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3033 Campus Drive North, Suite x75, Minneapolis, Minnesota 5544 I

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February 13, 2003

Mr. Mark Mailander

Area Engineer, Black Hills Area Office

U.S. Army Corps of Engineers

631 Saint Anne Street, Suite 101

Rapid City, South Dakota 57701-4667

Subject: Contract DACW45-94-D-0001, DO-48, WAD 1

Pride Hangar Source Remediation Options

Serial Letter 1-48-RA-362

FXBM 20027001

Dear Mr. Mailander:

Telephone

Earth Tech, Inc. (Earth Tech), has monitored an area of high trichloroethylene (TCE)

contamination near the northwest comer of the Pride Hangar. This letter summarizes the 763.55 x. _oo x

previously-reported results and evaluates remedial options for this area. By addressing this Facsimile

"hot spot" area as recommended herein, the required operational period of the remediation

system located down gradient may be shortened. 763.5 5_.2499

PRIDE HANGAR LIMITED DIRECT PUSH SOIL INVESTIGATION

Limited soil and ground water monitoring was conducted in May 2002 at the northwest

comer of the Pride Hangar to determine the extent of a potential source area identified during

previous direct push monitoring. The previous direct push work found TCE concentrations in

ground water samples of up to 30,000 ug/1. The May 2002 investigation consisted of the

advancement of nine direct push soil borings to further define the apparent source area. The

direct push soil borings were advanced to the water table. One ground water sample and up

to two soil samples were collected from each soil boring. The monitoring showed that the

main TCE source appears to be localized near the northwest comer of the Pride Hangar,

with another smaller source up gradient. The results of the supplemental investigation are

presented on Figure 1 and in Table 1 of the July 15, 2002, LTM report

(Serial Letter 1-43-RA-383).

EXISTING TREATMENT SYSTEM

The existing Pride Hangar total fluids extraction system consists of eight well points located

southeast of the Pride Hangar and a vacuum pump located in a below-grade equipment vault

immediately adjacent to the site. The system was designed to address a source area of

chlorinated volatile organic compounds (VOCs) near the Pride Hangar. Total fluids

(soil vapor andground water) are removed from the subsurface by applying a high vacuum to

the well points. Total fluids flow through underground vacuum conveyance piping to the

equipment vault. Vapor and liquid are separated in the vault. Soil vapor is discharged to the

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Mr. Mark Mailander

U.S. Army Corps of Engineers

February 13, 2003

Page 2

atmosphere through a vent stack. Ground water is pumped to the FRA Pump House No. 1

Treatment Facility.

The existing treatment system adequately addresses/prevents the down gradient migration of

the Pride Hangar TCE plume but does not rapidly address the contaminant hot spot located

northwest of the Pride Hangar.

SOURCE TREATMENT OPTIONS

Several remedial alternatives have been evaluated to determine the most practical and

effective method to treat the apparent source area that is located near the northwest comer of

the Pride Hangar. The main difficulty is the location on the operational side of the building,

which limits structures that can be placed in this area. Also, the size of the Pride Hangar and

the numerous underground utilities make an extension of the existing remediation system or

the installation of a new remediation system very difficult. For these reasons, we have

focused on remediation options that minimize utility and facility construction in this

congested area. The remediation strategies presented below address residual soil

contamination and ground water contamination.

GROUND WATER CONTAMINATION

Earth Tech recommends the application of Hydrogen Release Compound (HRC) to remediate

the area of elevated TCE in ground water. This is feasible because the source area is limited

in size, and there is ground water flow that will deliver the HRC to the down gradient portion

of the plume as it is being funneled to the Pride Hangar ground water extraction system.

Geology in this area is fairly uniform so the HRC can be injected through the entire thickness

of the aquifer. This is an innovative technique that has been used effectively elsewhere and

allows us to avoid the problems associated with utility and facility construction in this

congested area. The potential drawback of the technique is that, given the current

contaminant concentration, more than one HRC application may be required. Also, in rare

occasions ground water chemistry limits the effectiveness of HRC, but that concern can be

addressed prior to implementing the injection of HRC as the remedy. We will collect some

water chemistry information to determine the amount of HRC that would be required to

remediate the area of high TCE concentration, then design the HRC injection program based

on the water chemistry data.

SOIL CONTAMINATION

Previous monitoring indicates there is some residual contamination in the vadose zone and

capillary fringe in the "hot spot" area. The impacted area is currently covered with paving,

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Mr. Mark Mailander

U.S. Army Corps of Engineers

February 13, 2003

Page 3

which prevents water from infiltrating through the vadose zone and carrying contaminants

down to the ground water. However, as long as the TCE remains in the vadose zone, it is a

potential source of ground water contamination. Therefore, we recommend that this

contamination be remediated along with the ground water in the area of elevated

TCE concentration. Soil vapor extraction (SVE) would be the most effective remedial

technology. A temporary SVE system for this area would consist of two SVE wells, installed

in the delineated source area, and a small SVE blower. The wells would be connected via

above ground piping to the SVE blower and the off gas would be discharged to the

atmosphere. The SVE system would be run for a maximum one operational season running

from May to October. We believe the SVE system could be placed at the northwest comer

of the Pride Hangar with minimal disruption to operational activities in this area.

We will be prepared to discuss these recommendations with the RPM group during the

February 20 RPM meeting.

Thank you.

Very truly yours,

Earth Tech Environment & Infrastructure, Inc.

Kei ders .E.

Project Manager

File: 53039/01.2020

cc: Dell Petersen Len Havel

Jim Haggins Jeff Mashburn

Gary Schmidt Joane Lineburg

Dwight Pochant Greg Brooks

Mark Mercier Joe Odegaard

L:\WORK\TERC_53039_48\WPkDO-48 SERIAL LETTERS_48RA362.DOC

E A R T H _ T E C H

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