FINAL

Site Inspection Report 

Army Aviation Support Facility

Stockton, California

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) at ARNG Installations, 

Nationwide

November 2023

 

Prepared for:

Army National Guard Bureau

111 S. George Mason Drive

Arlington, VA 22204

UNCLASSIFIED
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The seal appearing on this document was authorized by Katharine Carr, P.G. 9315 on November 3, 2023, 

for the information contained herein for Stockton Army Aviation Support Facility. 
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Table of Contents

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

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

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

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

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

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

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

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

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

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..............................................................................................2-4

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

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

3.2 AOI 2 Wash Rack..................................................................................................3-1

3.3 AOI 3 Butler Building Parking Fire Training Area (FTA) .........................................3-1

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....................................................................................4-1

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

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

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

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

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

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

5.3 Temporary Well Installation and Groundwater Grab Sampling...............................5-3

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

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

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

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

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

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.........................................................6-2

6.3.3 AOI 1 Conclusions ......................................................................................6-3

6.4 AOI 2.....................................................................................................................6-3
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6.4.1 AOI 2 Soil Analytical Results .......................................................................6-3

6.4.2 AOI 2 Groundwater Analytical Results.........................................................6-3

6.4.3 AOI 2 Conclusions ......................................................................................6-4

6.5 AOI 3.....................................................................................................................6-4

6.5.1 AOI 3 Soil Analytical Results .......................................................................6-4

6.5.2 AOI 3 Groundwater Analytical Results.........................................................6-4

6.5.3 AOI 3 Conclusions ......................................................................................6-5

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

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

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

7.1.2 AOI 2...........................................................................................................7-2

7.1.3 AOI 3...........................................................................................................7-2

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

7.2.1 AOI 1...........................................................................................................7-3

7.2.2 AOI 2...........................................................................................................7-3

7.2.3 AOI 3...........................................................................................................7-3

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

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

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

9. References...................................................................................................................9-1
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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. Nonconformance and Corrective Action Report

Appendix C Photographic Log

Appendix D TPP Meeting Minutes

Appendix E Boring Logs

Appendix F Analytical Results

Appendix G Laboratory Reports

Figures

Figure 2-1 Facility Location

Figure 2-2 Facility Topography

Figure 2-3 Groundwater Features

Figure 2-4 Groundwater Elevations, July 2021

Figure 2-5 Surface Water Features

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 PFOA, PFOS, and PFBS Detections in Groundwater

Figure 6-7 PFHxS and PFNA Detections in Groundwater

Figure 7-1 Conceptual Site Model, AOI 1

Figure 7-2 Conceptual Site Model, AOI 2

Figure 7-3 Conceptual Site Model, AOI 3

Tables

Table ES-1 Screening Levels (Soil and Groundwater)

Table ES-2 Summary of Site Inspection Findings and Recommendations

Table 5-1 Site Inspection Samples by Medium

Table 5-2 Soil Boring Depths, Temporary Well Screen Intervals, and Groundwater 

Elevations

Table 6-1 Screening Levels (Soil and Groundwater)

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

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

Table 6-4 PFOA, PFOS, PFBS, PFNA, and PFHxS Results in Groundwater

Table 8-1 Summary of Site Inspection Findings and Recommendations
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Acronyms and Abbreviations

% percent

°C degrees Celsius

°F degrees Fahrenheit

μg/kg micrograms per kilogram

AASF Army Aviation Support Facility

AECOM AECOM Technical Services, Inc.

AFFF aqueous film-forming foam

amsl above mean sea level

AOI Area of Interest

ARNG Army National Guard

bgs below ground surface 

CAARNG California Army National Guard

CERCLA Comprehensive Environmental Response, Compensation, and Liability Act 

CoC chain of custody

CSM conceptual site model

CSMS Combined Support Maintenance Shop

DA Department of the Army

DoD Department of Defense

DPT direct push technology

DQO data quality objective

DUA data usability assessment

ECM Environmental Cost Management, Inc.

EA EA Engineering, Science, and Technology, Inc.

EDRTM Environmental Data Resources, Inc.TM

ELAP Environmental Laboratory Accreditation Program

EM Engineer Manual

FedEx Federal Express

FEMA Federal Emergency Management Agency

FMS Field Maintenance Shop

FTA Fire Training Area

GRPS Ground Penetrating Radar Systems

HAZMAT hazardous materials

HDPE high-density polyethylene 

HFPO-DA hexafluoropropylene oxide dimer acid

IDW investigation-derived waste

ITRC Interstate Technology Regulatory Council

LC/MS/MS liquid chromatography with tandem mass spectrometry

MIL-SPEC military specification

ND non-detect

NELAP National Environmental Laboratory Accreditation Program

ng/L nanograms per liter

NOAA National Oceanic and Atmospheric Administration

OSD Office of the Secretary of Defense
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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

PQAPP Programmatic UFP-QAPP

PVC polyvinyl chloride

QA quality assurance

QAPP Quality Assurance Project Plan

QC quality control

QSM Quality Systems Manual

RWQCB Regional Water Quality Control Board

SI Site Inspection

SL screening level

SOP standard operating procedure

TOC total organic carbon

TPP Technical Project Planning

UFP Uniform Federal Policy

US United States

USACE United States Army Corps of Engineers

USCS Unified Soil Classification System

USEPA United States Environmental Protection Agency

USFWS United States Fish and Wildlife Service
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Executive Summary

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

Inspections (SIs) on the current or potential historical use of per- and polyfluoroalkyl substances 

(PFAS) with a focus on the six compounds presented in the memorandum from the Office of the 

Secretary of Defense (OSD) dated 6 July 2022 (Assistant Secretary of Defense, 2022). The six 

compounds listed in the OSD memorandum include perfluorooctanesulfonic acid (PFOS), 

perfluorooctanoic acid (PFOA), perfluorononanoic acid (PFNA), perfluorohexanesulfonic acid 

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

, and perfluorobutanesulfonic acid 

(PFBS). These compounds are collectively referred to as “relevant compounds” throughout the 

document and the applicable screening levels (SLs) are provided in Table ES-1. 

The PA identified three Areas of Interest (AOIs) where PFAS-containing materials may have been 

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

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

compounds. This SI was completed at the Stockton Army Aviation Support Facility (AASF) in 

Stockton, California and determined further evaluation under the Comprehensive Environmental 

Response, Compensation, and Liability Act (CERCLA) is warranted for AOI 1and AOI 3; no further 

evaluation is warranted for AOI 2 at this time. The Stockton AASF will also be referred to as the 

“facility” throughout this document.

Stockton AASF is located at 2000 Stimson Road, Stockton, California 95206 and borders the 

Stockton Metropolitan Airport to the south. The Stockton Metropolitan Airport has a history of 

military use as a training installation and has been formerly referred to as Stockton Field and 

Sharpe Army Depot Field Annex. In 1973, the US Army vacated the airport; however, the 

California ARNG (CAARNG) presence remains. Stockton AASF is a CAARNG aviation 

maintenance facility for helicopters that has been leased from San Joaquin County since 1972 

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

The PA identified three AOIs for investigation during the SI phase. SI sampling results from the 

three AOIs were compared to OSD SLs. Table ES-2 summarizes the SI results for each AOI. 

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

Investigation (RI) for AOI 1 and AOI 3; no further evaluation is warranted for AOI 2 at this time.

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)

Analyteb

Residential

(Soil)

(μg/kg)a 

0-2 feet bgs

Industrial/ Commercial 

Composite Worker

(Soil)

(μg/kg)a 

2-15 feet bgs

Tap Water

(Groundwater)

(ng/L)a

PFOA 19 250 6

PFOS 13 160 4 

PFBS 1,900 25,000 601 

PFHxS 130 1,600 39

PFNA 19 250 6

Notes:

bgs = below ground surface; μg/kg = micrograms per kilogram; ng/L = nanograms per liter

a.) Assistant Secretary of Defense, 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. 6 July 2022. 

b.) 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 HFPODA 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 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 Airfield Proceed to RI 

2 Wash Rack No further action

at this time

3 Butler 

Building 

Parking FTA

Proceed to RI

Legend:

N/A = not applicable 

= detected; exceedance of the screening levels

= detected; no exceedance of the screening levels

= not detected
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1. Introduction

1.1 Project Authorization

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

(PAs) and Site Inspections (SIs) on the current or potential historical use of per- and 

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

memorandum from the Office of the Secretary of Defense (OSD) dated 6 July 2022 (Assistant 

Secretary of Defense, 2022). The six compounds listed in the OSD memorandum will be referred 

to as “relevant compounds” throughout this document and include perfluorooctanoic acid (PFOA), 

perfluorooctanesulfonic acid (PFOS), perfluorohexanesulfonic acid (PFHxS), perfluorononanoic 

acid (PFNA), hexafluoropropylene oxide dimer acid (HFPO-DA)1

, and perfluorobutanesulfonic 

acid (PFBS) at ARNG facilities nationwide. The ARNG performed this SI at the Stockton Army 

Aviation Support Facility (AASF) in Stockton, California. The Stockton AASF is also referred to as 

the “facility” throughout this document.

The SI project elements were performed in compliance with Comprehensive Environmental 

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

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

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

Department of the Army (DA) requirements and guidance for field investigations.

1.2 SI Purpose

A PA was performed at Stockton AASF (AECOM Technical Services, Inc. [AECOM], 2020) that

identified three Areas of Interest (AOIs) 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 AOIs 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

Stockton AASF is located at 2000 Stimson Road, Stockton, California 95206 and borders the 

Stockton Metropolitan Airport to the south. The Stockton Metropolitan Airport has a history of 

military use as a training installation and has been formerly referred to as Stockton Field and 

Sharpe Army Depot Field Annex. In 1973, the US Army vacated the airport; however, the 

California ARNG (CAARNG) presence remains. The AASF is situated in the Sacramento-San 

Joaquin Valley, within the Central Valley, approximately equidistant from the Pacific Ocean and 

Sierra Nevada Mountains (Figure 2-1). The geographic coordinates and surface elevation at the 

main gate of the AASF are 37°53’13.0” N; 121°14’37.2” W and 21 feet above mean sea level 

(amsl), respectively. 

Stockton AASF is a CAARNG aviation maintenance facility for helicopters. Impervious surfaces 

make up 82 percent (%) of the 22-acre facility and include roadways, parking lots, helipads, and 

taxi lanes. Physical structures include a hangar, maintenance shop, fuel tank farm, wash rack, 

and equipment storage areas. Stockton AASF has been leased from San Joaquin County since 

1972 (AECOM, 2020). 

Four state-owned divisional areas comprise the entire CAARNG facility, including the AASF, Field 

Maintenance Shop (FMS) #24, Armory, and Combined Support Maintenance Shop (CSMS). 

However, only the AASF was specifically evaluated for this SI. FMS #24 is a one-story 

maintenance building and the site of two former underground storage tanks. FMS #24 lies 

adjacent to the southwest of the AASF and has been extensively investigated as a source of 

petroleum hydrocarbons and related compounds in soil and groundwater (Adanta- Environmental 

Cost Management, Inc. [ECM] Joint Venture, 2018).

2.2 Facility Environmental Setting

The areas surrounding Stockton AASF comprise light and heavy industrial areas directly 

north/northwest and farmlands to the east, south, and west (Figure 2-2). Few residents reside in 

the rural farmlands within a 1-mile radius of the AASF, although the community of French Camp, 

California is located approximately 1.5 miles to the west. The facility sits at an average elevation 

of 26 feet amsl with a slight general topographic gradient to the west. There are no significant 

natural topographic features surrounding the facility.

2.2.1 Geology

Stockton AASF lies in the border of the Sacramento Valley and San Joaquin Valley. Deposits 

found within this region are composed of unconsolidated Quaternary sediments made up of 

alluvial and lake deposits. The Sierra Nevada Mountains to the east and the Sacramento Valley 

Coast Ranges to the west of the AASF are both contributing sources of deposits that originate 

from a variety of metamorphic and sedimentary rocks (Adanta-ECM Joint Venture, 2018). The 

uppermost geologic unit comprises marine and nonmarine (continental) sedimentary rocks 

(Figure 2-3). The Stockton Fault (trending northeast-southwest), forms the divide between the 

Sacramento and the San Joaquin Valleys, is located within Stockton city limits; the Midland Fault 

Zone (trending north) is located approximately 25 miles northwest of Stockton AASF (Oneida Total 

Integrated Enterprises [OTIE], 2010). 

During the SI, low to medium plasticity fines (silts) were observed as the dominant lithology of the 

unconsolidated sediments below the Stockton AASF. The borings were completed at depths 

between 45 and 49 feet below ground surface (bgs). Varying quantities of sand were noted, 
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specifically, isolated layers of silty sand, sandy silt, poorly graded sand with silt, and poorly graded 

sand were also observed in the borings with thicknesses ranging from a few inches to 11 feet. 

These observations are consistent with the reported fluvial depositional environment of the region.

2.2.2 Hydrogeology

Stockton AASF is located within the San Joaquin Groundwater Basin, which contains several 

water-bearing zones within the upper 700 to 800 feet of unconsolidated valley fill. Shallow 

groundwater primarily occurs in unconfined conditions, although it may occur in semi-confined 

conditions due to the distribution of clay lenses in the sediments. Based on historical 

investigations performed in the vicinity of the AASF, coarser sediments are generally 

discontinuous in the lateral and vertical direction above the groundwater table and more laterally 

continuous below the groundwater table (Versar, Inc., 2004; URS Corporation, 2007; OTIE, 2010, 

2012, 2013; ECM, 2015). 

According to the California Water Board Groundwater Ambient Monitoring and Assessment 

Program database, multiple public water system wells are located within a 4-mile radius (Figure 

2-3). A public water system well is defined as serving “15 or more connections or more than 25 

people per day” (California Water Board, 2019). Stockton AASF receives potable water from the 

Stockton Municipal Utilities Water Service, which derives approximately 25% of its water supply 

from groundwater wells, while the remaining water supply is from treated surface water supplied 

by the Stockton East Water District. The locations of Stockton Municipal Utilities Water Service 

wells are unknown. The Stockton East Water District also supplies surface water for agricultural 

irrigation in the area. The Stockton East Water District sources surface water from the New 

Melones Reservoir and the New Hogan Reservoir, located approximately 30 and 38 miles 

northeast of the AASF, respectively (City of Stockton, 2019; Stockton East Water District, 2019). 

According to the Stockton 2020 Water Quality Report, PFOS was measured by Stockton 

Municipal Utilities Water Service at concentrations ranging from non-detect (ND) to 7.0 

nanograms per liter (ng/L) (California Water Service, 2022). 

Depths to water measured in July 2021 during the SI ranged from 37.27 to 42.23 feet bgs. 

Groundwater elevation contours from the SI are presented on Figure 2-4 and indicate the 

groundwater flow direction at the AASF is primarily to the northeast, with local groundwater flow 

in the vicinity of the wash rack potentially to the southwest.

2.2.3 Hydrology

The nearest surface water feature to the facility is Rydberg Creek, which wraps around the 

southern and western boundary of the Stockton AASF facility, flows northwest, and connects with 

French Camp Slough. According to the National Wetlands Inventory, there are no wetland areas 

within the AASF property (Environmental Data Resources, Inc. [EDR]TM, 2019). The western 

portion of the AASF is in the Federal Emergency Management Agency (FEMA) 100-year flood 

zone. 

Stockton AASF lies within the Walker Slough-French Camp Slough Watershed. Storm water is 

diverted to storm drains inlets that are located on and around the AASF. The storm drains then 

discharge offsite through two discharge points into the City of Stockton storm sewer system, which 

eventually leads to the San Joaquin River. The western discharge point, however, has not been 

observed to have any flow. Surface water that is not captured in storm drains is drained south into 

an adjoining ditch canal (Rydberg Creek), which is a tributary to the San Joaquin River. The 

closest surface water intake is along the San Joaquin River, approximately 18 miles to the 

northwest. Based on Third Unregulated Contaminant Monitoring Rule (UCMR3) data, there were 

no detections of PFAS in surface water intakes for the cities of Stockton and Lathrop. The AASF 

has no water treatment system, and the oil water separator associated with the wash rack is 
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connected to the Stockton sanitary sewer system (HazCon, 2017). Surface water features are 

presented on Figure 2-5. 

2.2.4 Climate 

Stockton AASF is in a semi-arid, Mediterranean climate zone characterized by warm, dry 

summers and mild winters. The average annual rainfall is approximately 13.45 inches, with the 

majority of the rainfall occurring between late fall and early spring. Summer temperatures peak in 

July, with an average high of 93 degrees Fahrenheit (°F) and an average low of 60 °F. Winter 

temperatures are lowest in December to January and range from 53 °F during the day to 37 °F at 

night. Prevailing wind speeds are westerly or northwesterly for 9 months out of the year and 

southeasterly for 3 months out of the year (National Oceanic and Atmospheric Administration 

[NOAA], 2019.). 

2.2.5 Current and Future Land Use

The Stockton AASF serves as a CAARNG aviation maintenance facility for helicopters. The AASF 

includes a maintenance hangar, various storage buildings, and related infrastructure including 

parking lots, aircraft parking areas, wash rack, and refueling pads. The AASF is entirely fenced 

with restricted site access. Reasonably anticipated future land use is not expected to change from 

the current land use described above. 

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 amphibians, birds, crustaceans, fishes, plants, insects, mammals, and reptiles are 

federally endangered, threatened, proposed, and/ or are listed as candidate species in San 

Joaquin County, California (US Fish and Wildlife Service [USFWS], 2022). 

• Amphibians: California tiger Salamander, Ambystoma californiense (threatened);

California red-legged frog, Rana draytonii (threatened)

• Birds: Yellow-billed Cuckoo, Coccyzus americanus (threatened); Least Bell's vireo, Vireo

bellii pusillus (endangered); California clapper rail, Rallus longirostris obsoletus

(endangered)

• Crustaceans: Longhorn fairy shrimp, Branchinecta longiantenna (endangered); Vernal pool

fairy shrimp, Branchinecta lynchi (threatened); Conservancy fairy shrimp, Branchinecta

conservation (endangered); Vernal pool tadpole shrimp, Lepidurus packardi (endangered)

• Fishes: Delta smelt, Hypomesus transpacificus (threatened); longfin smelt, Spirinchus

thaleichthys (candidate)

• Flowering Plants: Greene's tuctoria, Tuctoria greenei (endangered); Sacramento Orcutt

grass, Orcuttia viscida (endangered); Ione manzanita, Arctostaphylos myrtifolia

(threatened); Large-flowered fiddleneck, Amsinckia grandiflora (endangered); Palmatebracted bird's beak, Cordylanthus palmatus (endangered); Fleshy owl's-clover, Castilleja

campestris ssp. succulenta (threatened)

• Insects: Monarch butterfly, Danaus plexippus (candidate); Valley elderberry longhorn

beetle, Desmocerus californicus dimorphus (threatened)
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• Mammals: San Joaquin kit fox, Vulpes macrotis mutica (endangered); Riparian brush 

rabbit, Sylvilagus bachmani riparius (endangered); Riparian woodrat (San Joaquin Valley), 

Neotoma fuscipes riparia (endangered)

• Reptiles: Giant garter snake, Thamnophis gigas (threatened); Alameda whipsnake (striped 

racer), Masticophis lateralis euryxanthus (threatened)

2.3 History of PFAS Use

Three AOIs were identified in the PA where AFFF may have been used, stored, disposed, or 

released historically at the Stockton AASF (AECOM, 2020). AFFF may have historically been 

released at the facility during familiarization training and fire training activities as early as 1992. 

Additional AFFF releases may also have occurred from incidental spills in the wash rack area. 

The potential release areas were grouped into three AOIs based on preliminary data and 

presumed groundwater flow directions. A description of each AOI is presented in Section 3. 
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12420 Milestone Center Drive

Germantown, MD 20876

Facility Location

Base Map: Sources: Esri, HERE, Garmin, USGS,

Intermap, INCREMENT P, NRCan, Esri Japan, METI,

0 0.5 1 2

Miles

^_

Legend

Facility Boundary

Stockton AASF

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Army Aviation Support Facility, Stockton, California

AECOM 3-1

3. Summary of Areas of Interest 

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

disposed, or released historically. Based on the PA findings, three potential release areas were 

identified at Stockton AASF and grouped into three AOIs (AECOM, 2020). The potential release 

areas are shown on Figure 3-1.

3.1 AOI 1 Airfield

AOI 1 is the airfield where controlled AFFF releases through familiarization training have occurred 

annually potentially as early as 1992 and then less frequently after 2010. The footprint of AOI 1 

also includes a former wash rack shown on historical figures as previously located south of the 

southwest corner of the airfield. There are no known releases at this former wash rack. 

3.2 AOI 2 Wash Rack

AOI 2 is the wash rack area and includes the hazardous materials (HAZMAT) locker with AFFF 

storage, located at the southwest corner of the wash rack. Controlled AFFF releases to the wash 

rack through activities related to fire training and familiarization training have occurred annually, 

potentially as early as 1992, and then less frequently after 2010. Potential AFFF releases from 

incidental spills in the wash rack area may have also occurred.

The wash rack drains lead to an oil water separator that connects to the Stockton sanitary sewer 

system. Therefore, discharges of AFFF in the wash rack would release directly into the sanitary 

sewer system; however, the HAZMAT locker is located near unpaved, grassy areas. Potential 

releases nearby the HAZMAT locker would drain via overland surface flow to the adjacent ditch 

canal (Rydberg Creek), which is a tributary to the San Joaquin River. 

3.3 AOI 3 Butler Building Parking Fire Training Area (FTA)

AOI 3 is a former FTA at what is now the parking area located east of the Butler Building. 

Controlled AFFF releases through fire training activities had occurred annually, potentially as early 

as 1992, until approximately 2010 to 2012, when the Butler Building was constructed. 

Ground-disturbing activities at AOI 3 have occurred. During the construction of the Butler Building, 

the soil beneath and in the vicinity of the parking lot was disturbed, and the area was regraded. 

The original soil was either left in place or used to fill in a depression west of the road next to the 

Butler Building. 
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Site Inspection Report

Army Aviation Support Facility, Stockton, California

AECOM 4-1

4. Project Data Quality Objectives

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

Assurance Project Plan (QAPP) Addendum (AECOM, 2021a), the objective of the SI is to identify 

whether there has been a release to the environment at the AOIs 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 presence or absence of relevant compounds at each of the sampled 

AOIs.

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 included:

• The PA for Stockton AASF (AECOM, 2020);

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

with the site-specific Uniform Federal Policy (UFP)-QAPP Addendum (AECOM, 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 by the property limits of the facility (Figure 2-2). Off-facility sampling 

was not included in the scope of this SI. If future off-facility sampling is required, the proper 

stakeholders will be notified, and necessary rights of entry will be obtained by ARNG with property 

owner(s). The SI scope was bounded vertically by the observed depths of the surficial groundwater 

table. Temporal boundaries were limited to the summer season, which was the earliest available time 

field resources were available to complete the study.

4.4 Analytical Approach

Samples were analyzed by Pace Analytical Gulf Coast, accredited under the Department of 

Defense (DoD) Environmental Laboratory Accreditation Program (ELAP; Accreditation Number 

74960) and the National Environmental Laboratory Accreditation Program (NELAP; Certificate

Number 01955). Data were compared to applicable SLs within this document and decision rules 

as defined in the SI QAPP Addendum (AECOM, 2021a).

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 
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AECOM 4-2

whether the collected data are of the right type, quality, and quantity to support the decisionmaking (DoD, 2019a; DoD, 2019b; USEPA, 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 SI QAPP Addendum (AECOM, 2021a). 
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AECOM 5-1

5. Site Inspection Activities

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

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

in accordance with the following approved documents:

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

(PQAPP) dated March 2018 (AECOM, 2018a);

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

• Final Preliminary Assessment Report, Stockton Army Aviation Support Facility, Stockton 

dated January 2020 (AECOM, 2020);

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

Army Aviation Support Facility, Stockton, California dated June 2021 (AECOM, 2021a); and

• Final Site Safety and Health Plan, Army Aviation Support Facility, Stockton, California dated 

June 2021 (AECOM, 2021b).

The SI field activities were conducted from 12 to 23 July 2021 and consisted of utility clearance,

direct push boring, soil sample collection, temporary monitoring well installation, grab groundwater 

sample collection, and land surveying. Field activities were conducted in accordance with the SI 

QAPP Addendum (AECOM, 2021a).

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

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

Systems Manual (QSM) 5.3 Table B-15 to fulfill the project DQOs:

• Thirty-six (36) soil samples from 16 boring locations;

• Ten (10) grab groundwater samples from 10 temporary well locations; 

• Nineteen (19) quality assurance (QA)/quality control (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 media. Field documentation is provided in Appendix B. A Log 

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

in Appendix B1. Sampling forms are provided in Appendix B2, land survey data are provided in 

Appendix B3, and Nonconformance and Corrective Action Reports are provided in Appendix 

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

5.1 Pre-Investigation Activities

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

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

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

5.1.1 Technical Project Planning

The US Army Corps of Engineers (USACE) TPP Process, Engineer Manual (EM) 200-1-2 

(USACE, 2016) defines four phases to project planning: 1.) defining the project phase; 2.) 

determining data needs; 3.) developing data collection strategies; and 4.) finalizing the data 

collection plan. The process encourages stakeholder involvement in the SI, beginning with 

defining overall project objectives, including DQOs, and formulating a sampling approach to 

address the AOIs identified in the PA. 
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AECOM 5-2

A combined TPP Meeting 1 and 2 was held on 24 February 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 include the ARNG, CAARNG, USACE, and California Regional Water 

Quality Control Board (RWQCB). Stakeholders were provided the opportunity to make comments 

on the technical sampling approach and methods at the combined TPP Meeting 1 and 2. The 

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

(AECOM, 2021a).

A TPP Meeting 3 was held on 27 June 2023 to discuss the results of the SI. Meeting minutes for 

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

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

5.1.2 Utility Clearance

AECOM’s drilling subcontractor, Cascade Technical Services, LLC. placed a ticket with the USA 

North 811 “Call Before You Dig” California utility clearance provider to notify them of intrusive work 

on 6 July 2021. Additionally, AECOM contracted Ground Penetrating Radar Systems (GPRS), a 

private utility location service, to perform utility clearance. GPRS performed utility clearance of 

the proposed boring locations on 12 July 2021 with input from the AECOM field team and Stockton

AASF facility staff. General locating services and ground-penetrating radar were used to complete 

the clearance. Additionally, the first 5 feet of each boring were pre-cleared using a hand auger to 

verify utility clearance in shallow subsurface where utilities would typically be encountered.

5.1.3 Source Water and Sampling Equipment Acceptability

Two potable water sources at Stockton AASF were sampled on 25 May 2021 to assess usability 

for decontamination of drilling equipment. Results of the sample collected at the wash rack spigot 

(S-DECON) confirmed this source to be acceptable for use in this investigation; therefore, it was 

used throughout the field activities. Specifically, the samples were analyzed for PFAS by 

LC/MS/MS compliant with QSM 5.3 Table B-15. The results of the decontamination water sample 

associated with the wash rack spigot source used 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 

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

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

(AECOM, 2021a). Prior to the start of field work each day, a Sampling Checklist was completed 

as an additional layer of control. The checklist served as a daily reminder to each field team 

member regarding the allowable materials within the sampling environment.

5.2 Soil Borings and Soil Sampling

Borings were installed in grass areas, where applicable, to avoid disturbing concrete or asphalt 

surfaces. Soil samples were collected via direct push technology (DPT), in accordance with the 

SI QAPP Addendum (AECOM, 2021a). A GeoProbe® 3126GT 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 feet of the boring, in accordance with AECOM utility clearance procedures. Surface 

soil samples collected with a hand auger between 0 and 2 feet bgs were backfilled with the parent 

soil that was removed from the hand auger. The soil boring locations are shown on Figure 5-1,

and depths are provided 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.
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AECOM 5-3

In general, three discrete soil samples were collected from the vadose zone for chemical analysis 

from each soil boring: one surface soil sample (0 to 2 feet bgs), one subsurface soil sample 

approximately 2 feet above the groundwater table, and one subsurface soil sample at the midpoint between the surface and the groundwater table. 

The soil cores were continuously logged for lithological descriptions by an AECOM field geologist 

using the Unified Soil Classification System (USCS). A photoionization detector (PID) was used 

to screen the breathing zone during boring activities as part of personal safety requirements. 

Observations and measurements were recorded on sampling forms (Appendix B2) and in a nontreated field logbook (i.e., composition notebook). Depth interval, recovery thickness, PID 

concentrations, moisture, relative density, color (using a Munsell soil color chart), and texture 

(using the USCS) were recorded. The boring logs are provided in Appendix E.

Soil borings completed during the SI found low to medium plasticity fines with varying levels of 

sand as the dominant lithology of the unconsolidated sediments below the Stockton AASF. The 

borings were completed at depths between 45 and 49 feet bgs. Isolated layers of silty sand, sandy 

silt, poorly graded sand with silt, and poorly graded sand were also observed in the boring logs at 

thicknesses ranging from a few inches to 11 feet. These observations are consistent with the 

understood fluvial depositional environment of the region.

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

(HDPE) bottles and labeled using a PFAS-free marker or pen. Samples were packaged on ice 

and transported via Federal Express (FedEx) under standard chain of custody (CoC) procedures 

to the laboratory and analyzed by LC/MS/MS compliant with QSM 5.3 Table B-15, total organic 

carbon (TOC) (USEPA Method 9060A) and pH (USEPA Method 9045D).

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

as the accompanying samples. Matrix spike (MS)/MS duplicates (MSDs) were collected at a rate 

of 5% and analyzed for the same parameters as the accompanying samples. In instances when 

non-dedicated sampling equipment was used, such as a hand auger for the shallow soil samples, 

equipment rinsate blanks were collected at a rate of 5% and analyzed for the same parameters 

as the soil samples. A temperature blank was placed in each cooler to ensure that samples were 

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

5.3 Temporary Well Installation and Groundwater Grab Sampling

DPT borings were converted to temporary wells. Temporary wells were installed using a 

GeoProbe® 3126GT dual-tube sampling system. Once the borehole was advanced to the desired 

depth, a temporary well was constructed of a 5-foot section of 1-inch Schedule 40 poly-vinyl 

chloride (PVC) screen with sufficient casing to reach ground surface. New PVC pipe and screen 

were used 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 bladder 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. 

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.
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AECOM 5-4

Each sample was collected into laboratory-supplied PFAS-free HDPE bottles and labeled using 

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

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

QSM 5.3 Table B-15 in accordance with the SI QAPP Addendum (AECOM, 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. One field reagent blank was collected in 

accordance with the PQAPP (AECOM, 2018a). A temperature blank was placed in each cooler to 

ensure that samples were preserved at or below 6°C during shipment.

Following well surveying (described below in Section 5.5), temporary wells were abandoned in 

accordance with the SI QAPP Addendum (AECOM, 2021a) by removing the PVC and backfilling 

the hole with neat cement grout. Upon completion of well abandonment, the ground surface at 

each location was patched to match existing surrounding conditions. A San Joaquin County Grout 

Inspector was onsite to inspect the grout and observe the decommissioning of the temporary 

monitoring wells.

5.4 Synoptic Water Level Measurements

Groundwater elevation measurements were collected from the ten new temporary monitoring

wells prior to groundwater purging and sampling. Water level measurements were taken from the 

northern side of the well casing. A groundwater flow contour map is provided in Figure 2-4.

Groundwater elevation data are provided in Table 5-2.

5.5 Surveying

The northern side of each well casing was surveyed by California-licensed land surveyors 

following guidelines provided in the SOPs provided in the SI QAPP Addendum (AECOM, 2021a). 

Survey data from the newly installed wells on the facility were collected on 22 July 2021 in the 

applicable Universal Transverse Mercator zone projection with California Coordinate System of 

1983 (CCS83) datum (horizontal) and North American Vertical Datum 1988 (vertical). Top of 

casing elevation and ground surface elevation were surveyed at each well. The surveyed well 

data are provided in Appendix B3.

5.6 Investigation-Derived Waste

As of the date of this report, the disposal of 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 SI QAPP Addendum (AECOM, 2019b) and with the DA Guidance for 

Addressing Releases of PFAS, Q18 (DA, 2018).

Soil IDW (i.e., soil cuttings) generated during the SI activities were contained in labeled, 55-gallon 

Department of Transportation (DOT)-approved steel drums and left onsite in a designated waste 

storage area. The soil IDW was not sampled and assumes the characteristics of the associated 

soil samples collected from that source location. 

Liquid IDW generated during SI activities (i.e., purge water, development water, and 

decontamination fluids) were contained in labeled, 55-gallon DOT-approved steel drums, and left 

onsite in a designated waste storage area. The liquid IDW was not sampled and assumes the 

characteristics of the associated groundwater samples collected from that source location.
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Management and disposal of containerized IDW is being handled by Engineering, Science, and 

Technology, Inc, PCB (EA) under a separate contract with USACE in accordance with SOP No. 

042A (EA, 2021). 

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 5.3 Table B-15 at Pace Analytical Gulf 

Coast in Baton Rouge, Louisiana, a DoD ELAP and NELAP certified laboratory. Soil samples 

were also analyzed for TOC using USEPA Method 9060A and pH by USEPA Method 9045D.

5.8 Deviations from SI QAPP Addendum

Three deviations from the SI QAPP Addendum were identified during review of the field 

documentation. 

• During DPT drilling activities, subsurface soil samples were collected from the mid-points of 

borings below 15 feet bgs, based on the total boring depths. The approved SI QAPP 

Addendum states that mid-point subsurface soil samples would be collected from 13 to 15 

feet bgs if total boring depth exceeded 30 feet bgs. The total boring depths at all boring 

locations exceeded 30 feet bgs, and the mid-point samples were inadvertently collected at 

depths greater than 15 feet bgs. Shallow subsurface soil samples collected at depths 

ranging from 18 to 22 feet bgs were therefore not compared to the industrial/commercial 

worker scenario SLs to provide a conservative assessment of that potential exposure route. 

These actions were documented in a nonconformance report dated July 2022 and are 

provided in Appendix B4. 

• AOI03-02 was offset by 110 feet southwest of the proposed boring location for utility 

avoidance and drill rig access purposes. 

• As noted in the field log, grab groundwater samples were initially collected on 16 July 2022 

without field readings. These samples were not sent to the laboratory for analysis. 

Groundwater samples and field parameters were collected on 21 and 22 July 2022 and 

samples were sent to the laboratory for analysis.
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Table 5-1

Site Inspection Samples by Medium

Site Inspection Report, Stockton AASF, California

Sample Identification

Sample

Collection

Date/Time

Sample Depth

(feet bgs)

LC/MS/MS compliant withQSM 5.3 Table B-15TOC(USEPA Method 9060A)pH(USEPA Method 9045D)

Comments

AOI01-01-SB-0-2 7/16/2021 9:15 0-2 x

AOI01-01-SB-20-21 7/16/2021 10:50 20-21 x

AOI01-01-SB-40-41 7/16/2021 10:45 40-41 x

AOI01-02-SB-0-2 7/16/2021 12:55 0-2 x

AOI01-02-SB-0-2-D 7/16/2021 12:55 0-2 x FD

AOI01-02-SB-20-21 7/16/2021 14:40 20-21 x

AOI01-02-SB-42-43 7/16/2021 14:35 42-43 x

AOI01-03-SB-0-2 7/19/2021 8:45 0-2 x

AOI01-03-SB-0-2-MS 7/19/2021 8:45 0-2 x MS

AOI01-03-SB-0-2-MSD 7/19/2021 8:45 0-2 x MSD

AOI01-03-SB-20-21 7/19/2021 10:55 20-21 x

AOI01-03-SB-41-42 7/19/2021 11:00 41-42 x

AOI01-04-SB-0-2 7/19/2021 11:20 0-2 x

AOI01-04-SB-21-22 7/19/2021 13:50 21-22 x

AOI01-04-SB-43-44 7/19/2021 13:55 43-44 x

AOI01-05-SB-0-2 7/19/2021 14:45 0-2 x

AOI01-05-SB-19-20 7/20/2021 8:05 19-20 x

AOI01-05-SB-39-40 7/20/2021 8:08 39-40 x

AOI01-06-SB-0-2 7/22/2021 7:25 0-2 x x x

AOI01-06-SB-0-2-MS 7/22/2021 7:25 0-2 x x MS

AOI01-06-SB-0-2-MSD 7/22/2021 7:25 0-2 x x MSD

AOI01-07-SB-0-2 7/22/2021 7:45 0-2 x

AOI01-07-SB-0-2-D 7/22/2021 7:45 0-2 x FD

AOI01-08-SB-0-2 7/22/2021 7:55 0-2 x

AOI01-09-SB-0-2 7/22/2021 8:10 0-2 x

AOI01-09-SB-0-2-MS 7/22/2021 8:10 0-2 x MS

AOI01-09-SB-0-2-MSD 7/22/2021 8:10 0-2 x MSD

AOI02-01-SB-0-2 7/20/2021 9:15 0-2 x x x

AOI02-01-SB-0-2-D 7/20/2021 9:15 0-2 x x FD

AOI02-01-SB-20-21 7/20/2021 10:30 20-21 x

AOI02-01-SB-42-43 7/20/2021 10:25 42-43 x

AOI02-02-SB-0-2 7/20/2021 12:20 0-2 x

AOI02-02-SB-20-21 7/20/2021 13:55 20-21 x

AOI02-02-SB-42-43 7/20/2021 14:00 42-43 x

AOI03-01-SB-0-2 7/21/2021 7:05 0-2 x x x

AOI03-01-SB-0-2-D 7/21/2021 7:05 0-2 x FD

AOI03-01-SB-18-19 7/21/2021 8:10 18-19 x

AOI03-01-SB-38-39 7/21/2021 8:15 38-39 x

AOI03-02-SB-0-2 7/21/2021 9:05 0-2 x

AOI03-02-SB-0-2-MS 7/21/2021 9:05 0-2 x MS

AOI03-02-SB-0-2-MSD 7/21/2021 9:05 0-2 x MSD

AOI03-02-SB-20-21 7/21/2021 10:03 20-21 x

AOI03-02-SB-42-43 7/21/2021 10:08 42-43 x

AOI03-03-SB-0-2 7/21/2021 11:45 0-2 x

AOI03-03-SB-20-21 7/21/2021 13:48 20-21 x

Soil Samples

AECOM 5-7
Table 5-1

Site Inspection Samples by Medium

Site Inspection Report, Stockton AASF, California

Sample Identification

Sample

Collection

Date/Time

Sample Depth

(feet bgs)

LC/MS/MS compliant withQSM 5.3 Table B-15TOC(USEPA Method 9060A)pH(USEPA Method 9045D)

Comments

AOI03-03-SB-41-42 7/21/2021 13:50 41-42 x

AOI03-04-SB-0-2 7/22/2021 8:30 0-2 x

AOI03-04-SB-0-2-D 7/22/2021 8:30 0-2 x FD

AOI03-05-SB-0-2 7/22/2021 8:45 0-2 x x x

AOI01-01-GW 7/21/2021 13:15 NA x

AOI01-02-GW 7/22/2021 8:30 NA x

AOI01-03-GW 7/22/2021 10:30 NA x

AOI01-03-GW-D 7/22/2021 10:30 NA x FD

AOI01-03-GW-MS 7/22/2021 10:30 NA x MS

AOI01-03-GW-MSD 7/22/2021 10:30 NA x MSD

AOI01-04-GW 7/22/2021 12:45 NA x

AOI01-05-GW 7/22/2021 14:25 NA x

AOI02-01-GW 7/22/2021 16:15 NA x

AOI02-02-GW 7/23/2021 7:50 NA x

AOI03-01-GW 7/23/2021 9:30 NA x

AOI03-02-GW 7/23/2021 10:55 NA x

AOI03-03-GW 7/23/2021 12:30 NA x

S-DECON 5/25/2021 13:25 NA x

S-DECON (RE) 5/25/2021 13:25 NA x re-extracted

S-DECON-2 5/25/2021 14:30 NA x

STOCK-ERB-01 7/20/2021 7:30 NA x LDPE Tubing

STOCK-ERB-02 7/23/2021 12:50 NA x Hand Auger

STOCK-ERB-03 7/23/2021 12:55 NA x

Stainless-steel

Bowl

STOCK-ERB-04 7/23/2021 13:00 NA x

Water Level

Meter

STOCK-FRB-01 7/23/2021 10:40 NA x

Notes:

AASF = Army Aviation Support Facility

bgs = below ground surface

ERB = equipment rinsate blank

FD = field duplicate

FRB = field reagent blank

LC/MS/MS = Liquid Chromatography Mass Spectrometry

LDPE = low-density polyethylene

MS/MSD = matrix spike/ matrix spike duplicate

QSM = Quality Systems Manual

TOC = total organic carbon

USEPA = United States Environmental Protection Agency

Quality Control Samples

Groundwater Samples

AECOM 5-8
Table 5-2

Soil Boring Depths, Temporary Well Screen Intervals, and Groundwater Elevations

Site Inspection Report, Stockton AASF, California

Area of

Interest

Boring

Location

Soil Boring

Depth

(feet bgs)

Temporary Well

Screen Interval

(feet bgs)

Top of Casing

Elevation

(feet NAVD88)

Ground Surface

Elevation

(feet NAVD88)

Depth to

Water

(feet btoc)

Depth to

Water

(feet bgs)

Groundwater

Elevation

(feet NAVD88)

AOI01-01 45 40 - 45 23.41 23.29 37.48 37.36 -14.07

AOI01-02 48 43 - 48 25.58 24.23 40.29 38.94 -14.71

AOI01-03 47 42 - 47 26.89 23.69 41.98 38.78 -15.09

AOI01-04 49 44 - 49 28.03 26.83 43.43 42.23 -15.4

AOI01-05 45 40 - 45 26.94 26.31 42.51 41.88 -15.57

AOI01-06 2 NA NA NA NA NA NA

AOI01-07 2 NA NA NA NA NA NA

AOI01-08 2 NA NA NA NA NA NA

AOI01-09 2 NA NA NA NA NA NA

AOI02-01 48 43 - 48 27.09 25.05 41.44 39.40 -14.35

AOI02-02 48 43 - 48 26.76 25.04 41.22 39.50 -14.46

AOI03-01 45 39 - 441 23.13 23.22 37.18 37.27 -14.05

AOI03-02 48 40.5 - 45.51 26.34 25.03 40.41 39.10 -14.07

AOI03-03 48 42 - 471 26.64 24.33 40.73 38.42 -14.09

AOI03-04 2 NA NA NA NA NA NA

AOI03-05 2 NA NA NA NA NA NA

Notes:

1

 Temporary well screen set above total depth to capture groundwater interface

AASF = Army Aviation Support Facility

bgs = below ground surface

btoc = below top of casing

NA = not applicable

NAVD88 = North American Vertical Datum 1988

3

1

2

AECOM 5-9
AECOM 5-10

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OM 5-10
­ Figure 5-1

CLIENT

REVISED

SCALE

PROJECT

ARNG

Site Inspection at Stockton AASF, CA

9/30/2022

9/30/2022

9/30/2022

MS

SS

CM

GIS BY

CHK BY

PM

Base Map: Source: Esri, Maxar, Earthstar Geographics, and the GIS User Community

9/30/2022

1:3,600

Site Inspection Sample Locations

12420 Milestone Center Drive

Germantown, MD 20876

!?

!?

!?

"S

"S

"S

"S

!?

!?

!?

!?

"S

"S

!?

!?

!?

AOI 2

AOI 1

AOI 3

Rydberg Creek

AOI01-03

AOI01-05

AOI01-01

AOI01-08

AOI01-09

AOI01-07 AOI01-06

AOI02-02

AOI02-01

AOI03-02

AOI03-01

AOI03-05 AOI03-04

AOI01-02

AOI01-04

AOI03-03

Legend

!? Soil Boring/Temporary Monitoring Well

"S Surface Soil Sampling

Area of Interest

Facility Boundary

Canal/Ditch

Surface Water Flow Direction

Groundwater Flow Direction

0 150 300 600

Feet

AECOM 5-11
5-12

THIS PAGE INTENTIONALLY BLANK
Site Inspection Report

Army Aviation Support Facility, Stockton, California

AECOM 6-1

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. A discussion of the results for each AOI is provided in Section 6.3

through Section 6.5. Table 6-2 through Table 6-4 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 dated 6 July 2022 

(Assistant Secretary of Defense, 2022). The ARNG program under which this SI was performed 

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

SLs established in the OSD memorandum, the 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 below.

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

Analyteb

Residential

(Soil)

(μg/kg)a 

0-2 feet bgs

Industrial/ 

Commercial 

Composite 

Worker

(Soil)

(μg/kg)a 

2-15 feet bgs

Tap Water

(Groundwater)

(ng/L)a

PFOA 19 250 6 

PFOS 13 160 4

PFBS 1,900 25,000 601 

PFHxS 130 1,600 39

PFNA 19 250 6 

Notes:

bgs = below ground surface; μg/kg = micrograms per kilogram; ng/L = nanograms per liter

a.) Assistant Secretary of Defense, 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. 6 July 2022. 

b.) 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 HFPODA 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. 

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 

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. However, as discussed in the SI 

QAPP Addendum deviation in Section 5.8, the industrial/commercial worker scenario was not 

applied to subsurface soil samples collected from the mid-point at the soil borings below 15 feet 

bgs (18 to 22 feet bgs) in each AOI. 
Site Inspection Report

Army Aviation Support Facility, Stockton, California

AECOM 6-2

6.2 Soil Physicochemical Analyses

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

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

of the TOC and pH sampling. 

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

appropriate, to assess fate and transport. According to the Interstate Technology Regulatory 

Council (ITRC), several important 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: Airfield. The soil and groundwater results are summarized on Table 6-2 through Table 6-

4. Soil and groundwater results are presented on Figure 6-1 through Figure 6-7.

6.3.1 AOI 1 Soil Analytical Results

Soil was sampled from surface soil (0 to 2 feet bgs) from boring locations AOI01-01 through 

AOI01-09. Soil was also sampled from two deep subsurface soil intervals (18 to 22 feet bgs and 

39 to 44 feet bgs) from boring locations AOI01-01 through AOI01-05. Figure 6-1 through Figure

6-5 present the ranges of detections in soil. Table 6-2 and Table 6-3 summarize the soil results.

PFOA, PFOS, PFHxS, PFNA, and PFBS were detected in soil at concentrations below their

respective SLs in surface soil. PFOA, PFOS, PFHxS, PFNA, and PFBS were detected at 

concentrations less than 6.61 micrograms per kilogram (μg/kg); all detected concentrations were 

below the SLs in surface soil. 

PFOS, PFHxS, and PFBS were detected in the deep subsurface soil intervals. PFOS was

detected at two of the five locations, with concentrations of 0.081 J μg/kg at 20 to 21 feet bgs at

AOI01-01 and 0.090 J μg/kg at 20 to 21 feet bgs at AOI01-03. PFHxS was detected in the midpoint sample (18 to 22 feet bgs) in four of the five locations and in the deepest sample (38 to 42 

feet bgs) in three of five locations, with concentrations ranging from 0.036 J μg/kg to 0.365 J 

μg/kg. PFBS was detected at 40 to 41 feet at location AOI01-01, with a concentration of 0.037 J 

μg/kg. PFOA and PFNA were not detected in either deep subsurface soil interval.

6.3.2 AOI 1 Groundwater Analytical Results

Figure 6-6 and Figure 6-7 present the ranges of detections in groundwater. Table 6-4

summarizes the groundwater results.

Groundwater was sampled from temporary monitoring wells AOI01-01 through AOI01-05. The 

following exceedances of the SLs were measured:

• PFOA was detected above the SL of 6 nanograms per liter (ng/L) in two of the five wells, 

with concentrations of 34.8 ng/L at AOI01-04 and 21.3 ng/L at AOI01-05. 
Site Inspection Report

Army Aviation Support Facility, Stockton, California

AECOM 6-3

• PFOS was detected above the SL of 4 ng/L at four of the five wells, with concentrations 

ranging from of 5.39 ng/L to 30.9 ng/L. 

• PFHxS was detected above the SL of 39 ng/L at all five wells, with concentrations 

ranging from of 45.2 ng/L to 536 ng/L. 

PFNA and PFBS were detected below their respective SLs.

6.3.3 AOI 1 Conclusions

Based on the results of the SI, PFOA, PFOS, PFHxS, PFNA, and PFBS were detected in soil 

below their respective SLs. PFOA, PFOS, and PFHxS were detected in groundwater at 

concentrations above their respective SLs. Based on the exceedances of the SLs in groundwater, 

further evaluation at AOI 1 is warranted.

6.4 AOI 2

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

AOI 2: Wash Rack. The results in soil and groundwater are summarized on Table 6-2 through 

Table 6-4. Soil and groundwater results are presented on Figure 6-1 through Figure 6-7.

6.4.1 AOI 2 Soil Analytical Results

Soil was sampled from surface soil (0 to 2 feet bgs) and two deep subsurface soil intervals (20 to 

21 feet bgs and 42 to 43 feet bgs) from boring locations AOI02-01 and AOI02-02. Figure 6-1

through Figure 6-5 present the ranges of detections in soil. Table 6-2 and Table 6-3 summarize 

the soil results.

PFOA, PFOS, PFHxS, and PFNA were detected in surface soil at concentrations less than 

0.683 J μg/kg and below the SLs. PFBS was not detected in surface soil. PFOA, PFOS, PFHxS, 

PFNA, and PFBS were not detected in either deep subsurface soil interval.

6.4.2 AOI 2 Groundwater Analytical Results

Figure 6-6 and Figure 6-7 present the ranges of detections in groundwater. Table 6-4

summarizes the groundwater results.

Groundwater was sampled from temporary monitoring wells AOI2-01 and AOI2-02. PFOA, PFOS, 

PFHxS, PFNA, and PFBS were all detected below their respective SLs in groundwater:

• PFOA was detected below the SL of 6 ng/L at concentrations of 5.32 ng/L and 1.47 J 

ng/L. 

• PFOS was detected below the SL of 4 ng/L at concentrations of 3.33 J ng/L and 3.15 J 

ng/L. 

• PFHxS was detected below the SL of 39 ng/L at concentrations of 2.72 J ng/L and 5.13 

ng/L. 

• PFNA was detected below the SL of 6 ng/L at AOI02-01, with a concentration of 

1.01 J ng/L. 

• PFBS was detected below the SL of 601 ng/L at AOI02-02, with a concentration of 

1.56 J ng/L.
Site Inspection Report

Army Aviation Support Facility, Stockton, California

AECOM 6-4

6.4.3 AOI 2 Conclusions

Based on the results of the SI, PFOA, PFOS, PFHxS, and PFNA were detected in soil below their 

respective SLs. PFOA, PFOS, PFHxS, PFNA, and PFBS were detected in groundwater at 

concentrations below their respective SLs. Therefore, further evaluation at AOI 2 is not warranted

at this time.

Due to the uncertainty regarding groundwater flow, this report resulted in only one location

(AOI02-01) with data associated with the wash rack at AOI 2. This single sample location yielded 

groundwater results below but very close to the groundwater SLs for PFOS and PFOA. Based on 

groundwater flow, the other sample location (AOI02-02) does not appear to be representative of 

AOI 2.

6.5 AOI 3

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

AOI 3: Butler Building Parking FTA. The results in soil and groundwater are presented in Table 6-

2 through Table 6-4. Soil and groundwater results are presented on Figure 6-1 through Figure 6-

7.

6.5.1 AOI 3 Soil Analytical Results

Soil was sampled from surface soil (0 to 2 feet bgs) from boring locations AOI03-01 through 

AOI03-05. Soil was also sampled from two deep subsurface soil intervals (18 to 21 feet bgs and 

38 to 43 feet bgs) from boring locations AOI03-01 through AOI03-03. Figure 6-1 through Figure

6-5 present the ranges of detections in soil. Table 6-2 and Table 6-3 summarize the soil results.

PFOS was detected above the SL of 13 μg/kg in surface soil at AOI03-02, with a concentration of 

26.2 J μg/kg. PFOA, PFHxS, PFNA, and PFBS were detected at concentrations less than 

1.46 μg/kg and below their respective SLs in surface soil. 

PFOS, PFHxS, and PFBS were detected in the deep subsurface soil intervals. PFOS was 

detected at a concentration of 4.12 μg/kg at 38 to 39 feet bgs at AOI03-01. PFHxS was detected 

in the mid-point samples (18 to 21 feet bgs) in two of three locations, AOI03-01 and AOI03-02 and 

in the deepest sample (38 to 39 feet bgs) in one location, AOI03-01 at concentrations ranging 

from 0.00622 J μg/kg to 0.822 J μg/kg. PFBS was detected at a concentration of 0.189 J μg/kg at 

38 to 39 feet bgs at AOI03-01. PFOA and PFNA were not detected in either deep subsurface soil

interval. 

6.5.2 AOI 3 Groundwater Analytical Results

Figure 6-6 and Figure 6-7 present the ranges of detections in groundwater. Table 6-4

summarizes the groundwater results.

Groundwater was sampled from temporary monitoring wells AOI03-01, AOI03-02, and AOI03-03. 

The following exceedances of the SLs were measured: 

• PFOA was detected above the SL of 6 ng/L in all three wells, with concentrations 

ranging from of 7.85 ng/L to 139 ng/L. 

• PFOS was detected above the SL of 4 ng/L at all three wells, with concentrations 

ranging from of 50.2 ng/L to 82.3 ng/L. 

• PFHxS was detected above the SL of 39 ng/L at all three wells, with concentrations 

ranging from of 56.8 ng/L to 112 ng/L. 
Site Inspection Report

Army Aviation Support Facility, Stockton, California

AECOM 6-5

• PFNA was detected above the SL of 6 ng/L at AOI03-01 with a concentration of 71.3 

ng/L 

PFBS was detected below the SL of 601 ng/L at all three wells, with a maximum concentration of 

16.0 ng/L. 

6.5.3 AOI 3 Conclusions

Based on the results of the SI, PFOS was detected in soil above the SL. PFOA, PFOS, PFHxS, 

and PFNA were detected in groundwater, at concentrations above their respective SLs. Based on 

the exceedances of the SLs in soil and groundwater, further evaluation at AOI 3 is warranted.
Site Inspection Report

Army Aviation Support Facility, Stockton, California

AECOM 6-6

THIS PAGE INTENTIONALLY BLANK
Table 6-2

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

Site Inspection Report, Stockton AASF

Analyte OSD Screening 

Level a

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

PFBS 1900 ND U ND U ND U ND U 0.037 J ND U ND U ND U ND U ND U

PFHxS 130 0.114 J 0.078 J 0.076 J 0.139 J 0.636 J 0.148 J 0.068 J ND U ND U 0.065 J

PFNA 19 0.052 J ND U ND U ND U ND U 0.056 J ND U 0.105 J 0.098 J 6.61

PFOA 19 0.213 J ND U ND U 0.251 J ND U 0.188 J ND U 0.136 J 0.130 J 1.42

PFOS 13 2.45 0.157 J 0.173 J 1.87 0.176 J 0.459 J 0.323 J 0.999 J 0.885 J 4.35

Grey Fill Detected concentration exceeded OSD Screening Levels Chemical Abbreviations

PFBS perfluorobutanesulfonic acid

References PFHxS perfluorohexanesulfonic acid

PFNA perfluorononanoic acid

PFOA perfluorooctanoic acid

PFOS perfluorooctanesulfonic acid

Interpreted Qualifiers Acronyms and Abbreviations

J = Estimated concentration AASF Army Aviation Support Facility

U = The analyte was not detected at a level greater than or equal to the adjusted DL AOI Area of Interest

D duplicate

DL detection limit

ft feet

HQ hazard quotient

ID identification

LCMSMS liquid chromatography with tandem mass spectrometry

LOD limit of detection

ND analyte not detected above the LOD

OSD Office of the Secretary of Defense

QSM Quality Systems Manual

Qual interpreted qualifier

SB soil boring

USEPA United States Environmental Protection Agency

μg/kg micrograms per kilogram

a. 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. Soil screening levels based on residential scenario for incidental ingestion of contaminated soil.

AOI01

AOI01-08-SB-0-2

07/22/2021

0-2 ft

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

AOI01-07-SB-0-2

07/22/2021

0-2 ft

AOI01-07-SB-0-2-D

07/22/2021

0-2 ft

AOI01-05-SB-0-2

07/19/2021

0-2 ft

AOI01-06-SB-0-2

07/22/2021

0-2 ft

AOI01-03-SB-0-2

07/19/2021

0-2 ft

AOI01-04-SB-0-2

07/19/2021

0-2 ft

AOI01-02-SB-0-2

07/16/2021

0-2 ft

AOI01-02-SB-0-2-D

07/16/2021

0-2 ft

Area of Interest

Sample ID

Sample Date

Depth

AOI01-01-SB-0-2

07/16/2021

0-2 ft

AECOM 6-7
Table 6-2

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

Site Inspection Report, Stockton AASF

Analyte OSD Screening 

Level a

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

PFBS 1900 ND U ND U ND U ND U ND U 0.045 J ND U ND U ND U ND U

PFHxS 130 0.035 J 0.161 J ND U 0.042 J 0.038 J 1.46 ND U 0.065 J 0.073 J 0.047 J

PFNA 19 0.025 J 0.059 J ND U 0.168 J 0.189 J ND U ND U 0.046 J 0.024 J 0.054 J

PFOA 19 0.145 J 0.355 J 0.200 J 0.365 J 0.339 J ND U 0.302 J 0.384 J 0.389 J 0.167 J

PFOS 13 0.243 J 0.683 J 0.065 J 0.247 J 0.263 J 26.2 J 0.946 J 1.26 0.675 J 1.25

Grey Fill Detected concentration exceeded OSD Screening Levels Chemical Abbreviations

PFBS perfluorobutanesulfonic acid

References PFHxS perfluorohexanesulfonic acid

PFNA perfluorononanoic acid

PFOA perfluorooctanoic acid

PFOS perfluorooctanesulfonic acid

Interpreted Qualifiers Acronyms and Abbreviations

J = Estimated concentration AASF Army Aviation Support Facility

U = The analyte was not detected at a level greater than or equal to the adjusted DL AOI Area of Interest

D duplicate

DL detection limit

ft feet

HQ hazard quotient

ID identification

LCMSMS liquid chromatography with tandem mass spectrometry

LOD limit of detection

ND analyte not detected above the LOD

OSD Office of the Secretary of Defense

QSM Quality Systems Manual

Qual interpreted qualifier

SB soil boring

USEPA United States Environmental Protection Agency

μg/kg micrograms per kilogram

a. 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. Soil screening levels based on residential scenario for incidental ingestion of contaminated soil.

AOI02 AOI03

AOI03-05-SB-0-2

07/22/2021

0-2 ft

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

AOI03-04-SB-0-2

07/22/2021

0-2 ft

AOI03-04-SB-0-2-D

07/22/2021

0-2 ft

AOI03-02-SB-0-2

07/21/2021

0-2 ft

AOI03-03-SB-0-2

07/21/2021

0-2 ft

AOI03-01-SB-0-2

07/21/2021

0-2 ft

AOI03-01-SB-0-2-D

07/21/2021

0-2 ft

AOI02-01-SB-0-2

07/20/2021

0-2 ft

AOI02-02-SB-0-2

07/20/2021

0-2 ft

Area of Interest

Sample ID

Sample Date

Depth

AOI01

AOI01-09-SB-0-2

07/22/2021

0-2 ft

AECOM 6-8
Table 6-3

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

Site Inspection Report, Stockton AASF

Area of Interest

Sample ID

Sample Date

Depth

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

PFBS ND U 0.037 J ND U ND U ND U ND U ND U

PFHxS 0.365 J 0.106 J 0.122 J ND U 0.075 J 0.036 J 0.058 J

PFNA ND U ND U ND U ND U ND U ND U ND U

PFOA ND U ND U ND U ND U ND U ND U ND U

PFOS 0.081 J ND U ND U ND U 0.090 J ND U ND U

Interpreted Qualifiers Chemical Abbreviations

J = Estimated concentration PFBS perfluorobutanesulfonic acid

U = The analyte was not detected at a level greater than or equal to the adjusted DL PFHxS perfluorohexanesulfonic acid

PFNA perfluorononanoic acid

PFOA perfluorooctanoic acid

PFOS perfluorooctanesulfonic acid

Acronyms and Abbreviations

AASF Army Aviation Support Facility

AOI Area of Interest

DL detection limit

ft feet

ID identification

LCMSMS liquid chromatography with tandem mass spectrometry

LOD limit of detection

ND analyte not detected above the LOD

QSM Quality Systems Manual

Qual interpreted qualifier

SB soil boring

μg/kg micrograms per kilogram

AOI01-04-SB-21-22

07/19/2021

21-22 ft

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

AOI01

AOI01-03-SB-20-21

07/19/2021

20-21 ft

AOI01-03-SB-41-42

07/19/2021

41-42 ft

AOI01-02-SB-20-21

07/16/2021

20-21 ft

AOI01-02-SB-42-43

07/16/2021

42-43 ft

AOI01-01-SB-20-21

07/16/2021

20-21 ft

AOI01-01-SB-40-41

07/16/2021

40-42 ft

AECOM 6-9
Table 6-3

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

Site Inspection Report, Stockton AASF

Area of Interest

Sample ID

Sample Date

Depth

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

PFBS ND U ND U ND U ND U ND U ND U ND U

PFHxS ND U ND U ND U ND U ND U ND U ND U

PFNA ND U ND U ND U ND U ND U ND U ND U

PFOA ND U ND U ND U ND U ND U ND U ND U

PFOS ND U ND U ND U ND U ND U ND U ND U

Interpreted Qualifiers Chemical Abbreviations

J = Estimated concentration PFBS perfluorobutanesulfonic acid

U = The analyte was not detected at a level greater than or equal to the adjusted DL PFHxS perfluorohexanesulfonic acid

PFNA perfluorononanoic acid

PFOA perfluorooctanoic acid

PFOS perfluorooctanesulfonic acid

Acronyms and Abbreviations

AASF Army Aviation Support Facility

AOI Area of Interest

DL detection limit

ft feet

ID identification

LCMSMS liquid chromatography with tandem mass spectrometry

LOD limit of detection

ND analyte not detected above the LOD

QSM Quality Systems Manual

Qual interpreted qualifier

SB soil boring

μg/kg micrograms per kilogram

AOI02-02-SB-42-43

07/20/2021

42-43 ft

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

AOI01 AOI02

AOI02-01-SB-42-43

07/20/2021

42-43 ft

AOI02-02-SB-20-21

07/20/2021

20-21 ft

AOI01-05-SB-39-40

07/20/2021

39-40 ft

AOI02-01-SB-20-21

07/20/2021

20-21 ft

AOI01-04-SB-43-44

07/19/2021

43-44 ft

AOI01-05-SB-19-20

07/20/2021

19-20 ft

AECOM 6-10
Table 6-3

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

Site Inspection Report, Stockton AASF

Area of Interest

Sample ID

Sample Date

Depth

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

PFBS ND U 0.189 J ND U ND U ND U ND U

PFHxS 0.062 J 0.822 J 0.00622 J ND U ND U ND U

PFNA ND U ND U ND U ND U ND U ND U

PFOA ND U ND U ND U ND U ND U ND U

PFOS ND U 4.12 ND U ND U ND U ND U

Interpreted Qualifiers Chemical Abbreviations

J = Estimated concentration PFBS perfluorobutanesulfonic acid

U = The analyte was not detected at a level greater than or equal to the adjusted DL PFHxS perfluorohexanesulfonic acid

PFNA perfluorononanoic acid

PFOA perfluorooctanoic acid

PFOS perfluorooctanesulfonic acid

Acronyms and Abbreviations

AASF Army Aviation Support Facility

AOI Area of Interest

DL detection limit

ft feet

ID identification

LCMSMS liquid chromatography with tandem mass spectrometry

LOD limit of detection

ND analyte not detected above the LOD

QSM Quality Systems Manual

Qual interpreted qualifier

SB soil boring

μg/kg micrograms per kilogram

AOI03

AOI03-03-SB-20-21

07/21/2021

20-21 ft

AOI03-03-SB-41-42

07/21/2021

41-42 ft

AOI03-02-SB-20-21

07/21/2021

20-21 ft

AOI03-02-SB-42-43

07/21/2021

42-43 ft

AOI03-01-SB-18-19

07/21/2021

18-19 ft

AOI03-01-SB-38-39

07/21/2021

38-39 ft

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

AECOM 6-11
Table 6-4

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

Site Inspection Report, Stockton AASF

Analyte OSD Screening

Level a

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

PFBS 601 107 15.4 17.8 17.8 9.73 6.39 ND U 1.56 J 16.0

PFHxS 39 536 45.2 168 166 130 132 2.72 J 5.13 81.3

PFNA 6 ND U ND U ND U ND U 4.85 ND U 1.01 J ND U 71.3

PFOA 6 5.80 ND U 5.69 5.48 34.8 21.3 5.32 1.47 J 139

PFOS 4 12.3 ND U 5.52 5.39 15.7 30.9 3.33 J 3.15 J 50.2

Grey Fill Detected concentration exceeded OSD Screening Levels Chemical Abbreviations

PFBS perfluorobutanesulfonic acid

References PFHxS perfluorohexanesulfonic acid

PFNA perfluorononanoic acid

PFOA perfluorooctanoic acid

PFOS perfluorooctanesulfonic acid

Interpreted Qualifiers Acronyms and Abbreviations

J = Estimated concentration AASF Army Aviation Support Facility

U = The analyte was not detected at a level greater than or equal to the adjusted DL AOI Area of Interest

D duplicate

DL detection limit

GW groundwater

HQ hazard quotient

ID identification

LCMSMS liquid chromatography with tandem mass spectrometry

LOD limit of detection

ND analyte not detected above the LOD

OSD Office of the Secretary of Defense

QSM Quality Systems Manual

Qual interpreted qualifier

USEPA United States Environmental Protection Agency

ng/l nanogram per liter

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

a. 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 levels based on residential scenario for direct ingestion of groundwater.

AOI01 AOI02

AOI02-02-GW

07/23/2021

AOI03

AOI03-01-GW

07/23/2021

AOI01-05-GW

07/22/2021

AOI02-01-GW

07/22/2021

AOI01-03-GW-D

07/22/2021

AOI01-04-GW

07/22/2021

AOI01-02-GW

07/22/2021

AOI01-03-GW

07/22/2021

Area of Interest

Sample ID

Sample Date

AOI01-01-GW

07/21/2021

AECOM 6-12
Table 6-4

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

Site Inspection Report, Stockton AASF

Analyte OSD Screening

Level a

Result Qual Result Qual

PFBS 601 7.71 9.40

PFHxS 39 112 56.8

PFNA 6 ND U ND U

PFOA 6 7.85 33.8

PFOS 4 60.2 82.3

Grey Fill Detected concentration exceeded OSD Screening Levels Chemical Abbreviations

PFBS perfluorobutanesulfonic acid

References PFHxS perfluorohexanesulfonic acid

PFNA perfluorononanoic acid

PFOA perfluorooctanoic acid

PFOS perfluorooctanesulfonic acid

Interpreted Qualifiers Acronyms and Abbreviations

J = Estimated concentration AASF Army Aviation Support Facility

U = The analyte was not detected at a level greater than or equal to the adjusted DL AOI Area of Interest

D duplicate

DL detection limit

GW groundwater

HQ hazard quotient

ID identification

LCMSMS liquid chromatography with tandem mass spectrometry

LOD limit of detection

ND analyte not detected above the LOD

OSD Office of the Secretary of Defense

QSM Quality Systems Manual

Qual interpreted qualifier

USEPA United States Environmental Protection Agency

ng/l nanogram per liter

AOI03-03-GW

07/23/2021

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

a. 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 levels based on residential scenario for direct ingestion of groundwater.

Area of Interest AOI03

Sample ID

Sample Date

AOI03-02-GW

07/23/2021

AECOM 6-13
AECOM 5-10

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OM 6-14
Rydberg Creek

AOI01-03

AOI01-05

AOI01-01

AOI01-08

AOI01-09

AOI01-07

AOI01-06

AOI02-02

AOI02-01

AOI03-02

AOI03-01

AOI03-05

AOI03-04

AOI01-02

AOI01-04

AOI03-03

Rydberg Creek

AOI01-03

AOI01-05

AOI01-01

AOI02-02

AOI02-01

AOI03-02

AOI03-01

AOI01-02

AOI01-04

AOI03-03

Rydberg Creek

AOI01-03

AOI01-05

AOI01-01

AOI02-02

AOI02-01

AOI03-02

AOI03-01

AOI01-02

AOI01-04

AOI03-03

12420 Milestone Center Drive

Germantown, MD 20876 Figure 6-1

PFOA Detections in Soil 

8/15/2022

8/15/2022

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Base Map: Source: Esri, Maxar, Earthstar Geographics, and

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1:3,600

Shallow

Intermediate

Deep

Exceedances of the OSD SL are depicted with a yellow halo.

*Depth intervals shown represent respective sampling position within a given soil boring location.

0 150 300 600

­

Feet

Facility Boundary

Canal/Ditch

PFOA Results (μg/Kg)

ND

>ND - 19

>19 - 250

>250 - 2,500

>2,500

Depth Intervals*

Shallow = 0-2 ft bgs

Intermediate = 18-22 ft bgs

Deep = 38-44 ft bgs

AECOM 6-15
Rydberg Creek

AOI01-03

AOI01-05

AOI01-01

AOI01-08

AOI01-09

AOI01-07

AOI01-06

AOI02-02

AOI02-01

AOI03-02

AOI03-01

AOI03-05 AOI03-04

AOI01-02

AOI01-04

AOI03-03

Rydberg Creek

AOI01-03

AOI01-05

AOI01-01

AOI02-02

AOI02-01

AOI03-02

AOI03-01

AOI01-02

AOI01-04

AOI03-03

Rydberg Creek

AOI01-03

AOI01-05

AOI01-01

AOI02-02

AOI02-01

AOI03-02

AOI03-01

AOI01-02

AOI01-04

AOI03-03

12420 Milestone Center Drive

Germantown, MD 20876 Figure 6-2

PFOS Detections in Soil 

8/15/2022

8/15/2022

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Site Inspection at Stockton AASF, CA

Base Map: Source: Esri, Maxar, Earthstar Geographics, and

the GIS User Community

1:3,600

Shallow

Intermediate

Deep

Exceedances of the OSD SL are depicted with a yellow halo.

*Depth intervals shown represent respective sampling position within a given soil boring location.

0 150 300 600

­

Feet

Facility Boundary

Canal/Ditch

PFOS Results (μg/Kg)

ND

>ND - 13

>13 - 160

>160 - 1,600

>1,600

Depth Intervals*

Shallow = 0-2 ft bgs 

Intermediate = 18-22 ft bgs 

Deep = 38-44 ft bgs

AECOM 6-16
Rydberg Creek

AOI01-03

AOI01-05

AOI01-01

AOI01-08

AOI01-09

AOI01-07

AOI01-06

AOI02-02

AOI02-01

AOI03-02

AOI03-01

AOI03-05

AOI03-04

AOI01-02

AOI01-04

AOI03-03

Rydberg Creek

AOI01-03

AOI01-05

AOI01-01

AOI02-02

AOI02-01

AOI03-02

AOI03-01

AOI01-02

AOI01-04

AOI03-03

Rydberg Creek

AOI01-03

AOI01-05

AOI01-01

AOI02-02

AOI02-01

AOI03-02

AOI03-01

AOI01-02

AOI01-04

AOI03-03

12420 Milestone Center Drive

Germantown, MD 20876 Figure 6-3

PFBS Detections in Soil 

8/15/2022

8/15/2022

MS 8/15/2022

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Site Inspection at Stockton AASF, CA

Base Map: Source: Esri, Maxar, Earthstar Geographics, and

the GIS User Community

1:3,600

Shallow

Intermediate

Deep

Exceedances of the OSD SL are depicted with a yellow halo.

*Depth intervals shown represent respective sampling position within a given soil boring location.

0 150 300 600

­

Feet

Facility Boundary

Canal/Ditch

PFBS Results (μg/Kg)

ND

>ND - 10

>10 - 1,900

>1,900 - 25,000

>25,000

Depth Intervals*

Shallow = 0-2 ft bgs 

Intermediate = 18-22 ft bgs 

Deep = 38-44 ft bgs

AECOM 6-17
Rydberg Creek

AOI01-03

AOI01-05

AOI01-01

AOI01-08

AOI01-09

AOI01-07

AOI01-06

AOI02-02

AOI02-01

AOI03-02

AOI03-01

AOI03-05

AOI03-04

AOI01-02

AOI01-04

AOI03-03

Rydberg Creek

AOI01-03

AOI01-05

AOI01-01

AOI02-02

AOI02-01

AOI03-02

AOI03-01

AOI01-02

AOI01-04

AOI03-03

Rydberg Creek

AOI01-03

AOI01-05

AOI01-01

AOI02-02

AOI02-01

AOI03-02

AOI03-01

AOI01-02

AOI01-04

AOI03-03

12420 Milestone Center Drive

Germantown, MD 20876 Figure 6-4

PFHxS Detections in Soil 

8/15/2022

8/15/2022

MS 8/15/2022

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Site Inspection at Stockton AASF, CA

Base Map: Source: Esri, Maxar, Earthstar Geographics, and

the GIS User Community

1:3,600

Shallow

Intermediate

Deep

Exceedances of the OSD SL are depicted with a yellow halo.

*Depth intervals shown represent respective sampling position within a given soil boring location.

0 150 300 600

­

Feet

Facility Boundary

Canal/Ditch

PFHxS Results (μg/Kg)

ND

>ND - 10

>10 - 130

>130 - 1,600

>1,600

Depth Intervals*

Shallow = 0-2 ft bgs 

Intermediate = 18-22 ft bgs 

Deep = 38-44 ft bgs

AECOM 6-18
Rydberg Creek

AOI01-03

AOI01-05

AOI01-01

AOI01-08

AOI01-09

AOI01-07

AOI01-06

AOI02-02

AOI02-01

AOI03-02

AOI03-01

AOI03-05

AOI03-04

AOI01-02

AOI01-04

AOI03-03

Rydberg Creek

AOI01-03

AOI01-05

AOI01-01

AOI02-02

AOI02-01

AOI03-02

AOI03-01

AOI01-02

AOI01-04

AOI03-03

Rydberg Creek

AOI01-03

AOI01-05

AOI01-01

AOI02-02

AOI02-01

AOI03-02

AOI03-01

AOI01-02

AOI01-04

AOI03-03

12420 Milestone Center Drive

Germantown, MD 20876 Figure 6-5

PFNA Detections in Soil 

8/15/2022

8/15/2022

MS 8/15/2022

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Site Inspection at Stockton AASF, CA

Base Map: Source: Esri, Maxar, Earthstar Geographics, and

the GIS User Community

1:3,600

Shallow

Intermediate

Deep

Exceedances of the OSD SL are depicted with a yellow halo.

*Depth intervals shown represent respective sampling position within a given soil boring location.

0 150 300 600

­

Feet

PFNA Results (μg/Kg)

ND

>ND - 19

>19 - 250

>250 - 2,500

>2,500

Facility Boundary

Canal/Ditch

Depth Intervals*

Shallow = 0-2 ft bgs 

Intermediate = 18-22 ft bgs 

Deep = 38-44 ft bgs

AECOM 6-19
Rydberg Creek

AOI01-03

AOI01-05

AOI01-01

AOI02-02

AOI02-01

AOI03-02

AOI03-01

AOI01-02

AOI01-04

AOI03-03

Rydberg Creek

AOI01-03

AOI01-05

AOI01-01

AOI02-02

AOI02-01

AOI03-02

AOI03-01

AOI01-02

AOI01-04

AOI03-03

Rydberg Creek

AOI01-03

AOI01-05

AOI01-01

AOI02-02

AOI02-01

AOI03-02

AOI03-01

AOI01-02

AOI01-04

AOI03-03

12420 Milestone Center Drive

Germantown, MD 20876 Figure 6-6

PFOA, PFOS, and PFBS 

Detections in Groundwater

10/5/2022

10/5/2022

MS 10/5/2022

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Site Inspection at Stockton AASF, CA

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the GIS User Community

1:3,600

Legend

Facility Boundary

Canal/Ditch

Groundwater Flow Direction

PFOA

PFOS

PFBS

PFOA Results (ng/L)

ND

>ND - 6

>6 - 40

>40 - 70

>70

PFOS Results (ng/L)

ND

>ND - 4

>4 - 40

>40 - 70

>70

PFBS Results (ng/L)

ND

>ND - 100

>100 - 601

>601 - 2,000

>2,000

0 150 300 600

­

Feet

Exceedances of the OSD SL are depicted with a yellow halo.

AECOM 6-20
Rydberg Creek

AOI01-03

AOI01-05

AOI01-01

AOI02-02

AOI02-01

AOI03-02

AOI03-01

AOI01-02

AOI01-04

AOI03-03

Rydberg Creek

AOI01-03

AOI01-05

AOI01-01

AOI02-02

AOI02-01

AOI03-02

AOI03-01

AOI01-02

AOI01-04

AOI03-03

12420 Milestone Center Drive

Germantown, MD 20876 Figure 6-7

PFHxS and PFNA

Detections in Groundwater

9/29/2022

9/29/2022

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1:3,600

PFHxS

PFNA

PFHxS Results (ng/L)

ND

>ND - 39

>39 - 100

>100 - 1,000

>1,000

PFNA Results (ng/L)

ND

>ND - 6

>6 - 100

>100 - 1,000

>1,000

0 150 300 600

­

Feet

Exceedances of the OSD SL are depicted with a yellow halo.

Legend

Facility Boundary

Canal/Ditch

Groundwater Flow Direction

AECOM 6-21
THIS PAGE INTENTIONALLY BLANK

spection Report

Aviation Support Facility, Stockton, California
Site Inspection Report

Army Aviation Support Facility, Stockton, California

AECOM 7-1

7. Exposure Pathways

The CSMs for each AOI, revised based on the SI findings, are presented on Figure 7-1 through 

Figure 7-3. Please note that while the CSM discussion assists in determining if a receptor may 

be impacted, the decision to move from SI to Remedial Investigation (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 site conditions with respect to known and suspected sources, potential transport mechanisms 

and migration pathways, and potentially exposed human receptors. A human exposure pathway 

is considered potentially complete when the following conditions are present:

1. Contaminant source;

2. Environmental fate and transport;

3. Exposure point;

4. Exposure route; and

5. Potentially exposed populations.

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

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

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

compounds above the SLs. Areas with an identified potentially complete pathway that have 

detections of the relevant compounds above the SLs may warrant further investigation. Although 

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

recommendation for future study in an 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 USEPA guidance for risk screening (USEPA, 2001). 

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

workers, trespassers, residents outside the facility boundary, and recreational users outside of 

the facility boundary.

7.1 Soil Exposure Pathway

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

between the source and potential receptors at AOI 1, AOI 2, and AOI 3 based on the 

aforementioned criteria.

7.1.1 AOI 1

AOI 1 is the Airfield, where controlled AFFF releases through familiarization training have occurred 

annually potentially as early as 1992 and then less frequently after 2010. 

PFOA, PFOS, PFHxS, PFNA, and PFBS were detected below the SLs in surface soil at AOI 1. 

Site workers, future construction workers, and trespassers could contact constituents in surface 
Site Inspection Report

Army Aviation Support Facility, Stockton, California

AECOM 7-2

soil via incidental ingestion and inhalation of dust. Therefore, the surface soil exposure pathway 

for site workers, future construction workers, and trespassers are potentially complete. Although 

shallow subsurface soil (between 2 and 15 feet bgs) samples were not collected, it is possible 

that construction workers could contact constituents in subsurface soil via incidental ingestion, as 

PFHxS, PFOS, and PFBS were detected in deep subsurface soil at AOI 1. Therefore, the 

subsurface soil exposure pathway for future construction workers is potentially complete. The 

CSM for AOI 1 is presented on Figure 7-1. 

7.1.2 AOI 2

AOI 2 is the wash rack area and includes the HAZMAT locker with AFFF storage, located at the 

southwest corner of the wash rack. Controlled AFFF releases to the wash rack through activities 

related to fire training and familiarization training have occurred annually at AOI 2, potentially as 

early as 1992, and then less frequently after 2010. Potential AFFF releases from incidental spills 

in the wash rack area may have also occurred. 

The wash rack drains lead to an oil water separator that connects to the Stockton sanitary sewer 

system. Therefore, discharges of AFFF in the wash rack would release directly into the sanitary 

sewer system; however, the HAZMAT locker is located near unpaved, grassy areas. 

PFOA, PFOS, PFHxS, and PFNA were detected below the SLs in surface soil at AOI 2. 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 site workers 

and future construction workers are potentially complete. Although shallow subsurface soil 

(between 2 and 15 feet bgs) samples were not collected, it is possible that construction workers 

could contact constituents in subsurface soil via incidental ingestion; therefore, the subsurface 

soil exposure pathway for future construction workers is potentially complete. The CSM for AOI 2 

is presented on Figure 7-2. 

7.1.3 AOI 3

AOI 3 is a former FTA at what is now the parking area located east of the Butler Building. 

Controlled AFFF releases through fire training activities have occurred annually at AOI 3, 

potentially as early as 1992, until approximately 2010 to 2012, when the Butler Building was 

constructed. 

PFOA, PFOS, PFHxS, PFNA, and PFBS were detected in surface soil at AOI 3, and PFOS 

exceeded the residential SL. Site workers, construction workers, and trespassers could contact 

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

soil exposure pathway for site workers, future construction workers, and trespassersare 

potentially complete. Although shallow subsurface soil (between 2 and 15 feet bgs) samples were 

not collected, it is possible that construction workers could contact constituents in subsurface soil 

via incidental ingestion, as PFOS, PFHxS, and PFBS were detected in deep subsurface soil at 

AOI 3. Therefore, the subsurface soil exposure pathway for future construction workers is 

potentially complete. The CSM for AOI 3 is presented on Figure 7-3.

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 based on the aforementioned criteria.
Site Inspection Report

Army Aviation Support Facility, Stockton, California

AECOM 7-3

7.2.1 AOI 1

PFOA, PFOS, and PFHxS were detected above their respective SLs in groundwater samples 

collected at AOI 1. Due to the presence of public water system wells within a 4-mile radius of the 

facility, the pathway for exposure to off-facility residents via ingestion of groundwater is considered 

potentially complete. Because the location of Stockton Municipal Utilities Water Service 

groundwater wells that may serve the AASF are unknown, the pathway for exposure to site

workers via ingestion of groundwater is also considered potentially complete. Depths to water 

measured during the SI at AOI 1 in July 2021 ranged from 37.36 to 42.23 feet bgs. Therefore, the 

ingestion exposure pathway for future construction workers is considered incomplete. The CSM 

for AOI 1 is presented on Figure 7-1. 

7.2.2 AOI 2

PFOA, PFOS, PFHxS, PFNA, and PFBS were detected in groundwater samples collected at AOI 

2, at concentrations below SLs. Due to the presence of public water system wells within a 4-mile 

radius of the facility, the pathway for exposure to off-facility residents via ingestion of groundwater 

is considered potentially complete. Because the locations of Stockton Municipal Utilities Water 

Service groundwater wells that may serve the AASF are unknown, the pathway for exposure to 

site workers via ingestion of groundwater is also considered potentially complete. Depths to water 

measured in July 2021 during the SI ranged from 39.4 to 39.5 feet bgs; therefore, the ingestion 

exposure pathway for future construction workers is considered incomplete. The CSM for AOI 2 

is presented on Figure 7-2. 

7.2.3 AOI 3

PFOA, PFOS, PFHxS, and PFNA were detected above their respective SLs in groundwater 

samples collected at AOI 3. Due to the presence of public water system wells within a 4-mile 

radius of the facility, the pathway for exposure to off-facility residents via ingestion of groundwater 

is considered potentially complete. Because the locations of Stockton Municipal Utilities Water 

Service groundwater wells that may serve the AASF are unknown, the pathway for exposure to 

site workers via ingestion of groundwater is also considered potentially complete. Depths to water 

measured in July 2021 during the SI ranged from 37.27 to 39.10 feet bgs; therefore, the ingestion 

exposure pathway for future construction workers is considered incomplete. The CSM for AOI 3 

is presented on Figure 7-3.

7.3 Surface Water and Sediment Exposure Pathway

The SI results in soil and groundwater, in combination with knowledge of the fate and transport 

properties of PFAS, were used to determine whether a potentially complete pathway exists 

between the source and potential receptors. 

Rydberg Creek wraps around the southern and western boundary of the facility and connects to

French Camp Slough, located approximately 2,000 feet away from AOI 2 and AOI 3. PFAS are 

water soluble and can migrate readily from soil to surface water via leaching and run-off. PFAS in 

runoff is also likely to flow into catchments that drain into these surface water features. The 

ingestion exposure pathways for surface water and sediment are potentially complete for 

trespassers and recreational users, based on the detections in groundwater and soil from AOI 1

through AOI 3. The ingestion exposure pathway for trespassers and recreational users is relevant 

to incidental ingestion during recreational use of Rydberg Creek and French Camp Slough, only, 

as surface water is not used as a drinking water source from these surface water features. The 

CSMs for AOI 1 through AOI 3 are presented in Figures 7-1 through 7-3, respectively.
Site Inspection Report

Army Aviation Support Facility, Stockton, California

AECOM 7-4

THIS PAGE INTENTIONALLY BLANK
Media

SOURCE PATHWAY RECEPTOR

AOI 1

Fire Training and

Mobile Fire

Extinguishers

Discharge

Surface soil

Human

activities

Precipitation/

run-off

Leaching/

infiltration

Airborne soil

particulate

Surface soil

at AOI

Surface

water/

sediment

Subsurface

soil

Shallow

groundwater

Source Release

Mechanism Media Transportand Migration Media ExposureRoutes

Inhalation of

dust

Ingestion

Ingestion

Ingestion

Ingestion

Human Receptors:

Current/ Future

Flow-Chart Continues

Partial/ Possible Flow

Flow-Chart Stops

Incomplete Pathway

Potentially Complete Pathway

Potentially Complete Pathway

with Exceedance of SL

/ / / /

Site

Worker

Construction 

Worker3 Resident1,2 Trespasser/ Recreational User1

Site

Worker

Construction 

Worker3 Resident1,2

/ / / /

/ / / /

/ / / /

/ / / /

LEGEND

Figure 7-1

Conceptual Site Model, AOI 1

Stockton AASF

Potential

Off-Facility

Source Not

under

Control of

ARNG

Trespasser/ 

Recreational User1

1. The resident and recreational users refer to offsite receptors.

2. Inhalation of dust for off-site receptors is likely

insignificant.

3. No current active construction at the facility.

Notes:

AECOM 7-5
Media

SOURCE PATHWAY RECEPTOR

AOI 2

Fire Training and

Mobile Fire

Extinguishers

Discharge

Surface soil

Human

activities

Precipitation/

run-off

Leaching/

infiltration

Airborne soil

particulate

Surface soil

at AOI

Surface

water/

sediment

Subsurface

soil

Shallow

groundwater

Source Release

Mechanism Media Transportand Migration Media ExposureRoutes

Inhalation of

dust

Ingestion

Ingestion

Ingestion

Ingestion

Human Receptors:

Current/ Future

Flow-Chart Continues

Partial/ Possible Flow

Flow-Chart Stops

Incomplete Pathway

Potentially Complete Pathway

Potentially Complete Pathway

with Exceedance of SL

/ / / /

Site

Worker

Construction 

Worker3 Resident1,2 Trespasser/ Recreational User1

Site

Worker

Construction 

Worker3 Resident1,2

/ / / /

/ / / /

/ / / /

/ / / /

LEGEND

Figure 7-2

Conceptual Site Model, AOI 2

Stockton AASF

Potential

Off-Facility

Source Not

under

Control of

ARNG

Trespasser/ 

Recreational User1

1. The resident and recreational users refer to offsite receptors.

2. Inhalation of dust for off-site receptors is likely

insignificant.

3. No current active construction at the facility.

Notes:

AECOM 7-6
Media

SOURCE PATHWAY RECEPTOR

AOI 3

Fire Training and

Mobile Fire

Extinguishers

Discharge

Surface soil

Subsurface

soil

Human

activities

Precipitation/

run-off

Leaching/

infiltration

Airborne soil

particulate

Surface soil

at AOI

Surface

water/

sediment

Subsurface

soil

Shallow

groundwater

Source Release

Mechanism Media Transportand Migration Media ExposureRoutes

Inhalation of

dust

Ingestion

Ingestion

Ingestion

Ingestion

Human Receptors:

Current/ Future

Flow-Chart Continues

Partial/ Possible Flow

Flow-Chart Stops

Incomplete Pathway

Potentially Complete Pathway

Potentially Complete Pathway

with Exceedance of SL

/ / / /

Site

Worker

Construction 

Worker3 Resident1,2 Trespasser/ Recreational User1

Site

Worker

Construction 

Worker3 Resident1,2

/ / / /

/ / / /

/ / / /

/ / / /

LEGEND

Figure 7-3

Conceptual Site Model, AOI 3

Stockton AASF

Potential

Off-Facility

Source Not

under

Control of

ARNG

Trespasser/ 

Recreational User1

1. The resident and recreational users refer to offsite receptors.

2. Inhalation of dust for off-site receptors is likely

insignificant.

3. No current active construction at the facility.

Notes:

AECOM 7-7
AECOM 5-10

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OM 7-8
Site Inspection Report

Army Aviation Support Facility, Stockton, California

AECOM 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 were conducted from 12 to 23 July 2021 and consisted of utility clearance, 

direct push boring, soil sample collection, temporary monitoring well installation, grab groundwater 

sample collection, and land surveying. Field activities were conducted in accordance with the SI 

QAPP Addendum (AECOM, 2021a).

• To fulfill the project DQOs set forth in the approved SI QAPP Addendum (AECOM, 2021a), 

samples were collected and analyzed for a subset of 18 compounds by LC/MS/MS 

compliant with QSM 5.3 Table B-15 as follows. 

• Thirty-six (36) soil samples from 16 boring locations; 

• Ten (10) grab groundwater samples from 10 temporary well locations; 

• Twenty-one (21) 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 AOIs 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 CSMs were refined to assess whether a potentially complete pathway 

exists between the source and potential receptors for potential exposure at the AOIs, which are 

described in Section 7.

8.2 Outcome 

Based on the results of this SI, further evaluation under CERCLA is warranted in an RI for AOI 1

and AOI 3; no further evaluation is warranted for AOI 2 at this time (see Table 8-1). Based on the 

CSMs developed and revised in light of the SI findings, there is potential for exposure to drinking 

water receptors from AOI 1 and AOI 3 from sources on the facility resulting from historical DoD 

activities. Sample analytical concentrations collected during the SI were compared against the

project SLs in soil and groundwater, as described in Table 6-1. A summary of the results of the SI 

data relative to the SLs is as follows:

• At AOI 1:

• PFOA, PFOS, and PFHxS in groundwater exceeded their respective SLs. PFOA 

exceeded the SL of 6 ng/L, with a maximum concentration of 34.8 ng/L at location 

AOI01-04. PFOS exceeded the SL of 4 ng/L, with a maximum concentration of 30.9 

ng/L at location AOI01-05. PFHxS exceeded the SL of 39 ng/L, with a maximum 

concentration of 536 ng/L at location AOI01-01. 

• The detected concentrations of PFOA, PFOS, PFHxS, PFNA and PFBS in soil at 

AOI 1 were below their respective SLs. 

• Based on the exceedances of the SLs in groundwater, further evaluation of AOI 1 is 

warranted in an RI.
Site Inspection Report

Army Aviation Support Facility, Stockton, California

AECOM 8-2

• At AOI 2:

• The detected concentrations PFOA, PFOS, PFHxS, PFNA, and PFBS in 

groundwater were below their respective SLs. 

• The detected concentrations of PFOA, PFOS, PFHxS, and PFNA in soil at AOI 2

were below their respective SLs. 

• Based on the results of the SI, no further evaluation of AOI 2 is warranted at this time.

• At AOI 3:

• PFOA, PFOS, PFHxS, and PFNA in groundwater exceeded their respective SLs.

PFOA exceeded the SL of 6 ng/L, with a maximum concentration of 139 ng/L at 

location AOI03-01. PFOS exceeded the SL of 4 ng/L, with a maximum concentration 

of 82.3 ng/L at AOI03-03. PFHxS exceeded the SL of 39 ng/L, with a maximum 

concentration of 112 ng/L at location AOI03-02. PFNA exceeded the SL of 6 ng/L,

with a maximum concentration of 71.3 ng/L at location AOI03-01. 

• PFOS in surface soil exceeded the SL of 13 μg/kg at AOI03-02, with a concentration 

of 26.2 J μg/kg.

• Based on the exceedances of the SLs in soil and groundwater, further evaluation of 

AOI 3 is warranted in the RI. 

Due to the uncertainty regarding groundwater flow, this report resulted in only one location

(AOI02-01) with data associated with the wash rack at AOI 2. This single sample location yielded 

groundwater results below but very close to the groundwater SLs for PFOS and PFOA. Based on 

groundwater flow, the other sample location (AOI02-02) does not appear to be representative of 

AOI 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.

Table 8-1 summarizes the SI results for soil and groundwater used to determine if an AOI should 

be considered for further investigation under CERCLA and undergo an RI. 
Site Inspection Report

Army Aviation Support Facility, Stockton, California

AECOM 8-3

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 Airfield Proceed to RI 

2 Wash Rack No further action

at this time

3 Butler 

Building 

Parking FTA

Proceed to RI

Legend:

= detected; exceedance of the screening levels

= detected; no exceedance of the screening levels

= not detected
Site Inspection Report

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AECOM 8-4

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Site Inspection Report

Army Aviation Support Facility, Stockton, California

AECOM 9-1

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