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30 3 3 C ampu s Dri v e N orth, Suite x75, M i nneap ol is, M innes o ta 55 44 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: C ontract D A CW4 5 -94-D-0 00 1, D O-48, WAD 1 Pride Hangar Source Remediation O ption s S e rial L e tte r 1- 4 8-RA- 36 2 FXBM 20027001 D ear Mr. Mailander: Tel ep h on e Earth T ec h, In c . (Earth T ec h) , ha s monitore d an are a of high trichl oro e thyl ene (T C E) c o ntaminati o n near the n orthw est c o mer 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 F ac si mi le "h ot sp ot" area as rec o mmen d e d he rein, th e require d op er atio nal peri od of the remed iatio n system l o c ated do wn gradi ent may b e sh ortened. 76 3.5 5 _. 24 99 PRIDE HANGAR LIMITED DIRE C T PUSH SOIL INVESTIGATION Limite d s oil an d grou nd wate r moni toring was c o n du c te d in May 20 0 2 at the northw es t c om e r of th e P rid e Han gar to d e te rmin e th e ex te nt of a p otential source are a id e ntifi e d duri n g pre viou s dire ct pus h mo nitoring. Th e p re vious direc t pus h w ork found T C E co nc entrations in groun d w ate r s amp l es of up to 3 0 , 0 00 u g / 1. Th e M ay 20 0 2 inv es tigati o n c on s i s te d of the ad vanc ement o f nine dire c t pu s h s oil boring s to f u rther d efine th e app are nt sou rce area. Th e direct p us h s oil borings we re adv an ced to the w ate r tab l e . One groun d w ate r samp le and u p to two s oil sampl e s w e re c oll ec te d fr om eac h s oil b o ring. T he mo nitorin g s ho we d that th e m ain T C E s ourc e app ears to b e lo c ali ze d n ear th e north wes t c om e r of the Prid e Hang ar, with an othe r small e r s ourc e u p gradi ent. Th e resul t s of th e s u pp l ement al inv es tigation are prese nte d o n Figure 1 an d in Tabl e 1 of the July 15, 20 0 2, LTM report (S e rial L e tter 1 -43 -RA- 383 ). EXISTING TREATMENT SYSTEM Th e ex i s ting Prid e Han g ar total fluid s extrac tion sy s tem consi s t s of eight w ell p oi nt s l oc ated s outheas t of th e Prid e Hang ar an d a vac uum pump l oc ated i n a belo w -gr ad e equip ment vault i mme diatel y adjacent to the s ite . T he s y s tem was d es ign e d to ad dress a s ource are a of chlorinate d v ol ati l e organi c c omp oun d s (VO Cs ) n e ar the Prid e Han g ar . Total fluids (so il v ap or and grou nd wate r) are remo ve d fro m the su bsu rfac e by app lying a high vac uum to th e w el l point s . T o tal fluid s fl ow throu gh un d e r grou nd vacuum c onv eyan ce p ip ing to the e qu ipm e nt vau lt. Vap o r an d liquid are sep arate d in the v ault. Soil v ap or i s di scharg e d to th e

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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 W

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TAMIN A TION 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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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

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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 SERI A L LETTERS_48R

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