan by May 29, 2026. This report should include the following elements at a minimum:
x Description of a VIMS to reduce the potential for vapor intrusion to occur, including plans and specifications for the proposed VIMS x A schedule for implementation of mitigation measures
x A monitoring plan to assess the effectiveness of the VIMS x A contingency plan if indoor air concentrations are greater than the ESL x Figures and iso-concentration maps showing locations for the mitigation measures, for existing soil vapor quality data
x Architectural drawings of the building(s) and VIMS
x Inventory, analysis, and monitoring of potential conduits such as utility penetrations, stairways, and elevator shafts
x Permanent sub-slab soil vapor monitoring wells for long-term monitoring of soil vapor concentrations beneath the buildings and VIMS
Attachment 16
3 2. A completion report for the Additional Soil Vapor Investigation by April 24, 2026.
The report should include the following at a minimum:
x Descriptions of fi eld activities
x Deviations from the scope of the Work Plan (if any)
x Figures illustrating the sampling locations and analytical results x Tabulated soil, soil vapor, and groundwater quality data that include historical data x Laboratory analytical reports
x Quality assurance/quality control sampling
x Boring logs, field sampling notes, permits
x Conclusions and recommendations for additional monitoring, investigation activities, and vapor intrusion mitigation, as necessary
The following documents provide information about how the Regional Water Board evaluates vapor intrusion. They also describe the information needed in various work plans and reports to support our evaluations and decisions. x Sections 6 - 7 of the Department of Toxic Substances Control October 2011 Vapor Intrusion Mitigation Advisory 3
x The USEPA 2015 OSWER Technical Guide for Assessing and Mitigating the Vapor Intrusion Pathway from Subsurface Vapor Sources to Indoor Air x Cal EPA June 2022, Vapor Intrusion Mitigation Guidance provides the relevant information
https://www.waterboards.ca.gov/rwqcb2/water_issues/programs/sitecleanup/202 2_VIM_Guidance.pdf
x Cal EPA February 2023, Supplemental Guidance: Screening and Evaluation Vapor Intrusion
GeoTracker regulations require TC to submit all documents in electronic format to the State Water Resources Control Board’s GeoTracker database, pursuant to California Code of Regulations, title 23, sections 3890-3895. See Electronic Submittal of Information for guidance on submitting documents to GeoTracker. Please note that this requirement includes all analytical data, monitoring well information (latitudes, longitudes, elevations, and water depth), site maps, and boring logs. All vapor intrusion sample location information should also be uploaded to GeoTracker. See Uploading Vapor Intrusion Information into GeoTracker for guidance on submitting sample location information. All data must be submitted as EDFs that comply with AB 2886 in accordance with the GeoTracker policy memo on estimated values and non-detect results.
Attachment 16
4 If you have any questions, please contact me at 510-622-2313 or emma.hoffman- [email protected].
Sincerely,
Emma Hoffman-Davies, P.G.
Engineering Geologist
cc:
Trent Weise, AEI, [email protected]
Tom Graf, GrafCon, [email protected]
Digitally signed by
Emma Hoffman-
Davies
Date: 2026.02.03
10:45:08 -08'00'
Attachment 16