DERR-2024-013252

6949 South High Tech Drive
Midvale, UT 84047
P [phone]
Terracon.com
March 9, 2023
Utah Department of Environmental Quality
Division of Environmental Response and Remediation
Voluntary Cleanup Program
P.O. Box 144840
195 North 1950 West
Salt Lake City, Utah 84114-4840
Attn: Mr. Chris Howell
P: [phone]
E: [e-mail]
RE: Responses to Comments for Site Characterization Report; dated January 18, 2023
Redwood Road Dump Pilot Phase Development VCP Site C121
1850 West Indiana Avenue
Salt Lake City, Utah
Terracon Project No. 61227342_5
Dear Mr. Howell:
Following are Terracon’s responses to DERR Comments on the above-referenced report.
The revised report is being submitted under separate cover. DERR comments are listed
below in black text with Terracon responses following in blue text.
General Comments:
1. A background value of 10.8 mg/kg for arsenic was established at the Redwood Road
Dump during the 1995 Site Investigation. This report was included as part of the
Environmental Assessment. Please revise the text and use this established background
value or propose further sampling for background. Final cleanup standards will be
established in the Remedial Action Plan (RAP).
Terracon Response: Text revised in several sections to acknowledge the background arsenic
concentration of 10.8 mg/kg for the VCP Site and to state that final cleanup standards will
be set forth in the RAP.
2. Conditions under the remaining concrete slab and in areas that have not been sampled
have an inherent uncertainty due to the history of this site. Please note this uncertainty in
the text and state that potential unknowns will be addressed in the RAP as fill and the
concrete slab are removed.
Terracon Response: Text revised in Section 6.1 to reflect this uncertainty.
Response to UDEQ Comments
Site Characterization Report dated January 18, 2023 Property State
March 9, 2023 | Terracon Project No. 61227342 Task 4.2
2
Specific Comments:
1. Section 1.2, page 2- On the third bullet, please revise the text to identify that TP-9
sample location was located outside of the “VCP Site” rather than outside the “Pilot Phase
area”. Additionally, please be consistent with this terminology throughout the text.
Terracon Response: Text revised throughout report using the term “VCP Site” to
refer to project.
2. Section 1.2, page 2- The DERR does not agree with the upper level background arsenic
value cited in the text. Please remove this reference throughout the text and see General
Comment #1 to address arsenic background concentrations.
Terracon Response: Text revised such that reference to regional background concentrations
for arsenic has been removed. The use of the site-specific background concentration of
10.8 mg/kg for arsenic is presented in Section 1.3.
3. Section 1.2, page 3- Please remove the “typical cleanup value for arsenic” in the text.
Cleanup levels will be established in the RAP.
Terracon Response: Text has been revised.
4. Section 1.2, page 4, bullet 1- Please specify in the text that this was the “March 2, 2022”
groundwater measurements that indicate a southeast flow direction.
Terracon Response: Text revised to state groundwater measurements from Jan 26, 2022,
were used to measure southeast flow direction.
5. Section 5, pages 15-26- Multiple quality assurance issues are noted where Data Quality
Objectives (DQO’s) were not met. The DERR agrees that they do not have a major impact on
the data; however, there are a few analytical validations that need to be revised. The DERR
and the author of the document met on February 9, 2023, to review and resolve technical
concerns. Please ensure the revised report incorporates these changes.
Terracon Response: Text in Section 5 has been revised.
6. Section 6.1.1, page 26- Please revise the text to assess if arsenic has exceeded site
background concentrations identified in General Comment #1.
Terracon Response: Text has been revised.
7. Section 6.3, page 28- Please rephrase this section to identify the Class III aquifer as
“Limited Use Ground Water” rather than “not be considered Drinking Water Quality”.
Remedial efforts should be employed to address the source of the contamination at MW- 05
(TDS concentrations exceeding 9,000 mg/L).
Terracon Response: Groundwater at the Site should be considered a Class III – Limited Use
Aquifer. A Class III aquifer has TDS levels ranging from 3,000 mg/L to 10,000 mg/L. The
concentrations reported in the investigation do not exceed the threshold for a Class III
Response to UDEQ Comments
Site Characterization Report dated January 18, 2023 Property State
March 9, 2023 | Terracon Project No. 61227342 Task 4.2
3
Limited Use Aquifer and remediation is not required. TDS is not considered a primary
pollutant nor a health hazard, and as the TDS concentration did not exceed the threshold for
a Class III Limited Use Aquifer, remediation is not required.
8. Section 7.0, page 29- The VCP applicant has indicated that additional sampling will be
proposed in the near future to better define the extent of impacted fill material that will
need to be managed in the RAP. This should include areas around the PAH and metals
impacts, at a minimum. Please revise the text to note this comment.
Terracon Response: Text in Section 7 has been revised.
9. Section 7.0, page 29- Please note that this paragraph includes risk management
statements that are better suited for the RAP. Please revise the text to note that a final
groundwater remedy will be proposed in the RAP. Additionally, groundwater monitoring
should be a component of the final remedy to ensure protection of human health and the
environment.
Terracon Response: Text has been revised to note the remedy will be presented in the RAP.
Terracon will propose institutional controls in the form of an Environmental Covenant
restricting access to groundwater at the site and engineering controls in the form of the
installation of the culinary water line above the groundwater level and vapor intrusion
mitigation system requirements for future structures. It is Terracon’s opinion that
groundwater monitoring post remedial action is not warranted.
The only constituent reported in groundwater at the site to exceed an MCL was dissolved
arsenic, which is naturally elevated in Utah and particularly in this location of the Salt Lake
Valley. The compound 1,4-dioxane was reported to exceed the Tap Water screening level,
and an MCL is not established for the compound. Two PFAS compounds were reported in
groundwater in concentrations that exceeded a Tap Water screening level. An MCL is not
established for PFAS and the two compounds reported do not meet the EPA criteria to be
considered a volatile chemical.
Implementation of the controls to be proposed in the RAP will ensure the protection of
future residents and the environment.
10. Section 6.3, page 28- Hexavalent chromium was detected exceeding the Tap water RSL
in MW-01. Please note this exceedance in the text along with the other reported groundwater
exceedances.
Terracon Response: Section 6.3.1 was added to note the hexavalent chromium exceedance
reported during the previous investigation.
11. Tables- Please include the Duplicate results in the analytical results Tables to ensure all
data is noted.
Response to UDEQ Comments
Site Characterization Report dated January 18, 2023 Property State
March 9, 2023 | Terracon Project No. 61227342 Task 4.2
4
Terracon response: It is Terracon’s report format to present QA/QC samples in a separate
table as they are not primary samples; however, the tables have been revised to include
duplicate results.
If you should have any questions or comments regarding these responses, please contact
either of the undersigned at [phone].
Sincerely,
Terracon Consultants, Inc.
Nancy Saunders Amy Austin
Project Manager Authorized Project Reviewer