EPA issued a fresh enforcement alert in September 2025 focused on Benzene Fenceline Monitoring at Petroleum Refineries, outlining EPA’s expectations for root cause analysis (RCA) and corrective actions for fenceline action level exceedances under the Refinery Maximum Available Control Technology (MACT) program (40 CFR 63 Subpart CC). Despite the current administration’s deregulatory push, the alert underscores EPA’s continued willingness to pursue penalties and injunctive relief—including new control equipment—where EPA believes refineries have fallen short.

Fenceline Monitoring Process and Pitfalls

Under the fenceline program, facilities must collect fenceline monitoring samples from a series of passive tube monitoring stations every 14 days and evaluate whether the annual average benzene concentration exceeds the action level within 45 days after the end of each sampling period. To make the action level determination, the facility must:

1. Calculate the facility’s theoretical benzene impact (Δc) as specified in 40 C.F.R. § 63.658(f)(1).
2. Compute the annual average Δc using the average of the 26 most recent 14-day sampling periods.
3. Compare the annual average Δc to the benzene action level to assess a potential exceedance.

Although the regulation only requires the 14-day delayed sampling, the absence of real-time data creates inherent challenges with identifying root causes, where the source of the benzene emissions may no longer be emitting at the time the investigation commences. This can be particularly challenging when the source of the exceedance originates from outside the facility fenceline. Further, the exceedance may be caused by emissions sources that are fully compliant with applicable regulations. However, regardless of the actual cause of the exceedance, the owner or operator must begin a root cause analysis within five days of making that determination, and no later than 50 days after the sampling period ends.

The root cause analysis must:

1. Identify the cause(s) of the exceedance for both primary and contributing factors.
2. Include an initial corrective action analysis to determine the measures needed to reduce the annual average benzene concentration (Δc) to or below 9 μg/m³.

The facility must complete the root cause analysis and implement initial corrective actions within 45 days of confirming the exceedance (40 C.F.R. § 63.658(g)).

In an attempt to navigate the inherent investigation challenges EPA notes the following best practices for finding root causes of action level exceedances:

• Monitors, such as gas chromatographs that can provide near real-time measurement, can be better correlated with wind direction than the two-week diffusion tube samples required by Method 325, and can specifically measure benzene at the parts per billion level;
• Forward looking infrared or “FLIR” optical gas imaging cameras to quickly screen large areas for emissions and leaks; and,
• Benzene-specific handheld photoionization detectors that can measure parts per billion benzene at emissions interfaces to pinpoint benzene emissions.

Consistent with previous alerts, these recommendations and suggested investigative tools go beyond the Refinery MACT requirements with EPA choosing to regulate through enforcement in lieu of revisions or clarifications to the Refinery MACT itself.

As a work around to the investigation challenges posed by offsite or exempt sources, the alert references the existing Refinery MACT provisions for site-specific fenceline monitoring plans may be requested for EPA approval. Specifically, the site specific monitoring provisions are mean to account for offsite upwind sources or onsite sources specified in 40 C.F.R. § 63.640(g) such as:

• Research and development facilities,
• Units processing natural gas liquids;
• Units that are used specifically for recycling discarded oil;
• Shale oil extraction units;
• Ethylene processes; and
• Process units and emission points subject to 40 CFR 63, Subparts F, G, H, and I

What EPA believes are common root causes

EPA believes that the following refinery emission sources are frequently causing benzene action level exceedances.

• Tanks storing high benzene content material such as pure benzene product, reformate, light naphtha, and pyrolysis gasoline from co-located ethylene plants.
• Marine vessel loading operations involving the high benzene content material listed above and the poor operation of loading arms; improper disposal of slop waste that drips from loading arms; barges that are not fully vapor tight; and/or barge pressure relief valves lifting during loading or while the barge is docked.
• Wastewater operations including uncontrolled dissolved air floatation systems and aerated biological treatment units, and uncontrolled junction boxes.

The continued emphasis on wastewater operations ties together EPA’s intensifying enforcement under the Benzene Waste Operations NESHAP (BWON, or 40 CFR 61 Subpart FF) program as well as recent enforcement activities focused on storage tank emissions.

BWON Overlap and Consent Decrees

The recent alert follows EPA’s February 2024 alert that centered on BWON noncompliance at refineries and ethylene plants, detailing perceived inspection pitfalls and control strategy gaps. Although the underlying BWON regulations have not been significantly revised since 1993, U.S. EPA was offered the opportunity to formally incorporate any enhanced requirements via the risk and technology review cycle for the refinery MACT regulations (40 CFR 63 Subparts CC and UUU). 40 CFR 63 Subpart CC includes wastewater requirements for refineries which cross reference the BWON. During this rulemaking, U.S. EPA decided to not adopt any additional requirements for refinery wastewater collection and treatment systems based partially on the concept that new benzene fenceline monitoring requirements would be sufficient to assure adequate control from wastewater sources. With this alert, U.S. EPA reiterates its intention to utilize benzene fenceline data under the Refinery MACT program to drive wastewater emissions control, and to pursue penalties and injunctive relief through Consent Decree regulation in conjunction with intensifying BWON enforcement.

The recent alert references a Spring 2025 enforcement case addressing violations of the Refinery MACT fenceline monitoring program. This case pertains to the HF Sinclair Navajo Refining LLC consent decree issued in January 2025, following two earlier BWON-related consent decrees — the 2024 Lima Refining Company and the 2023 BP Products North America Inc. cases. While the recent alert highlights the fenceline monitoring aspects of the HF Sinclair Navajo Refining LLC consent decree, the underlying noncompliance primarily involved BWON violations and the implementation of enhanced control requirements. However, unlike the prior BWON-focused consent decrees, the HF Sinclair decree also imposes additional requirements for non-wastewater sources (e.g., storage tanks), giving this case a distinct scope and character compared to its predecessors.

EPA’s current enforcement direction and willingness to pursue future consent decrees had appeared uncertain amid the current administration’s broader deregulatory agenda and push for unleashing American energy. However, with this recent alert, EPA has now publicly signaled its intent to continue pursuing enforcement at refineries, with expectation that this will include the pursuit of consent decrees based on alleged noncompliance with BWON requirements in parallel with the Refinery MACT fenceline monitoring program. Building on the recent consent decrees, future negotiations are expected to include capital investments in enhanced controls as part of injunctive relief, along with expanded monitoring, additional fenceline and community monitoring, and innovative or “novelty” testing requirements.

How Trinity can help

Trinity supports Fenceline Benzene Monitoring Services including Real-Time Fenceline Monitoring Services with Open Path Technologies. In addition, Trinity can support RCA facilitation and corrective action plan development and implementation, BWON/QQQ program audits, and site-specific monitoring plan preparation—aligning your internal compliance programs with EPA’s 2025 enforcement posture. Trinity has also been assisting a number of refineries and chemical plants in support of various stages of the current EPA enforcement process.

Please reach out to David Wall – Director Refinery Sector Services – for more information on current the enforcement initiative and how Trinity can help.