DSHW-2024-006469

DSHW-2024-006469 195 North 1950 West • Salt Lake City, UT
Mailing Address: P.O. Box 144880 • Salt Lake City, UT 84114-4880
Telephone [phone] • Fax [phone] • T.D.D. 711
www.deq.utah.gov
Printed on 100% recycled paper
State of Utah

SPENCER J. COX
Governor

DEIDRE HENDERSON
Lieutenant Governor
Department of
Environmental Quality

Kimberly D. Shelley
Executive Director

DIVISION OF WASTE MANAGEMENT
AND RADIATION CONTROL

Douglas J. Hansen
Director

May 29, 2024

Kris Blauer, Manager
Environmental Services
Northrop Grumman Systems Corporation
P.O. Box 707
Brigham City, UT 84302-0707

RE: 2023 M-136 LTTA Post-Closure Permit Annual Report
Northrup Grumman Systems Corporation (NGSC) – Promontory Facility
UTD009081357

Dear Mr. Blauer:

The Division of Waste Management and Radiation Control (Division) has reviewed the 2023 M-136
Liquid Thermal Treatment Area (LTTA) Post-Closure Permit Annual Report (Report) dated April 2024,
regarding the 2023 annual groundwater monitoring at the ATK Promontory facility. Based on this
review, the Division is providing the following comments:

1. General.

The Report does not include a comparison of the sampling results to appropriate screening
levels. The Groundwater Protection Standards listed in Table IV-1, Module IV Groundwater
Monitoring, of the ATK Launch Systems Promontory Post Closure Permit (Permit) were
established as screening levels. The Groundwater Protection Standards in the Permit are based
on 2017 maximum contaminant levels (MCL) and EPA Regional Screening Levels for Tapwater
(Tapwater RSL). As the MCLs and Tapwater RSLs may change over time, please use current
screening levels for comparison. Sampling results should be discussed in context of whether
constituents have or have not exceeded the current screening levels.

Please revise the Report to include a comparison of sampling results to current screening levels.

(Over)

Page 2 of 6

2. Section 1.0, Table 1 – 2023 Groundwater Monitoring List.

The list of wells monitored has changed from 2022 to 2023 and had appeared to change year to
year previously. As a result, it has been difficult to determine whether trends in plume
contaminant concentrations exist due to the time gaps in data for the wells that have not been
sampled consistently. Based on the letter from the Division dated July 19, 2023, key
groundwater wells in the source, mid-plume, and down-gradient areas will be sampled annually.
Annual sampling of the key wells will provide data to discern trends associated with the plumes
at the ATK Promontory facility. Once sufficient data exists, statistics will be analyzed to
determine concentration trends. Thank you for including the key wells outlined in the
July 19, 2023, letter in Table 1.

3. Section 2.2 Total Depth of Wells Completed in Unconsolidated Material.

Table 3 shows the total depth of the well (assumed to be total depth of the well at the time of
installation) and 2023 total depth of each well. The Report states that significant sediment
accumulation was not noted in any of the wells.

Please discuss what net change is considered “significant accumulation of sediment” in the wells
and what would trigger redevelopment of a well. Also, please include the well installation date
on this table to clarify the age of the wells and gauge sediment accumulation over time.

4. Section 3.1 Hotspots and Report Preface.

The Report Preface states that the Report includes identification of potential hotspots meriting
further investigation. Shotgun Spring, Pipe Spring, Well H-4 and Well H-5 are discussed as
hotspots in Section 3.1 of the Report. These sampling locations have exhibited elevated, but
fluctuating, concentrations of perchlorate and trichloroethene (TCE). The Report states that
ATK (now NGSC) will continue to sample these locations to document any trends. However,
it is noted that Well H-4 is not included in the list of key groundwater monitoring wells (Table 1)
that will be monitored consistently going forward.

The text further states that these values fluctuate and are significantly below what is predicted
into the out years in the contaminant model predictions. However, a historical review of the data
for the springs has shown increasing contamination, and this increasing trend has also been
confirmed with the modeling. As noted in the response to Comment No. 8 on the Human Health
Risk Assessment (letter dated December 21, 2023), “the groundwater model has been updated
and model results estimate an increase of perchlorate and TCE at both Pipe Springs and Shotgun
Springs, with projected concentrations at both springs of 107 µg/L [micrograms per liter] of TCE
and 950 µg/L of perchlorate in 30 years.” In addition, the response to Comment No. 5 on the
Ecological Risk Assessment (letter dated December 21, 2023) also acknowledges, “Since TCE
concentrations are predicted to increase at Pipe Spring and Shotgun Spring based on the 2023
Groundwater Model Update, it is possible that other COCs [contaminants of concern] related to
TCE could also increase at the Springs.”

As these locations are considered hotspots and of specific interest due to their location off-site,
a trend analysis of the data should be conducted to determine whether constituent concentrations
are increasing, decreasing, or stable over time.

Page 3 of 6

A non-parametric Mann-Kendall test is an appropriate method to use to determine whether a
statistically increasing, decreasing, or stable monotonic trend is observed. If a monotonic trend
is demonstrated using the Mann-Kendall test, and the trend appears to be linear, the Theil-Sen
line can be used to estimate the slope of the trend. The Theil-Sen test will determine the rate of
change.

Please conduct Mann-Kendall analysis, and Theil-Sen analysis if appropriate, on the data for
Shotgun Springs, Pipe Springs, and Well H-5 as part of the annual Report going forward. In
addition to the trend analysis, please include a discussion of the historical data review and
modeling predictions.

5. Section 3.1 Hot Spots

The Report identifies Shotgun Spring, Pipe Spring, Well H-4, and Well H-5 as hotspots that have
exhibited elevated concentrations of perchlorate and TCE.

a. The Report states that the perchlorate concentration in Shotgun Spring has increased
since the 2022 sampling event, while the TCE concentration is comparable to the
previous sampling events.

Perchlorate and TCE concentrations are currently 636 µg/L and 26.9 g/L, respectively.
The MCL for perchlorate is 15 g/L and for TCE is 5 g/L. The perchlorate and TCE
concentrations both exceed the applicable MCLs. Shotgun Spring is located outside the
ATK Promontory facility property boundaries, which indicates that contamination has
migrated offsite.

b. The Report also states that the constituent concentrations in Pipe Spring have shown
essentially no change when compared to 2022 data. Pipe Spring was sampled during
spring 2023, but not fall 2023. The spring 2023 TCE concentration is 13.1 g/L, which
exceeds the MCL of 5 g/L. The spring perchlorate concentration is not included in
Appendix B Perchlorate sample results but should be. Please include spring sampling
results for Pipe Springs in the results tables.

The fall 2022 perchlorate concentration, however, is 575 g/L. The most recent TCE and
perchlorate concentrations both exceed the applicable MCLs. Pipe Spring is located
outside the ATK Promontory facility property boundaries, which indicates that
contamination has migrated offsite.

c. The fall 2023 TCE and perchlorate concentrations in Well H-5 are 21.0 g/L and
748 g/L, respectively. These concentrations exceed the respective MCLs of 5 g/L and
15 g/L. Well H-5 is located outside the ATK Promontory facility property boundaries,
which indicates that contamination has migrated offsite.

d. Well H-4 was not sampled during fall 2023, as it is not listed as one of the key
monitoring wells to be annually monitored.

The Report states that NGSC will continue to monitor the springs on a semi-annual basis and
continue to monitor the wells. In accordance with R315-101-6(3)(i) of the Utah Administrative
Code, corrective action is required at a site when groundwater contamination concentrations
Page 4 of 6

have been shown to be above a corrective action level. In this case the corrective action level is
not limited to the MCL, as off-site exposure cannot be controlled and may also include exposure
to ecological and human receptors. The applicable screening level should include all potentially
complete exposure pathways and be protective of all exposure scenarios.

While continued monitoring is needed, both monitoring data and the 2023 Groundwater
Modeling indicate that contamination is migrating off-site and is anticipated to be at levels
exceeding appropriate corrective action levels. As discussed in the meeting dated
April 25, 2024, the Division’s concern is mitigating or retarding additional off-site migration of
COCs. As the TCE and perchlorate contamination has migrated offsite and exceeds the
respective screening level concentrations, please propose corrective measures to address these
contaminants and to prevent/minimize and further degradation of off-site resources.

6. Appendix A Analytical Results Table:

The table reporting analytical results for the sampled wells is formatted in a manner that does not
allow for efficient comparison of data within a single well, among wells, or across yearly reports
for trending analysis. The data are sorted by analyte rather than by well. This table should be
reformatted in a manner that allows the reader to quickly compare the data. The data should
instead be sorted by well, then analytes, then date. A possible format is provided below –
however, you may develop your own:

Analyte 1,1,1-ABC 1,1,2-
ABD
1,2-CD

TCE Perchlorate
Screening
Level
(MCL
5 µg/L)
(MCL
5 µg/L)
(MCL
2.5 µg/L)

(ETC) (ETC)
MDL /
MRL
0.6 (µg/L)
/ 1.0 (µg/L)
4.4 (µg/L)
/
10 (µg/L)

0.3(µg/L)
/
1.0
(µg/L)
X (µg/L)
/
X (µg/L)
X (µg/L) /
X (µg/L)

Well Sample
Date Groundwater Concentration
A-1 xx/xx/2023 2.3 15.7 4.1 Etc. Etc.
A-1 xx/xx/2024 ND 5.2 ND Etc. Etc.
A-2 xx/xx/2023 1.1 2.5 (CF) 0.6 J Etc. Etc.
A-2 xx/xx/2024 2.3 15.7 4.1 Etc. Etc.
B-1 xx/xx/2023 ND 5.2 ND Etc. Etc.
(CF)– Samples analyzed by Chemtech Ford
ND – non-detect
U – samples …
J – samples flagged as estimated
Bold – Result exceeding Screening Level

Please reformat the analytical results table to facilitate efficient comparison of data. Please
include applicable screening levels for the constituents monitored for data comparison.

Page 5 of 6

7. Appendix A – Analytical Data – Volatiles and Appendix B – Analytical Data - Perchlorate.

Pipe Spring appears not to have been sampled during the fall 2023 sampling event. The most
recent data included in the table is associated with the May 4, 2023, sampling event. According
to the ATK Launch Systems Groundwater Sampling and Analysis Plan and Post-Closure Permit,
Pipe Spring is required to be sampled semiannually – in the spring and fall.

Please ensure that Pipe Spring is sampled in accordance with the requirements. In addition, if,
for some reason, NGSC is not able to sample a well, the Division should be notified, and a
discussion should be included in the report as a deviation from the sampling plan.

8. Appendix C – Analytical Data Sheets.

An analytical data package and Quality Control (QC) report from Chemtech Ford for samples
collected on November 14 and 15, 2022 is included in the Report. It is assumed that the
inclusion of this information is in error. However, please explain the inclusion of these samples
and results. Please also review the Report to ensure that all information required for the
2023 sampling events is included.

9. Appendix C – Analytical Data Sheets.

Appendix C contains analytical information from both the ATK Launch Systems Analytical
Laboratory and Chemtech Ford. The analytical information should contain both the analytical
data package and the associated QC Report. The Chemtech Ford data package contains the
analytical data results and the QC Report. However, the data package from the ATK Launch
Systems Analytical Laboratory contains the analytical data results but not the Quality Control
Summary Reports for those data sets.

Please include the Quality Control Summary Reports for the samples analyzed by the
ATK Launch Systems Analytical Laboratory in a revised Report.

10. Appendix D – Plates and Report Narrative.

The isoconcentration maps provide a useful visualization of the plume size and concentration.
However, the isoconcentration maps should be accompanied with Report narrative that discusses
the status of the plumes and whether they are increasing, decreasing, stable, or migrating offsite.
The discussion should include information about changes in constituent concentrations.

Please include a discussion of the information in the isoconcentration maps.

11. Appendix D – Plates.

The 1,1-dichloroethane and chloroform isoconcentration maps should include concentration
contour lines representing the screening levels to aid in the visualization of the location of
screening level concentration exceedances in the plumes. To further aid in this visualization, a
contrasting color for the contour line representing the screening level concentrations should be
used on all isoconcentration maps.

Page 6 of 6

12. Appendix D – Plates.

The 1,1-Dichloroethene (DCE), trichloroethene (TCE), perchlorate, and chloroform
isoconcentration maps indicate that these constituents have migrated beyond the
ATK Promontory property boundary. These plumes have migrated beyond the western
boundary in the general locations of Well B-6 and/or Well F-1.

As the contamination has migrated offsite, please propose corrective measures to mitigate the
offsite migration of these constituents.

13. Appendix D – Plates.

The constituent concentrations associated with the well locations on the Plate 2 –
1,1-Dichloroethene isoconcentration map does not appear to have been updated since the
2022 report. For example, wells C-3, B-2, B-3, and B-4 are labeled with the respective
concentrations of 72.1 g/L, ND, 365 g/L, and 76 g/L – for both 2022 and 2023.
The 2023 analytical results for these wells are 74.7 g/L, 213 g/L, 262 g/L, and 32.1 g/L,
respectively. Please utilize the 2023 data and revise the map accordingly.

14. Appendix D – Plates.

Plate 3 – 1,1,1-Trichloroethane isoconcentration map does not include concentration values
associated with the contour lines. Please revise the map to include this information.

Please provide a written response to the comments and a revised Report within 90 days of the date of
this letter.

If you have any questions, please call Karen Wallner at 385-499-0218 or Hao Zhu at 801-558-9833.

Sincerely,

Paige Walton, Corrective Action Section Manager
Division of Waste Management and Radiation Control

PW/KHW/wa

c: Jordan Mathis, Health Officer, Bear River Health Department
Grant Koford, EHS, Environmental Health Director, Bear River Health Department
Tara Hubner, U.S. EPA, Region 8
Kris Blauer, Northrup Grumman Systems Corporation (Email and Hard Copy)
Tim Jimenez, Northrop Grumman Systems Corporation (Email)
Blair Palmer, Northrop Grumman Systems Corporation (Email)