Waste and Underground Tank Management Bureau

ExxonMobil Refining and Supply Company
Billings, Montana


ExxonMobil Refining and Supply Company Location Map Location Map 


The ExxonMobil Billings Refinery (Refinery) located northeast of Billings, Montana has been in operation since July 1949. The Refinery has the capacity to process approximately 60,000 barrels per day of domestic and Canadian crude oil into refined petroleum hydrocarbon products, by-products, and intermediate products.

Refinery operations are conducted on 367 acres of 770 acres owned by ExxonMobil; leaving approximately 403 acres of undeveloped land surrounding the Refinery operations.  The processing portion of the Refinery is bound to the south by railroad tracks and to the north by the Yellowstone River.  To the east, the processing and operations portion of the Refinery is bound by the former coke storage pile area, the Refinery’s wastewater treatment ponds, two inactive land treatment units, a former gravel quarry, and undeveloped land.  To the west of the Refinery is undeveloped land and an island of the Yellowstone River.  Beyond the property boundary are several businesses and residences.  The site is currently zoned for heavy industrial use.



Because ExxonMobil managed hazardous waste on-site they were required to obtain a hazardous waste permit under the Resource Conservation and Recovery Act (RCRA) and the Montana Hazardous Waste Act (MHWA).  RCRA is the federal law under which regulations concerning the management, treatment, storage, and disposal of hazardous waste are implemented.  The MHWA is the state equivalent to RCRA.

In 1988, ExxonMobil received a RCRA permit for the Refinery from the State of Montana, Department of Environmental Quality (DEQ).  The permit was issued for hazardous waste activities which included land treatment of oily hazardous wastes at three land treatment units (LTU), treatment of leaded tank bottoms in an open tank prior to off-site shipment, and storage of hazardous waste in containers prior to off-site shipment.  The permit imposed requirements for operation of these activities, monitoring, reporting, and corrective action for releases of hazardous waste or hazardous constituents.  Permits are issued for a term no longer than 10 years; ExxonMobil’s permit was reissued in 1999 and includes requirements for continued operation and/or closure of an operating land treatment unit and associated vehicle decontamination pad, an operating waste staging area, two land treatment units undergoing closure, and a lead weathering tank undergoing closure.



The Hazardous and Solid Waste Amendments of 1984 (HSWA) amended RCRA and included a requirement that owners and operators of hazardous waste facilities remediate releases of hazardous wastes or hazardous constituents from solid waste management units (SWMUs) and areas of concern (AOCs).  A SWMU is any unit that was used at any time to manage waste, regardless of whether the unit was intended for that purpose.  An AOC is any area at a facility having a probable release of a hazardous waste or hazardous constituent that may or may not be from a SWMU.  The HSWA corrective action requirements are established in Section 3004(u) of RCRA and 75-10-406(7) of Montana Code Annotated (MCA) of MHWA.  The requirements are codified in federal regulations at 40 Code of Federal Regulations (CFR) Section 264.101, as incorporated by reference in the Administrative Rules of Montana (ARM) 17.53.801.  Through HSWA, Congress required that permits issued to hazardous waste facilities contain corrective action requirements for SWMUs/AOCs.  The Montana Legislature directed the DEQ to adopt a state equivalent program.

ExxonMobil managed wastes in a number of SWMUs/AOCs at the Billings Refinery.  In 1999, the EPA along with DEQ, issued a HSWA permit for the Refinery which required ExxonMobil to investigate all facility SWMUs/AOCs and develop a corrective measures study for the SWMUs/AOCs which are contaminated above acceptable levels.  The DEQ was authorized as the lead for corrective action in 2000.



Before a cleanup plan can be developed, it is necessary to conduct a thorough investigation of a hazardous waste site to determine the types and amounts of contamination that may be there and the hazards they may present.

Investigating the Site.  Several assessments were conducted for the ExxonMobil site, the main investigations are listed below:

RCRA Facility Assessment (RFA)

Often the first activity in the corrective action process is the RFA. The objective of the RFA is to identify potential and actual releases from SWMUs/AOCs and make preliminary determinations about releases, the need for corrective action, and interim measures. The EPA completed the RFA for the Refinery in 1987.

RCRA Facility Investigation (RFI)

The RFI takes place when releases, or potential releases, have been identified and further investigation is necessary.  The purpose of the RFI is to gather enough data to fully characterize the nature, extent, and rate of migration of contaminants to determine the appropriate response action.

A Phase I RFI at the Refinery investigated 21 SWMUs/AOCs and a report was completed in November 1993.  The Phase I RFI was a preliminary study to develop a further understanding of the nature and extent of the contaminant releases at the Refinery.  A Phase II RFI report, completed in December 1998, further investigated the lateral and vertical extent of contaminants in soil and groundwater.

A site-wide risk assessment is also conducted as part of the RFI.  The risk assessment studies the health risks from potential exposure to the contaminants at the site.  ExxonMobil completed a site-wide risk assessment in February 1999.  Findings from the Risk Assessment are summarized below:

Summary of Site Risks.  The risk assessment for the Refinery was conducted in two phases.  In 1995, ExxonMobil produced a Risk Assessment Scoping Document for the Refinery.  In 1998, ExxonMobil produced the Final Risk Assessment for the Refinery.

For purposes of the risk assessment, the Refinery was divided into six exposure areas.  Exposure areas 1-5 were located within the confines of the Refinery and were evaluated according to human health standards and potential impacts to terrestrial organisms.  Exposure area 6 was the shoreline of the Yellowstone River and was evaluated using ecological criteria.

The chemicals of concern in the risk assessment were based on site samples collected during the RFI.  The chemicals of concern in the soil were arsenic, beryllium, lead, mercury, anthracene, benzene, chrysene, ethylbenzene, naphthalene, phenanthrene, toluene, and total xylenes.  The chemicals of concern in the surface water were antimony, arsenic, and toluene.  The chemicals of concern in the sediment were arsenic, lead, benzene, toluene, and xylenes.

Human Health Risk Characterization

In the human health evaluation in the risk assessment, ExxonMobil evaluated the potential risks to current and future on-site workers, future on-site construction workers, and current and future off-site residents for soil, groundwater, and sediment.  ExxonMobil evaluated both cancer and non-cancer health risks from exposure to the site chemicals.  Cancer risks are estimated as the increased change, over a lifetime, of a person developing cancer as a result of exposure to a potential cancer-causing chemical (carcinogen).  Non-cancer health risks were assessed by determining the hazard index (HI), or adverse affects of being exposed to several chemicals at one time.

The risk assessment stated that the potential site-wide cancer risks for the current on-site worker, future on-site worker, and future on-site construction workers at all exposure areas are well within or below the EPA acceptable range of one-in-ten-thousand (10-4) to one-in-a-million (10-6) probability of getting cancer.  All potential non-cancer risk estimates for current and future on-site workers were also found to be below the hazard index of 1, except for the future potential consumption of groundwater.

Ecological Risk Characterization

In the ecological risk evaluation in the risk assessment, both terrestrial and aquatic receptors that are likely to be found on the property were evaluated.  In an ecological risk assessment, when the Hazard Quotient (HQ) is less than one, it indicates that exposure is below a threshold level for toxicity, and it is unlikely that any adverse effects would occur.  When the HQ is above one, there is potential for adverse effects, although there is no linear dose-response relationship between the magnitude of the HQ and the observed or predicted effects.  For surface soil, the HQs for terrestrial receptors potentially contacting soil in the Refinery are all less than 1.0.  As a result, no adverse impacts would be expected to terrestrial wildlife residing on the Refinery proper.

For surface water, there were no HQs above 1.0 for exposure of birds, mammals, or aquatic life (fish, benthic invertebrates, or plants) to surface water in the Yellowstone River.  Therefore, it was determined that there is no potential impact to ecological receptors exposed to surface water near the Refinery.

Risk Assessment Conclusion for Surface Soils

Potential site-wide cancer risks estimated for the current on-site worker, future on-site worker, and future on-site construction workers at all of the Exposure Areas were all within or below the acceptable excess cancer risk range of 10-4 to 10-6 defined by U.S. Environmental Protection Agency (EPA).  Potential site-wide non-cancer risks estimated for these receptors are all below the target threshold hazard index of 1.0 for non-carcinogenic effects. 

For ecological receptors, it was concluded there is no apparent risk of adverse impacts to large or small terrestrial organisms potentially exposed to chemicals of concern in soil at each of the Exposure Areas. 

Based on the findings from the Risk Assessment, site surface soils were determined to be within acceptable levels and were not evaluated further in the corrective action process.

Risk Assessment Conclusion for Groundwater

The potential risks associated with the consumption of groundwater by an on-site worker, using the limited assumptions provided in the risk assessment, give risk estimates greater than 1E-06 for chemicals with cancer effects and greater than 1.0 for non-carcinogenic effects.  Therefore, groundwater corrective action continued to be evaluated in the corrective action process.

Corrective Measures Study (CMS)

After the RFI is completed and the regulatory agency determines that cleanup is necessary, the regulatory agency may require the owner/operator to conduct a CMS.  The purpose of the CMS is to identify and evaluate cleanup alternatives, called corrective measures, for releases at the facility.  The recommended measures are reviewed by the regulatory agency.  The regulatory agency then selects what it believes is the best remedy, given the site-specific considerations.  The CMS for the Refinery was completed in February 2005.

Media and Areas Evaluated for Corrective Action at the Refinery

With increasingly refined characterization of the ExxonMobil site, the six areas reviewed in the Risk Assessment were further refined into four remediation areas (the fire training area, interior refinery area, NAPL accumulation area, and river boundary area), see Figure 1, Appendix B.  The refined grouping allowed for a more focused and efficient study and implementation of any necessary corrective actions.

In 2000, the Fire Training Area was singled out by the EPA for a focused risk assessment.  The results of the risk assessment indicated no unacceptable risk to human health based on current and expected future uses of the property.  Therefore, corrective measures for the fire training area were not considered further in the CMS.

The proposed remedies for the three remaining areas (interior refinery area, NAPL accumulation areas, and river boundary area) were limited to the subsurface soils and groundwater and are described in further detail in Section IV below.  Findings during the risk assessment determined surface soils at the Refinery are below unacceptable risk values and remediation is not required.

a.   Interior Refinery Area (IRA).  Monitoring is conducted to identify changes in water quality in the IRA and is also used to monitor the thickness and changes in distribution for LNAPL.  Groundwater quality monitoring is conducted semi-annually and water level monitoring is conducted quarterly.

b.   NAPL Accumulation Area.  Vacuum enhanced recovery, a technology that uses pumps to remove various combinations of contaminated groundwater, separate-phase petroleum product, and hydrocarbon vapor from the subsurface, is conducted monthly in 21 wells located throughout the LNAPL accumulation areas.  NAPL thickness is also measured monthly in all 21 wells.  ExxonMobil has removed a total of approximately 321,281 gallons of hydrocarbon/water mixture from the groundwater.  Based on a 4.6% product to water ratio an estimated 14,749 gallons of NAPL has been recovered.

Five wells also currently pump groundwater in an effort to create a capture zone which is intended to prevent any contaminants from being released into the Yellowstone River.  Pumping wells SR95-1 and ERM-9B has been in operation since 1995.  Pumping wells SM95-2, MW06-2, MW06-4 were placed in operation in August 2006.  Pumping well MW06-1was also installed in August 2006, but is not used unless needed.  Fluids from the pumping wells are discharged to the refinery wastewater treatment system.

Two interceptor trenches have also been constructed, called the East Oil Interception Trench (EOIT) and West Oil Interceptor Trench (WOIT).  Total fluids are pumped from the two interceptor trench sumps into an API Separator.  Oil that accumulates on the water surface of the trenches that cannot be pumped to the API Separator is routinely removed as part of the vacuum enhanced recovery program.

c.   River Boundary Area.  ExxonMobil installed two pilot scale phytoremediation plots in May 2001.  Phytoremediation utilizes plants to remediate soil and water.  Phytoremediation works by utilizing photodegradation, enhanced rhizosphere biodegradation, hydraulic control, and phytovolatilization.  Despite drought conditions, the trees in the phytoremediation plots were considered established at the end of the 2004 growing season.  Monitoring and maintenance has been conducted each growing season since 2001.

A pilot air-sparge system was also installed in the River Boundary Area.  The air-sparge system consists of injecting atmospheric air into the groundwater, which provides oxygen for biodegradation and also physically strips volatile compounds from the groundwater.  The air-sparge system was installed directly upgradient of the West Phytoremediation Plot in March 1999 and operated continuously until November 2001.  The system was shut down during the winter of 2002, and restarted in June 2002, after which it has operated continuously.



Throughout the cleanup process, the DEQ will keep the public informed through notices of public meetings and public comment periods in area newspapers.



DEQ Contact: 
Ann M. Kron, Environmental Science Specialist
Montana Department of Environmental Quality
P.O. Box 200901
Helena, Montana 59620-0901
e-mail: akron@mt.gov