FINAL

Site Inspection Report 

Rapid City Army Aviation Support Facility

Rapid City, South Dakota 

Site Inspection for Perfluorooctanoic Acid (PFOA), Perfluorooctanesulfonic Acid (PFOS), 

Perfluorohexanesulfonic Acid (PFHxS), Perfluorononanoic Acid (PFNA), Hexafluoropropylene 

Oxide Dimer Acid (HFPO-DA), and Perfluorobutanesulfonic Acid (PFBS) 

ARNG Installations, Nationwide

October 2023 

Prepared for: 

Army National Guard Headquarters 

111 S. George Mason Drive 

Arlington, VA 22204 

UNCLASSIFIED
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TABLE OF CONTENTS

Page

LIST OF APPENDICES ................................................................................................................ iii

LIST OF FIGURES ....................................................................................................................... iv

LIST OF TABLES .......................................................................................................................... v

LIST OF ACRONYMS AND ABBREVIATIONS ...................................................................... vi

EXECUTIVE SUMMARY ...................................................................................................... ES-1

1. INTRODUCTION ............................................................................................... 1-1

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

1.2 Site Inspection 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 .................................................................. 2-2

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 Sensitive Habitat and Threatened/Endangered Species ... 2-3

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

3. SUMMARY OF AREAS OF INTEREST ........................................................... 3-1

3.1 AOI 1 – Overflow Area ........................................................................... 3-1

3.2 Adjacent Sources ..................................................................................... 3-2

3.2.1 Rapid City Fire Department ............................................. 3-2

3.2.2 Ellsworth Air Force Base ................................................. 3-2

3.2.3 Landfills ........................................................................... 3-2

4. PROJECT DATA QUALITY OBJECTIVES ..................................................... 4-1

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

4.2 Information Inputs ................................................................................... 4-1

4.3 Study Boundaries ..................................................................................... 4-1

4.4 Analytical Approach ................................................................................ 4-1

4.5 Data Usability Assessment ...................................................................... 4-2
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5. SITE INSPECTION ACTIVITIES...................................................................... 5-1

5.1 Pre-Investigation Activities ..................................................................... 5-2

5.1.1 Technical Project Planning .............................................. 5-2

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............................................................... 5-3

5.3 Temporary Well Installation and Groundwater Grab Sampling.............. 5-4

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

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

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

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

5.8 Deviations from SI UFP-QAPP Addendum ............................................ 5-6

6. SITE INSPECTION RESULTS .......................................................................... 6-1

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

6.2 Soil Physicochemical Analyses ............................................................... 6-2

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

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

6.3.2 AOI 1 Groundwater Analytical Results........................... 6-3

6.3.3 Conclusions...................................................................... 6-3

6.4 BOUNDARY SAMPLE LOCATIONS .................................................. 6-4

6.4.1 Boundary Sample Locations – Soil Analytical Results... 6-4

6.4.2 Boundary Sample Locations – Groundwater Analytical 

Results............................................................................. 6-4

6.4.3 Conclusions...................................................................... 6-4

7. EXPOSURE PATHWAYS.................................................................................. 7-1

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

7.1.1 AOI 1 .............................................................................. 7-2

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

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

7.3 Surface Water and Sediment Exposure Pathway..................................... 7-3

7.3.1 AOI 1 .............................................................................. 7-3

8. SUMMARY AND OUTCOME .......................................................................... 8-1
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8.1 SI Activities ............................................................................................. 8-1

8.2 Outcome ................................................................................................... 8-1

9. REFERENCES .................................................................................................... 9-1

LIST OF APPENDICES

Appendix A. Data Usability Assessment and Validation Reports 

Appendix B. Field Documentation 

B1. Log of Daily Notice of Field Activities

B2. Sampling Forms 

B3. Survey Data 

B4. Field Change Request Form 

Appendix C. Photographic Log 

Appendix D. Technical Project Planning Meeting Minutes 

Appendix E. Boring Logs and Well Construction Diagrams 

Appendix F. Analytical Results

Appendix G. Laboratory Reports 
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LIST OF FIGURES

Figure 2-1 Facility Location

Figure 2-2 Facility Topography

Figure 2-3 Groundwater Features

Figure 2-4 Surface Water Features

Figure 2-5 Groundwater Elevations, November 2021

Figure 3-1 Areas of Interest

Figure 5-1 Site Inspection Sample Locations

Figure 6-1 PFOA Detections in Soil

Figure 6-2 PFOS Detections in Soil

Figure 6-3 PFBS Detections in Soil

Figure 6-4 PFHxS Detections in Soil

Figure 6-5 PFNA Detections in Soil

Figure 6-6 PFOS, PFOA, and PFBS Detections in Groundwater

Figure 6-7 PFHxS and PFNA Detections in Groundwater

Figure 7-1 Conceptual Site Model, AOI 1 
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LIST OF TABLES

Table ES-1 Screening Levels (Soil and Groundwater)

Table ES-2 Summary of Site Inspection Findings and Recommendations

Table 5-1 Samples by Medium, Rapid City AASF, South Dakota, Site Inspection Report

Table 5-2 Soil Boring Depths and Temporary Well Screen Intervals, Rapid City AASF, 

South Dakota, Site Inspection Report

Table 5-3 Groundwater Elevation, Rapid City AASF, South Dakota, Site Inspection Report

Table 6-1 Screening Levels (Soil and Groundwater)

Table 6-2 PFOA, PFOS, PFBS, PFNA, and PFHxS Results in Surface Soil, Site Inspection 

Report, Rapid City AASF

Table 6-3 PFOA, PFOS, PFBS, PFNA, and PFHxS Results in Shallow Subsurface Soil, 

Site Inspection Report, Rapid City AASF

Table 6-4 PFOA, PFOS, PFBS, PFNA, and PFHxS Results in Deep Subsurface Soil, Site 

Inspection Report, Rapid City AASF

Table 6-5 PFOA, PFOS, PFBS, PFNA, and PFHxS Results in Groundwater, Site Inspection 

Report, Rapid City AASF

Table 8-1 Summary of Site Inspection Findings and Recommendations
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LIST OF ACRONYMS AND ABBREVIATIONS

°C Degrees Celsius

% Percent

μg/kg Microgram(s) per kilogram

μg/L Microgram(s) per liter

AASF Army Aviation Support Facility 

AECOM AECOM Technical Services, Inc. 

AFFF Aqueous Film Forming Foam

AFB Air Force Base

ANG Air National Guard

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

DLA Defense Logistics Agency

DoD Department of Defense

DPT Direct-push technology

DQI Data quality indicator

DQO Data quality objective

DUA Data Usability Assessment 

EA EA Engineering, Science, and Technology, Inc., PBC

EDR Environmental Data Resources, Inc.

EIS Extraction internal standards

ELAP Environmental Laboratory Accreditation Program

EM Engineer Manual

EB Equipment Blank 

FB Field blank

FedEx Federal Express

ft Foot (feet)

GIS geographic information systems

GPR ground penetrating radar

HDPE High-density polyethylene

HFPO-DA Hexafluoropropylene oxide dimer acid
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HQ Hazard Quotient

IDW Investigation-derived waste

ITRC Interstate Technology Regulatory Council

LC/MS/MS Liquid chromatography tandem mass spectrometry

LCS Laboratory control sample 

LCSD Laboratory control sample duplicate

LOQ Limit of quantification

MIL-SPEC military specification

MS Matrix spike

MSD Matrix spike duplicate

NELAP National Environmental Laboratory Accreditation Program 

ng/L Nanogram(s) per liter

No. Number

OSD Office of the Secretary of Defense

PA preliminary assessment

PFAS per- and polyfluoroalkyl substances

PFBS perfluorobutanesulfonic acid

PFHxS perfluorohexanesulfonic acid

PFNA perfluorononanoic acid

PFOA perfluorooctanoic acid

PFOS perfluorooctanesulfonic acid

PID photoionization detector

PVC polyvinyl chloride

QA Quality assurance

QAPP Quality Assurance Project Plan

QC Quality control

QSM Quality Systems Manual

RI Remedial investigation 

RPD Relative percent difference

SDARNG South Dakota Army National Guard

SD DANR South Dakota Department of Agriculture & Natural Resources

SI Site Inspection

SL Screening level

TCRA Time Critical Removal Action

TOC Total organic carbon

TPP Technical Project Planning
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UFP Uniform Federal Policy 

USACE U.S. Army Corps of Engineers

USEPA U.S. Environmental Protection Agency

UST Underground storage tank

Wood Wood Environment & Infrastructure Solutions, Inc.

WSP WSP USA Environment & Infrastructure, Inc.
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EXECUTIVE SUMMARY

The Army National Guard (ARNG) G-9 is performing Preliminary Assessments (PAs) and Site 

Inspections (SIs) at ARNG facilities nationwide based on the current or potential historical use of 

per- and polyfluoroalkyl substances (PFAS) with a focus on the six compounds presented in the 

memorandum regarding Investigating Per- and Polyfluoroalkyl Substances within the 

Department of Defense Cleanup Program (Assistant Secretary of Defense, 2022) from the Office 

of the Secretary of Defense (OSD) dated 6 July 2022. The six compounds listed in the OSD 

memorandum include perfluorooctanesulfonic acid (PFOS), perfluorooctanoic acid (PFOA), 

perfluorobutanesulfonic acid (PFBS), perfluorononanoic acid (PFNA), perfluorohexanesulfonic 

acid (PFHxS), and hexafluoropropylene oxide dimer acid (HFPO-DA)1

. These compounds are 

collectively referred to as “relevant compounds” throughout the document, and the applicable 

screening levels (SLs) are provided below in Table ES-1. 

The PA identified one Area of Interest (AOI) where PFAS-containing materials may have been

used, stored, disposed, or released historically (see Table ES-2 for AOI location). The objective 

of the SI is to identify whether there has been a release to the environment from the AOI 

identified in the PA and determine whether further investigation is warranted, a removal action is 

required to address immediate threats, or no further action is required based on SLs for the 

relevant compounds. This SI was completed at the Rapid City Army Aviation Support Facility 

(AASF) in Rapid City, South Dakota and determined additional investigation is warranted for 

AOI 1: Overflow Area. The Rapid City AASF will also be referred to as the “Facility” 

throughout this document.

The Facility, operated by the South Dakota ARNG (SDARNG), is constructed on approximately 

38 acres of land that is owned by the City of Rapid City and has been leased to the State of South 

Dakota Department of Military and Veterans Affairs since 1957 for a term of 99 years. The 

AASF includes several hangars, storage buildings, and administrative offices (AECOM 

Technical Services, Inc. [AECOM], 2020). 

The PA identified one AOI for investigation during the SI phase. SI sampling results from the 

AOI were compared to OSD SLs. Table ES-2 summarizes the SI results for the AOI. Based on 

the results of this SI, further evaluation under CERCLA is warranted in a Remedial Investigation 

(RI) for AOI 1: Overflow Area.

1 Of the six PFAS compounds presented in the 6 July 2022 OSD memorandum, HFPO-DA (commonly referred to as 

GenX) was not included as an analyte at the time of this SI. Based on the conceptual site model (CSM) developed 

during the PA and revised based on SI findings, the presence of HFPO-DA is not anticipated at the facility because 

HFPO-DA is generally not a component of military specification (MIL-SPEC) aqueous film forming foam (AFFF) 

and based on its history including distribution limitations that restricted use of GenX, it is generally not a component 

of other products the military used. In addition, it is unlikely that GenX would be an individual chemical of concern 

in the absence of other PFAS.
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Table ES-1. Screening Levels (Soil and Groundwater)

Analyte2

Residential

(Soil)

(μg/kg)1 

(0-2 feet bgs)

Industrial / Commercial

Composite Worker

(Soil)

(μg/kg) 1 

(2-15 feet bgs)

Tap Water

(Groundwater)

(ng/L)1 

PFOA 19 250 6

PFOS 13 160 4

PFBS 1,900 25,000 601

PFHxS 130 1,600 39

PFNA 19 250 6

Notes:

1. Assistant Secretary of Defense. July 2022. Risk Based Screening Levels in Groundwater and 

Soil using United States Environmental Protection Agency’s (USEPA’s) Regional Screening 

Level Calculator. Hazard Quotient (HQ)=0.1. May 2022. 

2. Of the six PFAS compounds presented in the 6 July 2022 OSD memorandum, HFPO-DA 

(commonly referred to as GenX) was not included as an analyte at the time of this SI. Based 

on the CSM developed during the PA and revised based on SI findings, the presence of 

HFPO-DA is not anticipated at the facility because HFPO-DA is generally not a component 

of MIL-SPEC AFFF and based on its history including distribution limitations that restricted 

use of GenX, it is generally not a component of other products the military used. In addition, 

it is unlikely that GenX would be an individual chemical of concern in the absence of other 

PFAS.

Abbreviations: 

μg/kg = microgram(s) per kilogram

bgs = below ground surface

ng/L = nanogram(s) per liter

Table ES-2. Summary of Site Inspection Findings and Recommendations

AOI

Potential Release 

Area

Soil – 

Source Area

Groundwater – 

Source Area

Groundwater – 

Facility Boundary Future Action

1 Overflow Area Proceed to RI

Legend:

 = Detected; exceedance of screening levels

 = Detected; no exceedance of screening levels

 = Not detected
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1. INTRODUCTION

1.1 PROJECT AUTHORIZATION

The Army National Guard (ARNG) G-9 is the lead agency in performing Preliminary 

Assessments (PAs) and Site Inspections (SIs) at ARNG facilities nationwide based on the current 

or potential historical use of per- and polyfluoroalkyl substances (PFAS) with a focus on the six 

compounds presented in the memorandum regarding Investigating Per- and Polyfluoroalkyl 

Substances within the Department of Defense Cleanup Program (Assistant Secretary of Defense, 

2022) from the Office of the Secretary of Defense (OSD) dated 6 July 2022. The six compounds 

listed in the OSD memorandum are referred to as “relevant compounds” throughout this 

document and include perfluorooctanesulfonic acid (PFOS), perfluorooctanoic acid (PFOA), 

perfluorobutanesulfonic acid (PFBS), perfluorononanoic acid (PFNA), perfluorohexanesulfonic 

acid (PFHxS), and hexafluoropropylene oxide dimer acid (HFPO-DA)1. The ARNG performed 

this SI at the Rapid City Army Aviation Support Facility (AASF) in Rapid City, South Dakota. 

The Rapid City AASF is also referred to as the “Facility” throughout this report. 

The SI project elements were performed in accordance with the Comprehensive

Environmental Response, Compensation, and Liability Act (CERCLA) (U.S. Environmental

Protection Agency [EPA], 1980), as amended, the National Oil and Hazardous Substances

Pollution Contingency Plan (40 Code of Federal Regulations Part 300; EPA, 1994), and in 

compliance with U.S. Department of Army (DA) requirements and guidance for field 

investigations.

1.2 SITE INSPECTION PURPOSE

A PA was performed at the Rapid City AASF (AECOM Technical Services, Inc. [AECOM],

2020) that identified one Area of Interest (AOI) where PFAS-containing materials may have 

been used, stored, disposed, or released historically. The objective of the SI is to identify whether 

there has been a release to the environment from the AOI identified in the PA and determine 

whether further investigation is warranted, a removal action is required to address immediate 

threats, or no further action is required based on screening levels (SLs) for the relevant 

compounds.

1 Of the six PFAS compounds presented in the 6 July 2022 OSD memorandum, HFPO-DA (commonly referred to as 

GenX) was not included as an analyte at the time of this SI. Based on the conceptual site model (CSM) developed 

during the PA and revised based on SI findings, the presence of HFPO-DA is not anticipated at the facility because 

HFPO-DA is generally not a component of military specification (MIL-SPEC) aqueous film forming foam (AFFF) 

and based on its history including distribution limitations that restricted use of GenX, it is generally not a component 

of other products the military used. In addition, it is unlikely that GenX would be an individual chemical of concern 

in the absence of other PFAS.
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2. FACILITY BACKGROUND

2.1 FACILITY LOCATION AND DESCRIPTION

The Rapid City AASF is in Pennington County, approximately 10 miles southeast of Rapid City, 

South Dakota and approximately 8 miles south of Ellsworth Air Force Base (AFB). The AASF is 

adjacent to Rapid City Regional Airport (Figure 2-1). The Facility is accessible from FAA Road 

from the north and Guard Road from the south (AECOM, 2020). 

The Facility is constructed on approximately 38 acres of land that is owned by the City of Rapid 

City and has been leased to the State of South Dakota Department of Military and Veterans 

Affairs since 1957 for a term of 99 years. The AASF includes several hangars, storage buildings, 

and administrative offices (AECOM, 2020). 

2.2 FACILITY ENVIRONMENTAL SETTING

The Rapid City AASF lies within the Black Hills regions, which is characterized as an isolated 

eroded mountain region; ancient rock removal by stream erosion produces this mountain setting. 

From a distance, the rounded hilltops, well-forested slopes, and deep valleys present a dark 

appearance, giving them their name. Rapid Creek is the main stream channel near the Facility 

(AECOM, 2020). The topography at the Site is shown on Figure 2-2.

2.2.1 Geology

The Facility is located within the eastern side of the Black Hills on an elliptically shaped dome 

created by the Laramide Orogeny. As a result of the Laramide Orogeny, crystalline basement 

rock and overlying Mesozoic and Paleozoic rocks were uplifted and exposed. Beneath the 

Facility lays Precambrian-age crystalline basement rocks that are overlain by Cambrian through 

Cretaceous deposits of dolomite, limestone, and sandstone. Within the Upper Cretaceous lies 

deposits of aged marine shales intertwined with beds of limestone and sandstone. This aged 

marine shale, also known as the Pierre Shale, has a range from 40 feet (ft) below ground surface 

(bgs) of the Facility down to 1,000 ft bgs. The Pierre Shale forms the bedrock of the Facility

(AECOM, 2020).

The Pierre Shale located on the Facility can be found as a light gray to dark gray, fragmented, 

organic-rich shale. The material is also noncalcic and can be easily changed by weathering that 

results in a change in color to an orange or brown shale. Within the Pierre Shale, bentonite beds 

are interlayered. Additionally, roughly one-foot-thick layers of ironstone are interbedded within 

the Pierre Shale. On the weathered parts of the Pierre Shale, selenite crystals and ironstone 

nodules can be found. The Pierre Shale can be found up to depths of 40 ft bgs, but at places of 

weathering, the depths can be shallower (Aerostar, 2019; AECOM, 2020).

During the SI, ten borings were advanced to groundwater saturation, with termination depths 

between 20 to 33 ft bgs. The soil was generally classified as sandy clay, sand, or clay. Nonnative fill material, namely well-graded sand, was identified in sample location AOI01-03 from 

surface to 5 ft bgs. Permeable lenses of gravel, sand, and silt of varying thickness were observed 

in most borings at depths ranging from 12 to 19 ft bgs. 
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2.2.2 Hydrogeology

The Black Hills area is an important recharge area for aquifers within the northern Great Plains. 

The Facility is located within the Williston Basin, where water flows into the Madison and 

Minnelusa aquifers. These aquifers are a part of the Paleozoic group, which occurs in areas that 

have high altitude and in uplift areas. The Madison aquifer has a siltstone, sandstone, limestone, 

and dolomite base. The water found in this location is typically in outcrop areas and flows to the 

recharge areas to the northeast. The discharge location occurs as a result of upward leakage to 

the lower Cretaceous aquifer located in central South Dakota. The Minnelusa aquifer has a 

limestone and sandstone base. The Minnelusa aquifer moves from areas of recharge to the 

northeast much like the Madison aquifer. A portion of the water will discharge upward by 

leakage into the lower Cretaceous aquifer. The lower Cretaceous aquifer is composed of 

sandstone and is confined by shale in areas where uplift can be found. Over one-half of the 

water found in these areas is moderately saline and can be briny in many parts. The salination of 

this water occurs from upward leakage of mineralized water from the Paleozoic aquifers (United 

States Geological Survey [USGS], 2002; AECOM, 2020).

During the SI, groundwater saturation was generally observed within permeable deposits at 

depths ranging from 12 and 19 ft bgs. Static depths to water were observed from 12 to 19 ft bgs. 

Well recharge rates varied by sample location. Temporary wells located at AOI01-04, AOI01-05, 

and RCAASF-02 had significant drawdown during purging while the other wells recharged with 

minimal drawdown during sampling.

Based on synoptic water level measurements taken from temporary wells during the SI fieldwork 

(see Section 5.4), groundwater flows toward the southeast (shown in Figure 2-5). No potable 

water wells are located within the boundary of the Facility. An Environmental Data Resources, 

Inc. (EDRTM) report conducted a well search for a 1-mile radius surrounding the Facility. Using 

additional online resources, such as state and local geographic information system (GIS)

databases, wells were researched to a 4-mile radius of the Facility (AECOM, 2020). From a 

review of the South Dakota Department of Agriculture and Natural Resources (SD DANR) 

Water Well Completion Reports online database, there are monitoring wells, and numerous 

domestic and irrigation wells located downgradient of the Facility within 1 to 2 miles (SD

DANR, 2022) (Figure 2-3).

Drinking water for the Facility is supplied by the Rapid City Water Division, which uses the 

Jackson Springs Gallery and the Girl Scouts Gallery as infiltration galleries along the Rapid 

Creek alluvium. Water is also drawn from the Minnelusa and Madison aquifers through eight 

wells. Surface water collects in the Rapid Creek, which collects water from the Deerfield and 

Pactola Reservoirs. This surface water supplies water for treatment to the Mountain View and 

Jackson Springs treatment plants which is then supplied for municipal use (AECOM, 2020). 

2.2.3 Hydrology

The Black Hills strongly influence the hydrology of western South Dakota. Many streams in 

western South Dakota originate in the Black Hills, where groundwater and surface water interact 

extensively. The base flow of most streams in the area comes from the higher altitudes and many

of the surrounding streams have headwater springs that originate from the Paleozoic carbonate 
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rocks. These steams generally flow eastward (USGS, 2002; AECOM, 2020). The Facility is 

located in the Rapid Creek Basin and surface water at the Facility flows south to southeast to 

unnamed tributaries of Rapid Creek as presented on Figure 2-4.

2.2.4 Climate

The climate at the Facility consists of four clearly separated seasons, with warm and clear 

summers and dry, freezing, cloudy, windy winters. Temperatures vary from average highs of 

59.1 degrees Fahrenheit (°F) to average lows of 33.5 °F. The average annual temperature is 46.3 

°F. Average precipitation is 18.32 inches of rain (AECOM, 2020). 

2.2.5 Current and Future Land Use

The Facility is adjacent to Rapid City Municipal Airport. The Facility consists of an 

administration building, office areas, storage buildings, and several hangars. Exterior features are 

vehicle parking areas and roads. Infrastructure improvements, land acquisitions, land use 

controls, and reasonably anticipated future land use is not anticipated to change (AECOM,

2020). The Facility is fenced and has restricted access areas.

2.2.6 Sensitive Habitat and Threatened/Endangered Species

A wildlife survey has not occurred at the Facility, and the Facility does not have any significant 

areas of habitat. The following species have not been identified at the Facility but may be present 

in the surrounding area.

The following species are listed as federally endangered, threatened, proposed, and/or candidate 

species in Pennington County, South Dakota (U.S. Fish and Wildlife Service [USFWS], 2021):

Birds: Whooping Crane, Grus americana (Endangered); and Red Knot, Calidris canutus rufa

(Threatened). 

Flowering Plants: Leedy’s Roseroot, Rhodiola integrifolia ssp. Leedyi (Threatened)

Insects: Monarch Butterfly, Danaus plexippus (Candidate)

Mammals: Northern Long-eared Bat, Myotis septentrionalis (Threatened).

2.3 HISTORY OF PFAS USE

One AOI was identified in the PA where aqueous film forming foam (AFFF) may have been 

used, stored, disposed, or released historically at the Rapid City AASF (AECOM, 2020). 

Interviews and records obtained during the PA indicate that AFFF may have been used, stored, 

disposed, or released historically at the Facility’s Main Hangar, which was constructed in 1999 

by the South Dakota Army National Guard (SDARNG) and was equipped with an AFFF fire 

suppression system (AECOM, 2020). Since 2002, the fire suppression system has been tested 

annually, to ensure the fire suppression system is functional. The potential PFAS release areas 
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were grouped into one AOI based on preliminary data and presumed groundwater flow 

directions. A description of the AOI is presented in Section 3.
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Figure 2-1

Facility Location

Rapid City AASF

3

Facility Data

Facility Boundary

^_

Data Sources: 

ESRI 2020

Date:.....................OCTOBER 2022

Prepared By:..........................WSP

Prepared For:.........................USACE

Projection:.......NAD 1983 StatePlane

G:\ANG\a_MXD\RapidCity, SD\Facility Location.mxd - megan.cameron - 2/17/2022 - 11:40:24 AM

SD

0 0.175 0.35 0.7 1.05 1.4

Miles
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Figure 2-2

Facility Topography

Rapid

City Fire

Department

Cold Storage Hanger

Main Hangar

AOI 1

Overflow Area

Old Hanger-Bay 1

3,000 Gallon

Bypass UST

New Readiness

Center

Former Wastewater

Lagoon

94

9

5

40

960

935

970

965

960

955

950

955

940

945

935

935

930

935

930

935

930

945

940

950

950

940

965

965

965

965

950

9

3

5

930

965

965

Facility Data

Facility Boundary

Building

Potential PFAS Release

Area of Interest

Data Sources:

ESRI 2020

AECOM 2020

Date:...................FEBRUARY 2022

Prepared By:.........................WOOD

Prepared For:........................USACE

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0 250 500 750 1,000

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Figure 2-3

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Mile Buffer

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Data Sources: 

ESRI 2020

AECOM 2020

Date:.....................OCTOBER 2022

Prepared By:..........................WSP

Prepared For:.........................USACE

Projection:.......NAD 1983 StatePlane

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(November 2021)

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Figure 2-4

Surface Water Features

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Facility Boundary

^_

Data Sources: 

ESRI 2020

AECOM 2020

Date:.....................OCTOBER 2022

Prepared By:..........................WSP

Prepared For:.........................USACE

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0 0.225 0.45 0.9 1.35 1.8

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Figure 2-5

Groundwater Elevations, November 2021

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City Fire

Department

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Main Hangar

AOI 1

Overflow Area

Old Hanger-Bay 1

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New Readiness

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^_

Data Sources: 

ESRI 2020

AECOM

Date:....................OCTOBER 2022

Prepared By:.........................WSP

Prepared For:........................USACE

Projection:......NAD 1983 StatePlane

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Interval (2 Foot)

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3. SUMMARY OF AREAS OF INTEREST

The PA evaluated areas where PFAS-containing materials may have been used, stored, disposed, 

or released historically. One potential release area was identified at the Rapid City AASF and 

identified as: AOI 1 Overflow Area. The AOI is shown on Figure 3-1.

3.1 AOI 1 – OVERFLOW AREA

The Main Hangar was constructed in 1999 by the SDARNG and was equipped with an AFFF 

fire suppression system. The hangar fire suppression system consists of two 1,100-gallon tanks 

filled with 3% AFFF concentrate. The AFFF tanks that supply the fire suppression system are 

housed in a separate room, adjacent to the hangar. When the fire suppression system is engaged, 

an automatic switch is activated that routes AFFF/water mixture from the trench drains to a 

3,000-gallon concrete bypass UST, shown on Figure 3-1. When the fire suppression system is 

not engaged, liquid in the trench drains is routed to an oil/water separator, which discharged to 

the former wastewater lagoon that was located on the west side of the airport. As of 17 July 

2023, the airport and facility sanitary sewer discharge to the Rapid City wastewater treatment 

plant. The airport wastewater lagoon will be reportedly decommissioned. The bypass UST is 

located on the southwest side of the hangar (AECOM, 2020). The former wastewater lagoon and 

the bypass UST are shown on Figure 2-2. 

The fire suppression system was installed and initially tested in December of 1999 by a 

contractor. A firehose was connected to the test header port just inside the hangar, and a 

foam/water mixture was analyzed. During this test, the large plastic container over-flowed and 

the foam/water mixture spilled onto the concrete. The contractor addressed the spill immediately; 

however, a small amount of the mixture flowed into the grassy area south of the Main Hangar, 

Figure 3-1. The plastic container held approximately 500-gallons of the foam/water mixture, 

which was taken off-site for disposal by the contractor (AECOM, 2020). 

Since 2002, the fire suppression system has been tested annually, to ensure the fire suppression 

system is functional. In general, the contractor will use a hose to bypass the hangar and empty

approximately 500-gallons of the AFFF/water mixture directly into the bypass UST during 

testing. During a single testing event that occurred in 2005, the bypass was not used and the 

AFFF mixture was discharged to the UST via the hangar trench drain. As a result, trace PFAS in 

the trench drain piping may be present. When the bypass UST becomes half full, a contractor 

removes the AFFF/water mixture. Historically the AFFF/water mixture was sent to the Rapid 

City Landfill and applied to the Municipal Solid Waste Compost to prevent fires. Based on 

records, a total of approximately 7,500-gallons of the AFFF/water mixture was disposed of at the 

Rapid City Landfill, with the first disposal occurring on January 6, 2006. In 2019, the 

AFFF/water mixture was disposed through a DLA contract. The AFFF/water mixture is no 

longer taken to the Rapid City Landfill. There were no reported leaks or releases from the bypass 

UST since installation; however, the bypass UST does not have a leak detection system 

(AECOM, 2020). 

There are six mobile Halon fire extinguishers that have been located on the aircraft ramp since 

1987. Prior to 2005, non-AFFF fire extinguishers were used on the flight line and ramp areas.
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In Spring of 2016, a portion of the concrete and grassy area southwest of the Main Hangar was

excavated to make the drainage ditch deeper to allow for better storm water drainage (AECOM,

2020). According to personnel, some of the soil that was removed from the area was temporarily 

stored south of the Readiness Center location (see Figure 3-1), until the construction of the 

Readiness Center in 2019. Reportedly, the soil was used as fill material during construction of 

the Readiness Center, however, there are no records indicating where the fill was placed. The use 

of this soil as fill made the Readiness Center a secondary AOI 1 source. 

3.2 ADJACENT SOURCES

Three potential off-Facility sources of PFAS are adjacent to the Facility and are not under the 

control of the SDARNG. A description of each off-Facility source is presented below and shown 

on Figure 3-1.

3.2.1 Rapid City Fire Department

The Rapid City Fire Department provides emergency response for the AASF. This department is 

an active fire station containing emergency response vehicles and equipment. Shown on Figure 

3-1, the Rapid City Fire Department is located north and upgradient of the Facility, and the 

history of AFFF used during emergency response actions and training was not included in the 

PA (AECOM, 2020). According to Rapid City Fire Department personnel, historical testing was 

conducted in the Rapid City Fire Department Station 8 parking lot and at the Ellsworth AFB and 

current training is conducted on the apron in front of Station 8 using a “no foam” testing system 

that has been in use since 2020.

3.2.2 Ellsworth Air Force Base

Ellsworth AFB is approximately 8 miles north and upgradient of the Rapid City AASF. In 2018, 

a Site Inspection was completed at Ellsworth AFB and 12 areas of concern were identified for 

potential PFAS contamination. The media impacted by PFAS at Ellsworth AFB include surface 

soil, groundwater, surface water, and sediment. The results of the SI indicated PFOA and PFOS 

concentrations above 70 parts per trillion (individually or combined) in groundwater for nine of 

12 areas investigated. Additionally, PFOA and PFOS were also detected in surface water and 

surface soil at four of twelve areas investigated (AECOM, 2020). Due to the distance to 

Ellsworth Air Force Base, it is not shown on the Figure 3-1.

3.2.3 Landfills

Interviewees indicated that the AFFF/water mixture from the bypass UST generated during fire 

suppression system testing was solely applied to Municipal Solid Waste Compost at the Rapid 

City Landfill by a contractor. The application of AFFF/water mixture was intended to prevent 

compost fires. Based on records, approximately a total 7,500-gallons of the AFFF/water mixture 

was disposed of at the Rapid City Landfill, with the first disposal occurring on January 6, 2006. 

In 2019, the AFFF/water mixture was disposed through a DLA contract and incinerated. The 

Rapid City Landfill is approximately 7 miles west of the AASF, hydraulically cross gradient of 

the Facility (AECOM, 2020). Due to the distance to the Rapid City Landfill, it is not shown on 

the Figure 3-1.
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Figure 3-1

Area of Interest

Rapid

City Fire

Department

Cold Storage Hanger

Main Hangar

AOI 1

Overflow Area

Old Hanger-Bay 1

3,000 Gallon

Bypass UST

New Readiness

Center

Temporary Fill

Storage Location

Facility Data

Facility Boundary

Building

Potential PFAS Release

Area of Interest

Data Sources:

ESRI 2020

AECOM 2020

Date:.....................FEBRUARY 2022

Prepared By:..........................WOOD

Prepared For:.........................USACE

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0 250

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Hydrology

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4. PROJECT DATA QUALITY OBJECTIVES

As identified during the Data Quality Objective (DQO) process and outlined in the SI Uniform 

Federal Policy (UFP)-Quality Assurance Project Plan (QAPP) Addendum (EA/Wood, 2021a), 

the objective of the SI is to identify whether there has been a release to the environment at the 

AOI identified in the PA. For each AOI, ARNG determines if further investigation is warranted, 

a removal action is required to address immediate threats, or whether no further action is 

warranted. This SI evaluated groundwater and soil for the presence or absence of relevant 

compounds at the sampled AOI.

4.1 PROBLEM STATEMENT

ARNG will recommend an AOI for remedial investigation (RI) if related soil and groundwater 

samples have concentrations of the relevant compounds above the OSD risk-based SLs. The SLs 

are presented in Section 6.1 of this Report.

4.2 INFORMATION INPUTS

Primary information inputs for the SI include the following:

• The PA Report for Rapid City AASF (AECOM, 2020);

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

accordance with the site-specific UFP-QAPP Addendum (EA/Wood, 2021a); and

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

parameters measured at the time of sampling.

4.3 STUDY BOUNDARIES

The scope of the SI was bounded horizontally by the property limits of the Facility (Figures 2-1 

and 2-2). Off-site sampling was not included in the scope of this SI. If future off-site sampling is 

required, the proper stakeholders will be notified, and necessary rights of entry will be obtained 

by ARNG with property owner(s). The scope of the SI was vertically bounded as follows: 

groundwater (approximately 17 ft bgs) and soil from direct-push technology (DPT) borings (33

ft bgs). Temporal boundaries were limited to the earliest available time field resources were 

available to complete the study.

4.4 ANALYTICAL APPROACH

Samples for PFAS, pH, and grain size analyses were analyzed by Eurofins, accredited under the 

DoD Environmental Laboratory Accreditation Program (ELAP; Accreditation Number 1.01) and 

the National Environmental Laboratory Accreditation Program (NELAP; Certificate Number 

021). Samples for total organic carbon (TOC) analyses were analyzed by CT Laboratories; 

accredited under the DoD ELAP Program (Certificate Number 3806.01). Data were compared to 

applicable SLs within this document and decision rules as defined in the UFP-QAPP Addendum 

(EA/Wood, 2021a). 
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4.5 DATA USABILITY ASSESSMENT

The Data Usability Assessment (DUA), which is provided in Appendix A, 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; EPA, 2017).

Based on the DUA, the environmental data collected during the SI were found to be acceptable 

and usable for this SI evaluation with the qualifications documented in the DUA and its 

associated data validation reports. These data are of sufficient quality to meet the objectives and 

requirements of the UFP-QAPP (EA 2020a).
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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 was 

implemented in accordance with the following approved documents. 

• Final Preliminary Assessment Report, Rapid City Army Aviation Support Facility, 

South Dakota, dated August 2020 (AECOM, 2020)

• Final Programmatic Uniform Federal Policy-Quality Assurance Project Plan,

Site Inspections for Per- and Polyfluoroalkyl Substances Impacted Sites, ARNG 

Installations, Nationwide, dated December 2020 (EA, 2020a)

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

Addendum, Rapid City Army Aviation Support Facility, South Dakota dated 

November 2021 (EA/Wood, 2021a)

• Final Programmatic Accident Prevention Plan, Revision 1, dated November 2020

(EA, 2020b)

• Final Site Safety and Health Plan, Rapid City Army Aviation Support Facility, 

South Dakota, dated Month 2021 (EA/Wood, 2021b). 

The SI field activities were conducted from 15 to 18 November 2021 and consisted of 

utility clearance, DPT boring and soil sample collection, temporary monitoring well 

installation, grab groundwater sample collection, and land surveying. Field activities 

were conducted in accordance with the UFP-QAPP Addendum (EA/Wood, 2021a), 

except as noted in Section 5.8.

The following samples were collected during the SI and analyzed for 24 compounds via 

liquid chromatography/tandem mass spectrometry (LC/MS/MS) compliant with QSM 

Version 5.3 Table B-15 to fulfill the project DQOs:

• Eighteen (18) soil samples from six locations;

• Nine (9) grab groundwater samples from nine temporary well locations;

• Eleven (11) quality assurance (QA)/QC samples.

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

presents the list of samples collected for each medium. 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. Sampling forms are provided in 

Appendix B2, land survey data are provided in Appendix B3, a Field Change Request 

form is provided in Appendix B4. Additionally, a photographic log of field activities is 

provided in Appendix C. 
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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 of these activities are presented below. 

5.1.1 Technical Project Planning

The USACE TPP Process, Engineers Manual (EM) 200-1-2 (Department of the Army [DA],

2016a) 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 DQOs, and formulating a sampling approach to address the AOI 

identified in the PA. 

A combined TPP Meeting 1 and 2 was held on 30 September 2021, prior to SI field activities. 

The combined TPP Meeting 1 and 2 was conducted in general accordance with EM 200-1-2. The 

stakeholders for this SI included ARNG, SDARNG, SD DANR, USACE, and representatives 

familiar with the Facility, the regulations, and the community. 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 UFP-QAPP Addendum (EA/Wood, 2021a). 

A TPP Meeting 3 was held after the field event 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 results and findings, and future actions, where warranted. 

5.1.2 Utility Clearance

WSP USA Environment & Infrastructure, Inc. (WSP), previously doing business as Wood 

Environment & Infrastructure Solutions, Inc. (Wood), contacted the South Dakota 811 Portal to 

notify them of intrusive work at the Facility. WSP contracted Blood Hound, LLC, a private 

utility location service, to perform utility clearance at the Facility. Utility clearance was 

performed at each of the proposed boring locations on 15 November 2021 with input from the 

WSP field team. General locating services and ground-penetrating radar (GPR) were used to 

complete the clearance. Additionally, the first 5 feet of each boring were pre-cleared by WSP’s 

drilling subcontractor, Dakota Drilling, Inc, using a hand auger to verify utility clearance in 

shallow subsurface where utilities would typically be encountered. 

5.1.3 Source Water and PFAS Sampling Equipment Acceptability

The potable water source used for decontamination of drilling equipment was confirmed to meet 

acceptability criteria, as defined in the UFP-QAPP Addendum, prior to the start of field 

activities. A sample from a potable water source at Rapid City AASF, was collected on 06

October 2021, prior to mobilization, and analyzed for PFAS by LC/MS/MS compliant with QSM 

5.3 Table B-15 (DoD, 2020). The results of the sample of the potable water source used for 
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decontamination of drilling equipment during the SI are provided in Appendix F. A discussion 

of the results is presented in the DUA (Appendix A).

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 appendix to the Programmatic 

UFP-QAPP (PQAPP) (EA, 2020a). 

5.2 SOIL BORINGS AND SOIL SAMPLING

Soil samples were collected via DPT drilling methods in accordance with Standard Operating 

Procedure 047 Direct-Push Technology Sampling (EA/Wood, 2021a). A Geoprobe® 7822DT 

dual-tube sampling system was used to collect continuous soil cores to the target depth. A hand 

auger was used to collect soil from the top 5 ft of the boring in compliance with utility clearance 

procedures. The soil boring locations are shown on Figure 5-1, and boring sample depths are 

provided in Table 5-1. Several boring locations were adjusted within a 50-feet offset for reasons 

including drill rig access, utility avoidance and bias toward sampling within observed drainage 

features.

Three discrete soil samples were collected for chemical analysis from each soil boring: one 

sample at the surface (0 to 1 ft bgs) and two subsurface soil samples. One subsurface soil sample 

was collected approximately 1 ft above the groundwater table, and one was collected at the midpoint between the surface and the groundwater table (not to exceed 15 ft bgs). Groundwater was 

encountered at depths ranging from 12.4 to 16.85 ft bgs during drilling. Total boring completion 

depths, to accommodate temporary well installation, ranged from 20 to 33 ft bgs.

During the drilling, the soil cores were continuously logged for lithological descriptions by a 

field geologist using the Unified Soil Classification System. A photoionization detector (PID) 

was used to screen the breathing zone during boring activities as a part of personal safety 

requirements. Observations and measurements were recorded on electronic sampling forms 

(Appendix B2) and in a non-treated field logbook. Depth interval, recovery thickness, PID 

concentrations, moisture, relative density, Munsell color, and Unified Soil Classification System

texture were recorded. The boring logs are provided in Appendix E. 

Each sample was collected into a laboratory-supplied PFAS-free high-density polyethylene 

(HDPE) bottle 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 (liquid chromatography tandem mass spectrometry 

[LC/MS/MS] compliant with QSM Version 5.3 Table B-15), TOC (EPA Method 9060A), pH 

(EPA Method 9045D), and grain size (ASTM Method D-422) in accordance with the UFPQAPP Addendum (EA/Wood, 2021a).

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

the accompanying samples. Matrix spike/matrix spike duplicate (MS/MSD) 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, one equipment blank (EB) was collected per day and analyzed for the same parameters 
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as the soil samples. A temperature blank was placed in each cooler for use in confirming that 

samples were preserved at or below 6 degrees Celsius (°C) during shipment. 

DPT borings were converted to temporary wells, which were subsequently abandoned after 

sampling and surveying in accordance with the UFP-QAPP Addendum (EA/Wood, 2021a). 

After removal of the casings, boreholes were abandoned using bentonite chips. Borings were 

installed in grass areas to avoid disturbing concrete or asphalt surfaces.

5.3 TEMPORARY WELL INSTALLATION AND GROUNDWATER GRAB 

SAMPLING

Temporary wells were installed using a GeoProbe® 7822DT dual-tube sampling system. Once 

the borehole was advanced to the desired depth, a temporary well was constructed of either 5-ft 

or 10-ft section of 1-inch Schedule 40 poly-vinyl chloride (PVC) screen with sufficient casing to 

reach the ground surface. New PVC pipe and screen were used at each location to avoid cross 

contamination between locations. The screen intervals for the temporary wells are provided in 

Table 5-2.

Groundwater samples were collected after a period of time following well installation to allow 

groundwater to infiltrate and recharge the temporary well screen intervals. After the recharge 

period, groundwater samples were collected using a peristaltic pump with PFAS-free HDPE 

tubing. The temporary wells were purged at a rate determined in the field to reduce turbidity and 

draw down prior to sampling. Water quality parameters (e.g., temperature, specific conductance, 

pH, dissolved oxygen, and oxidation-reduction potential) were measured using a water quality 

meter and recorded on the field sampling form (Appendix B2) before each grab sample was 

collected in a separate container. Additionally, a subsample of each groundwater sample was 

collected in a separate container, and a shaker test was completed to identify if there were any 

foaming. No foaming was noted in any of the groundwater samples.

Each sample was collected in 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 Version 5.3 Table B-15 in accordance with the UFP-QAPP Addendum (EA/Wood,

2021a). 

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. Three field blank (FB) samples were collected in 

accordance with the UFP-QAPP Addendum (EA/Wood, 2021a). A temperature blank was placed 

in each cooler for use in confirming that samples were preserved at or below 6°C during 

shipment. 

Following well surveying (described below in Section 5.7), temporary wells were abandoned in 

accordance with the SI UFP-QAPP Addendum (EA/Wood, 2021a) by removing the PVC and 

backfilling the hole with bentonite chips. 
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5.4 SYNOPTIC WATER LEVEL MEASUREMENTS

A synoptic groundwater gauging event was performed on 18 November 2021. Groundwater 

elevation measurements were collected from the nine new temporary monitoring wells. Water 

level measurements were taken from the survey mark on the northern side of the well casing.

Groundwater elevation data is provided in Table 5-3. A groundwater flow contour map is 

provided as Figure 2-4.

5.5 SURVEYING

The northern side of each new temporary well casing was surveyed using a Trimble R10 realtime kinematic differential global positioning system. Positions were collected in the applicable 

South Dakota State Plane North American Datum of 1983 (horizontal) and North American 

Vertical Datum 1988 (vertical). Surveying data were collected on 18 November 2021 and are

provided in Appendix B3.

5.6 INVESTIGATION-DERIVED WASTE

As of the date of this report, the disposal of PFAS investigation-derived waste (IDW) is not 

regulated federally. IDW generated during the SI is considered non-hazardous waste and was 

managed in accordance with the UFP-QAPP Addendum (EA/Wood, 2021a). 

Soil IDW (i.e., soil cuttings) and liquid IDW (i.e., purge water and decontamination fluids) 

generated during the SI activities were drummed separately in 55-gallon steel drums. The IDW 

drums were subsequently stored within secondary containment in a dedicated indoor area within 

the Rapid City AASF Facility. The soil IDW was not sampled and assumes the characteristics of 

the associated soil samples collected from that source location. The liquid IDW was not sampled 

and assumes the characteristics of the associated groundwater samples collected from that source 

location.

The IDW disposal is being managed separately under a contract with EA Engineering, Science, 

and Technology, Inc. Specifics on the disposal of solid and liquid IDW will be addressed in an 

IDW Technical Memorandum.

Other solids such as spent personal protective equipment, 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 by LC/MS/MS, compliant with QSM Version 5.3 Table B-15, at 

Eurofins in Lancaster, Pennsylvania, a DoD ELAP and NELAP-certified laboratory. 

Soil samples were also analyzed for TOC using EPA Method 9060A, pH by EPA Method 

9045D, and grain size using ASTM Method D-422.
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5.8 Deviations from SI UFP-QAPP Addendum

Deviations from the UFP-QAPP Addendum occurred based on conditions encountered during 

field activities. These deviations were discussed between WSP, EA, ARNG, and USACE. Three

deviations from the UFP-QAPP Addendum are noted below.

The location of AOI01-03 was chosen to assess an area of storm water runoff. When field 

verified, it was determined that the proposed location was not positioned in the target area, 

additionally a generator and a parking lot were in conflict with the originally proposed location. 

The sample location was shifted approximately 30 feet southeast to avoid the generator and 

parking lot and to assess the stormwater runoff area. A field change request was submitted on 16 

November 2021. The approved field change request form is included in Appendix B4.

While installing the boring at AOI01-05, the groundwater depth was not evident, and the boring 

was installed too deep to use the peristaltic pump. Another boring was installed approximately 5 

feet west of the original boring (identified as AOI01-05B). The soil samples were taken from the 

initial boring (AOI01-05), while the groundwater sample were taken from the step-out location 

(AOI01-05B). 

The location of the water table was not immediately evident in several of the borings (AOI01-02, 

AOI01-03, AOI01-06, and AOI01-07). As such, a 10 ft screen, rather than a 5 ft screen, was used 

to increase the likelihood that the temporary monitoring well was screened across the water 

table. 
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Table 5-1. Site Inspection Samples by Medium

Rapid City AASF, Rapid City, South Dakota

Site Inspection Report

Sample Identification

Sample 

Collection 

Date

Sample Depth

(ft bgs)

PFAS

(LC/MS/MS 

compliant with QSM 

5.3 Table B-15)

TOC

(EPA Method 9060A)

pH (EPA Method 

9045D)

Grain Size (ASTM 

D422)

Comments

Soil Samples

AOI01-01-SB-0-1 11/17/2021 0-1 X

AO101-01-TOC/pH/Moisture 11/17/2021 0-2 X X Parent Sample of 

AO101-01-

TOC/pH/Moisture

-DUP

AO101-01-TOC/pH/MoistureDUP11/17/2021 0-2 X X Field Duplicate

AO101-01-Grain Size 0-2 X

 AOI01-01-SB-6-8 11/17/2021 6-8 X

 AOI01-01-SB-10-11 11/17/2021 10-11 X

 AOI01-02-SB-0-1 11/17/2021 0-1 X Parent Sample of 

AOI01-DUP02-0-

1

 AOI01-DUP02-0-1 11/17/2021 0-1 X Field Duplicate

 AOI01-02-SB-5-6 11/17/2021 5-6 X

 AOI01-02-SB-13-14 11/17/2021 13-14 X

 AOI01-03-SB-5-6 11/17/2021 5-6 X

 AOI01-03-SB-6-7 11/17/2021 6-7 X

 AOI01-03-SB-9.5-11 11/17/2021 9.5-11 X

 AOI01-04-SB-0-1 11/16/2021 0-1 X

 AOI01-04-SB-6-7 11/16/2021 6-7 X

 AOI01-04-SB-13-14 11/16/2021 13-14 X

 AOI01-05-SB-0-1* 11/16/2021 0-1 X Parent Sample of 

AOI01-DUP01-0-

1

 AOI01-DUP01-0-1* 11/16/2021 0-1 X Field Duplicate

 AOI01-05-SB-6-7* 11/16/2021 6-7 X

 AOI01-05-SB-14-15* 11/16/2021 14-15 X

 AOI01-07-SB-0-1 11/16/2021 0-1 X

 AOI01-07-SB-7-8 11/16/2021 7-8 X

 AOI01-07-SB-14-15 11/16/2021 14-15 X

Groundwater Samples

 AOI01-01-GW 11/18/2021 X

AOI01-02-GW 11/18/2021 X

 AOI01-03-GW 11/18/2021 X

 AOI01-04-GW 11/18/2021 X

 AOI01-05-GW* 11/18/2021 X

AOI01-06-GW 11/18/2021 X

AOI01-07-GW 11/18/2021 X
Site Inspection Report

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Sample Identification

Sample 

Collection 

Date

Sample Depth

(ft bgs)

PFAS

(LC/MS/MS 

compliant with QSM 

5.3 Table B-15)

TOC

(EPA Method 9060A)

pH (EPA Method 

9045D)

Grain Size (ASTM 

D422)

Comments

RCAASF-01-GW 11/18/2021 X

 RCAASF-02-GW 11/18/2021 X Parent Sample of 

DUP-01-GW

 DUP-01-GW 11/18/2021 X Field Duplicate

Blank Samples

AOI01-Field Blank-01 11/16/2021 X Field Blank

AOI01-Equipment Blank-01 11/16/2021 X Equipment Blank 

Collected from 

Hand Auger

AOI01-Field Blank-02 11/17/2021 X Field Blank

AOI01-Equipment Blank-02 11/17/2021 X Equipment Blank 

Collected from 

Hand Auger

 AOI01-Equipment Blank-03 11/17/2021 X Equipment Blank 

Collected from 

Clean Tubing

 AOI01-Field Blank-03 11/18/2021 X Field Blank

 AOI01-Equipment Blank-04 11/18/2021 X Equipment Blank 

Collected from 

Clean Tubing

RCAASF-PW-01 10/6/2021 X Potable Water 

Sample

Notes:

*Soil samples collected from initial boring location (AOI101-05). Groundwater sample collected from off-set location (AOI01-

05B) and labeled as AOI01-05-GW. 

AASF = Army Aviation Support Facility

ASTM = American Society for Testing and Materials

bgs = below ground surface

LC/MS/MS = Liquid Chromatography Mass Spectrometry

QSM = Quality Systems Manual

TOC = total organic carbon

USEPA = United States Environmental Protection Agency
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Table 5-2. Soil Boring Depths and Temporary Well Screen Intervals

Rapid City AASF, Rapid City, South Dakota

Site Inspection Report

Area of Interest Boring Location

Soil Boring Depth

(ft bgs)

Temporary Well 

Screen Interval

(ft bgs)

1 

AOI01-01 20.0 15.0 – 20.0

AOI01-02 25.0 13.0 – 23.0

AOI01-03 25.0 15.0 – 25.0

AOI01-04 25.5 20.0 – 25.0

AOI01-05* 33.0 28.0 – 33.0

AOI01-05B* 20.0 15.0 – 20.0

AOI01-06 24.3 15.0 – 25.0

AOI01-07 25.0 15.0 – 25.0

RCAASF-01 25.0 20.0 – 25.0

RCAASF-02 23.0 18.0 – 23.0

Notes:

*Soil samples collected from initial boring location (AOI101-05). Groundwater sample collected 

from second location (AOI01-05B). 

bgs = below ground surface

ft = feet
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Table 5-3. Groundwater Elevation 

Rapid City AASF, Rapid City, South Dakota

Site Inspection Report

Monitoring Well 

ID

Top of Casing Elevation

(ft NAVD88)

Depth to Water

(ft btoc)

Depth to Water

(ft bgs)

Groundwater Elevation

(ft NAVD 88)

AOI01-01 3158.48 12.40 11.78 3146.08

AOI01-02 3159.58 13.35 12.72 3146.23

AOI01-03 3158.97 14.89 13.60 3144.08

AOI01-04 3153.90 15.88 15.34 3138.02

AOI01-05B* 3152.95 15.03 14.24 3137.92

AOI01-06 3154.38 16.85 16.01 3137.53

AOI01-07 3153.86 15.34 14.20 3138.52

RCAASF-01 3161.32 16.16 14.94 3145.16

RCAASF-02 3159.52 16.56 14.43 3142.96

Notes: 

*Soil samples collected from initial boring location (AOI101-05). Groundwater sample collected from second location (AOI01-

05B). Groundwater measurements not taken at AOI01-05

Bgs = below ground surface

btoc = below top of casing

NAVD88 = North American Vertical Datum 1988
Army National Guard Site Inspections

Site Inspection Report

Rapid City AASF, South Dakota

Figure 5-1

Site Inspection Sample Locations

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AOI01-01

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AOI01-03

RCAASF-02

Rapid

City Fire

Department

Cold Storage Hanger

Main Hangar

AOI 1

Overflow Area

Old Hanger-Bay 1

3,000 Gallon

Bypass UST

New Readiness

Center

3

^_

SD
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Site Inspection Report

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6. SITE INSPECTION RESULTS

This section presents the analytical results of the SI. The SLs used in this evaluation are 

presented in Section 6.1 and Table 6-1. A discussion of the results for the AOI is provided in 

Section 6.3. Tables 6-2 through 6-5 present results in soil or groundwater for the relevant 

compounds. 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 (Assistant Secretary 

of Defense, 2022). The ARNG program under which this SI was performed follows this DoD 

policy. Should the maximum concentration for sampled media exceed the SLs established in the 

OSD memorandum, the AOI will proceed to the next phase under CERCLA. The SLs 

established in the OSD memorandum apply to the five compounds presented on Table 6-1. 

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

Analyt,2

Residential

(Soil)

(μg/kg)1 

0-2 ft bgs

Industrial / Commercial

Composite Worker 

(Soil)

(μg/kg) 1 

2-15 ft bgs

Tap Water

(Groundwater)

(ng/L) 1 

PFOA 19 250 6

PFOS 13 160 4

PFBS 1,900 25,000 601

PFHxS 130 1,600 39

PFNA 19 250 6

Notes:

1. Assistant Secretary of Defense. July 2022. Risk Based Screening Levels in Groundwater and Soil using 

U.S. Environmental Protection Agency’s (EPA’s) Regional Screening Level Calculator. Hazard Quotient 

(HQ)=0.1. May 2022. 

2. Of the six PFAS compounds presented in the 6 July 2022 OSD memorandum, HFPO-DA (commonly 

referred to as GenX) was not included as an analyte at the time of this SI. Based on the CSM developed 

during the PA and revised based on SI findings, the presence of HFPO-DA is not anticipated at the facility 

because HFPO-DA is generally not a component of MIL-SPEC AFFF and based on its history including 

distribution limitations that restricted use of GenX, it is generally not a component of other products the 

military used. In addition, it is unlikely that GenX would be an individual chemical of concern in the 

absence of other PFAS.

Abbreviations:

μg/kg = microgram(s) per kilogram

bgs = below ground surface

ft = feet

ng/L = nanogram(s) per liter

The data in the subsequent sections are compared against the SLs presented in Table 6-1. The 

SLs for groundwater are based on direct ingestion. The SLs for soil are based on incidental 

ingestion and are applied to the depth intervals reasonably anticipated to be encountered by the 

receptors identified at the Facility: the residential scenario is applied to surface soil results (0 to 2 

feet bgs) and the industrial/commercial worker scenario is applied to shallow subsurface soil 
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results (2 to 15 feet bgs). The SLs are not applied to deep subsurface soil results (>15 feet bgs) 

because 15 feet is the anticipated limit of construction activities.

6.2 SOIL PHYSICOCHEMICAL ANALYSES

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

size, which are important for evaluating transport through the soil medium. Appendix E contains 

the results of the TOC, pH, and grain size sampling. 

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

appropriate, to assess fate and transport. 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 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: Overflow Area. The soil and groundwater results are summarized in Table 6-2 through 

Table 6-5. Soil and groundwater results are presented on Figures 6-1 through Figure 6-7.

6.3.1 AOI 1 Soil Analytical Results

Soil samples were collected from six boring locations associated with AOI 1 during the SI. 

Tables 6-2 and Table 6-4 summarize the soil results. Figure 6-1 through Figure 6-5 present the 

ranges of detections in soil.

Surface soil (0 to 1 ft bgs) was sampled from boring locations AOI01-01, AOI01-02, AOI01-04, 

AOI01-05 and AOI01-07, with duplicate surface soil samples being collected at AOI01-01 and 

AOI01-05. Due to the 2019 construction at the Rapid City AASF, in lieu of a surface soil sample

at AOI01-03, soil was collected at the first sign of native soil (5 to 6 ft bgs). Soil was also 

sampled from shallow subsurface soil (5 to 8 ft bgs) and deep subsurface soil intervals (9.5 to 15

ft bgs) from boring locations AOI01-01, AOI01-02, AOI01-03, AOI01-04, AOI01-05 and 

AOI01-07.

PFNA, PFOA and PFOS were detected in surface soil at concentrations below their respective 

SLs. PFNA was detected in surface soil at two of the five sample locations (AOI01-01 and 

AOI01-02 [and its duplicate]) and ranged in concentration from 1.9 μg/kg to 5.8 μg/kg. PFOA 

was detected in surface soil at two of the five sample locations (AOI01-01 and AOI01-02 [and its 

duplicate]) and ranged in concentration from 1.1 μg/kg to 13.0 μg/kg. PFOS was detected in
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surface soil at all five sample locations (AOI01-01, AOI01-02 [and its duplicate], AOI01-04, 

AOI01-05 [and its duplicate] and AOI01-07) and ranged in concentration from 1.1 J+ μg/kg to 

2.6 μg/kg. PFBS and PFHxS were not detected in the surface soil samples.

PFHxS, PFNA, PFOA and PFOS were detected in shallow subsurface soil at concentrations 

below their respective SLs. PFHxS was detected in shallow subsurface soil at one of six shallow 

subsurface sampling locations (AOI01-03) at a maximum concentration of 0.36 J μg/kg. PFNA 

was detected in shallow subsurface soil at one of six locations at a concentration of 0.54 J μg/kg

(AOI01-01). PFOA was detected in shallow subsurface soil at one of six locations at a 

concentration of 1.7 μg/kg (AOI01-01). PFOS was detected in shallow subsurface soil at one of 

six locations at a concentration of 0.75 μg/kg (AOI01-03). PFBS was not detected in shallow 

subsurface soil samples. 

PFNA, PFOA and PFOS were detected in deep subsurface soil at concentrations below their 

respective SLs. PFNA was detected in deep subsurface soil at one of six locations at a 

concentration of 1.9 μg/kg (AOI01-01). PFOA was detected in deep subsurface soil at one of six 

location at a concentration of 1.7 μg/kg (AOI01-01). PFOS was detected in deep subsurface soil 

in two of six locations and ranged in concentration from 0.88 μg/kg to 2.0 μg/kg (AOI01-01 and 

AOI01-02). PFBS and PFHxS were not detected in the deep subsurface soil samples.

6.3.2 AOI 1 Groundwater Analytical Results 

Groundwater samples were collected from seven temporary wells associated with AOI 1 during 

the SI. Figure 6-6 and Figure 6-7 presents the ranges of detections in groundwater. Table 6-5

summarizes the groundwater results.

Groundwater was sampled from temporary monitoring well locations AOI01-01 through AOI01-

07. PFHxS, PFNA, PFOA and PFOS were detected at concentrations exceeding their respective 

SLs. PFHxS was detected in all groundwater samples with concentrations ranging from 5.1 to 

280 ng/L, and it exceeded the SL at three locations (AOI01-01, AOI01-03, and AOI01-07). 

PFNA was detected in all groundwater samples with concentrations ranging from 0.41 J to 

140 ng/L, and it exceeded the SL at two locations (AOI01-01 and AOI01-02). PFOA was 

detected in all groundwater samples with concentrations ranging from 4.5 J- ng/L to 

1,300 J- ng/L, and it exceeded the SL at six locations (AOI01-01, AOI01-02, AOI01-03, AOI01-

04, AOI01-06, and AOI01-07). PFOS was detected in all groundwater samples with 

concentrations ranging from 1.4 J ng/L to 880 ng/L, and it exceeded the SL at five locations 

(AOI01-01, AOI01-02, AOI01-03, AOI01-06, and AOI01-07). PFBS was detected at 

concentrations below the respective SL. PFBS was detected in all locations and ranged in 

concentration from 2.8 J- ng/L to 21 J- ng/L. 

6.3.3 Conclusions

Based on the results of the SI, PFHxS, PFNA, PFOA, and/or PFOS were detected in 

groundwater at concentrations above their respective SLs at all locations except AOI01-05B.

Based on the exceedances of the SLs in groundwater, further evaluation at AOI 1 is warranted.

While no soil samples exceeded the SLs, PFHxS, PFNA, PFOS and PFOA were detected in 
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surface soil samples and shallow subsurface soil samples. PFNA, PFOA and PFOS were 

detected in deep subsurface soil at concentrations below their respective SLs.

6.4 BOUNDARY SAMPLE LOCATIONS

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

samples collected at the Facility boundary. The detected compounds are summarized in Tables 

6-2 through 6-5. Soil and groundwater results are presented on Figures 6-1 through 6-7.

6.4.1 Boundary Sample Locations – Soil Analytical Results

No soil samples were collected from boundary locations. 

6.4.2 Boundary Sample Locations – Groundwater Analytical Results 

Groundwater was sampled from RCAASF-01 and RCAASF-02. PFHxS was detected at 

concentrations exceeding its respective SL. PFHxS was detected at both locations at 

concentrations ranging from 19 ng/L to 430 ng/L [430 ng/L in the sample duplicate]; PFHxS 

exceeds the SL at one location (RCAASF-02). PFOA was detected at both locations with 

concentrations ranging from 8.9 J- ng/L to 87 J- ng/L [89 J- ng/L in the sample duplicate]; PFOA 

exceeded the screening level at both locations (RCAASF-01 and RCAASF-02). PFOS was 

detected at both locations with concentrations ranging from 11 ng/L to 310 ng/L [310 ng/L in the 

sample duplicate]; PFOS exceeded the SL at both locations (RCAASF-01 and RCAASF-02). 

PFBS and PFNA were detected in at both boundary locations at concentrations below their 

respective SLs. PFNA was detected at a concentration ranging from 1.5 J ng/L to 2.1 ng/L

(2.3 ng/L in the sample duplicate). PFBS was detected in both locations at concentrations 

ranging from 3.4 J- ng/L to 25 J- ng/L (24 J- ng/L in the sample duplicate).

6.4.3 Conclusions

Based on the results of the SI, PFHxS, PFOA and PFOS were detected in upgradient boundary 

groundwater samples at concentrations above their respective SLs. These results may indicate a 

potential off-Facility source upgradient of AOI 1.
Table 6-2

PFOA, PFOS, PFBS, PFNA, and PFHxS Results in Surface Soil

Site Inspection Report

Rapid City AASF

Analyte OSD Screening Level 1 Result Qual Result Qual Result Qual Result Qua

PFBS 1900 ND U ND U ND U ND U 

PFHxS 130 ND U ND U ND U ND U PFNA 19 5.8 2.0 1.9 ND U 

PFOA 19 13 1.1 1.7 ND U PFOS 13 2.6 1.2 2.0 1.1 J+ 

Notes ChemicGray Fill Detected concentration exceeded OSD Screening Levels PFBS 

PFHxS 

References PFNA 

1. Assistant Secretary of Defense, July 2022. Risk Based Screening Levels Calculated PFOA 

for PFOA, PFOS, PFBS, PFHxS, and PFNA in Groundwater or Soil using USEPA’s PFOS 

Regional Screening Level Calculator. HQ=0.1. May 2022. The screening levels for soil are

based on residential scenario for incidental ingestion of contaminated soil.

Interpreted Qualifiers

J+ = The result is an estimated quantity, but the result may be biased high.U = The analyte was not detected at a level greater than or equal to the adjusted DL.Acronyms and Abbreviationsμg/kg microgram(s) per kilogramAASF Army Aviation Support FacilityAOI Area of InterestDUP duplicateHQ Hazard QuotientID identificationLCMSMS liquid chromatography with tandem mass spectrometryLOD limit of detectionLOQ limit of quantitationND analyte not detected above the LOD (LOD values are presented in Appendix F)OSD Office of the Secretary of the DefenseQSM Quality Systems ManualPFAS per- and polyfluoroalkyl substancesSB soil boringUSEPA United States Environmental Protection AgencyQual interpreted qualifier

Soil, PFAS by LCMSMS compliant with QSM 5.3 Table B-15 (μg/kg)

Area of Interest

Depth

AOI01-01

0 - 1 ft 0 - 1 ft 0 - 1 ft 0 - 1 ft 

Sample Date 11/17/2021 11/17/2021 11/17/2021 11/16/2021

AO101-04-SB-0

Location ID AOI01-01 AOI01-02 AOI01-02 AOI01-04

Sample ID AO101-01-SB-0-1 AO101-02-SB-0-1 AO101-DUP02-0-1

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Table 6-3

PFOA, PFOS, PFBS, PFNA, and PFHxS Results in Shallow Subsurface Soil

Site Inspection Report

Rapid City AASF

Analyte OSD Screening Level 1 Result Qual Result Qual Result Qual Result Qual Result 

PFBS 25000 ND U ND U ND U ND U ND PFHxS 1600 ND U ND U 0.36 J 0.33 J ND PFNA 250 0.54 J ND U ND U ND U ND PFOA 250 1.7 ND U ND U ND U ND 

PFOS 160 ND U ND U ND U 0.75 ND 

Notes Chemical AbbreviationsGray Fill Detected concentration exceeded OSD Screening Levels PFBS p

PFHxS p

References PFNA p

1. Assistant Secretary of Defense, July 2022. Risk Based Screening Levels Calculated PFOA p

for PFOA, PFOS, PFBS, PFHxS, and PFNA in Groundwater or Soil using USEPA’s PFOS p

Regional Screening Level Calculator. HQ=0.1. May 2022. The screening levels for soil arebased on Industrial/Commercial Composite Worker scenario for incidental ingestion of contaminated soil.Interpreted Qualifiers

J = The result is an estimated quantity.U = The analyte was not detected at a level greater than or equal to the adjusted DL.Acronyms and Abbreviationsμg/kg microgram(s) per kilogramAASF Army Aviation Support FacilityAOI Area of InterestDUP duplicateHQ Hazard QuotientID identificationLCMSMS liquid chromatography with tandem mass spectrometryLOD limit of detectionLOQ limit of quantitationND analyte not detected above the LOD (LOD values are presented in Appendix F)OSD Office of the Secretary of the DefenseQSM Quality Systems ManualPFAS per- and polyfluoroalkyl substancesSB soil boringUSEPA United States Environmental Protection AgencyQual interpreted qualifier

Soil, PFAS by LCMSMS compliant with QSM 5.3 Table B-15 (μg/kg)

Area of Interest AOI01

Depth 6 - 8 ft 5 - 6 ft 5 - 6 ft 6 - 7 ft 6 - 7 

Sample ID AO101-01-SB-6-8 AO101-02-SB-5-6 AO101-04

Location ID AOI01-01 AOI01-02 AOI01

AOI01-03

AO101-03-SB-6-7

AOI01-03

AO101-03-SB-5-6

Sample Date 11/17/2021 11/17/2021 11/17/2021 11/17/2021 11/16/2

EA Engineering, Science, and Technology, Inc., PBCEA 
Site Inspection Report

Rapid City Army Aviation Support Facility, South Dakota

EA Engineering, Science, and Technology, Inc., PBC 6-8

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Table 6-4

PFOA, PFOS, PFBS, PFNA, and PFHxS Results in Deep Subsurface Soil

Site Inspection Report 

Rapid City AASF

Analyte OSD Screening Level 1 Result Qual Result Qual Result Qual Result Qual 

PFBS 25000 ND U ND U ND U ND U PFHxS 1600 ND U ND U ND U ND U PFNA 250 1.9 ND U ND U ND U PFOA 250 1.7 ND U ND U ND U PFOS 160 2.0 0.88 ND U ND U 

Notes Chemical AbbreviatioGray Fill Detected concentration exceeded OSD Screening Levels PFBS 

PFHxS 

References PFNA 

1. Assistant Secretary of Defense, July 2022. Risk Based Screening Levels Calculated PFOA 

for PFOA, PFOS, PFBS, PFHxS, and PFNA in Groundwater or Soil using USEPA’s PFOS 

Regional Screening Level Calculator. HQ=0.1. May 2022. The screening levels for soil arebased on Industrial/Commercial Composite Worker scenario for incidental ingestion of contaminated soil.Interpreted Qualifiers

U = The analyte was not detected at a level greater than or equal to the adjusted DL.Acronyms and Abbreviationsμg/kg microgram(s) per kilogramAASF Army Aviation Support FacilityAOI Area of InterestDUP duplicateHQ Hazard QuotientID identificationLCMSMS liquid chromatography with tandem mass spectrometryLOD limit of detectionLOQ limit of quantitationND analyte not detected above the LOD (LOD values are presented in Appendix F)OSD Office of the Secretary of the DefenseQSM Quality Systems ManualPFAS per- and polyfluoroalkyl substancesSB soil boringUSEPA United States Environmental Protection AgencyQual interpreted qualifier

Sample ID AO101-01-SB-10-11 AO101-02-SB-13-14 AO101-03-SB-9.5-11 AO101-04-SB-13-

Location ID AOI01-01 AOI01-02 AOI01-03 AOI01-04 

Soil, PFAS by LCMSMS compliant with QSM 5.3 Table B-15 (μg/kg)

Area of Interest

Depth

AOI01-01

10 - 11 ft 13 - 14 ft 9.5 - 11 ft 13 - 14 ft 

Sample Date 11/17/2021 11/17/2021 11/17/2021 11/16/2021 

EA Engineering, Science, and Technology, Inc., PBCEA 
Site Inspection Report

Rapid City Army Aviation Support Facility, South Dakota

EA Engineering, Science, and Technology, Inc., PBC 6-10

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Table 6-5

PFOA, PFOS, PFBS, PFNA, and PFHxS Results in Groundwater

Site Inspection Report

Rapid City AASF

Analyte OSD Screening Level 1 Result Qual Result Qual Result Qual Result Qual Result Qual Result Qual R

PFBS 601 8.3 J- 4.1 J- 21 J- 3.1 J- 2.8 J- 5.0 PFHxS 39 66 31 280 5.1 5.8 22 PFNA 6 140 26 5.1 0.41 J 0.9 J 0.63 J+ PFOA 6 1300 J- 470 J- 81 J- 7.0 J- 4.5 J- 6.4 PFOS 4 120 83 880 1.6 J 1.4 J 19 J+ 

Notes Gray Fill Detected concentration exceeded OSD Screening Levels 

References 

1. Assistant Secretary of Defense, July 2022. Risk Based Screening Levels Calculated 

for PFOA, PFOS, PFBS, PFHxS, and PFNA in Groundwater or Soil using USEPA’s 

Regional Screening Level Calculator. HQ=0.1 . May 2022. Groundwater screening levelsbased on residential scenario for direct ingestion of groundwater.Interpreted Qualifiers

J = Estimated concentration.J+ = The result is an estimated quantity, but the result may be biased high.J- = The result is an estimated quantity, but the result may be biased low.U = The analyte was not detected at a level greater than or equal to the adjusted DL.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.Acronyms and AbbreviationsAASF Army Aviation Support FacilityAOI Area of InterestDUP duplicateHQ Hazard QuotientID identificationLCMSMS liquid chromatography with tandem mass spectrometryLOD limit of detectionLOQ limit of quantitationND analyte not detected above the LOD (LOD values are presented in Appendix F)ng/L nanogram(s) per literOSD Office of the Secretary of the DefenseQSM Quality Systems ManualPFAS per- and polyfluoroalkyl substancesSB soil boringUSEPA United States Environmental Protection AgencyQual interpreted qualifier

AOI01-01 

11/18/2021 

AO101-04-GW

Sample Date 11/18/2021 11/18/2021 11/18/2021 11/18/2021

Area of Interest

Water, PFAS by LCMSMS compliant with QSM 5.3 Table B-15 (ng/l)

AOI01-05B AOI01-06

AO101-06-GW

11/17/2021

Location ID AOI01-01 AOI01-02 AOI01-03 AOI01-04

Sample ID AO101-01-GW AO101-02-GW AO101-03-GW AO101-05-GW

EA Engineering, Science, and Technology, Inc., PBCEA 
Site Inspection Report

Rapid City Army Aviation Support Facility, South Dakota

EA Engineering, Science, and Technology, Inc., PBC 6-12

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Figure 6-1

PFOA Detections in Soil

AOI01-01

AOI01-02

AOI01-04

AOI01-05

AOI01-07

0 150

Feet

Facility Data

Area of Interest

Buildings

Potential PFAS Release

Facility Boundary

SD

Data Sources:

ESRI 2020

AECOM 2020

Hydrology

Groundwater Flow Direction

Surface Water Flow Direction

G:\ANG\a_MXD\RapidCity, SD\Analytical results figures\PFOA Detections in Soil.mxd - USDJ716516 - 8/31/2023 - 12:08:22 PM

Army National Guard Site Inspections

Site Inspection Report

Rapid City AASF, South Dakota

Date:...............SEPTEMBER 2022

Prepared By:.......................WOOD

Prepared For:.....................USACE

Projection:......NAD 83 State Plane

Shallow (0-2')*

AOI01-01

AOI01-02

AOI01-03

AOI01-04

AOI01-05

AOI01-07

0 150

Feet

Intermediate (5-8')

AOI01-01

AOI01-02

AOI01-03

AOI01-04

AOI01-05

AOI01-07

0 150

Feet

Deep (9-15')

NOTES:

PFOA = perfluorooctanoic acid

ND = Non-Detect

(μg/Kg) = Microgram(s) per Kilogram

*At AOI01-03, surface samples were taken

at 5-6 feet bgs to account for known historic earthwork

Exceedances of the OSD SL are depicted

with a yellow halo.

PFNA Results

(ng/L)

ND

>ND - 19

>19 - 250

>250 - 2,500

>2,500
Site Inspection Report

Rapid City Army Aviation Support Facility, South Dakota

EA Engineering, Science, and Technology, Inc., PBC 6-14

This page intentionally left blank
Figure 6-2

PFOS Detections in Soil

AOI01-01

AOI01-02

AOI01-04

AOI01-05

AOI01-07

0 150

Feet

Facility Data

Area of Interest

Buildings

Potential PFAS Release

Facility Boundary

SD

Data Sources:

ESRI 2020

AECOM 2020

Hydrology

Groundwater Flow Direction

Surface Water Flow Direction

G:\ANG\a_MXD\RapidCity, SD\Analytical results figures\PFOS Detections in Soil.mxd - USDJ716516 - 8/31/2023 - 12:17:43 PM

Army National Guard Site Inspections

Site Inspection Report

Rapid City AASF, South Dakota

Date:..............SEPTEMBER 2022

Prepared By:.......................WOOD

Prepared For:.....................USACE

Projection:......NAD 83 State Plane

Shallow (0-2')*

AOI01-01

AOI01-02

AOI01-03

AOI01-04

AOI01-05

AOI01-07

0 150

Feet

Intermediate (5-8')

AOI01-01

AOI01-02

AOI01-03

AOI01-04

AOI01-05

AOI01-07

0 150

Feet

Deep (9-15)

NOTES:

PFOS = perfluorooctanesulfonic acid

ND = Non-Detect

(μg/Kg) = Microgram(s) per Kilogram

*At AOI01-03, surface samples were taken

at 5-6 feet bgs to account for known historic earthwork

Exceedances of the OSD SL are depicted

with a yellow halo.

PFOS Results

(ng/L)

ND

>ND - 13

>13 - 160

>160 - 1,600

>1,600
Site Inspection Report

Rapid City Army Aviation Support Facility, South Dakota

EA Engineering, Science, and Technology, Inc., PBC 6-16

This page intentionally left blank
Figure 6-3

PFBS Detections in Soil

AOI01-01

AOI01-02

AOI01-04

AOI01-05

AOI01-07

0 150

Feet

Facility Data

Area of Interest

Buildings

Potential PFAS Release

Facility Boundary

SD

Data Sources:

ESRI 2020

AECOM 2020

Hydrology

Groundwater Flow Direction

Surface Water Flow Direction

G:\ANG\a_MXD\RapidCity, SD\Analytical results figures\PFBS Detections in Soil.mxd - USDJ716516 - 8/31/2023 - 12:23:00 PM

Army National Guard Site Inspections

Site Inspection Report

Rapid City AASF, South Dakota

Date:...............SEPTEMBER 2022

Prepared By:.......................WOOD

Prepared For:.....................USACE

Projection:......NAD 83 State Plane

Shallow (0-2')*

AOI01-01

AOI01-02

AOI01-03

AOI01-04

AOI01-05

AOI01-07

0 150

Feet

Intermediate (5-8')

AOI01-01

AOI01-02

AOI01-03

AOI01-04

AOI01-05

AOI01-07

0 150

Feet

Deep (9-15')

NOTES:

PFBS = perfluorobutanesulfonic acid

ND = Non-Detect

(μg/Kg) = Microgram(s) per Kilogram

*At AOI01-03, surface samples were taken

at 5-6 feet bgs to account for known historic earthwork

Exceedances of the OSD SL are depicted

with a yellow halo.

PFBS Results (ng/L)

ND

>ND - 10

>10 - 1,900

>1,900 - 25,000

>25,000
Site Inspection Report

Rapid City Army Aviation Support Facility, South Dakota

EA Engineering, Science, and Technology, Inc., PBC 6-18

This page intentionally left blank
Figure 6-4

PFHxS Detections in Soil

AOI01-01

AOI01-02

AOI01-04

AOI01-05

AOI01-07

0 150

Feet

Facility Data

Area of Interest

Buildings

Potential PFAS Release

Facility Boundary

SD

Data Sources:

ESRI 2020

AECOM 2020

Hydrology

Groundwater Flow Direction

Surface Water Flow Direction

G:\ANG\a_MXD\RapidCity, SD\Analytical results figures\PFHxS Detections in Soil.mxd - USDJ716516 - 8/31/2023 - 12:27:52 PM

Army National Guard Site Inspections

Site Inspection Report

Rapid City AASF, South Dakota

Date:...............SEPTEMBER 2022

Prepared By:.......................WOOD

Prepared For:.....................USACE

Projection:......NAD 83 State Plane

Shallow (0-2')*

AOI01-01

AOI01-02

AOI01-03

AOI01-04

AOI01-05

AOI01-07

0 150

Feet

Intermediate (5-8')

AOI01-01

AOI01-02

AOI01-03

AOI01-04

AOI01-05

AOI01-07

0 150

Feet

Deep (9-15')

NOTES:

PFHxS = perfluorohexanesulfonic acid

ND = Non-Detect

(μg/Kg) = Microgram(s) per Kilogram

*At AOI01-03, surface samples were taken

at 5-6 feet bgs to account for known historic earthwork

Exceedances of the OSD SL are depicted

with a yellow halo.

PFHxS Results

(ng/L)

ND

>ND - 10

>10 - 130

>130 - 1,600

>1,600
Site Inspection Report

Rapid City Army Aviation Support Facility, South Dakota

EA Engineering, Science, and Technology, Inc., PBC 6-20

This page intentionally left blank
Figure 6-5

PFNA Detections in Soil

AOI01-01

AOI01-02

AOI01-04

AOI01-05

AOI01-07

0 150

Feet

Facility Data

Area of Interest

Buildings

Potential PFAS Release

Facility Boundary

SD

Data Sources:

ESRI 2020

AECOM 2020

Hydrology

Groundwater Flow Direction

Surface Water Flow Direction

G:\ANG\a_MXD\RapidCity, SD\Analytical results figures\PFNA Detections in Soil.mxd - USDJ716516 - 10/31/2023 - 1:58:56 PM

Army National Guard Site Inspections

Site Inspection Report

Rapid City AASF, South Dakota

Date:...............SEPTEMBER 2022

Prepared By:.......................WOOD

Prepared For:.....................USACE

Projection:......NAD 83 State Plane

Shallow (0-2')*

AOI01-01

AOI01-02

AOI01-03

AOI01-04

AOI01-05

AOI01-07

0 150

Feet

Intermediate (5-8')

AOI01-01

AOI01-02

AOI01-03

AOI01-04

AOI01-05

AOI01-07

0 150

Feet

Deep (9-15')

NOTES:

PFNA = perfluorononanoic acid

ND = Non-Detect

(μg/Kg) = Microgram(s) per Kilogram

*At AOI01-03, surface samples were taken

at 5-6 feet bgs to account for known historic earthwork

Exceedances of the OSD SL are depicted

with a yellow halo.

PFNA Results

(ng/L)

ND

>ND - 19

>19 - 250

>250 - 2,500

>2,500
Site Inspection Report

Rapid City Army Aviation Support Facility, South Dakota

EA Engineering, Science, and Technology, Inc., PBC 6-22

This page intentionally left blank
Figure 6-6

PFOA, PFOS, and PFBS Detections in Groundwater

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AOI01-01

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AOI01-05B AOI01-06

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RCAASF-01

RCAASF-02

3

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Area of Interest

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Potential PFAS ReleaseFacility Boundary

Hydrology

Groundwater Flow DirectionSurface Water Flow Direction

G:\ANG\a_MXD\RapidCity, SD\Analytical results figures\PFOA_PFOS_PFBS Detections in GW.mxd - megan.cameron - 9/26/2022 - 3:33:43 PM

Army National Guard Site Inspections

Site Inspection Report

Rapid City AASF, South Dakota

PFOA

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AOI01-01

AOI01-02

AOI01-03

AOI01-04

AOI01-05B AOI01-06

AOI01-07

RCAASF-01

RCAASF-02

3

0 150

Feet

PFOS

NOPFO

PFOPFBND ng/Exc

PFOA Results (ng/L)

!( ND

!( >ND - 6

!!( >6 - 40

!!( >40 - 70

!!( >70

PFOS Results (ng

!( ND

!( >ND - 4

!!( >4 - 40

!!( >40 - 70

!!( >70
Site Inspection Report

Rapid City Army Aviation Support Facility, South Dakota

EA Engineering, Science, and Technology, Inc., PBC 6-24

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Figure 6-7

PFHxS, and PFNA Detections in Groundwater

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AOI01-01

AOI01-02

AOI01-03

AOI01-04

AOI01-05B AOI01-06

AOI01-07

RCAASF-01

RCAASF-02

3

0 150

Feet

Facility Data

Area of Interest

! ! ! !

!

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Potential PFAS ReleaseFacility Boundary

Hydrology

Groundwater Flow DirectionSurface Water Flow Direction

G:\ANG\a_MXD\RapidCity, SD\Analytical results figures\PFHxS_PFNA_ Detections in GW.mxd - megan.cameron - 9/26/2022 - 3:37:20 PM

Army National Guard Site Inspections

Site Inspection Report

Rapid City AASF, South Dakota

PFHxS

3

NOPFH

PFNND ng/Excwith 

PFHxS Results (ng/L)

!( ND

!( >ND - 39

!!( >39 - 100

!!( >100 - 1,000

!!( >1,000
Site Inspection Report

Rapid City Army Aviation Support Facility, South Dakota

EA Engineering, Science, and Technology, Inc., PBC 6-26

This page intentionally left blank
Site Inspection Report

Rapid City Army Aviation Support Facility, South Dakota Version: FINAL

EA Engineering, Science, and Technology, Inc., PBC 7-1

7. EXPOSURE PATHWAYS

The Conceptual Site Model (CSM) for the AOI, revised based on the SI findings, is presented on 

Figure 7-1. Please note that while the CSM discussion assists in determining if a receptor may 

be impacted, the decision to move from SI to RI or interim action is determined based upon 

exceedances of the SLs for the relevant compounds and whether the release is more than likely 

attributable to the DoD. A CSM presents the current understanding of the Facility 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 figures use an empty 

circle symbol to represent an incomplete exposure pathway. Areas with no identified complete 

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

complete if the relevant compounds 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 the relevant compounds above the SLs. Areas with an identified potentially 

complete pathway and a complete pathway may warrant further investigation. Although the 

CSMs indicate whether potentially complete exposure pathways may exist, the recommendation 

for future study in a RI or no action at this time is based on the comparison of the SI analytical 

results for the relevant compounds to the SLs.

In general, the potential routes of exposure to the relevant compounds 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 toxicological study. The receptors 

evaluated are consistent with those listed in EPA guidance for risk screening (EPA, 2001). 

Receptors at the Facility include site workers (e.g., facility staff and visiting soldiers), 

construction workers, potential trespasser, offsite resident, and recreational users outside the 

Facility boundary.

7.1 SOIL EXPOSURE PATHWAY

The SI results in soil were used to determine whether a potentially complete pathway exists 

between the source and potential receptors at the AOI based on the aforementioned criteria. 
Site Inspection Report

Rapid City Army Aviation Support Facility, South Dakota Version: FINAL

EA Engineering, Science, and Technology, Inc., PBC 7-2

7.1.1 AOI 1 

AOI 1 is the Main Hangar, area south of the Main Hangar and immediate surrounding area. The 

Main Hangar, constructed in 1999, was equipped with an AFFF fire suppression system, 

consisting of two 1,100-gallon tanks filled with 3 percent AFFF concentrate. While the fire 

suppression system is engaged, an automatic switch activates to route the AFFF/water mixture to 

a 3,000-gallon concrete bypass underground storage tank. The system was initially tested in 

December of 1999 and during this test, the large plastic container over-flowed and released a 

foam/water mixture spill onto the concrete. The contractor addressed the spill, but a small 

amount flowed onto the grassy area south of the Main Hangar. In Spring of 2016, a portion of the 

concrete and grassy area southwest of the Main Hangar was excavated to make the drainage 

ditch deeper to allow for better storm water drainage (AECOM, 2020). According to personnel, 

some of the soil that was removed from the area was used as fill material, during the construction 

of the Readiness Center in 2019, making the Readiness Center a potential secondary AOI 1 

source. 

PFNA, PFOA and PFOS were detected in surface soil at concentrations below their respective 

SLs in the two borings located at the original release area (AOI01-01 and AOI01-02). 

Additionally, PFOS was detected in surface soil at concentrations below the SLs in the 

anticipated flow path (AOI01-07). PFOS was also detected in surface soil at concentrations

below the SLs in two soil borings in the estimated location of the fill material south of the 

Readiness Center (AOI01-04 and AOI01-05). Site workers and construction workers could 

contact constituents in surface soil via incidental ingestion and inhalation of dust. Therefore, the 

surface soil exposure pathway for facility workers and construction workers are potentially 

complete. PFHxS, PFNA, PFOA and PFOS were detected in shallow subsurface soil at 

concentrations below their respective SLs. Construction workers could contact constituents in 

shallow subsurface soil via incidental ingestion and inhalation of dust; therefore, the subsurface 

soil exposure pathway for construction workers is potentially complete. The CSM is presented in 

Figure 7-1.

7.2 GROUNDWATER EXPOSURE PATHWAY

The SI results in groundwater were used to determine whether a potentially complete pathway 

exists between the source and potential receptors at the AOI based on the aforementioned 

criteria. 

7.2.1 AOI 1 

AOI 1 is the Main Hangar, the area south of the Main Hangar and the immediate surrounding 

area. The aforementioned release of PFAS to the ground surface occurred in December of 1999 

south of the Main Hangar. Relevant compounds were detected in the groundwater at all sampling 

locations. The concentration of PFHxS in groundwater exceeded the SL at four of the nine 

temporary well locations. The concentration of PFNA in groundwater exceeded the SL at two of 

the nine temporary well locations. The concentration of PFOA in groundwater exceeded the SL 

at eight of the nine temporary well locations. The concentration of PFOS in groundwater 

exceeded the SL at seven of the nine temporary well locations. Based on the results of the SI at 

AOI 1, ground disturbing activities that extend to the water table (approximately 12-15 ft bgs) 
Site Inspection Report

Rapid City Army Aviation Support Facility, South Dakota Version: FINAL

EA Engineering, Science, and Technology, Inc., PBC 7-3

could result in construction worker exposure to constituents in groundwater via incidental 

ingestion. Potential residential receptors downgradient of the AOI could also be exposed by 

ingestion of groundwater. The concentration at the potential point of exposure for off-site 

residents is not known, therefore, the exposure pathway for ingestion is potentially complete for 

off-site residential receptors. Drinking water for the Facility is supplied by the City of Rapid 

City; therefore, the pathway for ingestion of shallow groundwater by a site worker or trespasser 

is incomplete. 

The CSM is presented in Figure 7-1.

7.3 SURFACE WATER AND SEDIMENT EXPOSURE PATHWAY

There is no surface water located at the AASF, however, stormwater at the Facility is managed 

by a series of grassed trenches and small retention basins before being ultimately discharged to 

Rapid Creek. Stormwater has the potential to transport AFFF or PFAS-impacted soils to water 

bodies. No surface water or sediment samples were collected as part of the SI.

7.3.1 AOI 1 

AOI 1 is the Main Hangar, the area south of the Main Hangar and the immediate surrounding 

area. The aforementioned release of PFAS to the ground surface occurred in December of 1999 

south of the Main Hangar. Relevant compounds were detected in the surface soil at 

concentrations below SLs at sample locations associated with the original release area and 

anticipated flow path (see Section 7.1). Due to the presence of relevant compounds in soil, there 

is the potential for stormwater to transport PFAS-impacted soil to Rapid Creek and expose the 

potential recreational user by ingestion of surface water. The CSM is presented in Figure 7-1. 
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Notes:

1. The resident and recreational user refers to 

off-site receptors.

2. Inhalation of dust for off-site receptors is 

highly unlikely. Figure 7-1

Conceptual Site Model

Rapid City AASF, South 

RECEPTO

Human Receptors

Current/Future

Site 

Worker

Construction 

Worker2 Resident1,2 

AOI 1 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

Ingestion

Ingestion

Ingestion

Ingestion

Potential 

Off-Facility 

Source Not 

under 

Control of 

ARNG

Potential release 

due to overflow of 

container during fire 

suppression system 

testing.

SOURCE

Source Release Mechanism Media

PATHWAY

Transport

and Migration Media Exposure Routes

Inhalation 

of Dust

Site 

Worker

Construction 

Worker Resident1,2 

LEGEND

Flow Chart ContinuesIncomplete PathwayPotentially Complete PathwayPotentially Complete Pathway with Exceedance of SL
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Site Inspection Report

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EA Engineering, Science, and Technology, Inc., PBC 8-1

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 abstracted from information contained in this 

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

the SLs. 

8.1 SI ACTIVITIES

The SI field activities at the Facility were conducted from 15 to 18 November 2021. The SI field 

activities included soil and groundwater sampling. Field activities were conducted in accordance 

with the UFP-QAPP Addendum (EA/Wood, 2021a), except as previously noted in Section 5.8. 

To fulfill the project DQOs set forth in the approved SI UFP-QAPP Addendum (EA/Wood,

2021a), samples were collected and analyzed for a subset of 24 PFAS compounds by LC/MS/MS 

compliant with QSM 5.3 Table B-15 as follows.

• Eighteen (18) soil samples from six locations;

• Nine (9) grab groundwater samples from nine temporary well locations;

• Eleven (11) quality assurance (QA)/QC samples.

An SI is conducted when the PA determines an AOI exists based on probable use, storage, and/or 

disposal of PFAS-containing materials. The SI includes multi-media sampling at the AOI to 

determine whether or not a release has occurred. The SI may conclude further investigation is 

warranted, a removal action is required to address immediate threats, or no further action is 

required. Additionally, the CSM was refined to assess whether a potentially complete pathway 

exists between the source and potential receptors for potential exposure at the AOI, which is

described in Section 7.

8.2 OUTCOME

Based on the results of this SI, further evaluation in the form of a RI is warranted for AOI 1. 

Based on the CSM developed and revised based on the SI findings, there is potential for 

exposure to receptors from AOI 1 from sources on the Facility resulting from historical DoD 

activities.

Sample chemical analytical concentrations collected during the SI were compared against the 

project SLs in soil and groundwater, as described in Table 6-1. The following bullets summarize 

the SI results relative to the SLs: 

At AOI 1:

• Relevant compounds were detected in groundwater in the AOI 1 source area. The 

concentration of PFHxS in groundwater exceeded the SL at three of the seven temporary 

well locations, with a maximum concentration of 280 ng/L. The concentration of PFNA 

in groundwater exceeded the SL at two of the seven temporary well locations with a 

maximum concentration of 140 ng/L. The concentration of PFOA in groundwater 
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exceeded the SL at six of the seven temporary well locations with a maximum 

concentration of 1,300 J- ng/L. The concentration of PFOS in groundwater exceeded the 

SL at five of the seven temporary well locations with a maximum concentration of 880 

ng/L. PFBS did not exceed the SLs. Based on the results of the SI, further evaluation of 

AOI 1 is warranted in the RI. 

• Relevant compounds were detected in soil at AOI 1 at concentrations below their 

respective SLs. 

Upgradient of AOI 1: 

• PFHxS, PFOA, and PFOS were detected in groundwater upgradient of the Facility in 

sample RCAASF-01 and RCAASF-02 above their respective SLs. The concentration of 

PFHxS in groundwater exceeded the SL at both temporary well locations, with a 

maximum concentration of 430 ng/L. The concentration of PFOA in groundwater 

exceeded the SL at both temporary well locations with a maximum concentration of 89 Jng/L. The concentration of PFOS in groundwater exceeded the SL both temporary well 

locations with a maximum concentration of 310 ng/L. PFBS and PFNA were detected in 

groundwater samples from both temporary well locations but did not exceed the SLs. 

Based on the results of the SI, there may be potential off-facility sources upgradient of 

the Rapid City AASF not related to DoD activities that have affected the groundwater. 

• Soil was not sampled at the two upgradient locations. 

Of the six PFAS compounds presented in the 6 July 2022 OSD memorandum, HFPO-DA 

(commonly referred to as GenX) was not included as an analyte at the time of this SI. Based on 

the CSM developed during the PA and revised based on SI findings, the presence of HFPO-DA 

is not anticipated at the facility because HFPO-DA is generally not a component of MIL-SPEC 

AFFF and based on its history including distribution limitations that restricted use of GenX, it is 

generally not a component of other products the military used. In addition, it is unlikely that 

GenX would be an individual chemical of concern in the absence of other PFAS.

Table 8-1. Summary of Site Inspection Findings and Recommendations 

AOI

Potential Release 

Area

Soil – 

Source Area

Groundwater – 

Source Area

Groundwater – 

Facility Boundary Future Action

1 Overflow Area Proceed to RI 

Legend:

 = Detected; exceedance of screening levels

 = Detected; no exceedance of screening levels

 = Not detected
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