California's progress removing aging single-walled storage tanks

California's regulation of underground storage tanks (USTs) predates federal standards. California passed the nation's first state UST statutes in 1983, with an effective date of January 1984. The federal regulations developed by US EPA did not become effective until December 1988 and incorporated aspects of California's innovative approach.

Underground storage tanks (USTs) are used to store large quantities of petroleum products (gasoline, diesel, oil, etc.) or other hazardous substances, such as industrial chemicals and pesticides. They generally are made from steel, fiberglass, or a combination of materials. While commonly found at current or former gas and service stations, they also are located at airports, industrial plants, municipal corporation yards, and military bases. UST systems also include piping and a pump device for moving the stored contents.

Until the 1980s, USTs only had one wall that acted as a barrier between their contents and the surrounding soil. Many USTs were made of bare steel, which is prone to rust and corrosion, causing leaks over time. Without secondary containment to capture leaking fluids, single-walled USTs are much more likely to leak into the surrounding soil, and potentially into groundwater. Aging components and improper installation, operation, or maintenance also increase the risk of a leak.

Yes, even double-walled USTs can leak because of improper installation, age and corrosion, damage from shifting soil, or improper maintenance. The outer (double) wall serves as a barrier to contain any leaks from the inner tank and a sensor between the two detects any leaks, allowing for a quick response to prevent any release of the tank contents to soil or groundwater.

Petroleum products and other hazardous substances that leak from USTs can pollute the soil and groundwater, and even an entire waterway, if the aquifer has been impacted. Relatively small leaks can have outsized effects on the environment and nearby drinking water sources.

A leaking UST also may emit vapors that can lead to the inhalation of toxic chemicals or even an explosion. These risks result from a process called "vapor intrusion" during which leaking chemicals can form vapors that travel through soil, accumulate under buildings and contaminate indoor air through cracks in foundations, utility lines, and crawl spaces.

Leaks from single-walled USTs frequently go undetected until they are removed or closed in place. Of the single-walled tanks closed so far over the past several years, about 30% were found to have leaked. While a UST can be removed or closed-in-place in a matter of months, it can take decades to clean up a leaking UST site.

According to state law, all single-walled USTs and components must be permanently closed by Dec. 31, 2025. Single-walled USTs that are not permanently closed by the end of 2025 will be considered out of compliance and cannot be operated legally. The State Water Resources Control Board is responsible for the regulation of USTs, while local agencies carry out enforcement (see below).

Beginning Jan. 1, 2026, owners/operators of non-compliant single-walled USTs or piping systems will be subject to enforcement action, which includes "red tagging" tanks to prevent fuel deliveries and dispensing, and civil penalties ranging from $500 – $5,000 per tank per day per violation. Per Health and Safety Code section 25292.05, there are no exceptions, exemptions, or extensions of this requirement.

Installation of single-walled USTs has been prohibited under California law since 1984 (with limited exceptions). Current law requires that all UST systems:

• Be constructed of materials that do not rust.
• Are "double-walled" or include secondary containment to protect soil and groundwater from leaks of hazardous substances.
• Include both leak prevention and detection measures, including corrosion-resistant piping, spill containment, and regular testing.
• Comply with various monitoring, inspection, and replacement standards.

In 1998, federal law required that all USTs be upgraded to prevent future releases due to corrosion or spills and that all owners and operators of petroleum USTs demonstrate financial responsibility and the resources necessary to clean up a site if a leak occurs. Federal law does not set a universal deadline for closing single-walled USTs but rather allows states to establish their own programs and set their own deadlines.

Efforts in California to regulate USTs predate those of the federal government. California passed the nation's first state UST statutes in 1983, with an effective date of January 1984. The federal regulations developed by US EPA did not become effective until December 1988 and incorporated aspects of California's innovative approach.

Closure of a UST system involves pre-closure, closure, and post-closure activities. Pre-closure activities involve obtaining local and state permits, securing funding (if needed), and hiring a qualified contractor to develop a closure plan. After all permits have been obtained and funding is in place, the actual closure may proceed.

Closure of a UST system involves emptying and cleaning the tank, after which it is either removed or closed-in-place. Closure also involves a site assessment to determine if the soil around the tank has been contaminated. If soil contamination is discovered, an extensive site investigation is performed, followed by remediation of the soil and groundwater, as necessary. When the closure is complete, a final report is submitted to the local Certified Unified Program Agency (CUPA), and the closure is reported through the California Environmental Reporting System (CERS) and GeoTracker.

The environmental damage caused by leaking USTs ranges from small soil-only impacts to large plumes of contamination that can travel long distances. Once hazardous substances are in the soil, they can become a long-term source of soil, groundwater and air pollution. Remediating groundwater pollution from a leaking UST is difficult, expensive, and can take anywhere from a few years to several decades to resolve.

There were approximately 50,000 single-walled UST systems installed in California before the Legislature prohibited the installation of new single-walled tanks in 1984. As of December 2025, over 99% of those single-walled USTs have been closed.

The State Water Board maintains updated information and a map on its website of remaining single-walled USTs and/or piping requiring closure by Dec. 31, 2025. As of Dec. 5, 2025, there still were 279 facilities with 753 single-walled components (tanks, tank piping, or system piping) requiring closure to prevent enforcement actions (Note: One facility may include multiple single-walled UST components).

Of these 279 facilities:

• 58 had not submitted a UST closure permit application to their local Certified Unified Program Agency (CUPA).
• 244, or 87%, are privately-owned.
• 35, or 13%, are government-owned.

The Board expects to update this information in February 2026.

Single-walled UST systems that still are in operation have a much higher chance of leaking due to their age. The State Water Board estimates that over 30 percent of the remaining single-walled UST systems have had or will have a release that requires cleanup. This cost of cleaning up these leaking single-walled USTs will continue to rise with each additional year that they remain in the ground.

Of the remaining 279 non-compliant UST sites (as of Dec. 5, 2025):

• 31 are within 1,000 feet of a public drinking water well.
• 101 are within one mile of a drinking water well.
• 147 are more than 1 mile from a drinking water well.

The risk to drinking water sources cannot be defined by proximity alone, because it depends heavily on soil type and groundwater flow.

About 60% of the facilities with single-walled tanks that have not been properly closed or removed are in economically disadvantaged or severely economically disadvantaged communities (DACs or SDACs). However, this does not necessarily mean that the owner/operator is disadvantaged. DAC and SDAC communities also tend to be disproportionately burdened by multiple sources of pollution, as measured by CalEPA's Office of Environmental Health Hazard Assessment's CalEnviroScreen.

Beginning Jan. 1, 2026, the State Water Board expects locally designated regulatory agencies (i.e. Certified Unified Program Agencies) to initiate enforcement actions against owners/operators of non-compliant single-walled UST components. These actions include "red tagging" tanks to prevent fuel deliveries and assessing civil penalties of $500 – $5,000 per tank per day per violation. The Board will work with local regulatory agencies to monitor enforcement until all single-walled USTs are closed.

At least eleven years. Effective Sept. 25, 2014, state law was amended to require that single-walled UST components be permanently closed by Dec. 31, 2025. The State Water Board and local regulatory agencies (Certified Unified Program Agencies, or CUPAs) have been working since 2014 to ensure that single-walled UST owners/operators are aware of this deadline and the funding opportunities available through RUST and the UST Cleanup Fund.

The State Water Board has primary responsibility for regulating USTs in California, including establishing state law, oversight, and enforcement. The Board also administers financial assistance programs related to petroleum USTs. In addition, the Board contracts with 81 local agencies throughout California, called Certified Unified Program Agencies (or CUPAs), to conduct regulatory oversight of UST cases. The CUPAs have day-to-day responsibility for regulating USTs, including permitting, oversight, and enforcement. However, CUPAs are not authorized to regulate the cleanup of leaks from USTs – this is overseen by the nine California Regional Water Quality Control Boards and the five State-certified Local Oversight Program (LOP) agencies.

CUPAs are generally housed within a division of the applicable county's environmental health department or hazardous materials division. Some cities have their own local permitting agencies, but the county CUPA generally oversees the process for cities within its jurisdiction.

In coordination with the applicable Certified Unified Program Agency, the nine California Regional Water Quality Control Boards (Regional Water Boards) and the five Local Oversight Program agencies (LOPs) investigate discharges from USTs that impact or have the potential to impact water quality in their respective regions. In such cases, the appropriate Regional Water Board or LOP oversees site investigation and cleanup. During cleanup, the Regional Water Board or LOP must ensure that any contaminated soil, groundwater, and/or soil vapor are remediated to the standards set forth in the State Water Board's Low-Threat UST Case Closure Policy (Policy).

When a site meets the requirements in the Low-Threat UST Case Closure Policy, the Regional Water Board or Local Oversight Program agency (LOP) will issue a pre-closure notification to inform the public of their intent to close the environmental case. The public has 60 days to comment on the proposed closure and provide any new information not previously presented to the Regional Water Board or LOP.

If no comments are received or the public comments do not provide new information, the Regional Water Board or LOP will issue a "uniform closure letter" (otherwise known as a "no further action" letter) to the owners, operators, or other responsible parties, as required by Health & Safety Code section 25296.10 (g).

The average cost to remove a single-walled UST ranges from $30,000 to $100,000. Should a UST leak be identified, the cost to clean up the contamination from a UST ranges from between $1.1 million and $1.5 million.

California has provided billions of dollars (detailed below) in grants and loans to owners/operators to remove, or remove and replace, their single-walled petroleum USTs. Most funding packages included a combination of grants and low-interest loans. The State Water Board manages both the RUST and the UST Cleanup Funds.

The Replacing, Removing, or Upgrading Underground Storage Tanks (RUST) Program was established in 1989, and the Board began administering the program in 2003. The RUST program provides state financing for up to 100 percent of the costs necessary to upgrade, remove, or replace petroleum USTs (Note: A single-walled UST cannot be upgraded, it only may be closed in place, removed, or removed and replaced).

• Since 2015, the RUST program has provided $183.1 million to approximately 630 owners/operators to remove, replace, or upgrade their petroleum USTs.
• Eligibility for RUST funding is limited to California-based, independently-owned-and-operated small businesses.
• Low-interest loans of between $10,000 to $750,000 have been provided for terms of 10 or 20 years.
• RUST grants do not need to be repaid. With very limited exceptions, the maximum grant that any entity could receive is $70,000.

The Underground Storage Tank Cleanup Fund (UST Cleanup Fund), established in 1989 and funded through a $.02 per gallon UST Petroleum Storage Fee, reimburses eligible owners/operators for the cost of cleaning up leaks from petroleum USTs. Since the early 1990s, the UST Cleanup Fund has provided over $5 billion for approximately 3,800 claims to remediate soil and groundwater contamination from leaking petroleum USTs.

• Eligibility for the UST Cleanup Fund is limited to California-based, independently-owned-and-operated businesses and local governments.
• Eligible reimbursement costs include cleanup costs, such as preliminary site assessment, soil and water investigations, remediation activities, monitoring, and regulatory technical assistance costs.
• The UST Cleanup Fund also reimburses court-approved claims for specified third party costs.
• The UST Cleanup Fund is set to sunset on Jan. 1, 2036, and the deadline for submitting a reimbursement claim is Dec. 31, 2034.

In 2024, the California Legislature extended the sunset date of the UST Cleanup Fund from Jan. 1, 2026, to Jan. 1, 2036. After Jan. 1, 2035, no new UST Cleanup Fund claims will be accepted for reimbursement. UST owners/operators may continue to use the UST Cleanup Fund to demonstrate financial responsibility for cleanup costs and third-party coverage as required by state and federal law until Jan. 1, 2035. Without this extension, petroleum UST owners/operators would have been required to secure other financial assurance mechanisms (like private insurance) that would have been prohibitively expensive.

Delays in UST closures have occurred for several reasons but are largely the result of incomplete applications or a need for the applicant to wait until funding is available. Delays also have resulted from very late action by some owners/operators. Since 1989, the owners/operators of nearly 99% of single-walled USTs have properly closed their USTs.

As previously noted, Certified Unified Program Agencies (CUPAs) are responsible for regulatory oversight of UST removal and closure activities, including permitting (but not including cleanup). The Board administers financial assistance for small businesses to support these activities through the RUST program.

Prior to June 2025, owner/operators could not begin their UST projects until the Board executed their RUST funding agreement. In June 2025, Senate Bill 124 revised the RUST statutes to allow eligible single-walled UST owner/operators to start their projects before their RUST agreement is executed - so long as they submitted a RUST application to the Board by Jun. 30, 2025.