DERR-2024-007863

Explanation of Significant Differences
Kennecott South Zone, Operable Unit 2
Southwest Jordan River Valley Ground Water Plumes
U.S. Environmental Protection Agency, Region 8
Utah Department of Environmental Quality
June 2007
Introduction
The Kenrlecott South Zone Site, proposed for the National Priorities List (WL) in 1994,
is located in southwestern Salt Lake County, Utah, about 10 miles southwest of Salt hike
City. Operable Unit 2 (OU2) of the Site, known as the Southwest Jordan River Valley
Ground Water Plumes, encornpasscs the groundwater beneath all. or portions of the
municipali t~es of West Jordan, South Jordan, Riverton, Herriman, and portions of
unincorporated Salt Lake County. A Record of Decision, selecting a remedy for OU2, is
dated December 13.2000.
The remedy was modified with an Explanation of Significant Differences (ESD) in
August 2003. This June 2007 Explanation of Significant Differences is the second ESD
to modify the original remedy. While the overall approach to this Site, and the ability to
meet stated objectives, remains unchanged, certain refinements to the original remedy (as
' modified by the first ESD) are necessary.
This Explanation of Significant Differences (ESD) describes the rationale for modifying
the remedy specified in the Record of Decision (ROD) and first ESD for Operable Unit 2
of the Kennecotl South Zone Site. Section P17(c) of CERCLA, 42 USC §9617(c), and
the National Contingency Plan VCP), 40 C.F.R. Section 30Q.435(c)(2)(i) quire that an
ESD be prepared when the dfferences in the Remedial Action significant1 y change but
do not fundamentally alter the remedy selected in the ROD with respect to scope,
performance, or cost.
This ESD is supported by and will become pan of the Admin~strative Record file for this
Site, in accordance with the NCP, Section 3001.823(a)(2), The Administrative Record is
available for review at UDEQ's office located at 168 North 1950 West, Salt Lake City,
Utah. Key documents and reports are also available for review at the City Recorder's
Office, City of West Jordan, 8000 South Redwood Rd, West Jordan, UT 84088.
Site KS~OIY
The Kennecott South Zone Site is composed of historic mining sites, of surface areas
contaminated by mining wastes which mi gated from sourcc areas downgradienr to cities
and towns, and of subsurface areas contaminated by acid leachates from the mining
district. The Kennecott South Zone Site is comprised of fifteen operable units.
The remedy selected for the Kennecott South Zone Operable Unit No. 2 - Southwest
Jordan Valley Groundwater Plumes, involves treatment and conlainment of contaminated
grovnd water. The principal sourccs which caused the pund water contamination have
been addressed in previous actions or are managed by Kennecott under provisions of a
Utah Ground Water Protection Permit.
The selected remedy, as modified by the first ESD, contains the following elements:
Continuation of sourcc control measures as administered through the State of
Utah Ground Water Protection Program.
- Prevent human exposure 20 unacceptable high concentrations of hazardous
substances andor pollutants or contaminants by limiting access to the
contaminated ground watet. The State Ground Water Management Plan,
issued by the State Engineer in June 2002, addresses issues specific to the
remediation effort and needcd restrictions in the area of the plumes. - Prevent human exposure to unacceptable high concentsations of hazardous
substances and/or pollutants or contaminant through point-of-use management
which includes providf ng in-house treatment units to residents with rmpacted
wells, replacement of their watet by hooking the properties up to municipal
drinking andor secondary supplies, andlor modifying their wells to reach
uncontaminated waters. - Contain the acid plume in Zone A by installation of her wells at the
leading edge of the contamination (1500 ppm sulfate or less), pump and weat
the waters to provide a hydraulic barrier to prevent further plume movement
while providing treated water for municipal use. The treatment technology for
the barrier we11 waters is reverse osmosis.
- Withdraw the heavily contaminated waters from the core of the acid plume in
Zone A and send it directly to the tailings line. Neutralization and metals
removal takes place in the tailings line. Neutralization can be augmented with
lime if needed.
Monitor the plume to follow the progress of natural attenuation for the
portions of the Zone A plume which contain sulfate in excess of the primary
drinking water standard for sulfate (500 ppm sulfate).
Disposal of acid water and reverse osmosis concentrates in existing pipline
used to slurry tailings to a tailings impoundment Wor to mine closure.
DeveIopment of a post-mine closure plan to manage extracted acid core water
and reverse osmosis treatment concentrates (derived hm the management
option selected for the water extracted at the beading edge wells) for use when
the mine and mi11 are no longer operating.
Basis for and Description of the Simificant Differences
A number of clarifications to the remedy are required to address barrier well water
management, source control measures for the Eastside Collection System and Bingham
Reservoir, and perEormance standards.
1) Water Management
The December 2000 ROD selcctcd treatment of barrier well water using reverse
osmosis and delivery of treated water to a municipal water purveyor. This clarification to
the remedy is to allow other management options for barrier well water including
continued use by Kennecott for industrial needs or the provision of raw or treated barrier
well water for any other lawful use that is both consistent with the quality of the water,
previous decision documents and acceptable to EPA and WQEQ.
2) Source Control Measures
The original semed y indicates Illat source control measures (i.e., Eastside
Collection Syaem, Bingharn Reservoir) are to be operated under State permits. As a
clrtrificati on, these permits are considered complimentary to the OU2 remedy and
management of the Southwest Jordan Valley Groundwater plumes. UDEQ will provide
routine reports to evaluate compliance with State per mi^. In the event that State permits
andor programs are ineffective in controlling potential sources of contamination to the
groundwater plume, additional Federal CEKCA response actions may be required. At a
minimum, Kennecott's compliance with applicable State permits will be evaluated no
less often than every live years pursuant to the CERCLA requirement to conduct a Five
Year Revicw whenever waste is left in place precludmg unrestricted use and unlimited
exposure.
3) Perfmnance Standards
There are three performance standards related to the sate of extraction from the core of
the plume in Zone A, plume containment, and cleanup levels to demonstrate the
effectiveness of the remedy.
A) Extraction Rate
Several wells have been installed for the extraction of heavily
contaminated water from the core of the acid plume in Zone A. The change in
this ESD is to define a rate of extraction to assure reduction in the size of the
contaminated plume. As of the time of the writing of this ESD, that extraction
rate has been established at a minimum of 1200 acre-feet per year from the core
of the acid plume, on a five-year rolling average. The extraction rate may be
modified pursuant to the Operation, Maintenance and Replacement IOM&R)
Plan.
B) Containment
Another change from the 2000 ROD is that a series of compliance points
has been established along the northern, eastern, and souhrn boundaries of the
Zone A Plume. These points of compliance are identified in the OM&R Plan.
The points of compliance may be modified pursuant to the OM&R Plan.
C) Cleanup Levels
The final cleanup levels for active remediation are given in the following table:
FINAL CLEANUP LEVELS FOR ACTIVE REMEDIATION
dissolved concentrations
* Once sulfate has reached 1500 mgll throughout the plume, active remediation may be
discontinued in favor of monitored natural attenuation until sulfate concentrations
tl-troughout the plume reach 500 mg1.
Nitrate has been deleted as a contaminant of concern since nitrate concentrations have
consistently been well below the groundwater protection limit.
Treatment levels for the reverse osmosis treatment plant have been deleted since the
water treatment plant is operating under a permit with the Utah Division of Drinking
Water.
The method for determining when final cleanup levels have been met will be identified in
the OM&R Plan when the groundwater quality in the plume approaches the final cleanup
levels.
Comments from Utah Department of Environmental Ouali ty
The Utah Department of Environmental Quality (LDEQ) supports EPA" decision to
modify the remedy for Operable Unit 2 of the Kennecott South Zone Site.
Pub1 ic Participation
EPA published a notice in the Deseret News and Salt Lake Tribune newspapers that
described thc ESD and its availability for review (under Swtion 1 171~) of CERCLA, 42
U.S.C. Section 9617). While a formal public comment period is not required when
issuing an ESD, EPA and ZTDEQ provided an opportunity for the pubIic to comment.
Following a 30-day comment period, a responsiveness summary was prepared in
response to comments received. This ESD, and all documents that support the changes
and clarifications, are contained in the Administrative Record of the Kennecott South
Zone Site (under 40 CFR, Section 300.43 5(c)(Z)(i)).
Under CERCZA Section 221, EPA must select a remedy that is protective of human
health and the environment, complies with Applicable or Relevant and Appropriate
Requirements ( ARARs), and is cost effective. EPA believes that the modifications to the
ROD for the Southwest Jordan River Valley groundwater plumes are appropriate and the
remedy will remain protective of human health and the environment. The selected
remedy will continue to comply with federal and state requirements that are applicable
and relevant and appropriate to the remedial action. This ESD does not fi~ndamentally
change Ihe remedy and is cost effective.
Section 121 aIso states that EPA must select a remedy that uses permanent solutions,
alternative treat men t tec bnologies, or resource recovery technologies to the maximum
extent practicable. In addition, CERCLA prefers remedies that include treatment that
permanently and signif cantly reduces the volume, toxicity, or mobiIity of hazardous
waste as a principal element of the remedy. The selected remedy uses treatment as a
principal element in remediation of the aquifer and meets the statutory requirement.
LZ-DT
Date
Ecosystems Protection and Remediation
US EPA Region 8
& Date
Executive Director
Utah Department of Environmental
Quality