## 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 co ntaminated 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 p ilot testing in May 2004 and intermittent operation of the system from July to November 2004. Implementation consisted of installation and operation of eight soil va por extraction (SVE) wells as temporary wells (grouted to the surface but w ithout surface protection). The SVE system was shut down November 15, 2004 because it was decided to test in-situ biodechlorination technology for trea tment 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) recomme nded “that the SVE system is not re-started until further ground water treatment progress is ma de.” 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 seve ral in-situ reductive treatment (IRT) zones have been injected. The IRT m onitoring results from this site have been reported by URS and the TCE concentration in groundwater has dro pped 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 ar ound the Pride Hangar during the 2002 direct push investigation (Earth Tech 2002). A map showi ng the 2002 direct push sample locations and the Pride Hangar AOC is presented in Figure 2 . A table containing the la boratory 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 th e 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 s ite, samples above 17 feet are consid ered 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. Tw o 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 northwe st and one was from near the UST abandonment location. The other soil samples from within th e 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 si x direct push borings ou tside the AOC, including two duplicate samples. Of these 13 samples, eight were collect ed near ground su rface (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 wa s 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 fr om the same boring at a depth of 16 feet to 16.5 feet (just above the water table) and the TCE concentrat ion was below detection limits. The combination of an asphalt cap on the surface and a silty clay surficial soil un it in this area helps to limit the potential for vertical downward migr ation 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 samp les collected within the AOC and the age of the samples (2002), additional sampling is recomm ended. 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 c onfirm 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 th e depth of groundwater (17 feet to 18 feet) and the selected sample depth will be based on th e 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 collec ted above a depth of 12 feet if a PID screening concentration above 5 ppm is measured. If multip le 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 DP 02PH06 and DP02PH10, samples will be collected from the 0 foot to 0.5 foot depth for comparis on to 2002 results. If higher PID readings are observed between 0.5 feet and 12 feet in these tw o borings, an additional soil sample will be collected. This could result in up to three soil samples from these two bo rings. All soil samples will be analyzed for VOC compounds by te st method USEPA SW846 Method 8260B with reporting limits of 5 μg/kg. Th e VOC compounds will include th e 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 lo gged using the Unified Soil Cl assification System from ground surface to the base of each bor ing. The samples will be collected using Encore® samplers in accordance with SOP No. 12 (URS 2006). The data collected during this propo sed investigation will be used to evaluate whether or not the vadose zone soil in the Pride Hangar AOC ma y act as a continuing source of groundwater contamination. If the investigation determin es 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, Ellswo rth 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, Ellswo rth 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 Qu ality Assurance Project Plan. Prepared for Ellsworth AFB/ACC- 4-Base PBC, Ellswo rth 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 Si gnificant 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 Ellsworth AFB/ACC 4-Base PBC FA8903-04-D-8679, DO 0053 Q:\1616\9962\Ellsworth\Tech Memos\PH Vadose Zone Soil Evaluation Ver 3\Pride Hangar Vadose Zone Soil Eval Ver 3.doc/Aug-10 /OMA List of F igures Figure 1 EAFB Base Boundary, Operable Units & Other Areas of Concern Figure 2 Vadose Zone Soil Sample Results

List of A ttachments 2002 Soil and Groundwater Sample Results Response to Comments

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OPERABLE

UNITS

_ OU ! s ) TO BE DELE ' [E D
OPERABLE

U NI TS (OU'S) AND O_ ER AREAS O F DQ N CERN _AD C'a) T O B E RET

_ JNE

_ g' 500' 1000 ' 2000' B A SE B O UNDARY

AS O F l i n go DABE: DECEMBER 8.2005 SCALE 

### F I GURE 1 EAFB BASE BOUNDARY OPERABLE UNITS & OTHER AREAS OF CONCERN ELLS W ORTH

AFB , SOUTH DAKOTA PREPARED FOR U , S , ARMY C ORPS OF ENG I NEERS



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## TECHNICAL MEMORANDUM Pride Hangar Vadose Zone Soil Evaluation

ATTACHMENT 1 2002 Soil and Groundwater Sample Results

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## TECHNICAL MEMORANDUM Pride Hangar Vadose Zone Soil Evaluation

ATTACHMENT 2 Comments and Responses

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6957 010RESPONSE 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, tran s-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 highe r 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 c ould potentially be collected. Response: Agree. The text will be revised to comply with this comment.

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