Document ID: EPA-HQ-SFUND-1990-0011-0178
Agency: epa
Document Type: Proposed Rule
Title: National Oil and Hazardous Substances Pollution Contingency Plan; National Priorities List: Deletion of the Mystery Bridge Rd./U.S. Highway 20 Superfund Site
Posted Date: 2019-07-23T04:00Z

[Federal Register Volume 84, Number 141 (Tuesday, July 23, 2019)]
[Proposed Rules]
[Pages 35360-35365]
From the Federal Register Online via the Government Publishing Office [www.gpo.gov]
[FR Doc No: 2019-15658]

-----------------------------------------------------------------------

ENVIRONMENTAL PROTECTION AGENCY

40 CFR Part 300

[EPA-HQ-SFUND-1990-0011; FRL-9997-03-Region 8]

National Oil and Hazardous Substances Pollution Contingency Plan; 
National Priorities List: Deletion of the Mystery Bridge Rd./U.S. 
Highway 20 Superfund Site

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 ([email protected]).
     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,

[[Page 35361]]

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: 
[email protected] 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: 
[email protected].

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.

[[Page 35362]]

    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

[[Page 35363]]

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 x 
10-4-1 x 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 x 
10-4-1 x 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.

[[Page 35364]]

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.

[[Page 35365]]

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.

    Dated: July 15, 2019.
 Gregory E. Sopkin,
Regional Administrator, Region 8.
[FR Doc. 2019-15658 Filed 7-22-19; 8:45 am]
 BILLING CODE 6560-50-P