TECHNICAL MEMORANDUM Pride Hangar Vadose Zone Soil Evaluation 

To: Ellsworth Air Force Base Remedial Project Managers

From: URS Group, Inc. 

Date: August 9, 2010

Subject: Evaluation of the Pride Hangar Vadose Zone Soil

1.0 Purpose 

The purpose of this technical memorandum is to evaluate the vadose zone soil located near the 

northwest corner of the Pride Hangar. This area is identified as the Pride Hangar Area of 

Concern (AOC) and is shown on Figure 1. The unsaturated soil at this location was excluded 

from the partial deletion of unsaturated soil (70318-70319 Federal Register/Vol. 71, No. 

232/December 4, 2006). This evaluation is being done to determine what activities may be 

needed to allow the Air Force to delete the Pride Hangar AOC. 

2.0 Background 

The Pride Hangar is located within Operable Unit (OU)-11 Area 1. Two former side-by-side 

waste solvent underground storage tanks (USTs) are the primary source of a trichloroethene 

(TCE) plume known as the Pride Hangar plume. These tanks were abandoned in 1992. The 

OU-11 Record of Decision (ROD) [United States Air Force (USAF) 1997] specified 

groundwater extraction and treatment in OU-11 Area 1, and a vacuum extraction system was 

installed to extract contaminated groundwater and operated southeast (downgradient) of the 

Pride Hangar. 

Additional soil and groundwater sampling was completed around the Pride Hangar in 2002 and 

was reported in an April 2002 LTM report (Earth Tech 2002). A recommendation to implement 

SVE at the northwest corner of the Pride Hangar was made in 2003 (Earth Tech 2003) to treat 

vadose zone soil. This action was implemented and consisted of SVE pilot testing in May 2004 

and intermittent operation of the system from July to November 2004. Implementation consisted 

of installation and operation of eight soil vapor extraction (SVE) wells as temporary wells 

(grouted to the surface but without surface protection). 

The SVE system was shut down November 15, 2004 because it was decided to test in-situ 

biodechlorination technology for treatment of contaminated groundwater in this area. Emulsified 

soy oil and culture was injected into groundwater in 2004 (Earth Tech 2005) and sodium lactate 

was injected in 2005. Earth Tech (2005) recommended “that the SVE system is not re-started 

until further ground water treatment progress is made.” This recommendation was made due to 

the potential aeration of groundwater and its detrimental effect on anaerobic groundwater 

treatment implemented in 2004 (Earth Tech 2005). 

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TECHNICAL MEMORANDUM Pride Hangar Vadose Zone Soil Evaluation 

The Pride Hangar groundwater plume is currently being treated using in-situ dechlorination in 

accordance with an Explanation of Significant Differences (USAF 2007). Operation of the 

vacuum extraction system ended in 2007, and several in-situ reductive treatment (IRT) zones 

have been injected. The IRT monitoring results from this site have been reported by URS and 

the TCE concentration in groundwater has dropped from nearly 3,500 μg/L in June 2007 to 

below the MCL of 5 μg/L (URS 2010). 

3.0 Vadose Zone Sampling Results 

Vadose soil samples were collected from around the Pride Hangar during the 2002 direct push 

investigation (Earth Tech 2002). A map showing the 2002 direct push sample locations and the 

Pride Hangar AOC is presented in Figure 2. A table containing the laboratory results of soil and 

groundwater samples, taken from the April 2002 LTM report, is provided in Attachment 1. The 

reporting limits for TCE for the soil samples listed in Attachment 1 ranged from 5.3 μg/kg to 

7.0 μg/kg. Twenty-seven soil samples, including two duplicate samples, were collected from 13 

direct push borings in this area. Seven of the direct push borings were completed within the 

AOC and six were completed outside the AOC. Based on depth to groundwater measurements 

of 17 feet to 18 feet at this site, samples above 17 feet are considered to be vadose zone soil. 

Nine of 14 soil samples collected within the AOC were from the vadose zone. Seven of these 

vadose zone samples were from a depth of 0 feet to 0.5 feet (just beneath pavement) and the TCE 

results ranged from < 6 μg/kg to 120 μg/kg. Two of these vadose zone samples were from a 

depth between 15.5 feet and 16.5 feet and the results were < 5 μg/kg and < 6 μg/kg. One of 

these samples was from the area to the northwest and one was from near the UST abandonment 

location. The other soil samples from within the AOC were collected from the saturated zone 

below the water table and TCE was detected at concentrations ranging from 3.9J (estimated 

below reporting limit) to 79 μg/kg. 

Thirteen soil samples were collected from six direct push borings outside the AOC, including 

two duplicate samples. Of these 13 samples, eight were collected near ground surface (within 3 

feet) and only one sample was collected near the depth of groundwater. That sample was 

collected from a depth of 14 feet to 14.5 feet and the result was below detection limits. The 

other samples were collected from the saturated zone below the water table. 

4.0 Conclusions and Recommendations 

Vadose zone or unsaturated soil sample results from this site do not indicate that there is a 

significant source of contaminated soil that would threaten groundwater. The highest detected 

TCE concentration in the vadose zone was 120 μg/kg. This sample was collected just beneath 

surface pavement. A soil sample was collected from the same boring at a depth of 16 feet to 

16.5 feet (just above the water table) and the TCE concentration was below detection limits. The 

combination of an asphalt cap on the surface and a silty clay surficial soil unit in this area helps 

to limit the potential for vertical downward migration of any TCE that may be present just 

beneath the asphalt. 

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TECHNICAL MEMORANDUM Pride Hangar Vadose Zone Soil Evaluation 

Due to the low number of vadose zone soil samples collected within the AOC and the age of the 

samples (2002), additional sampling is recommended. Eight direct push soil borings are 

proposed at the locations shown on Figure 2 within the AOC. One additional direct push soil 

boring will be completed outside the AOC near DP02PH10 to confirm the 2002 results. Up to 

two soil samples should be collected from each of the nine borings and a photoionization 

detector (PID) should be used to screen the soil samples. One soil sample from each boring will 

be collected within 5 feet of the depth of groundwater (17 feet to 18 feet) and the selected sample 

depth will be based on the highest PID screening concentration within this interval. If no PID 

detections are measured from this interval, the sample will be collected from just above the water 

table. One additional soil sample will be collected above a depth of 12 feet if a PID screening 

concentration above 5 ppm is measured. If multiple intervals have PID concentrations above 5 

ppm, this sample will be collected from the interval of the highest PID screening concentration. 

For direct push borings near old borings DP02PH06 and DP02PH10, samples will be collected 

from the 0 foot to 0.5 foot depth for comparison to 2002 results. If higher PID readings are 

observed between 0.5 feet and 12 feet in these two borings, an additional soil sample will be 

collected. This could result in up to three soil samples from these two borings. All soil samples 

will be analyzed for VOC compounds by test method USEPA SW846 Method 8260B with 

reporting limits of 5 μg/kg. The VOC compounds will include the following: 1,1-DCA; cis-1,2-

DCE; trans-1,2-DCE; PCE; TCE; 1,1,1-TCA; and vinyl chloride. 

The soil samples will be collected in accordance with SOP No. 6 (URS 2006). The borings will 

be continuously sampled and logged using the Unified Soil Classification System from ground 

surface to the base of each boring. The samples will be collected using Encore® samplers in 

accordance with SOP No. 12 (URS 2006). 

The data collected during this proposed investigation will be used to evaluate whether or not the 

vadose zone soil in the Pride Hangar AOC may act as a continuing source of groundwater 

contamination. If the investigation determines the vadose zone soils do not threaten 

groundwater, then deletion of this AOC will be recommended. 

5.0 References 

Earth Tech. 2002. Serial Letter 1-43-RA-383 to U. S. Army Corps of Engineers, LTM, 

OU-11/April 2002 Quarterly Report, Ellsworth AFB, South Dakota. July 15. 

Earth Tech. 2003. Serial Letter 1-54-RA-301 to U. S. Army Corps of Engineers, Pride Hangar 

Source Remediation Recommendation, Ellsworth AFB, South Dakota. September 3. 

Earth Tech. 2005. Serial Letter ERS-3-RA-158 to U. S. Army Corps of Engineers, Pride 

Hangar Source Area Treatment/Updated Work Plan, Ellsworth AFB, South Dakota. 

February 21. 

URS Group, Inc. (URS). 2006. Final Basewide Quality Assurance Project Plan. Prepared for 

Ellsworth AFB/ACC- 4-Base PBC, Ellsworth AFB, South Dakota. December. 

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TECHNICAL MEMORANDUM Pride Hangar Vadose Zone Soil Evaluation 

United States Air Force (USAF). 1997. Final Record of Decision for Remedial Action at 

Operable Unit 11, Ellsworth Air Force Base, South Dakota. April. 

USAF. 2007. Explanation of Significant Differences to the Record of Decision for Operable 

Units 1, 2, 3, 4, 5, 6, 7, 8, 11, and 12. May 1. 

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TECHNICAL MEMORANDUM Pride Hangar Vadose Zone Soil Evaluation 

Final Pride Hangar Vadose Zone Soil Evaluation/Ver. 3.0 Page 5 of 5

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List of Figures 

Figure 1 EAFB Base Boundary, Operable Units & Other Areas of Concern 

Figure 2 Vadose Zone Soil Sample Results 

List of Attachments 

2002 Soil and Groundwater Sample Results 

Response to Comments 
t'

OPERABLEUNITS_OU!s)TO BE DELE'[ED

OPERABLEUNITS(OU'S) AND O_ER AREASOF DQNCERN_ADC'a)TO BE RET_JNE_ g' 500' 1000' 2000'

BASE BOUNDARYAS OF lingo DABE: DECEMBER 8.2005 SCALE

FIGURE 1

EAFB BASE BOUNDARY

OPERABLE UNITS & OTHER AREAS OF CONCERN

ELLSWORTHAFB, SOUTH DAKOTA

PREPARED FOR U,S, ARMY CORPS OF ENGINEERS

3056 001

TECHNICAL MEMORANDUM Pride Hangar Vadose Zone Soil Evaluation 

ATTACHMENT 1 

2002 Soil and Groundwater Sample Results

Final Pride Hangar Vadose Zone Soil Evaluation/Ver. 3.0

Ellsworth AFB/ACC 4-Base PBC 

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TECHNICAL MEMORANDUM Pride Hangar Vadose Zone Soil Evaluation 

ATTACHMENT 2 

Comments and Responses 

Final Pride Hangar Vadose Zone Soil Evaluation/Ver. 3.0

Ellsworth AFB/ACC 4-Base PBC 

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RESPONSE TO COMMENTS 

DRAFT REPORT 

PRIDE HANGAR VADOSE ZONE SOIL EVALUATION 

ELLSWORTH AFB, SOUTH DAKOTA 

Comments by Joane Lineburg, SDDENR, dated July 1, 2010 

Comments on Section 4.0 

Comment 1. Add text to specify what parameters the samples will be analyzed for. I 

recommend the samples be analyzed for the same parameters as the 2002 direct push 

investigation (1,1-DCA, cis-1,2-DCE, trans-1,2-DCE, PCE, TCE, 1,1,1-TCA, and vinyl 

chloride). 

Response: Agree. We will also add the test method (USEPA SW846 Method 

8260B) and reporting limits (5 μg/kg). 

Comment 2. For the proposed borings near DP02PH06 and DP02PH10, I recommend 

samples be collected from 0-0.5 feet bgs, regardless of PID readings, in order to have results that 

are directly comparable to the 2002 results. If there are higher PID readings in the interval 

between 0.5 feet and 12 feet, a soil sample should also be collected from that interval. For these 

two borings, three soil samples could potentially be collected. 

Response: Agree. The text will be revised to comply with this comment. 

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