FINAL

Site Inspection Report 

Helena Army Aviation Support Facility

Helena, Montana

Perfluorooctanesulfonic Acid (PFOS) and 

Perfluorooctanoic Acid (PFOA) Impacted Sites

ARNG Installations, Nationwide

August 2021

Prepared for:

Army National Guard Bureau

111 S. George Mason Drive

Arlington, VA 22204

UNCLASSIFIED
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Table of Contents 

Executive Summary ................................................................................................................ ES-1 

1. Introduction .................................................................................................................... 1-1 

1.1 Project Authorization ............................................................................................... 1-1 

1.2 SI Purpose .............................................................................................................. 1-1 

2. Facility Background ........................................................................................................ 2-1 

2.1 Facility Location and Description ............................................................................ 2-1 

2.2 Facility Environmental Setting ................................................................................. 2-1 

2.2.1 Geology ........................................................................................................ 2-1 

2.2.2 Hydrogeology ............................................................................................... 2-1 

2.2.3 Hydrology ..................................................................................................... 2-2 

2.2.4 Climate ......................................................................................................... 2-3 

2.2.5 Current and Future Land Use ....................................................................... 2-3 

2.2.6 Critical Habitat and Threatened/ Endangered Species................................. 2-3 

2.3 History of PFAS Use ............................................................................................... 2-3 

2.4 Potable Water Sampling ......................................................................................... 2-3 

3. Summary of Areas of Interest ......................................................................................... 3-1 

3.1 AOI 1 ....................................................................................................................... 3-1 

3.1.1 60 Hangar ..................................................................................................... 3-1 

3.1.2 47 Hangar ..................................................................................................... 3-1 

3.1.3 Tri-MaxTM Spill Area and Frozen Extinguisher Release Area ....................... 3-1 

4. Project Data Quality Objectives ..................................................................................... 4-1 

4.1 Problem Statement ................................................................................................. 4-1 

4.2 Goals of the Study .................................................................................................. 4-1 

4.3 Information Inputs ................................................................................................... 4-2 

4.4 Study Boundaries ................................................................................................... 4-2 

4.5 Analytical Approach ................................................................................................ 4-2 

4.6 Data Usability Assessment ..................................................................................... 4-3 

4.6.1 Precision ....................................................................................................... 4-3 

4.6.2 Accuracy ....................................................................................................... 4-4 

4.6.3 Representativeness ...................................................................................... 4-5 

4.6.4 Comparability ................................................................................................ 4-5 

4.6.5 Completeness ............................................................................................... 4-5 

4.6.6 Sensitivity ..................................................................................................... 4-6 

5. Site Inspection Activities ................................................................................................. 5-1 

5.1 Pre-Investigation Activities ...................................................................................... 5-1 

5.1.1 Technical Project Planning ........................................................................... 5-1 

5.1.2 Utility Clearance ........................................................................................... 5-2 

5.1.3 Source Water and PFAS Sampling Equipment Acceptability ....................... 5-2 

5.2 Soil Borings and Soil Sampling ............................................................................... 5-2 

5.3 Permanent Well Installation and Groundwater Sampling ....................................... 5-3 

5.4 Synoptic Water Level Measurements ..................................................................... 5-4 

5.5 Surveying ................................................................................................................ 5-4 

5.6 Investigation-Derived Waste ................................................................................... 5-4 

5.7 Laboratory Analytical Methods ................................................................................ 5-4 

5.8 Deviations from SI QAPP Addendum ..................................................................... 5-5 
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6. Site Inspection Results...................................................................................................6-1

6.1 Screening Levels ....................................................................................................6-1

6.2 Soil Physicochemical Analyses...............................................................................6-1

6.3 AOI 1.......................................................................................................................6-2

6.3.1 AOI 1 Soil Analytical Results ........................................................................6-2

6.3.2 AOI 1 Groundwater Analytical Results .........................................................6-2

6.3.3 AOI 1 Conclusions........................................................................................6-2

7. Exposure Pathways .......................................................................................................7-1

7.1 Soil Exposure Pathway...........................................................................................7-1

7.1.1 AOI 1 ............................................................................................................7-1

7.2 Groundwater Exposure Pathway ............................................................................7-2

7.2.1 AOI 1 ............................................................................................................7-2

8. Summary and Outcome .................................................................................................8-1

8.1 SI Activities .............................................................................................................8-1

8.2 SI Goals Evaluation ................................................................................................8-1

8.3 Outcome .................................................................................................................8-2

9. References.....................................................................................................................9-1

Appendices

Appendix A Data Validation Reports

Appendix B Field Documentation

B1. Log of Daily Notice of Field Activities

B2. Well Construction Logs

B3. Well Development Forms

B4. Groundwater Sampling Forms

B5. Survey Data

Appendix C Photographic Log

Appendix D TPP Meeting Minutes/Montana DEQ Memorandum

Appendix E Boring Logs and Well Construction Diagrams

Appendix F Analytical Results

Appendix G Laboratory Reports
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Figures

Figure 2-1 Facility Location

Figure 2-2 Groundwater Features

Figure 2-3 Groundwater Elevations - July 12, 2020 

Figure 2-4 Surface Water Features

Figure 2-5 Potable Well Sampling Areas

Figure 3-1 Areas of Interest

Figure 5-1 Site Inspection Sample Locations 

Figure 6-1 PFOS Detections in Soil

Figure 6-2 PFOA Detections in Soil

Figure 6-3 PFOA and PFOS Detections in Groundwater

Figure 7-1 Conceptual Site Model, AOI 1

Tables

Table ES-1 Screening Levels (Soil and Groundwater)

Table ES-2 Summary of Site Inspection Findings

Table ES-3 Site Inspection Recommendations

Table 2-1 PFAS Detections in Potable Wells

Table 5-1 Site Inspection Samples by Medium

Table 5-2 Boring Depths and Permanent Well Screen Intervals

Table 5-3 Depth to Water and Groundwater Elevations

Table 6-1 Screening Levels (Soil and Groundwater)

Table 6-2 PFAS Detections in Surface Soil

Table 6-3 PFAS Detections in Subsurface Soil

Table 6-4 PFAS Detections in Groundwater

Table 8-1 Summary of Site Inspection Findings

Table 8-2 Site Inspection Recommendations
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Acronyms and Abbreviations

6:2 FTS 6:2 Fluorotelomer sulfonate

8:2 FTS 8:2 Fluorotelomer sulfonate

μg/Kg micrograms per Kilogram

°C degrees Celsius

°F degrees Fahrenheit

% percent

AASF Army Aviation Support Facility

AECOM AECOM Technical Services, Inc.

AFFF aqueous film forming foam

AOI Area of Interest

ARNG Army National Guard

bgs below ground surface 

btoc below top of casing

CERCLA Comprehensive Environmental Response, Compensation, and Liability Act 

CoC chain of custody

CSM conceptual site model

DA Department of the Army

DoD Department of Defense

DO dissolved oxygen

DQI data quality indicator

DQO data quality objective

DUA data usability assessment

DVR data validation report 

EIS extraction internal standards

ELAP Environmental Laboratory Accreditation Program

EM Engineers Manual

FedEx Federal Express

HDPE high-density polyethylene 

HRAA Helena Regional Airport Authority

HSA hollow stem auger

IDW investigation-derived waste

ITRC Interstate Technology Regulatory Council

LC/MS/MS liquid chromatography tandem mass spectrometry

LCS laboratory control spike

LCSD laboratory control spike duplicate

LOQ limit of quantitation

MBMG Montana Bureau of Mines and Geology

MDL method detection limit

mph miles per hour

MS matrix spike

MSD matrix spike duplicate

MTARNG Montana Army National Guard

MTDEQ Montana Department of Environmental Quality
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NELAP National Environmental Laboratory Accreditation Program

NEtFOSAA N-ethyl perfluorooctanesulfonamidoacetic acid

ng/L nanograms per liter

NMeFOSAA N-methyl perfluorooctanesulfonamidoacetic acid

ORP oxidation-reduction potential

OSD Office of the Secretary of Defense

PA Preliminary Assessment

PFAS per- and polyfluoroalkyl substances

PFBA perfluorobutyrate

PFBS perfluorobutanesulfonic acid

PFCs perfluorinated compounds

PFDA perfluorodecanoic acid

PFDoA perfluoroheptanoic acid

PFHpA perfluoroheptanoic acid

PFHxA perfluorohexanoic acid

PFHxS perfluorohexanesulfonic acid

PFNA perfluorononanoic acid

PFOA perfluorooctanoic acid

PFOS perfluorooctanesulfonic acid

PFPeA perfluoropentanoic acid

PFTeDA perfluorotetradecanoic acid

PFTrDA perfluorotridecanoic acid

PFUdA perfluoroundecanoic acid

PID photoionization detector

Pioneer Pioneer Technical Services, Inc

PPE personal protective equipment

PQAPP Programmatic UFP-QAPP

PVC polyvinyl chloride

QA quality assurance

QAPP Quality Assurance Project Plan

QC quality control

QSM Quality Systems Manual

RI Remedial Investigation

RPD relative percent differences

SI Site Inspection

SL screening level

SOP standard operating procedure

TOC total organic carbon

TPP Technical Project Planning

UCMR Unregulated Contaminant Monitoring Rule

UFP Uniform Federal Policy

US United States

USACE United States Army Corps of Engineers

USCS Unified Soil Classification System

USEPA United States Environmental Protection Agency
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USFWS United States Fish and Wildlife Service

USGS United States Geological Survey
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Executive Summary

The Army National Guard (ARNG) G9 is performing Preliminary Assessments (PAs) and Site 

Inspections (SIs) at per- and polyfluoroalkyl substances (PFAS)-impacted sites at ARNG facilities 

nationwide. The objective of the SI at each facility is to identify whether there has been a release 

to the environment from the Area of Interest (AOI) identified in the PA and determine the presence 

or absence of perfluorooctanoic acid (PFOA), perfluorooctanesulfonic acid (PFOS), and 

perfluorobutanesulfonic acid (PFBS) at or above screening levels (SLs). An SI was completed at 

the Helena Army Aviation Support Facility (AASF) in Helena, Montana. The Helena AASF will be 

referred to as the “facility” throughout this document. 

The facility is on a 75-acre parcel of land adjacent to the Helena Regional Airport in Lewis and 

Clark County. The AASF is on the eastern city limits of Helena, east of Interstate Highway 15, 

south of Canyon Ferry Road, and north of the Burlington Northern Railroad Tracks. The PFAS PA 

Report identified two potential release areas which were grouped into one AOI and investigated 

during the SI (AECOM, 2018c; AECOM, 2020b). The SI field activities were conducted from 6 to 

13 July 2020 and included the collection of soil and groundwater samples. 

To fulfill the project Data Quality Objectives (DQOs) set forth in the approved SI Quality Assurance 

Project Plan (QAPP) Addendum (AECOM, 2020b), samples were collected and analyzed for a 

subset of 18 PFAS by liquid chromatography tandem mass spectrometry (LC/MS/MS) compliant 

with Quality Systems Manual (QSM) 5.1 Table B-15. The 18 PFAS analyzed as part of the ARNG 

SI program are specified in Section 5.7 of this Report.

The Department of Defense (DoD) has adopted a policy to retain facilities in the Comprehensive 

Environmental Response, Compensation, and Liability Act (CERCLA) process based on riskbased SLs for soil and groundwater, as described in a memorandum from the Office of the 

Secretary of Defense (OSD) dated 15 October 2019 (Assistant Secretary of Defense, 2019). The 

ARNG PFAS SIs follow this DoD policy and, when the maximum site concentration for sampled 

media exceed the SLs, the site will proceed to a Remedial Investigation (RI), the next phase under 

CERCLA. The SLs apply to three compounds, PFOA, PFOS, and PFBS, for both soil and 

groundwater, as presented in Table ES-1. All other results presented in this Report are considered 

informational in nature and serve as an indication as to whether soil, groundwater, sediment, and 

surface water contain or do not contain the 18 PFAS analyzed within the boundaries of the facility. 

Sample chemical analytical concentrations were compared against the project SLs as described 

in Table ES-1. A summary of the results of the SI data relative to the SLs is as follows: 

• PFOA, PFOS, and PFBS were detected in groundwater at AOI 1 and PFOS exceeded the

individual SL of 40 nanograms per liter (ng/L), with maximum concentrations of 775 ng/L

(814 ng/L duplicate) and 175 ng/L at locations HAASF-MW005 and HAASF-MW003, 

respectively. Based on the results of the SI, further evaluation of AOI 1 is warranted in the 

RI.

• Based on the SL exceedances and well information from the Montana Bureau of Mines 

and Geology (MBMG) database, a potentially complete pathway exits to off-facility 

residential wells. 

• The detected concentrations of PFOA, PFOS, and PFBS in soil samples from the AOI 

were below the SLs. 

Table ES-2 summarizes the SI results for soil and groundwater at AOI 1: 60 and 47 Hangar Fire

Suppression System Release and Tri-MaxTM Spill/Release Area. Based on the conceptual site 
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model (CSM) developed and revised in light of the SI findings, there is potential for PFOS

exposure to drinking water receptors caused by DoD activities.

Table ES-3 summarizes the rationale used to determine if an AOI should be considered for further 

investigation under CERCLA and undergo an RI. Based on the results of this SI, further evaluation 

is warranted in the RI for AOI 1: 60 and 47 Hangar Fire Suppression System Release and TriMaxTM Spill/Release Area.

Table ES-1: Screening Levels (Soil and Groundwater)

Analyte

Residential 

(Soil)

(μg/kg)a,b

0-2 feet bgs

Industrial/ Commercial 

Composite Worker

(Soil)

(μg/kg)a,b

Tap Water

(Groundwater)

(ng/L)a,b

PFOA 130 1,600 40

PFOS 130 1,600 40

PFBS 130,000 1,600,000 40,000

Notes:

a.) Assistant Secretary of Defense, 2019. Risk Based Screening Levels Calculated for PFOS, PFOA, PFBS in Groundwater and Soil 

using United States Environmental Protection Agency’s (USEPA’s) Regional Screening Level Calculator. Hazard Quotient (HQ) = 0.1. 

15 October 2019. 

b.) USEPA, 2021. Risk Based Screening Levels Calculated for PFBS in Groundwater and Soil using USEPA’s Regional Screening Level 

Calculator. HQ = 0.1. 8 April 2021.

Table ES-2: Summary of Site Inspection Findings

AOI Potential PFAS 

Release Area

Soil –

Source Area

Groundwater –

Source Area

Groundwater –

Facility 

Boundary

1

60 and 47 Hangar Fire 

Suppression System 

Release and Tri-MaxTM

Spill/Release Area

Legend:

= detected; exceedance of the screening levels

= detected; no exceedance of the screening levels

= not detected

Table ES-3: Site Inspection Recommendations

AOI Description Rationale Future Action

1

60 and 47 Hangar 

Fire Suppression 

System Release and 

Tri-MaxTM

Spill/Release Area

Exceedances of SLs in groundwater at 

source area and downgradient facility 

boundary. No exceedances of SLs in soil. 

Proceed to RI 
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1. Introduction

1.1 Project Authorization

The Army National Guard (ARNG) G9 is the lead agency in performing Preliminary Assessments 

(PAs) and Site Inspections (SIs) for Perfluorooctanesulfonic acid (PFOS) and Perfluorooctanoic 

acid (PFOA) Impacted Sites at ARNG Installations Nationwide. This work is supported by the 

United States (US) Army Corps of Engineers (USACE) Baltimore District and their contractor, 

AECOM Technical Services, Inc. (AECOM), under Contract Number W912DR-12-D-0014, Task 

Order W912DR17F0192, issued 11 August 2017. The ARNG performed this SI at Helena Army 

Aviation Support Facility (AASF) in Helena, Montana. The Helena AASF is referred to as the 

“facility” throughout this document. 

The SI project elements were performed in compliance with Comprehensive Environmental 

Response, Compensation, and Liability Act (CERCLA; US Environmental Protection Agency 

[USEPA], 1980), as amended, the National Oil and Hazardous Substances Pollution Contingency 

Plan (40 Code of Federal Regulations Part 300; USEPA, 1994), and in compliance with US 

Department of the Army (DA) requirements and guidance for field investigations, including specific 

requirements for sampling for PFOA, PFOS, and perfluorobutanesulfonic acid (PFBS), and the 

group of related compounds known in the industry as per- and polyfluoroalkyl substances (PFAS). 

The term PFAS is used throughout this Report to encompass all PFAS chemicals being evaluated, 

including PFOA, PFOS, and PFBS, which are the key components of the suspected releases 

being evaluated, and the other 15 related compounds listed in the task order.

1.2 SI Purpose

A PA (AECOM, 2018c) that identified two potential PFAS release areas, which were grouped into 

one Area of Interest (AOI), was performed at the facility. The objective of the SI is to identify 

whether there has been a release to the environment from the AOI and determine the presence 

or absence of PFOA, PFOS, and PFBS at or above screening levels (SLs). 

As stated in the Federal Facilities Remedial Site Inspection Summary Guide (USEPA, 2005), an 

SI has five goals: 

1. Develop information to potentially eliminate a release from further consideration because 

it is determined that it poses no significant threat to human health or the environment;

2. Determine the potential need for a removal action;

3. Collect or develop data to evaluate potential release;

4. Collect data to better characterize the release for more effective and rapid initiation of a 

Remedial Investigation (RI), if determined necessary; and

5. Collect data to determine whether the release is more than likely the result of activities 

associated with the Department of Defense (DoD).

In addition to the USEPA-identified goals of an SI, the ARNG SI also identifies whether there are 

potential off-facility PFAS sources.
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2. Facility Background

2.1 Facility Location and Description

The AASF is adjacent to the Helena Regional Airport (Figure 2-1) in Lewis and Clark County in 

Helena, Montana. The AASF is on the eastern city limits of Helena, east of Interstate Highway 15,

south of Canyon Ferry Road, and north of the Burlington Northern Railroad Tracks. The 

communities of Helena, East Helena, Clancy, and Jefferson City lie within 15 miles of the AASF

(Montana ARNG [MTARNG], 1994). 

In 1998, due to insufficient space, the AASF relocated to its present location on the north-central 

portion of the Helena Regional Airport property, approximately 750 feet north of Runway 9/27.

The facility includes operation, maintenance, and repair for ARNG rotary-winged aircraft (60 

Hangar and 47 Hangar), administrative offices, and classrooms (Helena Regional Airport Authority 

[HRAA], 2018). The two rotary-winged hangars are equipped with independent fire suppression 

systems. The facility also includes an armory and a fixed-wing aircraft hangar. The armory and 

the fixed-wing aircraft hangar does not have a fire suppression system or portable aqueous film 

forming foam (AFFF) extinguishers as of the date of this SI Report.

2.2 Facility Environmental Setting

The facility is located on the edge of the Helena Valley. The valley is bounded on the west by the 

Scratchgravel Hills, on the south by the Elkhorn Mountains, on the north by the Big Belt 

Mountains, and on the east by the Spokane Bench (MTARNG, 1994). The elevation of the facility 

is approximately 3,825 feet above mean sea level. The Continental Divide is located 15 miles 

west of the valley. The western part of the valley is gently sloping, while the eastern portion of the 

valley consists of low-rolling hills. The terrain around the AASF can be characterized as the 

transition between the rolling foothills of Mount Ascension and the flats of the Helena Valley

(Pioneer Technical Services, Inc. [Pioneer], 2009).

2.2.1 Geology

Helena lies within the Northern Rocky Mountains physiographic province. Quaternary-age 

sediments fill the valley and form a northeast-sloping alluvial plain. The sedimentary plain is 

bounded by broad pediments and alluvial fans of the Elkhorn Mountains, the Scratchgravel Hills, 

and the Big Belt Mountains (Pioneer, 2009). 

The AASF is situated on Quaternary-age alluvium derived from carbonate rocks and shale 

(Pioneer, 2009). A slope wash deposit, approximately 20 feet thick, underlies the soil at the AASF. 

This deposit consists of beds of coarse gravel interlayed with thin irregular beds and lenses of silt 

and clay. The gravel, in a matrix of sandy and silty clay, is composed of fragments of quartzite, 

shale, and limestone (US Geological Survey [USGS], 1986). Sedimentary bedrock from the Late 

Cretaceous to Middle Proterozoic Age underlies the slope wash and stream deposits. The 

bedrock layer is several thousand meters thick and is made up of sandstone, shale, limestone, 

and dolomite (MTARNG, 1994).

2.2.2 Hydrogeology

The facility is located along the southern boundary of the Helena Valley-Fill Aquifer System. This 

aquifer system is a major source of domestic water for local residents, with the majority of 

domestic water wells at a depth of less than 70 feet (MTARNG, 1994). Groundwater flow is 

generally from the southern, western, and northern margins of the valley, toward Lake Helena. 
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Based on a Helena AASF groundwater study (Pioneer, 2009), groundwater flow directions at the

facility vary from due north to due east (Figure 2-2). During the SI, depth to water ranged from

40.91 feet below top of casing (btoc) to 56.78 feet btoc. Groundwater elevations were calculated,

and an updated groundwater flow map indicated groundwater flows northeast (Figure 2-3).

Lateral discontinuity of fine-grained layers allows hydraulic interconnection of water-yielding 

zones that function as one complex aquifer (USGS, 1992). Aquifer recharge is through infiltration 

of streamflow, leakage from irrigation canals, infiltration of excess irrigation water, and inflow from 

fractures in bedrock. Discharge is through leakage to streams and drains, upward leakage to Lake 

Helena, and withdrawals from wells (MTARNG, 1994). 

No potable water wells are located on the facility; however, a review of the Montana Bureau of 

Mines and Geology (MBMG) database indicated as many as 3,842 wells exist within a 4-mile

radius of the facility (MBMG, 2020), as shown on Figure 2-2. A query of the MBMG database 

showed a public supply well on the eastern boundary of the AASF; however, the MTARNG has 

no knowledge of a well on the property boundary, and the well could not be located during the PA.

The MBMG database classifies wells based on their use: domestic, commercial, or industrial. Of 

the 3,842 wells within 4 miles of the facility, 805 potential domestic wells exist in the downgradient 

direction of the facility (north of the facility), some as close as 0.5 miles from the facility boundary

(MBMG, 2020). The majority of these downgradient domestic wells range in depth from 50 to over 

100 feet below ground surface (bgs) and are cased off to the bottom of the well. However, a small 

percentage of the 805 domestic wells were screened shallower (less than 50 feet). Drinking water 

for the facility is supplied by the City of Helena. The City of Helena uses groundwater and surface 

water as water sources for its residents (Helena Water Utilities Public Water System, 2004). More 

information is provided in Section 2.2.3. Additionally, the City of Helena was selected to participate 

in the USEPA Third Unregulated Contaminant Monitoring Rule (UCMR) assessment monitoring. 

Results from the sampling indicated the six PFAS contaminants analyzed were below the method 

detection limit (USEPA, 2017a; MTDEQ, 2020).

2.2.3 Hydrology

Surface water was diverted around the AASF during construction; therefore, no surface water 

currently enters the facility. The largest stream and the closest to the facility is Prickly Pear Creek, 

about 2 miles to the east of the facility, which flows towards the north (Figure 2-4). A detention 

pond near the northeast corner of the AASF collects runoff from most of the facility. The detention 

pond was originally approximately 3 feet deep and seeded with vegetation (MTARNG, 1994). The 

detention pond was reconfigured once in 2005 or 2006 and recontoured during construction in 

2017. If soil were removed during the 2005 or 2006 reconfiguration, the disposition of the soil 

would be unknown. Per the project manager for the 2017 construction, if soil were removed during 

the recontouring, it was likely re-used elsewhere at the facility during the construction project or 

removed by the contractor (Bullock Construction) and used at a construction yard in Boulder, 

Montana, or another construction site in Lakeside, Montana. Unprocessed surface water is used 

for irrigation in the fields near the facility, but exact details are currently unavailable on this water 

usage.

Regional surface water features include Lake Helena, the Missouri River, and the Helena Valley 

Reservoir. Surface water stored in the Helena Valley Reservoir provides one source of drinking 

and irrigation water used by the City of Helena (the other source includes groundwater). Water

from the Reservoir is distributed across the city through the Helena Valley Canal. The Canal is 

31.7 miles long and flows in a clockwise direction from the Helena Valley Reservoir to its 

termination at Lake Helena (US Bureau of Reclamation, 2017). The 31.7 miles of the canal is 

lined, with the exception of a 10.2 mile stretch. Information provided by the Helena Valley Irrigation 

District indicated that the section of canal immediately downgradient of the facility is lined with 

asphalt. The facility is not located within a mapped floodplain area.
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2.2.4 Climate

The climate at the AASF is northern desert with large daily temperature fluctuations and an 

average temperature of 58.3 degrees Fahrenheit (°F). Seasonally, temperatures vary from 

summer highs of 86°F to winter lows of 14°F (World Climate, 2018). Average annual precipitation

is 11.2 inches of rain and 38 inches of snow (World Climate, 2018). Factors affecting the climate 

include invasions of maritime air masses from the Pacific Ocean and drainage of cool air into the 

valley from the surrounding mountains. The prevailing wind is westerly, averaging 7 to 8 miles per 

hour (mph), with gust speeds of 55 to 65 mph.

2.2.5 Current and Future Land Use

The AASF is a controlled access facility with public roads and is adjacent to the Helena Regional 

Airport. The land is owned by the Department of the Army and leased to the State of Montana 

(MTARNG). The Helena Regional Airport is owned and operated by the HRAA and provides 

commercial and general air service to the Helena area and west-central Montana. The HRAA

owns a number of land parcels that have been subdivided and zoned to allow for commercial 

development with restriction (HRAA, 2018). Future land use is not anticipated to change.

2.2.6 Critical Habitat and Threatened/ Endangered Species

The following birds, plants, mammals, and reptiles are federally endangered, threatened, 

proposed, and/or are listed as candidate species in Lewis and Clark County, Montana (US Fish 

and Wildlife Service [USFWS], 2020).

• Mammals: Grizzly Bear, Ursus arctos horribilus (threatened)

• Mammals: Canada Lynx, Lynx canadensis (threatened)

• Mammals: North American Wolverine, Gulo luscus (proposed threatened)

• Fish: Bull Trout, Salvenlinus confluentus (threatened)

• Bird: Red Knot, Calidris canutus rufa (threatened)

• Plants: Whitebark Pine, Pinus albicaulis (candidate)

2.3 History of PFAS Use

Four potential PFAS release areas were identified at the Helena AASF during the PA (AECOM, 

2018c). Two potential releases were from fire suppression system tests performed at the 60 and 

47 Hangar. The other two releases were from portable Tri-MaxTM fire extinguishers that leaked or 

spilled onto the asphalt surrounding the AASF. The two Tri-MaxTM releases occurred in the same 

general location. All four potential releases eventually entered the detention pond on the northeast 

side of the AASF through the storm water drain. Findings from the PA did not indicate any other 

activity at the facility contributed AFFF or PFAS-containing material to the environment. A more 

thorough description of the releases is presented in Section 3.

2.4 Potable Water Sampling

Due to the historical releases of AFFF, the potential exists for exposure to offsite drinking water 

receptors immediately north of the facility boundary. Though not included in the original scope, 

programmatic contingencies are in place to add off-facility sampling if SI results deem the 

sampling is warranted. Based on the magnitude and location of the groundwater exceedances, 

the project team agreed that off-facility sampling was necessary to evaluate the potential impact 
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to off-facility receptors. Prior to sampling, approval was obtained from the Deputy Assistant 

Secretary of the Army for Environment, Safety and Occupational Health. Potable water samples 

were collected from five potable wells located in closest proximity to the facility boundary 

(downgradient of AOI 1). Sample results are provided below and in Table 2-1: 

• PFOA – Detections ranged from non-detect to 1.94 J nanograms per liter (ng/L) (HAASFPOTABLE-04).

• PFOS – Detections ranged from non-detect to 8.57 ng/L (HAASF-POTABLE-04).

• PFBS – Detections ranged from non-detect to 4.81 ng/L (HAASF-POTABLE-04).
Table 2-1

PFAS Detections in Potable Wells

Site Inspection Report, Helena AASF

Analyte USEPA HA a Result Qual Result Qual Result Qual Result Qual Result Qual Result Qual Result Qual

Water, PFAS by LCMSMS Compliant with QSM 5.3 Table B-15 (ng/L)6:2 FTS - ND ND ND ND ND 1.11 J ND

FOSA - 1.49 J 1.18 J ND UJ ND 1.66 J 1.38 J 1.18 J

NMeFOSAA - ND ND ND ND ND 1.07 J ND UJ

PFBS - ND ND ND ND 4.81 0.907 J ND UJ

PFDA - ND ND ND ND ND 0.898 J ND UJ

PFHpA - ND ND ND ND 1.46 J 1.02 J ND UJ

PFHxA - ND ND ND ND 4.65 1.53 J ND UJ

PFHxS - 1.04 J ND ND ND 16.2 1.03 J ND UJ

PFNA - ND ND ND ND ND 0.834 J ND UJ

PFNS - ND ND ND ND ND 0.787 J ND UJ

PFOA 70 ND ND ND ND 1.94 J 1.36 J ND UJ

PFOS 70 ND ND ND 0.984 J 8.57 2.57 J ND UJ

PFPeA - ND ND ND ND 4.31 1.01 J ND UJ

PFPeS - ND ND ND ND 3.32 J 0.883 J ND UJ

Total PFOA+PFOS 70 ND ND ND 0.984 10.5 3.93 ND

Grey Fill Detected concentration exceeded USEPA HA Chemical Abbreviations

6:2 FTS 6:2 

References FOSA Per

NMeFOSAA N-mPFBS perPFDA perPFHpA per

Interpreted Qualifiers PFHxA perJ = Estimated concentration PFHxS per

UJ = The analyte was not detected at a level greater than or equal to the adjusted detection limit (DL). However, the reported adjusted DL is approximate and may be inaccurate or imprecise. PFNA per

PFNS per

Acronyms and Abbreviations PFOA per

AASF Army Aviation Support Facility PFOS per

DUP Duplicate PFPeA per

HA Health Advisory PFPeS per

LCMSMS Liquid Chromatography Mass SpectrometryLOD Limit of DetectionLOQ Limit of QuantitationND Analyte not detected above the LODQual Interpreted QualifierUSEPA United States Environmental Protection Agencyng/L nanogram per liter- Not applicable

02/16/2021

Area of Interest

Sample ID

Sample Date

HAASF-POTABLE-01

02/16/2021

a. United States Environmental Protection Agency. 2016. Drinking Water Health Advisory for PFOA. Office of Water (4304T). Health and Ecological Criteria Division, Washington, DC 20460. EPA Document Number: 822-R-16-005. May 2016. / EPA. 2016. Drinking Water Health Advisory for PFOS. Office of Water (4304T). Health and Ecological Criteria Division, Washington, DC 20460. EPA Document Number: 822-R-16-004. May 2016.

POTABLE

HAASF-POTABLE-05

04/29/2021

HAASF-POTABLE-05 DUP

04/29/2021

HAASF-POTABLE-03

04/29/2021

HAASF-POTABLE-04

04/30/2021

HAASF-POTABLE-02

02/16/2021

HAASF-POTABLE-02-DUP

AECOM 
Site Inspection Report

Helena Army Aviation Support Facility, Helena, Montana 

THIS PAGE INTENTIONALLY BLANK

AECOM 
CLIENT

REVISED

SCALE

ARNG

GIS BY

CHK BY

MS

AB

10/15/2020

10/15/2020

NOTES Site Inspection for PFAS at Helena AASF, MT

10/15/2020

1:63,360 ­ Figure 2-1 PM RG 10/15/2020

C:\Users\stankevichm\OneDrive - AECOM Directory\ARNG_PFAS_GIS_60552172\MXDs\MT\Helena_AASF\Helena_AASF_SI_Figures\SI_Report\Fig_2-1_Helena_Facility_Location.mxd

12420 Milestone Center Drive

Germantown, MD 20876

Facility Location

^_

Base Map: Copyright:© 2013 National Geographic

Society, i-cubed

0 0.5 1 2

Miles

Legend

Facility Boundary

Helena AASF

AECOM 2-7
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Helena Regional Airport

TenmileCr eek

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M cClellan Creek

Helena Vall ey Canal

CLIENT

REVISED

SCALE

ARNG

GIS BY

CHK BY

MS

AB

10/15/2020

10/15/2020

NOTES Site Inspection for PFAS at Helena AASF, MT

10/15/2020

1:95,040 ­ Figure 2-2 PM RG 10/15/2020

C:\Users\stankevichm\OneDrive - AECOM Directory\ARNG_PFAS_GIS_60552172\MXDs\MT\Helena_AASF\Helena_AASF_SI_Figures\SI_Report\Fig_2-2_Helena_Groundwater_Features.mxd

12420 Milestone Center Drive

Germantown, MD 20876

Groundwater Features

Source: Esri, Maxar, GeoEye, Earthstar Geographics,

CNES/Airbus DS, USDA, USGS, AeroGRID, IGN, and

0 0.75 1.5 3

Miles

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AECOM 2-8
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Figure 2-3

Groundwater Elevation - July 12, 2020

12420 Milestone Center Drive ­ Germantown, MD 20876

CLIENT

REVISED

SCALE

PROJECT

ARNG

Site Inspection for PFAS at Helena AASF, MT

2/3/2021

2/3/2021

2/3/2021

MS

AB

CM

GIS BY

CHK BY

PM

Base Map: Source: Esri, Maxar, GeoEye, Earthstar Geographics, CNES/Airbus DS,

USDA, USGS, AeroGRID, IGN, and the GIS User Community

2/3/2021

1:3,000

C:\Users\stankevichm\OneDrive - AECOM Directory\ARNG_PFAS_GIS_60552172\MXDs\MT\Helena_AASF\Helena_AASF_SI_Figures\SI_Report\Fig_2-3_Helena_SI_Groundwater_Contours.mxd

!? Soil Boring/Temporary Monitoring Well

!( Storm Drain Inlet

Storm Drain Line

Facility Boundary

Canal/Ditch

Groundwater Contour

Inferred Groundwater Contour

Groundwater Flow Direction

AECOM 2-9
Site Inspection Report

Helena Army Aviation Support Facility, Helena, Montana 

THIS PAGE INTENTIONALLY BLANK

AECOM 2-10
Last Chance

Gulch Watershed

Lower Prickley Pear

Gulch Watershed

Lower Tenmile

Creek Watershed

Lower Silver

Creek Watershed

Lake Helena

Watershed

Helena Valley Canal

Pric klyP ear Creek

Tenmi le Creek

CLIENT

REVISED

SCALE

ARNG

GIS BY

CHK BY

MS

AB

2/3/2021

2/3/2021

NOTES Site Inspection for PFAS at Helena AASF, MT

1:63,360

2/3/2021 ­ Figure 2-4 PM CM 2/3/2021

C:\Users\stankevichm\OneDrive - AECOM Directory\ARNG_PFAS_GIS_60552172\MXDs\MT\Helena_AASF\Helena_AASF_SI_Figures\SI_Report\Fig_2-4_Helena_Surfacewater_Features.mxd

12420 Milestone Center Drive

Germantown, MD 20876

Surface Water Features

Source: Esri, Maxar, GeoEye, Earthstar Geographics,

CNES/Airbus DS, USDA, USGS, AeroGRID, IGN, and

0 0.5 1 2

Miles

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Concrete Lined

Canal/Ditch

Canal/Ditch

Pipeline

River/Stream

Inferred Surface Water

Flow Direction

Helena AASF

AECOM 2-11
AECOM 2-12

Site Inspection Report

Helena Army Aviation Support Facility, Helena, Montana 

THIS PAGE INTENTIONALLY BLANK
Boeing

Rocky Mountain Emergency Services

Helena AASF

Airport Road

Bompart Lane

Canyon Ferry Road

0 250 500 1,000

Feet

CLIENT

REVISED

SCALE

PROJECT

ARNG

Site Inspection for PFAS at Helena AASF, MT

7/28/2021

7/28/2021

7/28/2021

MS

AB

CM

GIS BY

CHK BY

PM

Base Map: Source: Esri, Maxar, GeoEye, Earthstar Geographics, CNES/Airbus

DS, USDA, USGS, AeroGRID, IGN, and the GIS User Community

7/28/2021

Figure 2-5

Potable Well Sampling Area

12420 Milestone Center Drive

Germantown, MD 20876 1:6,000 ­ Legend

Potable Well Sampling Area

Facility Boundary

Groundwater Flow Direction

AECOM 2-13
Site Inspection Report

Helena Army Aviation Support Facility, Helena, Montana 

THIS PAGE INTENTIONALLY BLANK

AECOM 2-14
Site Inspection Report

Helena Army Aviation Support Facility, Helena, Montana 

AECOM 3-1

3. Summary of Areas of Interest 

This section presents a summary of each potential PFAS release area by AOI. The two potential 

PFAS release areas were grouped into one AOI (AOI 1) based on proximity and direction of 

groundwater flow (Figure 3-1). 

3.1 AOI 1 

AOI 1 consists of four potential PFAS release areas, as described below. 

3.1.1 60 Hangar

The 60 Hangar is located on the western side of the AASF. The 60 Hangar was built in 1999 and 

houses rotary-winged aircraft. Originally, AFFF was stored at the 60 Hangar in a 400-gallon 

aboveground storage tank which supplied the fire suppression system. During the PA interviews, 

it was originally determined that no AFFF was released from the 60 Hangar. However, subsequent 

interviews were performed which revealed that the AFFF fire suppression system was tested 

shortly after installation. Specific details regarding the volume, chemical composition, and 

concentration of the AFFF released during the test are not known, but interviewees confirmed that 

after the test was completed, AFFF was coming out of the bay and settled on the apron in front of 

the 60 Hangar. It is believed that AFFF entered the floor drains inside the 60 Hangar which go to 

the Helena Publicly Owned Treatment Works and storm drains outside the 60 Hangar which flow 

to the onsite retention basin.

In 2011 the fire suppression system was retrofitted. During the renovation, the AFFF was removed 

by Tyco SimplexGrinnell and replaced with Jet-X High Expansion Foam. The Jet-X High 

Expansion Foam system was tested in 2012 during which all material from the new suppression 

system flowed into a floor drain that runs the length of the 60 Hangar and discharged to the Helena 

Publicly Owned Treatment Works. 

3.1.2 47 Hangar

The 47 Hangar is located adjacent to the 60 Hangar on the eastern side of the AASF. The 47 

Hangar was constructed in 2006 and houses rotary-winged aircraft. According to interviewees, 

the 47 Hangar contains a fire suppression system supplied with Jet-X High Expansion Foam and 

was tested once in 2006. For the test, 60 gallons of Jet-X concentrate was mixed with 1940 

gallons of water. All the released Jet-X High Expansion Foam flowed into a floor drain that runs 

the length of the 47 Hangar and discharged to the Helena Publicly Owned Treatment Works. 

3.1.3 Tri-MaxTM Spill Area and Frozen Extinguisher Release Area

PFAS were potentially released once to a concrete surface at AOI 1 by the MTARNG in the early2000s. During filling of fire extinguishers, a 5-gallon jug of Tri-MaxTM 30 spilled onto the concrete 

behind the most eastern end of the 60 Hangar. The spilled Tri-MaxTM 30 possibly ran into a drain 

that empties into a detention pond to the northeast of the 47 Hangar. Additionally, a second 

release occurred during the winter of 1998 or 1999 in which a fire extinguisher stored outside 

froze, split, and released its contents. The exact location of this release is unknown, but it is 

assumed to have occurred in the same general location as the 5-gallon AFFF spill. A spill was not 

noted; however, it is likely the contents were released to the concrete surface. As a corrective 

action, fire extinguishers are now stored in the hangars. No specific information regarding the 

exact location, contents of the extinguisher, or the volume released was available at the time of 

the PA or SI. It is unknown if fire extinguishers with AFFF were used during training. Further, it is 

unknown how fire extinguishers at the AASF are emptied and/or disposed.
Site Inspection Report

Helena Army Aviation Support Facility, Helena, Montana 

AECOM 3-2

The detention pond is approximately 5 feet deep and collects runoff from most of the facility, 

including industrial stormwater runoff. Drainages have been diverted around the AASF, and 

unprocessed surface water is not used in the area, except for irrigation. Drinking water is supplied 

by the City of Helena; however, domestic wells are located downgradient of AOI 1, within 4 miles 

of the AASF.
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AOI1

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Figure 3-1

Areas of Interest

12420 Milestone Center Drive ­ Germantown, MD 20876

CLIENT

REVISED

SCALE

PROJECT

ARNG

Site Inspection for PFAS at Helena AASF, MT

12/7/2020

12/7/2020

12/7/2020

MS

AB

RG

GIS BY

CHK BY

PM

Base Map: Source: Esri, Maxar, GeoEye, Earthstar Geographics, CNES/Airbus DS,

USDA, USGS, AeroGRID, IGN, and the GIS User Community

12/7/2020

1:8,400

C:\Users\stankevichm\OneDrive - AECOM Directory\ARNG_PFAS_GIS_60552172\MXDs\MT\Helena_AASF\Helena_AASF_SI_Figures\SI_Report\Fig_3-1_Helena_Areas_of_Interest.mxd

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A! Monitoring Well

!P Domestic Well

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AECOM 3-3
Site Inspection Report

Helena Army Aviation Support Facility, Helena, Montana 

AECOM 3-4

THIS PAGE INTENTIONALLY BLANK
Site Inspection Report

Helena Army Aviation Support Facility, Helena, Montana 

AECOM 4-1

4. Project Data Quality Objectives

Project Data Quality Objectives (DQOs) are qualitative and quantitative statements that specify 

the quality of data and define the level of certainty required to support project decision-making

process. The specific DQOs established for this facility are described below. These DQOs were 

developed in accordance with the USEPA’s seven-step iterative process (USEPA, 2006).

4.1 Problem Statement

The following problem statement was developed during project planning:

The presence of PFAS, which may pose a risk to human health or the environment, in 

environmental media at the facility is currently unknown. PFAS are classified as emerging 

environmental contaminants that are garnering increasing regulatory interest due to their potential 

risks to human health and the environment. The regulatory framework for managing PFAS at both 

the federal and state level continues to evolve. 

The DoD has adopted a policy to retain facilities in the CERCLA process based on risk-based 

SLs for soil and groundwater, as described in a memorandum from the Office of the Secretary of 

Defense (OSD) dated 15 October 2019 (Assistant Secretary of Defense, 2019). The ARNG 

program under which this SI was performed follows this DoD policy. Should the maximum site 

concentration for sampled media exceed the SLs established in the OSD memorandum, the site 

will proceed to the next phase under CERCLA. The SLs established in the OSD memorandum 

apply to three compounds: PFOS, PFOA, and PFBS. The SLs are presented in Section 6.1 of 

this Report. 

The following quotes from the DA policy documents form the basis for this project (DA, 2016; DA, 

2018): 

• “The Army will research and identify locations where PFOS- and/or PFOA-containing 

products, such as AFFF, are known or suspected to have been used. Installations shall 

coordinate with installation/facility fire response or training offices to identify AFFF use or 

storage locations. The Army will consider fire training areas, AFFF storage locations, 

hangars/buildings with AFFF suppression systems, fire equipment maintenance areas, and 

areas where emergency response operations required AFFF use as possible source areas. 

In addition, metal plating operations, which used certain PFOS-containing mist 

suppressants, shall be considered possible source areas.” 

• “Based on a review of site records...determine whether a CERCLA PA is appropriate for 

identifying PFOS/PFOA release sites. If the PA determines a PFOS/PFOA release may 

have occurred, a CERCLA SI shall be conducted to determine presence/absence of 

contamination.” 

• “Identify sites where perfluorinated compounds are known or suspected to have been 

released, with the priority being those sites within 20 miles of the public systems that tested 

above USEPA HA levels.” (USEPA, 2016a; USEPA, 2016b).

4.2 Goals of the Study

The following goals were established for this SI:

1. Determine the presence or absence of PFOA, PFOS, and PFBS at or above SLs.

2. Develop information to potentially eliminate a release from further consideration because 

it is determined that it poses no significant threat to human health or the environment.
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3. Determine the potential need for a removal action. 

4. Collect data to better characterize the release areas for more effective and rapid initiation 

of a RI.

5. Identify within 4 miles of the installation other potential PFAS sources (fire stations, major 

manufacturers, other DoD facilities) and receptors, including both groundwater and 

surface water receptors, to determine whether the ARNG is the likely source of PFAS, or 

whether there is an off-facility source of PFAS responsible for installation detections of 

PFAS (USEPA, 2005).

6. Determine whether a potentially complete pathway exists between the source and 

potential receptors and whether ARNG is the likely source of the contamination. 

4.3 Information Inputs

Primary information inputs included the following:

• The PA for the Helena AASF (AECOM, 2018c);

• Analytical data from groundwater and soil samples collected as part of this SI in accordance 

with the site-specific Uniform Federal Policy (UFP)-Quality Assurance Project Plan (QAPP)

Addendum (AECOM, 2020b); and

• Field data collected during the SI, including groundwater elevation and water quality 

parameters measured at the time of sampling.

• Analytical data from potable water samples collected from five potable wells located in 

closest proximity to the facility boundary downgradient of AOI1.

4.4 Study Boundaries

The scope of the SI was bounded by the property limits of the facility (Figure 2-1). Off-facility sampling 

was performed at potable wells within 0.5 miles of the facility boundary.

4.5 Analytical Approach

Samples were analyzed by Pace Analytical Gulf Coast, accredited under the DoD Environmental 

Laboratory Accreditation Program (ELAP; Accreditation Number 74960) and the National 

Environmental Laboratory Accreditation Program (NELAP; Certificate Number 01955). Data were 

compared to applicable SLs and decision rules as defined in the SI QAPP Addendum (AECOM, 

2020b). These rules governed response actions based on the results of the SI sampling effort.

The decision rules described in the Worksheet #11 of the SI QAPP Addendum identify actions 

based on the following:

Groundwater:

• Is there a human receptor within 4 miles of the facility?

• What is the concentration of PFOA, PFOS, and PFBS at the potential release areas?

• What is the concentration of PFOA, PFOS, and PFBS at the facility boundary upgradient 

and downgradient of the potential release areas?

• What does the conceptual site model (CSM) suggest in terms of source, pathway, and 

receptor? 
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Soil:

• What is the concentration of PFOA, PFOS, and PFBS in shallow surface soil (0 to 2 feet 

bgs)?

• What is the concentration of PFOA, PFOS, and PFBS in deep soil (i.e., capillary fringe)?

• What does the CSM suggest in terms of source, pathway, and receptor? 

Soil and groundwater samples were collected from each of the potential release areas. 

Groundwater was encountered at approximately 40 to 56 feet bgs.

4.6 Data Usability Assessment

The Data Usability Assessment (DUA) is an evaluation at the conclusion of data collection 

activities that uses the results of both data verification and validation in the context of the overall 

project decisions or objectives. Using both quantitative and qualitative methods, the assessment 

determines whether project execution and the resulting data have met installation-specific DQOs. 

Both sampling and analytical activities are considered to assess whether the collected data are 

of the right type, quality, and quantity to support the decision-making (DoD, 2019a; DoD, 2019b; 

USEPA, 2017b).

Data Quality Indicators (DQIs) (Precision, Accuracy, Representativeness, Comparability, 

Completeness and Sensitivity) are important components in assessing data usability. These DQIs 

were evaluated in the subsequent sections and demonstrate that the data presented in this SI 

Report are of high quality. Although the SI data are considered reliable, some degree of 

uncertainty can be associated with the data collected. Specific factors that may contribute to the 

uncertainty of the data evaluation are described below. The Data Validation Report (DVR) 

(Appendix A) presents explanations for all qualified data in greater detail.

4.6.1 Precision

Precision is the degree of agreement among repeated measurements of the same characteristic 

on the same sample or on separate samples collected as close as possible in time and place. 

Field sampling precision is measured with the field duplicate relative percent differences (RPD); 

laboratory precision is measured with calibration verification, internal standard recoveries, 

laboratory control spike (LCS) and matrix spike (MS) duplicate RPD.

Extraction internal standards (EIS) were added by the laboratory during sample extraction to 

measure relative responses of target analytes and used to correct for bias associated with matrix 

interferences and sample preparation efficiencies, injection volume variances, mass spectrometry 

ionization efficiencies, and other associated preparation and analytical anomalies. Several field 

samples displayed EIS area counts outside the quality control (QC) limits of 50-150 percent (%). 

The non-detect field sample results associated with EIS area counts less than 10% were initially 

flagged “X” but should be considered for inclusion in the data set. Since PFAS compounds are 

quantitated based on a normalized 100% internal standard percent recovery for this method and 

in MS pairs with low area counts and the target compounds were shown to be able to be 

recovered. The data points flagged “X” were non-detect results for perfluorotetradecanoic acid 

(PFTeDA) and perfluorotridecanoic (PFTrDA). The non-detect field sample results associated 

with the remaining EIS area counts less than the lower QC limit of 50% but greater than 20% 

were qualified “UJ”. The qualified field sample results associated with a negative bias should be 

considered usable as estimated values and as likely true negatives.

Calibration verifications were performed routinely to ensure that instrument responses for all 

calibrated analytes were within established QC criteria. The calibration verifications were within 

the project established precision limits presented in the SI QAPP Addendum (AECOM, 2020b).
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LCS/LCS duplicate (LCSD) pairs were prepared by addition of known concentrations of each 

analyte in a matrix-free media known to be free of target analytes. LCS/LCSD pairs were analyzed 

for every analytical batch to demonstrate the ability of the laboratory to detect similar 

concentrations of a known quantity in matrix-free media. The LCS/LCSD samples were within the 

project established precision limits presented in the SI QAPP Addendum (AECOM, 2020b).

MS/MS duplicate (MSD) samples were prepared, analyzed, and reported for all preparation 

batches. MS/MSD samples demonstrated that the analytical system was in control for the matrix 

being tested. MS/MSD samples were submitted to the laboratory for analysis at a rate of 5%. The 

MS/MSD performed on parent sample AOI01-01-SB-55-57 displayed an RPD greater than the 

QC limit of 30% for PFTrDA at 63%. The associated parent sample result was non-detect; 

therefore, no data qualifying action was required, and the associated parent sample result should 

be considered usable as reported. 

Field duplicate samples were collected at a rate of 10% to assess the overall sampling and 

measurement precision for this sampling effort. The field duplicate samples were analyzed for 

PFAS and general chemistry parameters. The field duplicate samples were within the project 

established precision limits presented in the SI QAPP Addendum (AECOM, 2020b).

Laboratory duplicate samples were prepared and analyzed to assess the overall laboratory 

analytical method and measurement precision for this sampling effort. The laboratory duplicates 

were analyzed for total organic carbon (TOC). The laboratory duplicate pair performed on samples 

AOI01-03-SB-20-22 and AOI01-03-SB-20-22-D displayed an RPD greater than the QC limit of 

25% for TOC at 47% and 38%, respectively. The positive results in the associated batch were 

qualified “J” and should be considered as estimate.

4.6.2 Accuracy

Accuracy is a measure of confidence in a measurement. The smaller the difference between the 

measurement of a parameter and its "true" or expected value, the more accurate the 

measurement. The more precise or reproducible the result, the more reliable or accurate the 

result. Accuracy is measured through percent recoveries in the LCS/LCSD, MS/MSD, and 

surrogates.

LCS/LCSD samples were prepared by addition of known concentrations of each analyte in a 

matrix-free media known to be free of target analytes. LCS/LCSD samples were analyzed for 

every analytical batch and demonstrated that the analytical system was in control during sample 

preparation and analysis, with a limited number of exceptions. PFTrDA displayed an LCSD 

recovery outside the QC limits of 70%-130% at 68% for batch 688084. The field sample results 

associated with a negative bias were non-detect and were qualified “UJ”. The qualified field 

sample results should be considered usable as estimated values. The polyfluorinated compound 

6:2 fluorotelomer sulfonate (6:2 FTS) displayed LCSD recovery outside the QC limits at 132% for 

batch 687724. The field sample results associated with a positive bias were non-detect; no dataqualifying action was required, and results should be considered usable as reported. 

MS/MSD samples were prepared, analyzed, and reported at a rate of 5%. MS/MSD samples 

demonstrated that the analytical system was in control for the matrix being tested, with the 

following exceptions. The MS/MSD performed on parent sample AOI01-01-SB-55-57RE 

displayed percent recoveries less than the lower QC limit of 70% for PFTrDA at 63%. The parent 

sample results associated with a negative bias were qualified “UJ” and should be considered 

usable as estimated values with a negative bias. The MS/MSD performed on parent sample 

AOI01-01-SB-55-57 displayed percent recoveries greater than the upper QC limit of 130% for 

PFTrDA at 183%. The parent sample results associated with a positive bias were non-detect; no 

data-qualifying action was required, and the results should be considered usable as reported.
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4.6.3 Representativeness

Representativeness qualitatively expresses the degree to which data accurately reflect site 

conditions. Factors that affect the representativeness of analytical data include appropriate 

sample population definitions, proper sample collection and preservation techniques, analytical 

holding times, use of standard analytical methods, and determination of matrix or analyte 

interferences. 

Relating to the use of standard analytical methods, the laboratory followed the method as 

established in PFAS by liquid chromatography tandem mass spectrometry (LC/MS/MS)

Compliant with Quality Systems Manual (QSM) 5.1 Table B-15, including the specific preparation 

requirements (i.e. ENVI-Carb or equivalent used), mass calibration, spectra, all the ion transitions 

identified in Table B-15 were monitored, standards that contained both branch and linear isomers 

when available were used, and isotopically-labeled standards were used for quantitation.

Field QC samples were collected to assess the representativeness of the data collected. Field 

duplicates were collected at a rate of 10% for all field samples, while MS/MSD samples were 

collected at a rate of 5%. All preservation techniques were followed by the field staff, and all 

technical and analytical holding times were met by the laboratory, with the exception of pH. For 

the pH analysis, the holding time is “immediate”. The associated field sample results were 

qualified “J” and should be considered usable as estimated values. The laboratory used approved 

standard methods in accordance with the SI QAPP Addendum (AECOM, 2020b) for all analyses.

Instrument blanks and method blanks were prepared by the laboratory in each batch as a negative 

control. All associated instrument blanks and method blanks were non-detect for all target 

analytes. 

Equipment blanks and field blanks were also collected for groundwater and soil samples. All 

equipment blanks and field blanks were non-detect for all target analytes.

Overall, the data are usable for evaluating the presence or absence of PFAS at the facility. 

Sufficient usable data were obtained to meet the objectives of the SI.

4.6.4 Comparability

Comparability is the extent to which data from one study can be compared directly to either past 

data from the current project or data from another study. Using standardized sampling and 

analytical methods, units of reporting, and site selection procedures help ensure comparability. 

Standard field sampling and typical laboratory protocols were used during the SI and are 

considered comparable to ongoing investigations.

4.6.5 Completeness

Completeness is a measure of the amount of valid data obtained from a measurement system 

compared to the amount of data expected under normal conditions. The laboratory provided data 

meeting system QC acceptance criteria for all samples tested. Project completeness was 

determined by evaluating the planned versus actual quantities of data. Percent completeness per 

parameter is as follows and reflects the exclusion of “X” flagged data:

• PFAS in groundwater by LC/MS/MS compliant with QSM 5.1 Table B-15 at 100%;

• PFAS in soil by LC/MS/MS compliant with QSM 5.1 Table B-15 at 98.8%;

• pH in soil by USEPA Method 9045D at 100%; and

• TOC by USEPA Method 9060 at 100%
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4.6.6 Sensitivity

Sensitivity is the capability of a test method or instrument to discriminate between measurement 

responses representing different levels (e.g., concentrations) of a variable of interest. Examples 

of QC measures for determining sensitivity include laboratory fortified blanks, a method detection 

limit (MDL) study, and calibration standards at the limit of quantitation (LOQ). In order to meet the 

needs of the data users, project data must meet the measurement performance criteria for 

sensitivity and project LOQs specified in the SI QAPP Addendum (AECOM, 2020b). The 

laboratory provided the requested MDL studies and provided applicable calibration standards at 

the LOQ. In order to achieve the DQOs for sensitivity outlined in the SI QAPP Addendum 

(AECOM, 2020b), the laboratory reported all field sample results at the lowest possible dilution. 

Additionally, any analytes detected below the LOQ and above the MDL were reported and 

qualified “J” as estimated values by the laboratory.
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5. Site Inspection Activities

This section describes the environmental investigation and sampling activities that occurred as 

part of the SI. The SI sampling approach was based on the findings of the PA and implemented 

in accordance with the following approved documents:

• Final Preliminary Assessment Report Army Aviation Support Facility, Helena, Montana 

dated October 2018 (AECOM, 2018c).

• Final Site Inspection Programmatic Uniform Federal Policy-Quality Assurance Project Plan 

dated March 2018 (AECOM, 2018a);

• Final Site Inspection Quality Assurance Project Plan Addendum, Helena Army Aviation 

Support Facility, Helena, Montana dated July 2020 (AECOM, 2020b);

• Final Programmatic Accident Prevention Plan dated July 2018 (AECOM, 2018b); and

• Final Site Safety and Health Plan, Helena Army Aviation Support Facility, Helena, Montana 

dated June 2020 (AECOM, 2020a).

SI field activities were conducted from 6 to 13 July 2020 and included soil sampling, permanent 

groundwater monitoring well installation, development, and low-flow groundwater sampling. Field 

activities were conducted in accordance with the SI QAPP Addendum (AECOM, 2020b), except as 

noted in Section 5.8. 

The following samples were collected during the SI and analyzed for a subset of 18 PFAS by 

LC/MS/MS compliant with QSM 5.1 Table B-15 to fulfill the project DQOs:

• 17 soil grab samples from 7 boring locations; and

• 5 groundwater samples from 5 permanent monitoring well locations.

Figure 5-1 provides the sample locations for all media across the facility. Table 5-1 presents the 

list of samples collected for each media. Field documentation is provided in Appendix B. A Log 

of Daily Notice of Field Activity was completed throughout the SI field activities, which is provided 

in Appendix B1. Additionally, a photographic log of field activities is provided in Appendix C. 

5.1 Pre-Investigation Activities

In preparation for the SI field activities, project team members participated in Technical Project 

Planning (TPP) meetings, performed utility clearance, and sampled decontamination source 

water. Details for each of these activities are presented below.

5.1.1 Technical Project Planning

The USACE TPP Process, Engineers Manual (EM) 200-1-2 (USACE, 2016) defines four phases 

to project planning: 1.) defining the project phase; 2.) determining data needs; 3.) developing data 

collection strategies; and 4.) finalizing the data collection plan. The process encourages 

stakeholder involvement in the SI, beginning with defining overall project objectives, including 

quantitative and qualitative DQOs, and formulating a sampling approach to address the AOI 

identified in the PA. 

A combined TPP Meeting 1 and 2 was held on 29 April 2020, prior to SI field activities. Meeting 

minutes are provided in Appendix D. TPP meetings 1 and 2 were conducted in general 

accordance with EM 200-1-2.
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The stakeholders for this SI include the ARNG G9, MTARNG, USACE, and Montana Department 

of Environmental Quality (MTDEQ). Stakeholders were provided the opportunity to make 

comments on the technical sampling approach and methods at the combined TPP Meeting 1 and 

2. The outcome of the combined TPP Meeting 1 and 2 was memorialized in the SI QAPP 

Addendum (AECOM, 2020b). 

TPP Meeting 3 was held on 15 January 2021 to discuss the results of the SI. Meeting minutes for 

TPP 3 are included in Appendix D of this report. Future TPP meetings will provide an opportunity 

to discuss the results and findings, and future actions, where warranted.

5.1.2 Utility Clearance

Utility clearance was conducted by Montana811 and facilitated by MTARNG. MTARNG contacted 

Montana811 one-call utility clearance contractor to notify them of intrusive work. AECOM field 

staff were onsite during the utility locate. Additionally, the first 5 feet of each boring were advanced

using hand augering methods to verify utility clearance in shallow subsurface where utilities would 

typically be encountered.

5.1.3 Source Water and PFAS Sampling Equipment Acceptability

Under normal circumstances, a potable water sample would have been collected from the facility 

during TPP Meeting 1 and 2; however, a virtual meeting was held instead. As a result, potable 

water used for decontamination of drilling equipment was taken from Fort William Henry Harrison 

which has been previously sampled and confirmed to be PFAS-free. The results of the potable 

well sample are provided in Appendix F.

Materials that were used within the sampling zone were confirmed as acceptable for use in the 

PFAS sampling environment. The checklist of acceptable materials for use in the PFAS sampling 

environment was provided in the Standard Operating Procedures (SOPs) appendix to the SI

QAPP Addendum (AECOM, 2020b). Prior to the start of field work each day, a PFAS Sampling 

Checklist was completed as an additional layer of control. The checklist served as a daily reminder 

to each field team member regarding the allowable materials within the sampling environment. 

5.2 Soil Borings and Soil Sampling

Soil samples were collected via hollow stem auger (HSA) in accordance with the SI QAPP 

Addendum (AECOM, 2020b). A CME-75 auger rig with 18-inch split-spoon was used to collect 

one core every 5 feet. A hand auger was used to collect soil from the top 5 feet of the boring to 

be compliant with utility clearance procedures.

Three discrete soil samples were collected from the vadose zone for chemical analysis from each 

soil boring. One surface soil sample and two subsurface soil samples (one approximately 1 foot 

above the groundwater table and one at the mid-point between the ground surface and the 

groundwater table) were collected at each boring using HSA. 

The soil boring locations are shown on Figure 5-1 and depths are provided Table 5-1. The soil 

boring locations were selected based on the AOI information as agreed on through TPP and SI

QAPP Addendum review.

The soil cores were logged for lithological descriptions by a field geologist using the Unified Soil 

Classification System (USCS). A photoionization detector (PID) was used to screen the breathing 

zone during boring activities as part of personal safety requirements. Observations and 

measurements were recorded on sampling forms (Appendix B2) and in a non-treated field 

logbook (i.e., composition notebook). Depth interval, recovery thickness, moisture, relative 
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density, color (using a Munsell soil color chart), and texture (using the USCS) were recorded. The 

boring logs are provided in Appendix E.

Lithology observed during the SI was consistent with descriptions from previous investigations at 

the facility and surrounding area. Borings advanced in the subsurface consisted of sands, silts, 

and clays with lenses of small, subangular gravel. Sand layers varied from brown, yellow, and, 

gray; generally-poorly sorted; sub-angular to rounded grains. Silt and clay layers were 

encountered, but did not terminate drilling at any locations. Generally, silts and clays intervals 

were described as brown, cohesive, with low to medium plasticity and containing trace to some 

fine-grained sand. Calcium carbonate (derived from the surrounding sedimentary bedrock) was 

observed in most of the borings and confirmed by testing using dilute acid. Each soil sample was 

collected into laboratory-supplied PFAS-free high-density polyethylene (HDPE) bottles and 

labeled using a PFAS-free marker or pen. Samples were packaged on ice and transported via 

Federal Express (FedEx) under standard chain of custody (CoC) procedures to the laboratory 

and analyzed for PFAS (LC/MS/MS compliant with QSM 5.1 Table B-15), TOC (USEPA Method 

9060A) and pH (USEPA Method 9045D) in accordance with the SI QAPP Addendum (AECOM, 

2020b).

Field duplicate samples were collected at a rate of 10% and analyzed for the same parameters 

as the accompanying samples. MS/MSDs were collected at a rate of 5% and analyzed for the 

same parameters as the accompanying samples. In instances when non-dedicated sampling 

equipment was used, such as a hand auger for the shallow soil samples, equipment rinsate blanks

were collected at a rate of 5% and analyzed for the same parameters as the soil samples. A 

temperature blank was placed in each cooler to ensure that samples were preserved at or below 

4 degrees Celsius (°C) during shipment.

5.3 Permanent Well Installation and Groundwater Sampling

A CME-75 was used to install five 2-inch diameter monitoring wells. The monitoring wells were

constructed with Schedule 40 polyvinyl chloride (PVC), flush threaded 10-foot sections of riser, 

0.010-inch slotted well screen, and a threaded bottom cap. The location of the permanent wells 

were based on proximity to potential PFAS sources and to determine PFAS concentrations at the 

facility boundary. The depth of the permanent wells were determined in the field based on 

observations made by the field geologist, targeting zones where wet soils were observed. A filter 

pack of 20/40 silica sand was installed in the annulus around the well screen to a minimum of 2-

foot above the well screen. A 2-foot thick bentonite seal was placed above the filter sand and 

hydrated with water. Bentonite chips were placed in the well annulus from the top of the bentonite 

seal to approximately 6 inches bgs and hydrated with water. All monitoring wells were completed 

with flush mount well vaults. Well construction diagrams are provided in Appendix B3. The screen 

interval of each of the groundwater monitoring wells is provided in Table 5-2.

Development and sampling of wells was completed in accordance with the SI QAPP Addendum 

(AECOM, 2020b). The newly installed monitoring wells were developed no sooner than 24 hours 

following installation by pumping and surging using a variable speed submersible pump. Well 

development records are provided in Appendix B4. Samples were collected no sooner than 24 

hours following development via low-flow sampling methods using a Geotech bladder pump

(using a polytetrafluoroethylene bladder) with disposable PFAS-free, HDPE tubing. New tubing

and bladder was used at each well and the pumps were decontaminated between each well. The 

wells were purged at a rate determined in the field to reduce draw down prior to sampling. Water 

quality parameters (e.g., temperature, specific conductance, pH, dissolved oxygen [DO], 

oxidation-reduction potential [ORP], and turbidity) were measured using a water quality meter and 

recorded on the field sampling form (Appendix B5). Water levels were measured to the nearest 

0.01 inch and recorded. Additionally, a subsample of each groundwater sample was collected in 

a separate container and a shaker test was completed to identify if there was any foaming. No 

foaming was noted in any of the groundwater samples.
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Each sample was collected into laboratory-supplied PFAS-free HDPE bottles and labeled using 

a PFAS-free marker or pen. Samples were packaged on ice and transported via FedEx under 

standard CoC procedures to the laboratory and analyzed for PFAS by LC/MS/MS Compliant with 

QSM 5.1 Table B-15 in accordance with the SI QAPP Addendum (AECOM, 2020b).

Field duplicate samples were collected at a rate of 10% and analyzed for the same parameters 

as the accompanying samples. MS/MSDs were collected at a rate of 5% and analyzed for the 

same parameters as the accompanying samples. One field reagent blank was collected in 

accordance with the programmatic QAPP (PQAPP) (AECOM, 2018a). A temperature blank was 

placed in each cooler to ensure that samples were preserved at or below 4°C during shipment.

5.4 Synoptic Water Level Measurements

A synoptic groundwater gauging event was performed on 12 July 2020. Depth to water 

measurements were collected from the 5 new monitoring wells from the northern side of the well 

casing. A groundwater flow contour map is provided in Figure 2-3. Calculated groundwater 

elevation data is provided in Table 5-3.

5.5 Surveying

The northern side of each well casing was surveyed by Montana-Licensed land surveyor following 

guidelines provided in the SI QAPP Addendum SOPs (AECOM, 2020b). Survey data from the 

newly installed wells on the facility were collected on 13 July 2020 in the North American Datum 

of 1983 Montana State Plane. The surveyed well data are provided in Appendix B6.

5.6 Investigation-Derived Waste

Soil investigation-derived waste (IDW) (i.e., soil cuttings) and liquid IDW (purge and 

decontamination water) generated during the SI activities were containerized in 24, separate 55-

gallon drums (19 soil and 5 liquid) and stored on the facility. The soil and liquid IDW was not 

sampled and assumes the PFAS characteristics of the associated soil samples collected from 

that source location. 

Other solids such as spent personal protective equipment (PPE), plastic sheeting, tubing, rope, 

unused monitoring well construction materials, and other environmental media generated during 

the field activities were disposed of at a licensed solid waste landfill.

5.7 Laboratory Analytical Methods

Samples were analyzed for a subset of 18 PFAS by LC/MS/MS compliant with QSM 5.1 Table B15 at Pace Analytical Gulf Coast in Baton Rouge, Louisiana, a DoD ELAP and NELAP certified 

laboratory. The 18 PFAS analyzed as part of the ARNG SI program include the following: 

• 6:2 fluorotelomer sulfonic acid (6:2 

FTS)

• 8:2 fluorotelomer sulfonic acid (8:2 

FTS)

• N-ethyl 

perfluorooctanesulfonamidoacetic acid 

(NEtFOSAA)

• Perfluorohexanoic acid (PFHxA)

• Perfluorohexanesulfonic acid (PFHxS)

• Perfluorononanoic acid (PFNA)

• Perfluorooctanoic acid (PFOA)

• Perfluorooctanesulfonic acid (PFOS)

• Perfluoropentanoic acid (PFPeA)

• Perfluorotetradecanoic acid (PFTeDA)

• Perfluorotridecanoic acid (PFTrDA)
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• N-methyl 

perfluorooctanesulfonamidoacetic acid 

(NMeFOSAA)

• Perfluorobutyrate (PFBA)

• Perfluorobutanesulfonic acid (PFBS)

• Perfluorodecanoic acid (PFDA)

• Perfluorododecanoic acid (PFDoA)

• Perfluoroundecanoic acid (PFUdA)

• Perfluoroheptanoic acid (PFHpA)

Soil samples were also analyzed for TOC using USEPA Method 9060A and pH by USEPA Method 

9045D.

5.8 Deviations from SI QAPP Addendum

One deviation was identified after completion of the field work during the reporting stage and 

therefore a Nonconformance and Corrective Action Report was not completed. The deviation from 

the SI QAPP Addendum is noted below:

• While advancing the borehole at HAASF-MW002, split-spoon samples were collected 

continuously the entire length of the borehole. Given that the depth to water was deeper 

than anticipated and in order to maintain the field schedule, the team determined that splitspoons would be collected once every five feet (one per five-foot auger run).
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Table 5-1

Site Inspection Samples by Medium

Site Inspection Report, Helena AASF

Sample Identification

Sample

Collection 

Date

Sample Depth 

(feet bgs)

PFAS by LC/MS/MS compliant with QSM 5.1 Table B-15TOC(USEPA Method 9060A)pH (USEPA Method 9045D)

Comments

AOI01-01-SB-00-02 7/8/2020 0-2 x

AOI01-01-SB-25-27 7/8/2020 25-27 x

AOI01-01-SB-55-57 7/8/2020 55-57 x

AOI01-02-SB-00-02 7/7/2020 0-2 x

AOI01-02-SB-28-30 7/7/2020 28-30 x

AOI01-02-SB-55-57 7/7/2020 55-57 x

AOI01-03-SB-00-02 7/9/2020 0-2 x

AOI01-03-SB-20-22 7/9/2020 20-22 x

AOI01-03-SB-20-22-D 7/9/2020 20-22 x x x Field Duplicate

AOI01-03-SB-44-46 7/9/2020 44-46 x

AOI01-04-SB-00-02 7/8/2020 0-2 x

AOI01-04-SB-20-22 7/8/2020 20-22 x

AOI01-04-SB-39-41 7/8/2020 39-41 x

AOI01-05-SB-00-02 7/9/2020 0-2 x

AOI01-05-SB-25-27 7/9/2020 25-27 x

AOI01-05-SB-50-52 7/9/2020 50-52 x

AOI01-05-SB-50-52-D 7/9/2020 50-52 x Field Duplicate

AOI01-06-SB-00-02 7/8/2020 0-2 x

AOI01-06-SB-00-02-D 7/8/2020 0-2 x Field Duplicate

AOI01-07-SB-00-02 7/8/2020 0-2 x

HAASF-MW001 7/12/2020 58.5 x

HAASF-MW002 7/11/2020 57.0 x

HAASF-MW003 7/12/2020 45.0 x

HAASF-MW004 7/12/2020 43.0 x

HAASF-MW005 7/12/2020 51.5 x

HAASF-MW005-D 7/12/2020 51.5 x Field Duplicate

Soil Samples

Groundwater Samples

Notes:

AOI = Area of Interest

bgs = below ground surface

D = duplicate

HAASF = Helena Army Aviation Support Facility

MW = monitoring well

PFAS = per- and polyfluoroalkyl substances

pH = potential for hydrogen

SB = soil boring

TOC =total organic carbon

USEPA = United States Environmental Protection Agency

AECOM 5-7
Table 5-2

 Boring Depths and Permanent Well Screen Interval

Site Inspection Report, Helena AASF

Area of 

Interest Soil Boring ID Monitoring Well 

ID

Soil Boring 

Depth 

(feet bgs)

Permanent 

Well Screen 

Interval

(feet bgs)

AOI01-01 HAASF-MW001 60.3 50.3-60.3

AOI01-02 HAASF-MW002 62 52-62

AOI01-03 HAASF-MW003 50 40-50

AOI01-04 HAASF-MW004 44.1 34-44

AOI01-05 HAASF-MW005 56.5 45-55

AOI01-06 NA 2 NA

AOI01-07 NA 2 NA

Notes:

AOI = Area of Interest

bgs = below ground surface

HAASF = Helena Army Aviation Support Facility

ID = identification

MW = monitoring well

NA = not applicable

AOI 1

AECOM 5-8
Table 5-3

Depth to Water and Groundwater Elevation

Site Inspection Report, Helena AASF

Location ID Ground Surface 

Elevation (ft amsl)

Depth to Water 

(ft btoc)

Groundwater 

Elevation (ft amsl)

HAASF-MW001 3833.67 56.78 3776.89

HAASF-MW002 3812.79 44.22 3768.57

HAASF-MW003 3808.01 40.90 3767.11

HAASF-MW004 3808.36 40.71 3767.65

HAASF-MW005 3815.22 45.62 3769.60

AOI01-06 3808.62 NA NA

AOI01-07 3807.94 NA NA

Notes:

AOI = Area of Interest

amsl = above mean sea level

btoc = below top of casing

ft = feet

HAASF = Helena Army Aviation Support Facility

ID = identification

MW = monitoring well

NA = not applicable

AECOM 5-9
Site Inspection Report

Helena Army Aviation Support Facility, Helena, Montana 

AECOM 5-10

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!(

!(

!(

!(

!(

!(

!(

!(

!(

!(

!(

!(

!( !(

!( !(

!(

!(

!?

!?

!?

!?

"S "S

!?

Detention

Pond RMESTC

HAASF-MW002

HAASF-MW003

HAASF-MW004

HAASF-MW005

AOI01-06

AOI01-07

HAASF-MW001

Helena Valley Canal

AOI1

0 125 250 500

Feet

Figure 5-1

Site Inspection Surveyed Sample Locations

12420 Milestone Center Drive ­ Germantown, MD 20876

CLIENT

REVISED

SCALE

PROJECT

ARNG

Site Inspection for PFAS at Helena AASF, MT

12/7/2020

12/7/2020

12/7/2020

MS

AB

RG

GIS BY

CHK BY

PM

Base Map: Source: Esri, Maxar, GeoEye, Earthstar Geographics, CNES/Airbus DS,

USDA, USGS, AeroGRID, IGN, and the GIS User Community

12/7/2020

1:3,000

C:\Users\stankevichm\OneDrive - AECOM Directory\ARNG_PFAS_GIS_60552172\MXDs\MT\Helena_AASF\Helena_AASF_SI_Figures\SI_Report\Fig_5-1_Helena_SI_Sample_Surveyed_Locations.mxd

!? Soil Boring/Permanent Monitoring Well

"S Surface Soil Boings (via Hand Auger)

!( Storm Drain Inlet

Storm Drain Line

Area of Interest

Facility Boundary

Canal/Ditch

Inferred Surface Water Flow Direction

Groundwater Flow Direction

AECOM 5-11
Site Inspection Report

Helena Army Aviation Support Facility, Helena, Montana 

AECOM 5-12

THIS PAGE INTENTIONALLY BLANK
Site Inspection Report

Helena Army Aviation Support Facility, Helena, Montana 

AECOM 6-1

6. Site Inspection Results 

This section presents the analytical results of the SI for each AOI. The SLs used in this evaluation 

are presented in Section 6.1. A discussion of the results for the AOI is provided in Section 6.3. 

Table 6-2 through Table 6-4 present PFAS results for samples with detections in soil or 

groundwater; only constituents detected in one or more samples are included. Tables that contain 

all results are provided in Appendix F, and the laboratory reports are provided in Appendix G.

6.1 Screening Levels

The DoD has adopted a policy to retain facilities in the CERCLA process based on risk-based 

SLs for soil and groundwater, as described in a memorandum from the OSD dated 15 October 

2019 (Assistant Secretary of Defense, 2019). The ARNG program under which this SI was 

performed follows this DoD policy. Should the maximum site concentration for sampled media 

exceed the SLs established in the OSD memorandum, the site will proceed to an RI, the next 

phase under CERCLA. The SLs apply to three compounds, PFOA, PFOS, and PFBS, for both 

soil and groundwater, as presented in Table 6-1. 

All other results presented in this Report are considered informational in nature and serve as an 

indication as to whether soil, groundwater, sediment, and surface water contain or do not contain 

PFAS within the boundaries of the facility.

Table 6-1: Screening Levels (Soil and Groundwater)

Analyte

Residential 

(Soil)

(μg/kg)a,b

0-2 feet bgs

Industrial/ Commercial 

Composite Worker

(Soil)

(μg/kg)a,b

Tap Water

(Groundwater)

(ng/L)a,b

PFOA 130 1,600 40

PFOS 130 1,600 40

PFBS 130,000 1,600,000 40,000

Notes:

a.) Assistant Secretary of Defense, 2019. Risk Based Screening Levels Calculated for PFOS, PFOA, PFBS in Groundwater and Soil 

using United States Environmental Protection Agency’s (USEPA’s) Regional Screening Level Calculator. Hazard Quotient (HQ) = 0.1. 

15 October 2019. 

b.) USEPA, 2021. Risk Based Screening Levels Calculated for PFBS in Groundwater and Soil using USEPA’s Regional Screening Level 

Calculator. HQ = 0.1. 8 April 2021.

6.2 Soil Physicochemical Analyses

To provide basic soil parameter information, soil samples were analyzed for TOC and pH, which 

are important for evaluating transport through the soil medium. Appendix F contains the results 

of the TOC and pH sampling. 

The data collected in this investigation will be used in subsequent investigations, where 

appropriate, to assess fate and transport of PFAS contaminants. According to the Interstate 

Technology Regulatory Council (ITRC), several important PFAS partitioning mechanisms include 

hydrophobic and lipophobic effects, electrostatic interactions, and interfacial behaviors. At 

relevant environmental pH values, certain PFAS are present as organic anions and are therefore 

relatively mobile in groundwater (Xiao et al., 2015), but tend to associate with the organic carbon 

fraction that may be present in soil or sediment (Higgins and Luthy 2006; Guelfo and Higgins, 

2013). When sufficient organic carbon is present, organic carbon normalized distribution 

coefficients (Koc values) can help in evaluating transport potential, though other geochemical 
Site Inspection Report

Helena Army Aviation Support Facility, Helena, Montana 

AECOM 6-2

factors (for example, pH and presence of polyvalent cations) may also affect PFAS sorption to 

solid phases (ITRC, 2018).

6.3 AOI 1

This section presents the analytical results for soil and groundwater in comparison to SLs for AOI 

1, which includes the 60 and 47 Hangar Fire Suppression System Releases and the Tri-MaxTM

Spill/Release Area. The detected compounds in soil and groundwater are summarized on Table 

6-2 through Table 6-4. The detections of PFOS and PFOA in soil and groundwater are presented

on Figure 6-1 through Figure 6-3.

6.3.1 AOI 1 Soil Analytical Results

PFOA, PFOS, and PFBS did not exceed the SLs in soil at AOI 1. Figure 6-1 and Figure 6-2

present detections in soil for PFOS and PFOA. The detected compounds in soil are summarized 

on Table 6-2 and Table 6-3.

Soil was sampled from seven locations at AOI 1, the shallow interval (0 to 2 feet bgs), intermediate 

interval (20 to 30 feet bgs), and deep interval (39 to 57 feet bgs) from boring locations HAASFMW001 through HAASF-MW005. Additionally, two shallow interval (0 to 2 feet bgs) samples were 

collected from AOI01-06, and AOI01-07. PFOA and PFBS were not detected in any soil samples. 

PFOS were detected in soil at concentrations several orders of magnitude lower than the SLs. In 

the shallow interval, PFOS was detected at one location (HAASF-MW003) at a concentration of 

0.208 J micrograms per Kilogram (μg/Kg). In the intermediate interval, PFOS was detected at one 

location (HAASF-MW005) at a concentration of 0.219 J μg/Kg. In the deep interval, PFOS was

detected at one location (HAASF-MW005) at a concentration of 1.72 μg/Kg (2.37 μg/Kg

duplicate). All the soil detections of PFOS occurred at locations HAASF-MW003 and HAASFMW005 which correspond to the elevated detections of PFOS (175 ng/L and 775 ng/L [814 ng/L 

duplicate]) from the groundwater samples collected at the same locations.

6.3.2 AOI 1 Groundwater Analytical Results

PFOS exceeded the SLs in groundwater at AOI 1. PFOA and PFBS were detected in groundwater 

did not exceed the SLs at AOI 1. Figure 6-3 present the ranges of detections for PFOS and PFOA.

The detected compounds in groundwater are summarized in Table 6-4. 

Groundwater at AOI 1 was sampled from five permanent monitoring well locations HAASFMW001 through HAASF-MW0005. The SL of 40 ng/L for PFOS was exceeded at HAASF-MW003 

and HAASF-MW005 at maximum concentrations of 175 ng/L and 775 ng/L (814 ng/L duplicate), 

respectively. PFOA was detected below the SL of 40 ng/L at three well locations, with 

concentrations ranging from 1.89 J ng/L to 9.59 J ng/L. PFBS was detected below the SL of 

40,000 ng/L at four well locations, with concentrations ranging from 1.92 J ng/L to 3.61 J ng/L.

6.3.3 AOI 1 Conclusions

Based on the results of the SI, PFOS was detected in soil at AOI 1; however, the detected 

concentrations were several orders of magnitude lower than the soil SLs. PFOS was detected in 

groundwater at concentrations exceeding the individual SL of 40 ng/L at two well locations. PFOA 

and PFBS were detected in groundwater in several locations but at concentrations below SLs. 

Based on the exceedance of the SL for PFOS in groundwater, further evaluation at AOI 1 is 

warranted. 
Table 6-2

PFAS Detections in Surface Soil

Site Inspection Report, Helena AASF

Analyte OSD Screening 

Level a

Result Qual Result Qual Result Qual Result Qual Result Qual Result Qual Result Qual Result Qual

Soil, PFAS by LCMSMS Compliant with QSM 5.1 Table B-15 (μg/Kg)

PFOS 130 ND ND UJ 0.208 J ND ND ND UJ ND ND

Grey Fill Detected concentration exceeded OSD Screening Levels Chemical Abbreviations

PFBS perfluorobutanesulfonic acid

PFOA perfluorooctanoic acid

PFOS perfluorooctanesulfonic acid

References

Acronyms and Abbreviations

AOI Area of Interest

D Duplicate

DL detection limit

ft feet

HQ Hazard quotient

ID identification

Interpreted Qualifiers LCMSMS Liquid Chromatography Mass Spectrometry

J = Estimated concentration ND Analyte not detected above the LOD

UJ = The analyte was not detected at a level greater than or equal to the adjusted DL. However, the reported adjusted DL is approximate and may be inaccurate or imprecise. OSD Office of the Secretary of Defense

PFAS per- and polyfluoroalkyl substances

QSM Quality Systems Manual

Qual Interpreted Qualifier

SB Soil boring

USEPA United States Environmental Protection Agency

μg/Kg micrograms per Kilogram

AOI01-07-SB-00-02

07/08/2020

0 - 2 ft

a. Assistant Secretary of Defense, 2019. Risk Based Screening Levels Calculated for PFOS, PFOA, PFBS in Groundwater or Soil using USEPA’s

Regional Screening Level Calculator. HQ=0.1. 15 October 2019. Soil screening levels based on residential scenario for direct ingestion of contaminated 

soil.

AOI01-05-SB-00-02

07/09/2020

0 - 2 ft

AOI01-03-SB-00-02

07/09/2020

0 - 2 ft

AOI01-04-SB-00-02

07/08/2020

0 - 2 ft

AOI01-01-SB-00-02

Area of Interest

Sample ID

Sample Date

Depth

AOI01-06-SB-00-02

07/08/2020

0 - 2 ft

07/08/2020

0 - 2 ft

AOI01-02-SB-00-02

07/07/2020

0 - 2 ft

AOI 1

AOI01-06-SB-00-02-D

07/08/2020

0 - 2 ft

AECOM 6-3
Table 6-3

PFAS Detections in Subsurface Soil

Site Inspection Report, Helena AASF

Area of Interest

Sample ID

Sample Date

Depth

Analyte Result Qual Result Qual Result Qual Result Qual Result Qual Result Qual Result Qual Result Qual Result Qual Result Qual Result Qual

PFOS ND ND ND ND ND ND ND ND 0.219 J 1.72 2.37

Interpreted Qualifiers Chemical Abbreviations

J = Estimated concentration PFOS perfluorooctanesulfonic acid

Acronyms and Abbreviations

AOI Area of Interest

D Duplicate

ft feet

ID identification

LCMSMS Liquid Chromatography Mass Spectrometry

ND Analyte not detected above the LOD

PFAS per- and polyfluoroalkyl substances

QSM Quality Systems Manual

Qual Interpreted Qualifier

SB Soil boring

μg/Kg micrograms per Kilogram

AOI01-01-SB-25-27

07/08/2020

25 - 27 ft

AOI01-01-SB-55-57

07/08/2020

55 - 57 ft

AOI01-02-SB-28-30

07/07/2020

28 - 30 ft

AOI01-02-SB-55-57

07/07/2020

55 - 57 ft 39 - 41 ft

AOI01-03-SB-20-22

07/09/2020

20 - 22 ft

AOI01-03-SB-44-46

07/09/2020

44 - 46 ft

AOI 1

50 - 52 ft 50 - 52 ft

Soil, PFAS by LCMSMS Compliant with QSM 5.1 Table B-15 (μg/Kg)

AOI01-05-SB-50-52 AOI01-05-SB-50-52-D

07/09/2020 07/09/2020

AOI01-05-SB-25-27

07/09/2020

25 - 27 ft

AOI01-04-SB-20-22

07/08/2020

20 - 22 ft

AOI01-04-SB-39-41

07/08/2020

AECOM 6-4
Table 6-4

PFAS Detections in Groundwater

Site Inspection Report, Helena AASF

Analyte OSD Screening 

Level a

Result Qual Result Qual Result Qual Result Qual Result Qual Result Qual

6:2 FTS - ND ND 16.0 ND 13.2 16.8

PFBA - 2.84 J 2.24 J 9.11 J 2.91 J 19.6 20.0

PFBS 40000 3.61 J ND 1.96 J 3.12 J 1.92 J 1.80 J

PFHpA - ND ND 11.6 ND 11.6 10.5

PFHxA - 3.23 J 4.01 J 15.9 7.85 J 30.1 31.1

PFHxS - 9.49 J ND 74.2 26.4 36.7 37.8

PFNA - ND ND ND ND 2.40 J 2.50 J

PFOA 40 1.89 J ND 9.07 J ND 9.59 J 10.7

PFOS 40 ND ND 175 ND 775 814

PFPeA - ND 3.33 J 4.14 J 6.23 J 21.3 21.7

Grey Fill Detected concentration exceeded OSD Screening Levels Chemical Abbreviations

6:2 FTS 6:2 fluorotelomer sulfonate

PFBA perfluorobutanoic acid

PFBS perfluorobutanesulfonic acid

References PFHpA perfluoroheptanoic acid

PFHxA perfluorohexanoic acid

PFHxS perfluorohexanesulfonic acid

PFNA perfluorononanoic acid

PFOA perfluorooctanoic acid

PFOS perfluorooctanesulfonic acid

PFPeA perfluoropentanoic acid

Interpreted Qualifiers Acronyms and Abbreviations

J = Estimated concentration D Duplicate

HAASF Helena Army Aviation Support Facility

HQ Hazard quotient

ID identification

LCMSMS Liquid Chromatography Mass Spectrometry

MW monitoring well

ND Analyte not detected above the LOD

OSD Office of the Secretary of Defense

PFAS per- and polyfluoroalkyl substances

QSM Quality Systems Manual

Qual Interpreted Qualifier

USEPA United States Environmental Protection Agency

ng/L nanogram per liter

- Not applicable

a. Assistant Secretary of Defense, 2019. Risk Based Screening Levels Calculated for PFOS, PFOA, PFBS in Groundwater or Soil using USEPA’s

Regional Screening Level Calculator. HQ=0.1. 15 October 2019. Groundwater screening levels based on residential scenario for direct ingestion of

groundwater.

Area of Interest

Sample ID

Sample Date

HAASF-MW001

07/12/2020

Water, PFAS by LCMSMS Compliant with QSM 5.1 Table B-15 (ng/L)

HAASF-MW005

07/12/2020

HAASF-MW004

07/12/2020

HAASF-MW003

07/12/2020

HAASF-MW002

07/11/2020

AOI 1

HAASF-MW005-D

07/12/2020

AECOM 6-5
Site Inspection Report

Helena Army Aviation Support Facility, Helena, Montana 

AECOM 6-6

THIS PAGE INTENTIONALLY BLANK
HAASF-MW002

HAASF-MW003

HAASF-MW004

HAASF-MW005

HAASF-MW001

HAASF-MW002

HAASF-MW003

HAASF-MW004

HAASF-MW005

HAASF-MW001

HAASF-MW002

HAASF-MW003

HAASF-MW004

HAASF-MW005

AOI01-06

AOI01-07

HAASF-MW001

0 215 430 860

Feet

CLIENT

REVISED

SCALE

PROJECT

ARNG

Site Inspection for PFAS at Helena AASF, MT

12/7/2020

12/7/2020

12/7/2020

MS

AB

CM

GIS BY

CHK BY

PM

Base Map: Source: Esri, Maxar, GeoEye, Earthstar Geographics, CNES/Airbus

DS, USDA, USGS, AeroGRID, IGN, and the GIS User Community

12/7/2020

1:5,160 Figure 6-1

PFOS Detections in Soil

12420 Milestone Center Drive

­ Germantown, MD 20876

Shallow Intermediate Deep

Facility Boundary

Inferred Surface Water Flow Direction

Groundwater Flow Direction

PFOS Results (μg/Kg)

ND

>ND - 10

>10 - 130

>130 - 1,600

>1,600

AECOM 6-7
HAASF-MW002

HAASF-MW003

HAASF-MW004

HAASF-MW005

HAASF-MW001

HAASF-MW002

HAASF-MW003

HAASF-MW004

HAASF-MW005

HAASF-MW001

HAASF-MW002

HAASF-MW003

HAASF-MW004

HAASF-MW005

AOI01-06

AOI01-07

HAASF-MW001

Facility Boundary

Inferred Surface Water Flow Direction

Groundwater Flow Direction

PFOA Results (μg/Kg)

ND

>ND - 10

>10 - 130

>130 - 1,600

>1,600

CLIENT

REVISED

SCALE

PROJECT

ARNG

Site Inspection for PFAS at Helena AASF, MT

12/7/2020

12/7/2020

12/7/2020

MS

AB

CM

GIS BY

CHK BY

PM

Base Map: Source: Esri, Maxar, GeoEye, Earthstar Geographics, CNES/Airbus

DS, USDA, USGS, AeroGRID, IGN, and the GIS User Community

12/7/2020

1:5,160 Figure 6-2

PFOA Detections in Soil

12420 Milestone Center Drive

­ Germantown, MD 20876

Shallow Intermediate Deep

0 215 430 860

Feet

AECOM 6-8
HAASF-MW002

HAASF-MW003

HAASF-MW004

HAASF-MW005

HAASF-MW001

HAASF-MW002

HAASF-MW003

HAASF-MW004

HAASF-MW005

HAASF-MW001

0 140 280 560

Feet

CLIENT

REVISED

SCALE

PROJECT

ARNG

Site Inspection for PFAS at Helena AASF, MT

12/7/2020

12/7/2020

12/7/2020

MS

AB

CM

GIS BY

CHK BY

PM

Base Map: Source: Esri, Maxar, GeoEye, Earthstar Geographics, CNES/Airbus

DS, USDA, USGS, AeroGRID, IGN, and the GIS User Community

12/7/2020

Figure 6-3

PFOA and PFOS Detections 

in Groundwater

12420 Milestone Center Drive

Germantown, MD 20876

PFOA PFOS

PFOA Results (ng/L)

ND

>ND - 10

>10 - 40

>40 - 70

>70 - 400

>400

PFOS Results (ng/L)

ND

>ND - 10

>10 - 40

>40 - 70

>70 - 400

>400

1:3,360 ­ Legend

Facility Boundary

Inferred Surface Water Flow Direction

Groundwater Flow Direction

AECOM 6-9
Site Inspection Report

Helena Army Aviation Support Facility, Helena, Montana 

AECOM 6-10

THIS PAGE INTENTIONALLY BLANK
Site Inspection Report

Helena Army Aviation Support Facility, Helena, Montana 

AECOM 7-1

7. Exposure Pathways

The CSM for AOI 1, revised based on the SI findings, is presented on Figure 7-1. A CSM presents 

the current understanding of the site conditions with respect to known and suspected sources, 

potential transport mechanisms and migration pathways, and potentially exposed human 

receptors. A human exposure pathway is considered potentially complete when the following 

conditions are present:

1. Contaminant source;

2. Environmental fate and transport;

3. Exposure point;

4. Exposure route; and

5. Potentially exposed populations.

If any of these elements are missing, the pathway is incomplete. The CSM figure uses an empty 

circle symbol to represent an incomplete exposure pathway. Areas with an incomplete pathway 

generally warrant no further action. However, the pathway is considered potentially complete if 

PFOA, PFOS, or PFBS are detected, in which case the CSM figure uses a half-filled circle symbol 

to represent a potentially complete exposure pathway. Additionally, a completely filled circle symbol 

is used to indicate when a potentially complete exposure pathway has detections of PFOA, PFOS, 

or PFBS above the SLs. Areas with an identified potentially complete pathway may warrant further 

investigation. 

In general, the potential PFAS exposure pathways are ingestion and inhalation. Human exposure 

via the dermal contact pathway may occur, and current risk practice suggests it is an insignificant 

pathway compared to ingestion; however, exposure data for dermal pathways are sparse and 

continue to be the subject of PFAS toxicological study. The receptors evaluated are consistent 

with those listed in USEPA guidance for risk screening (USEPA, 2001). Receptors at the facility

include site workers (e.g., facility staff and visiting soldiers), construction workers, trespassers, 

residents outside the facility boundary, and recreational users outside of the facility boundary. 

7.1 Soil Exposure Pathway

The SI results for PFOA, PFOS, and PFBS in soil were used to determine whether a potentially 

complete pathway exists between the source and potential receptors at AOI 1 based on the 

aforementioned criteria. 

7.1.1 AOI 1

AFFF was released to soil from four separate releases/spills within AOI 1. PFOA and PFBS were 

not detected in soil. PFOS was detected in soil at AOI 1 and confirms the release of PFAS to soil 

in AOI 1. Ground-disturbing activities could potentially result in site worker and construction 

worker exposure to PFOS via inhalation of dust or ingestion of surface soil. Ground-disturbing 

activities could also potentially result in construction worker exposure to subsurface soil. 

Additionally, off-facility residents, off-facility recreational users (nearby walking path), and 

trespassers could potentially be exposed to PFOS via inhalation of dust caused by on-facility 

ground disturbing activities, although this exposure is likely insignificant. No construction is 

occurring at AOI 1. The CSM is presented on Figure 7-1. 
Site Inspection Report

Helena Army Aviation Support Facility, Helena, Montana 

AECOM 7-2

7.2 Groundwater Exposure Pathway

The SI results for PFOA, PFOS, and PFBS in groundwater were used to determine whether a 

potentially complete pathway exists between the source and potential receptors at AOI 1 based 

on the aforementioned criteria.

7.2.1 AOI 1

PFOA, PFOS, and PFBS were detected in groundwater from permanent monitoring wells at AOI 

1 and exceeded the SL for PFOS in two permanent monitoring wells (one source location, one 

facility boundary location). According to the MBMG database, approximately 805 domestic, 

commercial, or industrial wells exist within 4 miles of the facility in the downgradient direction, with 

some as close as 0.5 miles away. However, the database did not further classify domestic wells 

into subcategories for agriculture, ranching, or drinking water use. Due to these uncertainties, five 

potable wells downgradient of AOI 1 were sampled in 2021. PFOA, PFOS, and PFBS were 

detected in groundwater, but were below SLs. Therefore, the ingestion exposure pathway for 

groundwater is considered potentially complete for offsite residents. The facility is on city water, 

which has been tested and confirmed to be PFAS-free (see Section 2.2.2); therefore, the 

ingestion pathway is incomplete for site workers. Further, due to the depth of groundwater, the 

ingestion pathway for construction workers, off-facility recreational users, and trespassers is also 

considered incomplete. The CSM is presented on Figure 7-1. 
Media

SOURCE PATHWAY RE

AOI 1 Accidental Spill PFAS in Surface Soil

Human 

Activities

Precipitation/ 

Run-Off

Leaching/ Infiltration

Airborne Soil 

Particulate

Surface Soil 

at AOI

Surface 

Water/ 

Sediment

Subsurface 

Soil

Shallow 

Groundwater

Source Release Mechanism Media Transport and Migration Media Exposure Routes

Inhalation of 

Dust

Ingestion

Ingestion

Ingestion

Hum

C

Flow-Chart ContinuesPartial / Possible FlowFlow-Chart StopsIncomplete PathwayPotentially Complete PathwayPotentially Complete Pathway with Exceedance of SL

Site 

Worker

Cons

Wo

Site 

Worker

Cons

W

LEGEND

Figure 7

Conceptual Si

AOI 1

Potential 

Off-Facility 

Source Not 

under 

Control of 

ARNG

/

/ 

/ 

Ingestion 

NOTES:1. The resident and recreational user

receptors refer to an off-site resident or

recreational user.

2. Dermal contact exposure pathway is

incomplete for PFAS.

// 
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8. Summary and Outcome

This section summarizes SI activities and findings. The most significant findings are summarized 

in this section and are reproduced directly or extracted from information contained in this Report. 

The outcome provides general and comparative interpretations of the findings relative to the SLs. 

8.1 SI Activities

SI field activities were conducted from 6 to 13 July 2020 and included soil sampling, permanent 

groundwater monitoring well installation, development, and low-flow groundwater sampling. Field 

activities were conducted in accordance with the SI QAPP Addendum (AECOM, 2020b), except as 

noted in Section 5.8. 

To fulfill the project DQOs set forth in the approved SI QAPP Addendum (AECOM, 2020b),

samples were collected and analyzed for a subset of PFAS by LC/MS/MS compliant with QSM 

Table B-15, as follows. The 18 PFAS analyzed as part of the ARNG SI program are specified in 

Section 5.7 of this Report.

• 17 soil grab samples from 7 boring locations; and

• 5 groundwater samples from 5 permanent monitoring well locations.

The information gathered during this investigation was used to determine if PFOA, PFOS, and/or

PFBS were present at or above SLs. Additionally, the CSM was refined to assess whether 

potentially complete pathways, which are described in Section 7, exist between the source and 

potential receptors for potential exposure to PFOA, PFOS, and PFBS at AOI 1. 

8.2 SI Goals Evaluation

As described in Section 4.2, the SI activities were designed to achieve six main goals or DQOs. 

This section describes the SI goals and the conclusions that can be made for each based on the 

data collected during this investigation. 

1. Determine the presence or absence of PFOA, PFOS, and PFBS at or above SLs.

PFOA, PFOS, and PFBS were detected at the facility in soil and groundwater; however,

only PFOS in groundwater exceeded the SL. PFOS was detected both at the source area,

as well as at the facility boundary between source area and potential off-facility drinking

water receptors. The detected concentrations of PFOA and PFBS in groundwater

samples, as well as, PFOA, PFOS, and PFBS in soil samples from AOI 1 were below the

SLs.

2. Develop information to potentially eliminate a release from further consideration because

it is determined that it poses no significant threat to human health or the environment.

PFOA, PFOS, and PFBS were detected in groundwater and PFOS exceeded the SL at

the source area and facility boundary. The exceedance at the facility boundary is

immediately downgradient of the AOI 1 source area. As a result, no release area can be

eliminated from further consideration at this point in the investigation.

3. Determine the potential need for a removal action.

As described in Section 2.4, in 2021, the offsite wells were sampled due to exceedances

of SLs observed in groundwater in monitoring wells at the AASF during the SI. Five

properties were selected to be sampled due to their proximity to the facility. PFOA, PFOS,
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Helena Army Aviation Support Facility, Helena, Montana 

AECOM 8-2

and PFBS were detected in groundwater, but were below SLs. A removal action is not 

needed at this time because the potable water sample results were below the SLs.

4. Collect data to better characterize the release areas for more effective and rapid initiation

of a RI.

The geological data collected as part of the SI indicate the facility is underlain by

unconsolidated, heterogeneous valley fill deposits, dominated by well-graded sand with

thin lenses of silt and clay and thin beds of small gravel.

The observations from the borings advanced during the SI are consistent with the surficial

geology of the area. The Helena Valley consists of material eroded from the surrounding

mountains and hills. The sands, silts, and clays are yellow to brown, well-graded, and

mixed with subangular gravel. Most of these deposits originate from the surrounding

sedimentary bedrock. The interlayering of these lenses provides communication from the

ground surface to the top of the valley aquifer.

Depth to water at the facility ranged from approximately 40.71 to 56.78 feet bgs.

Groundwater flow direction is north-northeast, towards Lake Helena and the Missouri

River. These geologic and hydrogeologic observations inform development of technical

approach for the RI.

5. Identify within 4 miles of the installation other potential PFAS sources (fire stations, major

manufacturers, other DoD facilities) and receptors, including both groundwater and

surface water receptors, to determine whether the ARNG is the likely source of PFAS, or

whether there is an off-facility source of PFAS responsible for installation detections of

PFAS (USEPA, 2005).

Based upon the evaluation of groundwater and soil results in comparison to SLs, in

combination with the groundwater flow direction analysis, the results of the SI indicate that

the source of detected concentrations of PFOA, PFOS, and PFBS at the facility is likely

attributable to ARNG activities. The two locations with PFOS exceedances in groundwater

were found at the identified source area and immediately downgradient. Results of the PA

did not find any other adjacent source that could have contributed to these groundwater

results. Furthermore, the upgradient (HAASF-MW001) and cross-gradient (HAASFMW004) monitoring wells installed did not suggest any adjacent contributing PFAS source

potentially migrating within the boundaries of the facility. As such, ARNG will evaluate AOI

1 further in an RI.

6. Determine whether a potentially complete pathway exists between the source and

potential receptors and whether ARNG is the likely source of the contamination.

As described in Section 2.4, in 2021, offsite potable wells were sampled due to the

exceedance of SLs observed in groundwater during the SI. Five properties were selected

to be sampled due to their proximity to the facility. PFOA, PFOS, and PFBS were detected

in groundwater, but were below SLs. A removal action is not needed at this time because

the potable sample results were below the SLs. Based on these results, a potentially

complete pathway exists to potential receptors.

8.3 Outcome 

Based on the CSM developed and revised in light of the SI findings, there is potential for exposure 

to drinking water receptors from sources on the facility resulting from historical DoD activities at 

AOI 1. Sample chemical analytical concentrations collected during and after the SI were 

compared against the project SLs for PFOA, PFOS, and PFBS in soil and groundwater, as 

described in Table 6-1. The following bullets summarize the SI results: 
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AECOM 8-3

• PFOA, PFOS, and PFBS were detected in groundwater at AOI 1 and PFOS exceeded the

individual SL of 40 ng/L, with maximum concentrations of 775 ng/L (814 ng/L duplicate) and

175 ng/L at locations HAASF-MW005 and HAASF-MW003; respectively. Based on the

results of the SI, further evaluation of AOI 1 is warranted in the RI.

• Offsite wells were sampled due to exceedances of SLs observed in groundwater in

monitoring wells at the AASF during the SI and well information from the MBMG database,

a potentially complete pathway exits to off-facility residential wells.

• The detected concentrations of PFOA, PFOS, and PFBS in soil samples from AOI 1 were

below the SLs.

Table 8-1 summarizes the SI results for soil and groundwater. Based on the CSMs developed 

and revised in light of the SI findings, there is potential for exposure to drinking water receptors 

caused by DoD activities at or adjacent to the facility.

Table 8-2 summarizes the rationale used to determine if an AOI should be considered for further 

investigation under CERCLA and undergo an RI. Based on the results of this SI, further evaluation 

is warranted in the RI for AOI 1: 60 and 47 Hangar Fire Suppression System Release and TriMaxTM Spill/Release Area. 

Table 8-1: Summary of Site Inspection Findings

AOI Potential PFAS 

Release Area

Soil – 

Source Area

Groundwater – 

Source Area

Groundwater –

Facility 

Boundary

1 

60 and 47 Hangar Fire 

Suppression System 

Release and Tri-MaxTM 

Spill/Release Area

Legend:

= detected; exceedance of the screening levels

= detected; no exceedance of the screening levels

= not detected

Table 8-2: Site Inspection Recommendations

AOI Description Rationale Future Action

1

60 and 47 Hangar 

Fire Suppression 

System Release and 

Tri-MaxTM 

Spill/Release Area

Exceedances of SLs in groundwater at 

source area and downgradient facility 

boundary. No exceedances of SLs in soil. 

Proceed to RI 
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