AGENCY:

Environmental Protection Agency (EPA).

ACTION:

Proposed rule; notice of intent.

SUMMARY:

The Environmental Protection Agency (EPA) Region 8 is issuing a Notice of Intent to Delete the Mystery Bridge Rd./U.S. Highway 20 Superfund Site (Site) located in Evansville, WY, from the National Priorities List (NPL) and requests public comments on this proposed action. The NPL, promulgated pursuant to the Comprehensive Environmental Response, Compensation, and Liability Act (CERCLA) of 1980, as amended, is an appendix of the National Oil and Hazardous Substances Pollution Contingency Plan (NCP). The EPA and the State of Wyoming, through the Wyoming Department of Environmental Quality (WDEQ), have determined that all appropriate response actions under CERCLA, [other than maintenance of institutional controls and five-year reviews], have been completed. However, this deletion does not preclude future actions under Superfund.

DATES:

Comments must be received by August 22, 2019.

ADDRESSES:

Submit your comments, identified by Docket ID no. EPA-HQ-SFUND-1990-0011, by one of the following methods:

• http://www.regulations.gov. Follow on-line instructions for submitting comments. Once submitted, comments cannot be edited or removed from Regulations.gov. The EPA may publish any comment received to its public docket. Do not submit electronically any information you consider to be Confidential Business Information (CBI) or other information whose disclosure is restricted by statute. Multimedia submissions (audio, video, etc.) must be accompanied by a written comment. The written comment is considered the official comment and should include discussion of all points you wish to make. The EPA will generally not consider comments or comment contents located outside of the primary submission (i.e. on the web, cloud, or other file sharing system). For additional submission methods, the full EPA public comment policy, information about CBI or multimedia submissions, and general guidance on making effective comments, please visit http://www2.epa.gov/​dockets/​commenting-epa-dockets.
• Email: Andrew Schmidt (schmidt.andrew@epa.gov).
• Mail: Andrew Schmidt, Remedial Project Manager, 8SEM-RB-SA, Environmental Protection Agency, Region 8, 1595 Wynkoop Street, Denver, CO 80202.
• Hand delivery: Andrew Schmidt, Remedial Project Manager, 8SEM-RB-SA, Environmental Protection Agency, Region 8, 1595 Wynkoop Street, Denver, CO 80202.

Instructions: Direct your comments to Docket ID no. EPA-HQ-SFUND-1990-0011. EPA's policy is that all comments received will be included in the public docket without change and may be made available online at http://www.regulations.gov, including any personal information provided, unless the comment includes information claimed to be Confidential Business Information (CBI) or other information whose disclosure is restricted by statute. Do not submit information that you consider to be CBI or otherwise protected through http://www.regulations.gov or email. The http://www.regulations.gov website is an “anonymous access” system, which means EPA will not know your identity or contact information unless you provide it in the body of your comment. If you send an email comment directly to EPA without going through http://www.regulations.gov, your email address will be automatically captured and included as part of the comment that is placed in the public docket and made available on the internet. If you submit an electronic comment, EPA recommends that you include your name and other contact information in the body of your comment and with any disk or CD-ROM you submit. If EPA cannot read your comment due to technical difficulties and cannot contact you for clarification, EPA may not be able to consider your comment. Electronic files should avoid the use of special characters, any form of encryption, and be free of any defects or viruses.

Docket: All documents in the docket are listed in the http://www.regulations.gov index. Although listed in the index, some information is not publicly available, e.g., CBI or other information whose disclosure is restricted by statute. Certain other material, such as copyrighted material, will be publicly available only in the hard copy. Publicly available docket materials are available either electronically in http://www.regulations.gov or in hard copy at: U.S. EPA Region 8, Superfund Records Center and Technical Library, 1595 Wynkoop Street, Denver, CO 80202. Viewing hours: 8 a.m. to 4 p.m., Monday through Thursday, excluding holidays; Contact: Andrew Schmidt; (303) 312-6283, email: schmidt.andrew@epa.gov and Natrona County Public Library, Reference Desk, 307 East 2nd Street, Casper, WY 82601-2593, (307) 577-7323, Hours: Monday through Thursday: 9 a.m. to 7 p.m., Friday and Saturday: 9 a.m. to 5 p.m.

FOR FURTHER INFORMATION CONTACT:

Andrew Schmidt, Remedial Project Manager, U.S. Environmental Protection Agency, Region 8, SEM-R8-SA, 1595 Wynkoop St., Denver, CO 80211, (303) 312-6283, email: schmidt.andrew@epa.gov.

SUPPLEMENTARY INFORMATION:

Table of Contents

I. Introduction

II. NPL Deletion Criteria

III. Deletion Procedures

IV. Basis for Intended Site Deletion

I. Introduction

EPA Region 8 announces its intent to delete the Mystery Bridge Rd./U.S. Highway 20 Superfund Site from the National Priorities List (NPL) and requests public comment on this proposed action. The NPL constitutes appendix B of 40 CFR part 300 which is the National Oil and Hazardous Substances Pollution Contingency Plan (NCP), which EPA promulgated pursuant to section 105 of the Comprehensive Environmental Response, Compensation and Liability Act (CERCLA) of 1980, as amended. EPA maintains the NPL as the list of sites that appear to present a significant risk to public health, welfare, or the environment. Sites on the NPL may be the subject of remedial actions financed by the Hazardous Substance Superfund (Fund). As described in 40 CFR 300.425(e)(3) of the NCP, sites deleted from the NPL remain eligible for Fund-financed remedial actions if future conditions warrant such actions.

EPA will accept comments on the proposal to delete this site for thirty (30) days after publication of this document in the Federal Register.

Section II of this document explains the criteria for deleting sites from the NPL. Section III discusses procedures that EPA is using for this action. Section IV discusses the Mystery Bridge Rd./U.S. Highway 20 Superfund Site and demonstrates how it meets the deletion criteria.

II. NPL Deletion Criteria

The NCP establishes the criteria that EPA uses to delete sites from the NPL. In accordance with 40 CFR 300.425(e), sites may be deleted from the NPL where no further response is appropriate. In making such a determination pursuant to 40 CFR 300.425(e), EPA will consider, in consultation with the State, whether any of the following criteria have been met:

i. Responsible parties or other persons have implemented all appropriate response actions required;

ii. All appropriate Fund-financed response under CERCLA has been implemented, and no further response action by responsible parties is appropriate; or

iii. The remedial investigation has shown that the release poses no significant threat to public health or the environment and, therefore, the taking of remedial measures is not appropriate.

Pursuant to CERCLA section 121(c) and the NCP, EPA conducts five-year reviews to ensure the continued protectiveness of remedial actions where hazardous substances, pollutants, or contaminants remain at a site above levels that allow for unlimited use and unrestricted exposure. EPA conducts such five-year reviews even if a site is deleted from the NPL. EPA may initiate further action to ensure continued protectiveness at a deleted site if new information becomes available that indicates it is appropriate. Whenever there is a significant release from a site deleted from the NPL, the deleted site may be restored to the NPL without application of the hazard ranking system.

III. Deletion Procedures

The following procedures apply to deletion of the Site:

(1) The EPA consulted with the State before developing this Notice of Intent to Delete.

(2) The EPA has provided the State 30 working days for review of this notice prior to publication of it today.

(3) In accordance with the criteria discussed above, EPA has determined that no further response is appropriate;

(4) The State of Wyoming, through the WDEQ, has concurred with deletion of the Site, from the NPL.

(5) Concurrently with the publication of this Notice of Intent to Delete in the Federal Register, a notice is being published in the Casper Star-Tribune.

(6) The EPA placed copies of documents supporting the proposed partial deletion in the deletion docket, made these items available for public inspection, and copying at the Site information repositories identified above.

If comments are received within the 30-day public comment period on this document, EPA will evaluate and respond appropriately to the comments before making a final decision to delete. If necessary, EPA will prepare a Responsiveness Summary to address any significant public comments received. After the public comment period, if EPA determines it is still appropriate to delete the Site, the Regional Administrator will publish a final Notice of Deletion in the Federal Register. Public notices, public submissions and copies of the Responsiveness Summary, if prepared, will be made available to interested parties and in the site information repositories listed above.

Deletion of a site from the NPL does not itself create, alter, or revoke any individual's rights or obligations. Deletion of a site from the NPL does not in any way alter EPA's right to take enforcement actions, as appropriate. The NPL is designed primarily for informational purposes and to assist EPA management. Section 300.425(e)(3) of the NCP states that the deletion of a site from the NPL does not preclude eligibility for future response actions, should future conditions warrant such actions.

IV. Basis for Intented Site Deletion

The following information provides EPA's rationale for deleting the Site from the NPL:

Site Background and History

The Site is in Natrona County, Wyoming northeast of Casper, Wyoming and one mile east of Evansville. The Site is bordered on the north by the North Platte River, on the west by the Sinclair Refinery (formerly known as the Little America Refining Company or LARCO), on the south by U.S. Highway 20 and on the east by Mystery Bridge Road. The northern two thirds of the Site contain residential housing units built primarily between 1973 and 1983. The Tallgrass Energy Partners, LP facility (formerly owned by KN Energy and KM Upstream LLC and referred to as the former KMI Property) and the adjacent DOW/DSI property comprise the southern third of the Site. The site is underlain by two aquifers, a shallow alluvial aquifer and a deeper bedrock aquifer. Activities at the site impacted the shallow alluvial aquifer, which was historically utilized by residences downgradient for domestic water supply purposes.

Site investigations, initiated due to resident complaints of poor water and air quality, were completed in 1986 and 1987 and identified a benzene, toluene, ethylbenzene and xylenes (BTEX) plume originating from the former KMI Property and a volatile halogenated organic chemicals (VHOs) plume originating from the DOW/DSI property moving northeast towards the North Platte River. The Site was proposed for listing on the National Priorities List (NPL) June 24, 1988 (53 FR 23978), and was listed on the NPL on August 30, 1990 (55 FR 35502). The former KMI property was partially deleted from the NPL on August 29, 2017 (82 FR 29764). Potential releases at the Sinclair Refinery (formerly LARCO) facility are currently being addressed under a Resource Conservation and Recovery Act (RCRA) 3008(h) order.

KM Upstream LLC and its predecessors operated a natural gas fractionation, compression, cleaning, odorizing, and transmission facility at the Site beginning in 1965. During the plant start-up, an underground pipe burst, injecting 5,000 to 10,000 gallons of absorption oil into the subsurface. In addition, an earthen flare pit was initially used to collect spent material generated by the facility. Absorption oil, emulsions, anti-foulants, and anti-corrosive agents, crude oil condensate, liquids accumulated in the flare stack, potassium hydroxide treated waste, and lubrication oils and blowdown materials from plant equipment were all possibly collected in the flare pit. In 1984, a concrete-lined flare pit was constructed and put into operation. Leaks from the earthen flare pit, the initial absorption oil spill, and a catchment area that collected surface water run-off from the facility are all believed to have contributed to the BTEX soil and groundwater impacts.

The DOW/DSI facility conducted oil and gas production enhancement services starting in the 1950's. Contamination originating from the DOW/DSI facility is believed to have come from the truck wash water disposal system (believed to have contained chlorinated solvents) and the toluene storage area on the northern end of the facility.

EPA is the lead agency for the Site and WDEQ is the support agency. Pursuant to the 1991 Consent Decree, KN Energy, its successor KMI, and DOW/DSI have jointly conducted and funded the remediation work at the Site. The former KMI Property is in continued operation as mid-stream gas processing facility and is now owned and operated by Tallgrass Energy Partners.

Removal Actions

In August 1986, nearby residents complained of poor air and water quality. The Agency for Toxic Substances and Disease Registry (ATSDR) then issued an advisory after VHOs were detected in area drinking water wells. Studies determined that a contaminated groundwater plume from the nearby industrial facilities was responsible for the poor water quality.

Starting in 1987, EPA searched to identify potentially responsible parties (PRPs) for the contaminated groundwater. EPA also oversaw a removal action in January 1987 for immediate installation of 25 groundwater monitoring wells and alternative drinking water provisions for area residents until permanent alternatives could be established. By July 1987, EPA identified KMI and Dow/DSI as the companies responsible for generating the contaminated plumes emanating from the industrial area. By December 1987, KMI and Dow/DSI entered separate Administrative Orders on Consent (AOCs) to perform immediate removal actions to control the sources of contamination and inhibit further migration of the existing groundwater plumes into the residential subdivision. Each PRP was required to prepare an Engineering Evaluation/Cost Analysis (EE/CA) of its property to document the extent and nature of the contaminants present and to support proposals of expedited removal actions.

EPA's initial response actions also included extending a water transmission line from the Town of Evansville and connecting residents after detecting elevated levels of contaminants in drinking water wells. In addition, the Evansville water treatment plant received a new water intake and related upgrades. This work was completed in January 1989.

KMI Property

An investigation was conducted as part of the EE/CA for removal actions at the KMI property. The investigation included a soil vapor survey and borings to collect soil and groundwater samples. The soil vapor survey was conducted near the flare pit and soil and groundwater samples were also collected. The investigation lead to the discovery of impacted soil, groundwater, and light non-aqueous phase liquids (LNAPLs) in the subsurface.

The EE/CA prepared by KMI evaluated removal technologies and recommended a removal action. KMI began the recommended removal action in November 1989. The removal action consisted of groundwater pump and treat (PAT) and soil vapor extraction (SVE) systems to remove BTEX contaminants in three phases: LNAPL, groundwater, and soil vapor. The SVE system extracted vapor phase hydrocarbons from the unsaturated interval between the water table and the ground surface. The PAT system pumped groundwater to the surface where volatile hydrocarbons were removed by air stripping. LNAPL, when present, was removed from the groundwater extraction wells when the PAT system was in operation.

The removal actions on the KMI property were also selected as part of the OU1 remedy in the Record of Decision (ROD); thus, operation of the PAT and SVE systems continued into the remedial phase. The completion of removal activities for the KMI parcel, including confirmation sampling data and QA/QC activities are documented in the OU2 Phase I Report, KN Energy Gas Compressor Station, dated June 27, 1995.

DOW/DSI Property

In accordance with the AOC, DOW/DSI prepared an EE/CA report to document the nature and extent of the releases of contaminants, and to support proposals of expedited removal actions to control migration of contaminants and eliminate sources of contaminants beneath and adjacent to the property. As a result of the investigative activities conducted to support the EE/CA at the DOW/DSI property, several volatile halogenated organic (VHO) soil contaminants were identified in the groundwater and soil near the abandoned chlorinated gravel leach sump area.

The EE/CA prepared by DOW/DSI evaluated removal technologies and recommended a removal action. Beginning in late 1987 and continuing through part of 1988, the removal action was conducted by DOW/DSI. The removal action consisted of removals of a buried wash water disposal system, an empty and out-of-service underground storage tank (UST), and approximately 440 cubic yards of soil and debris from an older abandoned sump area. The excavations were backfilled with clean sand and gravel. In addition, two SVE systems were installed on the property to remove volatile halogenated organic (VHO) chemicals from the abandoned sump area and aromatic hydrocarbons (toluene, xylenes and ethylbenzene) from the former toluene storage area.

Confirmatory subsurface soil sampling prior to shutdown of the SVE systems in 1988 showed that the SVE systems had lowered in-situ concentrations of soil contaminants below the Soil Action Levels (SALs) developed by EPA for the Site in support of the expedited removal actions. The completion of removal activities for the DOW/DSI parcel, including confirmation sampling data and QA/QC activities are documented in the Phase I Summary Report for the DSI Property under OU2 of the Brookhurst/Mystery Bridge Superfund Site, dated February 22, 1994.

Remedial Actions

In December 1987, the AOC signed by DOW/DSI and KMI also required the two PRPs to perform a Remedial Investigation/Feasibility Study (RI/FS) of the Brookhurst Subdivision, which is located north and east of the KMI and DOW/DSI properties and hydrologically downgradient. The RI/FS report, which was completed in June 1990, concluded that two plumes of contaminated groundwater originated in the industrial area south of the subdivision and were migrating through the subdivision in a northeast direction. The first of these plumes was contaminated with VHO compounds (referred to as the VHO plume) and extended from the DOW/DSI property to the North Platte River. The second plume was contaminated with BTEX compounds (referred to as the BTEX plume) and extended from the KMI property to the adjacent Burlington Northern Rail Road property and possibly into the subdivision directly north of the KMI property. In addition, LNAPL originating at the KMI property and extending slightly into the subdivision, was found floating on the groundwater. The RI/FS suggested that VHO and BTEX plumes were not commingled in the area downgradient from the DOW/DSI and KMI facilities.

As part of the RI/FS, a Baseline Risk Assessment (BRA) was conducted in 1989. The BRA assessed carcinogenic risks and the potential for non-cancer health effects of eleven chemicals resulting from direct ingestion of contaminated groundwater under residential homes. Risks were also calculated for the hypothetical scenario where the DOW/DSI and KMI facilities were redeveloped for residential use. The BRA concluded that ecological risks due to the releases from the industrial areas were not expected to be significant, but that human health cancer risks presented by the VHO and BTEX plumes in alluvial groundwater under the residential scenario were unacceptable. Human health non-cancer risks due to the VHO and BTEX plumes were determined to be below a level of concern.

The Site was divided into two OUs, OU1 was designated to address contaminated groundwater, and OU2 was designated to address contaminant source areas on the industrial properties. The creation of the two OUs was done to ensure that the principle threat to human health and the environment, groundwater (OU1), was dealt with immediately, and to allow further assessment of the soil source areas to ensure adequate cleanup.

The OU1 ROD was signed on September 24, 1990. The remedial action objectives (RAOs) were to: Prevent ingestion of water containing trans-1,2 dichloroethylene (DCE), 1,1,1 trichloroethane (TCA), trichloroethylene (TCE), tetrachloroethylene (PCE), benzene, toluene, ethylbenzene, or xylene at concentrations that either a) exceed MCLs or proposed MCLs, or b) present a total carcinogenic risk greater than 1 × 10⁻⁴−1 × 10⁻⁶; and restore the alluvial aquifer to concentrations that both a) meet the MCLs or proposed MCLs for trans-1,2 DCE, 1,1.1 TCA, TCE, PCE, benzene, toluene, ethylbenzene, and xylene, and b) present a total carcinogenic risk less than 1 × 10⁻⁴−1 × 10⁻⁶.

The agency selected a combination of alternatives to address the VHO plume and the BTEX plume. Common elements included source area groundwater treatment, soil removal and soil vapor extraction, monitoring of groundwater, and implementation of institutional controls.

KMI Property (BTEX Plume)

The selected remedy included continued operation of the KMI removal action. Specifically, the selected remedy included extraction of ground water with concentrations of BTEX compounds above MCLs or proposed MCLs throughout the plume; treatment of contaminated groundwater with an on-site air stripping facility; and reinjection of treated water into the aquifer to provide additional hydraulic control of the BTEX plume and to minimize any potential impact from the BTEX remediation efforts on the RCRA and VHO plumes.

DOW/DSI Property (VHO Plume)

The selected remedy included continued operation and enhancements to the DOW/DSI removal actions. The remedy included extraction of groundwater with concentrations of VHOs above MCLs or proposed MCLs in the upgradient portion of the plume (i.e., on and/or near the DOW/DSI facility); treatment of contaminated groundwater with an on-site air stripping facility; reinjection of treated water into the aquifer to provide additional hydraulic containment of the upgradient portion of the VHO plume being extracted, minimize any impact from the VHO remediation efforts on the RCRA plume and BTEX plume, and enhance the natural attenuation process in the downgradient portions of the VHO plume; and reliance on natural processes for reduction of VHO levels in downgradient portions of the VHO plume.

The OU2 ROD was signed on September 30, 2010 and determined that removal actions taken at each parcel treated or excavated all soils exceeding industrial use standards. The soils at these properties are acceptable for industrial uses. The remedy selected for OU2 is institutional controls to limit the use of KMI and DOW/DSI parcels to industrial use, to govern the handling of excavated soils on each parcel and to restrict groundwater use.

Remedy Implementation

Following the OU1 ROD, a Consent Decree (CD) was signed with both DOW/DSI and KMI in October 1991, in which the parties agreed to implement the OU1 ROD remedy.

Requirements for the KMI Industries BTEX plume remedial design (RD) included groundwater monitoring to determine whether additional groundwater extraction or monitoring points downgradient of the KMI facility were needed. During the RD, it was determined that contamination above MCLs had not migrated beyond the facility boundary and no system expansion was needed. Since no expansion was needed, no addition remedial construction was performed.

Requirements for the DOW/DSI VHO plume included construction of a groundwater extraction and treatment system. The system installation was completed in August 1993 and included three extraction wells, a water treatment unit, and an infiltration gallery.

In conjunction with the OU2 ROD, a special warranty deed was recorded for the KMI property, and a restrictive covenant was recorded for the DOW/DSI property in Natrona County in September 2010. Both the KMI warranty deed and the DOW/DSI restrictive covenant limit use of the properties to industrial use, govern the management of excavated soils on each property, prevent the use of groundwater on each property for any use other than sampling and monitoring, and ensure that no use of the properties will jeopardize the selected remedies.

Attainment of Cleanup Levels

KMI Property

The October 1991 CD for remedial design and remedial action entered by the Court required the following groundwater performance standards for the KMI property:

1. Remediate groundwater so that concentrations shall not exceed MCLs and proposed MCLs, as set forth in the ROD for BTEX.

2. The area of attainment shall include the entire BTEX plume, including those areas of the plume inside and outside the KMI facility.

Concentrations of ethylbenzene, toluene, and xylenes were not historically measured above the MCLs. As a result, the groundwater remediation evaluation focused on benzene as the indicator contaminant of concern.

A KMI Groundwater Monitoring Plan (GWMP) was developed in 1993 to evaluate the effectiveness of the Remedial Action (RA), to evaluate groundwater post-RA and determine compliance with the performance standards. Specifically, the KMI GWMP established that following shut down of the remediation system and after 12 months of groundwater sampling with results below the MCL, post-RA monitoring would begin.

The KMI remediation system operated continuously between November 1989 and August 1996 when EPA approved KMI's request to cease active remediation. The pre-certification inspection was completed on July 16, 1997, and approval of Remedial Action Completion was provided on August 20, 1997. After active treatment was shut down, attainment monitoring was conducted to evaluate post remediation conditions. Achievement of RAOs under post-RA monitoring was determined to have been met after a minimum of eight quarterly sampling events were conducted in which, for each well, the 90 percent one-tailed upper confidence limit (UCL90) concentrations for benzene, ethylbenzene, toluene and total xylenes were below the MCLs for each chemical. Compliance with RAOs for the KMI plume was achieved in November 2009. The completion of remedial activities, including statistical analyses of groundwater data were documented in The Completion Report for Groundwater Remediation Activities at the Casper Compressor Station letter report dated June 22, 2010, and the Summary of the Previous Eight Quarters of Post Remedial Action Groundwater Monitoring at the Casper Compressor Station, Technical Memorandum, dated February 12, 2010.

Because the KMI parcel met RAOs for the source area and in groundwater, and because the necessary institutional controls were in place to prevent unacceptable exposure to site contaminants, the KMI property was partially deleted from the NPL on August 29, 2017.

DOW/DSI Property

The October 1991 CD for remedial design and remedial action required the following performance standards for the DOW/DSI parcel:

1. Remediate groundwater so that concentrations shall not exceed MCLs and proposed MCLs, as set forth in the ROD for VHOs.

2. The area of attainment shall include the entire VHO plume, including those areas of the plume inside and outside the DOW/DSI facility.

Similar to the KMI property, a Groundwater Monitoring Plan (GWMP) was developed in 1993 to evaluate the effectiveness of the Remedial Action (RA) and to evaluate groundwater post-RA and determine compliance with the performance standards. In accordance with post-RA groundwater monitoring requirements, RAOs would not be achieved until the 85 percent upper confidence limit (UCL85) of the arithmetic mean for four consecutive quarters of groundwater monitoring data did not exceed the remedial performance goals. The test would be performed for each monitoring well in the contaminant plume.

The DOW/DSI remediation system operated continuously between November 1993 and April 2001 when EPA approved DOW/DSI's request to cease active remediation. The request was based on the appearance of a temporary petroleum sheen entering the groundwater treatment equipment and measurable light non-aqueous phase liquid (LNAPL) near the north boundary of the DOW/DSI property, which the treatment system was not designed to handle. The observance of LNAPL in some of the wells was temporary and in subsequent sampling events was not observed.

On September 18, 2015, DOW/DSI submitted a letter detailing achievement of RAOs for the VHO plume based on requirements detailed in the 1993 DOW/DSI GWMP. Due to the age of the document and new guidance, EPA asked DOW/DSI to evaluate groundwater data using a more stringent statistical test. In October 18, 2017, DOW/DSI submitted a report detailing achievement of RAOs for the VHO plume based on using a more stringent 95% upper confidence level on the mean and using a minimum of eight data points. Completion of the remedial action was documented in the Completion of Remedial Action and Completion of Work Report, Mystery Bridge Rd./US Highway 20 Superfund Site Consent Decree for Remedial Design and Remedial Action (OU1) and Administrative Order for Removal Action on Consent (OU2), dated October 18, 2017.

Additional sampling was conducted in 2018 to confirm conclusions made in the 2017 Completion of Remedial Action and Completion of Work Report. Sampling results confirmed that concentrations of dissolved phase VHOs were below MCLs in the wells sampled. Results of additional confirmation sampling are documented in the Revised 2018 Well Redevelopment and Groundwater Sampling Report, Mystery Bridge Rd/US Highway 20 Superfund Site Consent Decree for Remedial Design and Remedial Action (OU1), dated September 28, 2018. The Completion of Remedial Action and Completion of Work Report, Mystery Bridge Rd./US Highway 20 Superfund Site Consent Decree for Remedial Design and Remedial Action (OU1) and Administrative Order for Removal Action on Consent (OU2), was subsequently updated on April 5, 2019.

Operation and Maintenance

No operation or maintenance is required for the Site; however, the effectiveness and presence of the environmental covenants will be evaluated every five years as part of the Five-Year Review process.

Five-Year Review

Five Year Review Reports (FYR) are required for the Mystery Bridge Site. FYRs are required because hazardous substances, pollutants, or contaminants remain at the site above levels that allow for unlimited use and unrestricted exposure (UU/UE). The next FYR is due five years from the signing of the July 2019 FYR.

The last (5th) FYR was completed in July 2019. The 5th FYR concluded that the remedies at OU1 and OU2 are protective of human health and the environment, and there were no Issues or Recommendations noted. The FYR noted that Institutional Controls (ICs) are in place restricting the use of drinking water beneath the industrial properties, governing the management of soils on each of properties, and limiting future development to industrial use.

Community Involvement

Public participation activities have been satisfied as required in CERCLA Section 113(k), 42 U.S.C. 9613(k) and CERCLA Section 117, 42 U.S.C. 9617. Documents in the deletion docket, which the EPA is relying on for the proposed deletion from the NPL, are available to the public in the information repositories, and a notice of availability of the Notice of Intent to Delete has been published in the Casper Star-Tribune to satisfy public participation procedures required by 40 CFR 300.425(e)(4).

Determination That the Site Meets the Criteria for Deletion in the NCP

The EPA and the State have followed procedures detailed in 40 CFR 300.425(e) in order to propose deletion of this Site from the NPL. The Site has achieved all Remedial Action Objectives specified in the ROD for both soil and groundwater, and all RAOs are consistent with EPA policy and guidance. EPA in consultation with the State of Wyoming has determined that no further Superfund response action is necessary in order to protect human health and the environment.

List of Subjects in 40 CFR Part 300

• Environmental protection
• Air pollution control
• Chemicals
• Hazardous substances
• Hazardous waste
• Intergovernmental relations
• Penalties
• Reporting and recordkeeping requirements
• Superfund
• Water pollution control
• Water supply

Authority: 33 U.S.C. 1321(d), 42 U.S.C. 9601-9657; E.O. 13626, 77 FR 56749, 3 CFR 2013 Comp., p. 306; E.O. 12777, 56 FR 54757, 3 CFR, 1991 Comp., p. 351; E.O. 12580, 52 FR 2923, 3 CFR, 1987 Comp., p. 193.