DSHW-2025-000618

February5, 2025
Dean P. Anderson
Vice President
Tesoro Refining & Marketing Company, LLC
474 West 900 North
Salt Lake City, Utah 84103-1494
RE:Approval ofthePartial Groundwater Recovery Shutdown and Monitoring Plan;
Marathon, Salt Lake City RefineryEPA #UTD000826362
Dear Mr. Anderson:
The Division of Waste Management and Radiation Control (Division) has received a revisedPartial Groundwater Recovery Shutdown and Monitoring Plan; Marathon, Salt Lake City Refinery(Plan)
submitted on January 20, 2025, by Tesoro Refining and Marketing Company, LLC(Marathon) for the Marathon Salt Lake City Refinery (Site).
The initial Plan was submitted on December 17, 2024, and outlined actions to shut down the pump-and-treat system (System)and gauge light non-aqueous phase liquid (LNAPL) in groundwater
at the Site. The Division responded to the initial Plan with comments on January 2, 2024 (DSHW-2025-000022) suggesting that the System be shut down and that the proposed LNAPL evaluation
criteria be modified in a revised Plan.
The revised Plan includes the Data Evaluation Criteria and Tools section, proposing performance metrics to evaluate LNAPL post-System shutdown. The Plan indicates the threshold LNAPL
thickness of 1 foot will trigger a bail down test to evaluate transmissivity. If transmissivity is below 0.8 square feet per day (ft2/day) then gauging will continue and no additional
action will be performed. However, if transmissivity is above 0.8ft2/day additional action will be required to evaluate and address the LNAPL. The Division’s concerns are 1.) determination
of whether the LNAPL is residual or a new source, 2.) migration of LNAPL regardless of rate, and 3.) whether the LNAPL is contributing to the dissolved plume on-Site.
The Division agrees with the proposedgroundwater and LNAPL gauging proposed in the Plan. The gauging results should dictate whether actions should be taken to evaluate the LNAPL thickness
or migration. If a considerable increase in LNAPL thickness is observed, the Division reserves the right to request investigation into the source and/or transmissivity rate of LNAPL.

Additionally, a Post Corrective Action Plan (CAP), Annual Groundwater Monitoring Plan dated July 16, 2024 (DSHW-2024-007597) was submitted to the Division. After identification of the
LNAPL and the proposal to restart the System so that maintenance of the underground sewer line could be conducted, the Division recommended waiting to move forward with the Post CAP
Annual Groundwater Monitoring Plan. Now that the maintenance has been performed the Division will move forward in reviewing the Post CAP Annual Groundwater Monitoring Plan to address
the dissolved phase plume. In addition to evaluating the dissolved phase plume, the Post CAP Annual Groundwater Monitoring plan will need to be revised to include the monitoring of the
LNAPL thickness and whether LNPAL is contributing to the dissolved phase plume both on-Site and in sentinel wells off-Site. If it is determined that the LNAPL is migrating off-Site or
contributing to the dissolved phase plume, corrective action and/or additional monitoring will be required.
The Division has no comments or questions concerning thisplan. The Plan is hereby approved.
If you have any questions, please call Ethan Upton at [phone].
Sincerely,
Douglas J. Hansen, Director
Division of Waste Management and Radiation Control
DJH/EAU/
c:Angela C. Dunn, MD, MPH, Health Officer, Salt Lake County Health Dept.
Dorothy Adams, Deputy Director, Salt Lake County Health Dept.
Ron Lund, Environmental Health Director, Salt Lake County Health Dept.
James Beardall, Marathon Petroleum Company, ([e-mail])
Chris Kaiser, Marathon Petroleum Company, ([e-mail])
Scarlett Ball, Antea Group, ([e-mail])