AMENDMENT TO THE RECORD OF DECISION FOR PHASE I … · 2021. 3. 18. · Larry Sparks 2/24/2016 DOE...

U.S. Department of Energy Oak Ridge Office of

Environmental Management

Document Number: DOE/OR/01-2697&D2 Febmary 2016

Amendment to the Record of Decision for Phase I Interim Source Control Actions in the

Upper East Fork Poplar Creek Characterization Area, Oak Ridge, Tennessee

Water Treatment at Outfall 200

APPROVED FOR PUBLIC RELEASE

This document has been approved for release to the public by:

Larry Sparks 2/24/2016 DOE Oak Ridge Office Classification Officer Date

Amendment to the Record of Decision for Phase I Interim Source Control Actions in the Upper East Fork Poplar Creek Characterization Area, Oak Ridge. Tennessee, Water Treatment at Outfall 200

Document Nuraher; DOE/OR/01-2697&D2

Page: i

APPROVALS

Amendment to the Record of Decision for Phase I Interim Source Control Actions in the Upper East Fork Poplar Creek Characterization Area, Oak Ridge, Tennessee:

Water Treatment at Outfoll 200

DOE/OR/01-2697&D2

Susan M. Carige, Manager Oak Ridge Office of Environmental Management

Shari L. Me{ Deputy Commissioner Tennessee Departni^t of Enviror ent and Conservation

E. Hill, Director Superfund Division U.S. Environmental Protection Agency, Region 4

Date

=3^

S'A'^/W6 Date

Amendment to the Record of Decision for Phase I Interim Source Document Number: DOE/OR/01-2697&D2 Control Actions in the I'pper East Eork Poplar Creek Characterization Area, Oak Ridge, Tennessee, Water Treatment at Outfall 200 Page: 11

CONTENTS

APPROVALS i

CONTENTS ii

FIGITRES iii

TABLES iii

ACRONAMS iv

PREFACE vi

1.0 SITE NAME AND LOCATION 1

2.0 INTRODUCTION AND PITRPOSE 1

3.0 SITE HISTORY, CONTAMINATION, and SELECTED REMEDY 2

3.1 OVERVIEW OF Y-12 NATIONAL SECITRITY COMPLEX AND liPPER EAST FORK POPLAR CREEK 2

3.2 SUMALARY OF THE UEFPC PHASE I ROD 6

4.0 BASIS FOR THE DOCUMENT 10

4.1 CONCEPTUAL SITE MODEL 12

5.0 DESCRIPTION OF SIGNIFICANT DIFFERENCES 13

5.1 REMEDIAL ACTION OBJECTIVES 13

5.2 IDENTIFICATION OF NEW REMEDIAL ACTION ALTERNATIVES 15

5.2.1 .Alternative 1. No Further .Action 15

5.2.2 .Alternative 2. Water Treatment at Outfall 200 16

5.2.2.1 .Alternative 2a. Water Treatment at Outfall 200 witli 1500 gpm Treatment Capacity and No Stoimwater Storage 18

5.2.2.2 .Alternative 2b. Water Treatment at (!)utfall 200 with 3000 gpm Treatment Capacity and No Stoimwater Storage 19

5.2.2.3 .Alternative 2c. Water Treatment at (!)utfall 200 witli 3000 gpm Treatment Capacity and 2 Million Gallons Stoimwater Storage 19

5.2.2.4 .Alternative 2d. Water Treatment at (!)utfall 200 with 3000 gpm Treatment Capacity and 10 Million Gallons Stoimwater Storage 20

5.2.3 Change in Facility Location 21

6.0 EVALUATION OF ALTERNATIVES 22

6.1 COMP ARATIVE AN ALYSIS OF ALTERN ATIVES 22

6.1.1 (Xerall Pi otection of Human Health and tlie Envii onment 22

6.1.2 Compliance with .ARARs 23

6.2 SELECTED REMEDY 26

6.3 CONCLUSION 31

7.0 EPA AND TDEC COMMENTS 31

Amendment to the Record of Decision for Phase I Interim Source Document Number: DOE/OR/01-2697&D2 Control Actions in the I'pper East Eork Poplar Creek Characterization Area, Oak Ridge. Tennessee. Water Treatment at Outfall 200 Page: ill

8.0 STATUTORY DETERMINATIONS 31

9.0 PUBLIC P ARTICIPATION COMPLIANCE 32

lO.O REFERENCES 32

APPENDIX A. RESPONSIVENESS SUM^LARY A-I

APPENDIX B. APPLICABLE OR RELEVANT AND APPROPRIATE REQUIREMENTS B-I

FIGURES

Figure I. Location of the Upper East Fork Poplar Creek Watershed 4

Figure 2. Suspected Areas of Mercury Contamination in UEFPC Watershed 5

Figure 3. Mercury Concentration in UEFPC Surface Water at Station 17 and in Fish II

Figure 4. Outfall 200 Water Treatment System Preliminary System Process Flow Diagram 27

Figure 5. Outfall 200 Water Treatment System Preliminary Site Location 28

TABLES Table I. Remedial Action Objectives 15

Table 2. Comparative Analysis of Remedial .Alternatives 24

Table 3. Summarv of Remedial .Actions under the Selected Remedy for Modification of the UEFPC Phase I ROD .". 29

Table 4. Cost Estimate for Outfall 200 Selected Remedy 29

Amendment to the Record of Decision for Phase I Interim Source Control Actions in the I'pper East Fork Poplar Creek Characterization Area, Oak Ridge, Tennessee, Water Treatment at Outfall 200

Document Number: DOE/OR/01-2697&D2

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ACRONYMS

.\EA Atomic Energv Act of 1954

As Low As Reasonably Achievable

.\R.\R Applicable or Relevant and Appropriate Requirement

.\RRA .American Recoverv and Reinvestment .Act of 2009

.A\VQ)C .Ambient Water Q)uality Criteria

BSWTS Big Spring Water Treatment System

C.A Characterization .Area

CERCL.A Comprehensive Env ironmental Response. Compensation, and Liability .Act of 1980

CFR Code of Federal Regulations

COC Contaminant of Concern

D&D Decontamination and Decommissioning

DOE U.S. Depaitment of Energy

ENIWMF Env ironmental Management Waste Management Facility

EP.A U.S. Environmental Piotection .Agency

LSD Explanation of Significant Differences

ETTP East Tennessee Technology Park

FF.A Federal Facility .Agreement

FES Focused Feasibility Study

FS Feasibility Study

gpm Gallons Per Minute

LEW Low-Level Waste

LUC Land Use Contiol

LUCIP Land Use Contiol Implementation Plan

Mgd Million Gallons Per Day

NCP National Oil and Hazardous Substances Pollution Contingency Plan

NEP.A National Env ironmental Policy .Act of 1969

NNS.A National Nuclear Security .Administration

NPDES National Pollutant Discharge Elimination System

NP(!) Y-12 National Nuclear Security .Administration Pioduction (!)ffice

NSC Non-Signrficant Change Notice

(!)&M (!)perations and Maintenance

(!)RNL (!)ak Ridae National Laboratorv

Amendment to the Record of Decision for Phase I Interim Source Control Actions in the I'pper East Fork Poplar Creek Characterization Area, Oak Ridge, Tennessee, Water Treatment at Outfall 200


Pase: v

ORR (!)ak Ridge Resen ation

PCB Polychlorinated Biphenyl

PIDAS Y-12 Perimeter Intrusion Detection and Assessment System

ppt Parts Per Trillion

¥L\0 Remedial Action (!)bjective

RCR.\ Resource Conserv ation and Recov ery Act of 1976

RDWP Remedial Design W ork Plan

RI Remedial Investigation

ROD Record of Decision

S&M Surveillance and Maintenance

TBC To Be Considered

TCA Tennessee Code .Annotated

TDEC Tennessee Department of Env ironment and Conserv ation

ITEFPC Upper East Eork Poplar Creek

UCOR ITRS 1 CH2M Oak Ridge LLC

WAC W aste .Acceptance Criteria

WENU W est End Mercury .Area

Y-12 Y-12 National Security Complex

Amendment to the Record of Decision for Phase I Interim Source Document Number: DOE/OR/01-2697&D2 Control Actions in the I'pper East Eork Poplar Creek Characterization Area, Oak Ridge, Tennessee, Water Treatment at Outfall 200 Page: \i

PREFACE

Amendment to the Record of Decision for Phase I Interim Source Control Actions in the Upper East Fork Poplar Creek Characterization Area. Oak Ridge. Tennessee (D(!)E (!)R 01-2697&D2) was prepared in accordance with requirements of tlie Comprehensive Env ironmental Response. Compensation, and Liabilitv Act of 1980 (CERCLA) to document tlie modification of the selected remedy for enviionmental remediation of contaminated areas w ithin the Upper East Fork Poplar Creek (L^FPC) characterization area (CA). This record of decision (R(!)D) amendment documents a modification to the selected interim remedy agreed on by tlie U.S. Department of Energy (D(!)E). the Tennessee Department of Environment and ConseiAation (TDEC). and tlie U.S. Enviionmental Piotection .Agency (EP.\). This modification includes tlie constiaiction and operation of a new water treatment facility to furtlier reduce mercuiv discharges from tlie Y-12 National Security Complex to LT!FPC surface water. Tliis modification will supplement tlie response actions already included in the selected remedy - which address mercury-contaminated sediment in stoiTn sewers, point groundwater discharges, and mercury-contaminated sediment in LT!FPC and Lake Reality, each of which contributes to contamination of suiface water w ithin the LT!FPC watershed - to achieve further reductions in mercuiv concentiations in LT!FPC surface water and releases to tlie offsite env ironment. This remedy will be implemented to the extent practicable while minimizing disruption of the continuing mission of tlie Y-12 National Security Complex (Y-12. formerly the (!)ak Ridge Y-12 Plant). This decision is based on tlie .Administiative Record file for this project, which includes the following principal documents:

• Report on the Remedial Investigation of the Upper East Fork Poplar Creek Characterization Area at the Oak Ridge Y-12 Plant. Oak Ridge. Tennessee. DOE OR 01-1641 V1-V4&D2 (DOE 1998):

• Feasibility Study for the Upper East Fork Poplar Characterization Area at the Oak Ridge Y-12 Plant. Oak Ridge. Tennessee. DOE OR 01-1747&D2 (DOE 1999):

• Addendtim to the Feasibility Study for the Upper East Fork Poplar Creek Characterization Area at the Oak Ridge Y-12 Plant. Oak Ridge. Tennessee. DOE OR 01-1747&D3 .A3 (DOE 2000):

• Record of Decision for Phase I Interim Source Control Actions in the Upper East Fork Poplar Creek Characterization Area. Oak Ridge. Tennessee. DOE OR 01-1839&D3 (DOE 2001):

• Focused Feasibility Study for Water Treatment at Outfall 200 tinder the Record of Decision for Phase I Interim Source Control Actions in the Upper East Fork Poplar Creek Characterization Area. Oak Ridge. Tennessee. DOE OR 01-2660&D3 (DOE 2015a): and

• Proposed Plan for Water Treatment at Outfall 200 under the Record of Decision for Phase I Interim Source Control Actions in the Upper East Fork Poplar Creek Characterization Area. Oak Ridge. Tennessee. DOE OR 01-2661&D2 (DOE 2015b).

These documents and otlier information supporting the selected remedial action can be found at tlie D(!)E InfoiTnation Center. 1 Science.aov Wav. Oak Ridae. Tennessee 37830. (865) 241-4780.

Amendment to the Record of Decision for Phase I Interim Source Document Number: DOE/OR/01-2697&D2 Control Actions in the I'pper East Eork Poplar Creek Characterization Area, Oak Ridge, Tennessee, Water Treatment at Outfall 200 Page: 1

1.0 SITE NAME AND LOCATION

U.S. Depaitment of Energy (DOE) (;)ak Ridge ReseiA ation ((;)RR) Upper East Fork Poplar Creek Characterization .Area (L^FPC) Oak Ridge. Tennessee CERCLIS ID IN 1890090003

2.0 INTRODUCTION AND PURPOSE

This decision document presents a modification to the selected remedy for interim remedial actions for remediation of specified areas witliin tlie Upper East Fork Poplar Creek (LT!FPC) Characterization .\i ea (C.\) at the Y-12 National Security Complex (Y-12. formerly the (Yak Ridge Y-12 Plant) on the U.S. Depaitment of Energy (D(!)E) (Yak Ridge Resenation ((!)RR) in (Yak Ridge. Tennessee. Remediation of the LT!FPC W atershed is being conducted tluough a multi-phase remedial action program under the Comprehensive Env iionmental Response. Compensation, and Liability .Act of 1980 (CERCL.A). The fiist stage of tliis program was documented in tlie Record of Decision for Phase I Interim Source Control Actions in the Upper East Fork Poplar Creek Characterization Area. Oak Ridge. Tennessee (D(!)E 2002) (herein refened to as the LT!FPC Phase I R(9D). which was signed on May 2. 2002. by D(9E. the Tennessee Depaitment of Environment and Conseivation (TDEC). and tlie U.S. Enviionmental Piotection .Agency (EP.A). The LT!FPC Phase 1 R(!)D selected remedy included a series of interim source conti ol actions designed to address the most significant sources of mercuiv contamination in LT!FPC for which sufficient data existed at tliat time to support appropriate remedy selection decisions. Some of the remedial actions selected in the LT!FPC Phase 1 R(!)D have been successfully completed while otliers are still scheduled for future implementation.

Wliile the actions completed to date have achieved significant reductions in tlie mercuiv releases from the site, tlie lev el of mercury in LT!FPC suiface water remains abov e the interim goal established in the LT!FPC Phase 1 R(!)D and applicable regulatoiv criteria. Therefore, this R(!)D amendment modifies tlie selected remedy to include constiaiction and operation of a new water treatment facility designed to achieve fuitlier reductions in mercury releases and concentrations in the offsite env ironment. This new water tieatment system will treat discharges from tlie stoiTn sewer system adjacent to tlie foiTner mercuiv-use buildings in the W est End Mercury .Aiea (WEM.A). which cunently constitutes the largest source of mercury releases to LT!FPC. The integration point for the WENLA storm sewer network is a location designated (!)utfall 200. The new water tieatment facility will be constiaicted to treat discharges from (!)utfall 200 with a treatment capacity of 3000 gallons per minute (gpm) or 4.3 million gallons per day (Mgd) of inlluent suiface water and storage capacity for stoiTnwater flows in excess of ti eatment capacity up to 2 million gallons to manage stieam flows up to 40.000 gpm.

Constiuction and operation of tliis new water treatment facility to treat discharges from tlie WEM.A stoiTn sewer sy stem constitutes a fundamental change to the selected remedy in the LT!FPC Phase 1 R(!)D. and requires documentation under tlie R(!)D amendment process, pursuant to CERCL.A Section 117 and Section 300.435(c)(2)(ii) of the National (!)il and Hazardous Substances Pollution Contingency Plan (NCP). (!)ther components of tlie remedial actions identified in the LT!FPC Phase 1 R(!)D selected remedy are unaffected by tliis R(!)D amendment. No otlier fundamental changes to tlie LT!FPC Phase 1 R(!)D remedial actions are being proposed, and tlierefore. other components of tlie LT!FPC Phase 1 R(!)D selected remedy are not witliin tlie scope of this R(!)D amendment.

Amendment to the Record of Decision for Phase I Interim Source Document Number: DOE/OR/01-2697&D2 Control Actions in the I'pper East Eork Poplar Creek Characterization Area, Oak Ridge. Tennessee. Water Treatment at Outfall 200 Page: 2

This modified set of remedial actions for tlie LT!FPC watershed was chosen in accordance w itli the requirements of CERCL.A. as amended by the Superfund .Amendments and Reauthorization .Act of 1986 (S.AR.A) (42 United States Code Sect. 9601 et seq.). and. to the extent practicable, the NCR [40 Code of Federal Regulations (CFR) 300]. The Federal Facility .Agreement (FF.A) (D(9E 1992) for (9ak Ridge was developed to integrate the requiiements of CERCL.A and the Resource Consenation and Recoveiy .Act of 1976 (RCR.A) and to prov ide a legal framework for remediation activ ities at (!)RR. This integrated approach extends to preparation of decision documents under CERCL.A and RCR.A. In addition. National Env ironmental Policy .Act of 1969 (NEP.A) values are incoiporated in tlie documents prepared for tliis project in accordance witli tlie Secretarial Policy Statement on the National Environmental Policy Act of 1969 (D(9E 1994). This policy states that D(9E will rely on the CERCL.A process for rev iew of actions taken under CERCL.A and will address and incoiporate NEP.A v alues in CERCL.A ev aluations to the extent practicable. .Although not included as part of the selected remedy in tlie LT!FPC Phase 1 R(!)D. feasibility study alternatives that included water treatment at (!)utfall 200 were evaluated under CERCL.A and the National Enviionmental Policy .Act of 1969 (NEP.A). and deemed to be protective of human health and the env ironment.

This R(!)D amendment and other inlbiTnation supporting the selected remedy is part of the .Administi ativ e Record file for tlie LT!FPC W atershed, and is available tluough tlie D(!)E InlbiTnation Center. 1 Science.gov W ay. (!)ak Ridge. Tennessee 37830. from 8:00 am to 5:00 pm Monday tlirough Friday (telephone 865-241-4780). Documentation in tliis file includes the focused feasibility study (FFS) (DOE 2015a) and the proposed plan (DOE 2015b) developed specifically for evaluation of this modification of the selected remedy, as well as the original LT!FPC remedial investigation (RI) report (D(!)E 1998). draft feasibility study (FS) (DOE 1999). FS .Addendum (DOE 2000). proposed plan (DOE 2001) and LT!FPC Phase 1 R(!)D (D(!)E 2002). In addition. D(!)E has considered all comments received on tlie proposed plan in preparing this R(!)D amendment.

This document is issued by D(!)E. as tlie lead agency . The EP.A and TDEC are support agencies as parties of the FF.A for this response action.

3.0 SITE HISTORY, CONTAMINATION, AND SELECTED REMEDY

3.1 OVERVIEW OE V-12 N.ATION.AL SECURITY COMPLEX .AND UPPER E AST EORK POPL AR CREEK

The 34.000-acre (!)RR is located w ithin and adjacent to the coiporate limits of tlie city of (!)ak Ridge. Tennessee, in Roane and .Anderson Counties. The (!)RR is bounded to the east and north by tlie dev eloped portion of the city of (!)ak Ridge. The (!)RR hosts tluee major industrial research and production facilities originally constmcted as part of the W orld W ar 11-era Manhattan Pioject: East Tennessee Teclinology Park (ETTP). Oak Ridge National Laboratory (ORNL). and the Y-12 National Security Complex (Y-12. foiTnerly the Oak Ridge Y-12 Plant) (Figure 1).

The boundaries of the LT!FPC w atershed, which includes approximately 1170 acres that encompasses tlie industrialized area of Y-12. extend along the top of Pine Ridge to the noitli. tlie top of Chestnut Ridge to the soutli. tlie eastern boundaiy of tlie Bear Creek \'alley w atershed to the west, and the D(!)E property line to the east (Figure 1). Major features of LT!FPC with respect to mercury contamination are summarized in Figure 2.

The Y-12 National Security Complex was built by tlie U.S. .Anny Coips of Engineers in 1943 as part of the W orld W ar 11-era Manhattan Pioject. and remains an active manufacturing and developmental


engineering facilih. It occupies approximately 600 acres within Bear Creek \'alley near the northeastern corner of the (!)RR. adjacent to tlie city of (!)ak Ridge. The original mission of tlie facility w as to chemically separate and produce fissile from using an electromagnetic separation process (alpha process) and to manufacture weapons components as part of the national effort to produce tlie atomic bomb. .\s otlier uranium enricliment processes were dev eloped and implemented at otlier installations, tlie role of Y-12 expanded to include weapon components manufactui ing and precision machining, research and development, lithium isotope separation, and special nuclear materials storage and management. The cunent mission of the installation is multifaceted and includes tlie follow ing National Nuclear Security .Administration (NNS.A) assignments: manufacturing and reworking nuclear weapons components, dismantling nuclear weapons components, sen ing as tlie nation's stockpile for special nuclear materials, and prov iding special production support to otlier programs.

Historic manufacturing processes, programs, and waste management practices associated witli Y-12*s mission hav e resulted in tlie contamination of soil, suiface water, sediment, and gi oundwater. These processes included chemical separation techniques: weapons manufacturing: research and dev elopment: waste storage, management, and disposal: and physical plant maintenance activ ities. These processes also resulted in the release of large quantities of mercury to tlie env ii onment. .\s a result of tliese historical releases, mercuiy contamination is present in onsite soils, sediments and building stmctures. and in offsite surface water, sediments and biota. Because of the contaminant releases at Y-12 and otlier D(!)E facilities, tlie (!)ak Ridge Resei^ation was placed on the U.S. Environmental Piotection .Agency's (EP.A's) National Piiorities List established under tlie Comprehensive Env ironmental Response. Compensation, and Liability .Act of 1980 (CERCL.A) [54 Federal Register 48184. November 21. 1989].

Remediation of the LT!FPC w atershed is being conducted in stages using a phased approach. The Record of Decision for Phase I Interim Source Control Actions in the Upper East Fork Poplar Creek Characterization Area. Oak Ridge. Tennessee (D(!)E 2002) constitutes tlie initial phase and adckesses inteiim actions for remediation of principal-tlireat. mercury-contaminated soil, sediment, and point groundwater discharges tliat contribute contamination to suiface water. The Record of Decision for Phase II Interim Remedial A ct ions for Contaminated Soils and Scrapyard in Upper East Eork Poplar Creek. Oak Ridge. Tennessee (D(9E 2006a) (LT!FPC Phase 11 R(9D) was issued in 2006 for the remediation of tlie balance of contaminated soil, scrap, and buried materials at the Y-12 site. Decisions regarding final land use and final goals for surface water, groundwater, and soil for tlie watershed will be addressed in future decision documents.

.As shown in Figure 2. LT!FPC flows directly from Y-12 into the City of (!)ak Ridge. The stoiTn sewer network seiA icing tlie foiTner mercuiy processing buildings in tlie W est End Mercuiy .Aiea has become contaminated from mercuiy contamination in soil and groundwater. This contaminated storm drain netw ork discharges tlu ough a series of outfalls into LT!FPC suiface water. Cun ent and historical contaminant releases from the LT!FPC watershed exit tlie (!)RR v ia suiface water (LT!FPC at Station 17) and aroundwater (east into Union \'allev).

Amendment to the Record of Decision for Phase I Interim Source Control Actions in the Upper East Fork Poplar Creek Characterization Area, Oak Ridge, Tennessee, Water Treatment at Outfall 200


Page: 4

Figure 1. Location of the Upper East Fork Poplar Creek Watershed

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3.2 SUM^URV OF THE UEFPC PHASE I ROD

The LT!FPC Phase I R(!)D considered tliree alternativ es for remediation of specified areas within the LT!FPC watershed. Tliese alternatives all included similar source contiol actions (e.g.. hydraulic isolation of contaminated soils and cleanout relining of contaminated sewer lines in tlie W est End Mercuiy .\i ea. excavation of contaminated sediments from LT!FPC and Lake Reality), land use controls, and monitoring, but tliev differed primarily in the extent of water ti eatment operations proposed. .Alternativ e 3a ("Source Control") included consti uction of a relativ ely small (300 gpm. 0.43 Mgd) water tieatment facility at Building 9201-2 to treat water from in-leakage of groundw ater into the basement of this building and the adjacent Outfall 51. while .Alternative 4a ("Migration Control Using W ater Treatment at Station 17") called for constmction of a much larger (10.400 gpm. 15 Mgd) water treatment facility at Station 17 (near the location where LT!FPC exits tlie A'-12 site), and .Alternative 6a ("Migration and Source Contiol witli W ater Treatment at Outfall 200") called for constiuction of an inteiTnediate capacity (3000 gpm. 4.3 Mgd) water treatment facility at (!)utfall 200.

.All tliree alternatives were deteiTnined to meet tlie CERCL.A tlireshold criteria of protectiveness and compliance with .AR.ARs (witli an interim waiver for tlie .AW"Q)C in-stream standard for mercuiv). and .Alternativ e 3a was deteiTnined to prov ide the best balance of trade-offs among the alternativ es with respect to tlie CERCL.A balancing and modify ing criteria. .Alternative 3a was estimated to have tlie lowest cost of all alternativ es, at less than one-half tlie estimated cost of tlie otlier action alternativ es, and to have the least uncertainties regarding implementability and long-term effectiveness. In particular. TDEC expressed concerns regarding the selection of a remedy that was dependent on the ability of a large-scale water tieatment facility to consistently achieve the desired levels for mercuiv. and expressed a preference to proceed with source control actions and to defer decisions on large-scale water tieatment to future decision documents.

.As reflected by the inclusion of "Phase I" in the title, the LT!FPC Phase I R(!)D was designed to be the First stage of a multi-stage remediation program. The LT!FPC Phase I R(!)D selected .Alternativ e 3a. Source Contiol. as the response action that best met the CERCL.A evaluation criteria to achieve tlie Remedial .Action (!)bjective (R.A(!)): "restore suiface water to human healtli recreational risk-based values at Station 17". The selected alternativ e focused on a series of interim source contiol actions designed to reduce the release of mercuiv to tlie offsite env ironment. These actions were designed to address the most significant sources of mercury contamination in LT!FPC for which sufficient data existed at that time to support appropriate remedy selection decisions tluough the CERLC.A process. The interim source control actions selected in tlie LT!FPC Phase I R(!)D included:

• Hy draulic isolation of contaminated soils and cleanout relining of contaminated sewer lines in the W est End Mercury .Aiea:

• Excav ation of contaminated sediments from LT!FPC and Lake Reality :

• Consti uction and operation of a water treatment sy stem at Building 9201-2 to treat discharge from (9utfall 51:

• Continued operation of prev iously existing water tieatment systems [i.e.. Central Mercury Treatment Sy stem (CMTS) and East End Mercuiv Treatment Sy stem (EEMTS)] as needed:

• Land use controls to prevent fish consumption and to restrict access to contaminated areas: and

• Surface water monitorina to evaluate reductions in contaminant concentiations.


In addition to these source control actions, the selected remedy also included tluee short-term studies and two long-term studies to evaluate potential additional response actions:

• The teclinical feasibility of a horizontal groundwater capture well as an additional component of hydraulic isolation of the WENLA:

• The depth and mobility of contamination and alternativ e technologies for in situ treatment of mercury-contaminated soil at tlie Building 81-10 site:

• Treatment and disposal options for soil and sediment that fail to meet tlie W aste Acceptance Criteria (W"AC) for tlie Enviionmental Management W aste Management Facility (EMWMF):

• Ev aluation of the v iability of large-scale ti eatment of mercury-contaminated suiface water in LT!FPC: and

• Groundw ater studies to facilitate a better understanding of the dynamics of tlie groundw ater plumes underlying the LT!FPC watershed.

Piev ious modifications to tlie LT!FPC Phase I R(!)D have included tliree non-significant change (N-SC) notices and the Explanation of Significant Differences for the Record of Decision for Phase I Interim Source Control Actions in the Upper East Fork Poplar Creek Characterization Area (D(!)E 2012a). which documented more significant changes:

• A non-significant change was documented in 2006 (D(!)E 2006b) to modify the LT!FPC suiface water monitoring requirements to upgrade sampling equipment at Station 200A6 for collection of continuous mercuiy flux samples as 7-day composites and discontinue sampling at Outfalls 150. 160. 163. and 169 until one year prior to the WEMA remedial actions.

• non-significant change was documented in 2006 (D(!)E 2007) to discontinue ti eatment of water collected in sumps at Building 9201-5 (.Alpha 5) at the Central Mercuiy Treatment System due to tlie leakage of brine solution from cooling sy stems into the building sumps: metlianol in tlie brine solution was found to contribute to enlianced bacterial growth at CMTS which negativ ely impacted the system ti eatment efficiency. W ater is being allowed to accumulate in tlie basement of Building 9201-5.

• non-significant change was documented in 2014 (DOE 2014a) to clarify tliat monitoring requii ements and sampling protocols for LT!FPC will be documented in the East Eork Poplar Creek and Chestnut Ridge A dministrative U 'atersheds Remedial A ction Report Comprehensive Monitoring Plan (D(9E 2014b). rather than tlie LT!FPC Remedial Design Work Plan (RDWP) as stated in the LT!FPC Phase 1 R(9D.

• .\n ESD was approved in 2012 (DOE 2012a) to modify components of the selected remedy: (1) the constiaiction of interim asphalt caps ov er approximately 3.5 acres of unpav ed areas at WEMA was eliminated: (2) tlie schedule for excav ation of contaminated sediments from LT!FPC and Lake Reality was rev ised to be consistent witli the overall remediation stiategy to conduct remedial action for LT!FPC in a generally upgradient-to-downgradient sequence: and (3) two treatability studies that are no longer considered useful (ev aluations of horizontal groundwater capture well, and in-situ treatment of soils at 81-10 area) were eliminated.

Some of tlie response actions selected in the LT!FPC Phase I R(!)D hav e been successfully completed while others are still scheduled for future implementation. The following response actions selected in tlie LT!FPC Phase I R(!)D have been completed or are cunently in operation:


• W ater Treatment at (!)utlall 51 - The Big Spring W ater Treatment System (BSWTS) was constructed in FY2005 to treat mercur\-contaminated discharge from (!)utfall 51 (including the large-volume spring designated Big Spring located near the southeast corner of Building 9201-2) and water from the Building 9201-2 sumps. BSWTS has a treatment capacity of 300 gpm. Mercuiy contamination witliin shallow groundwater beneatli and adjacent to Building 9201-2 discharges at this spring. Following constmction in FY2005. mercury-contaminated water was rerouted from (!)utfall 51 and the Building 9201-2 sumps to the BSWTS in December 2006. Influent previously treated at the East End Mercuiv Treatment System also was rerouted to tlie BSWTS at tliis time and the EEMTS operation was discontinued. During FY2014. the av erage concentration of mercuiv in BSWTS influent was 4.2 pg L and 0.026 pg L in system effluent: only one of the weekly composite samples exceeded the peifoiTnance goal of 200 ppt (0.2 pg L) specified in the R(!)D. During FY2014. the v olume of water treated at BSWTS was approximately 99 million gallons, and tlie total mercuiv flux discharged in tlie tieated effluent was approximately 6 grams. In addition, water bypassing tieatment during periods of high flow during FY2014 contributed an estimated mercury flux of approximately 10 grams. (D(9E 2015c)

• Hydraulic isolation actions in tlie W est End Mercuiv .\i ea (WTM.\) - Cleaning and repaii" of the stoiTn sewer network in WENLA was initiated in FY2009. More than 20.000 linear-feet of storm sewer lines were inspected using v ideotape to deteiTnine their condition. During FY2011. more than 8.000 linear-feet of tliese sewer lines were cleaned and approximately 1.200 linear-feet were re-lined. The constmction of temporaiv asphalt caps over approximately 3.5 acres of unpaved areas in WTNL\ was eliminated under the ESD (DOE 2012a): this component of hydraulic isolation for WTM.\ soils was determined to be no longer needed, due to the acceleration of the schedule for demolition of the WTNL\ foiTner mercuiv-use buildings, making contaminated soils in these areas accessible for excav ation, where appropriate, under tlie Phase 11 R(!)D.

• Continued operation of prev iouslv existing water tieatment systems - The selected remedy included the continued operation of prev iouslv existing treatment systems for treatment of mercuiv contaminated waters as needed. These included tlie East End Mercury Treatment System (EEMTS). which continued operation only until the new BSWTS was constmcted. and is no longer in operation: and the Central Mercury Treatment System (CMTS). which continues operation today. CMTS was designed to ti eat mercury contaminated water collected in sumps at the WTNL\ buildings, most notably Buildings 9201-4 and 9201-5. with a treatment capacity of 50 gpm. Treatment of water from the sumps in Building 9201-5 was discontinued follow ing an accidental introduction of methanol from a leaking cooling system in 2005 that inteifered witli mercury treatment, but treatment of sump water from Building 9201-4 (a much larger source of mercuiv) continues. The total volume of water treated during FY2014 was approximately 2.6 million gallons: no effluent sample exceeded tlie goal of 200 ppt and the total mercury discharge was estimated at less than 2 mg (D(!)E 2015c).

• Land Use Controls - Land use contiols (LUCs). including postings and periodic patrols of LTFPC. hav e been implemented to ensure protection of potential human receptors. LUCs will be implemented in accordance witli the Land Use Contiol Implementation Plan (LUCIP) developed as part of the Remedial .Action Report Comprehensive Monitoring Plan (DOE 2014b). LUCs are checked regularly for protectiv eness and any issues or changes are reported in tlie annual Remediation Effectiveness Report. DOE is committed to implementing and maintaining LUCs to ensure tliat the selected remedy remains protectiv e of human healtli and tlie env ii onment.

• Suiface W ater Monitoring - Suiface water monitoring is conducted in accordance witli the R.\(!) to evaluate reductions in contaminant concentrations and flux. LTFPC suiface water is monitored at Station 17. where LTFPC exits Y-12. as well as at the midpoint of the LTFPC channel, at stoiTn sewer outfalls, and at tieatment system effluents. The objective of tliis monitoring is to deteiTnine attainment of tlie interim goal for mercuiv of 200 ppt in LTFPC at Station 17. Monitoring also is peifoiTned to assess reduction of mercuiv in fish and to assess the effectiv eness of indiv idual actions.


Remedial actions completed to date have achieved reductions in mercury tlux and concentrations in L^FPC surface water but have not achieved the interim goal. The daily median mercurv tlux at Station 17 was measured at 11.4 g d from FY2000-FY2005 and at 7.0 g d from FY2006-FY2010. Mercurv tlux increased during FY2011-FY2013 due to the WENLA storm drain cleanout project, and then began to decline toward tlie pre-stoiTn drain cleanout levels in FY2014 (D(!)E2015c). (!)nlv in 2007 did tlie average mercurv concentration at Station 17 achieve tlie interim goal of 200 ng L. No reduction of mercuiy concentration in fish tissue was obsened during this period.

The following response actions selected in the LT!FPC Phase 1 R(!)D are scheduled for future implementation:

• Remov al of contaminated sediments from LT!FPC and Lake Reality - Contaminated sediments will be removed from LT!FPC and Lake Reality to reduce mercuiy and PCB levels in fish and to protect recreational surface water users. Sediment will be remov ed from the LT!FPC sti eambed to bedrock (1 to 6 feet deep). Contaminated soil will also be removed from the LT!FPC banks. .\n estimated total v olume of 8000 v d3 of contaminated material will be remov ed. The sti eambed will be backfilled and graded to restore the creek channel. .Approximately 1 foot of contaminated sediment (approximately 4000 vd3) will be remov ed from Lake Reality. The total estimated v olume of contaminated sediment and soil is 12.000 vd3. Contaminated soils and sediments tliat meet the waste acceptance criteria (\V.\C) of tlie onsite Environmental Management Waste Management Facility (LMWMF) or successor facility will be disposed of at that facility, while materials tliat exceed these criteria will be sent off-site for treatment and disposal. The schedule for implementation of this action was rev ised under the LSD (DOE 2012a) to follow completion of upstream decontamination and decommissioning actions and remedial actions, for consistency witli the overall remediation strategy to conduct remedial actions for LT!FPC in a generally upgradient-to-downgradient sequence and reduce risk of recontamination in remediated areas.

This R(!)D amendment identifies an additional response action to be taken to supplement tlie actions aheady included in the LT!FPC Phase I R(!)D selected remedy to achieve further reductions in mercuiy concentiations in LT!FPC surface water and releases to the offsite env iionment. This R(!)D amendment modifies tlie selected remedy to include constmction and operation of a new water tieatment facility to treat discharges from tlie storm sewer system adjacent to the foiTner mercury-use buildings in the West End MercuiT .\iea (WENLA). which cunently constitutes the largest source of mercuiy releases to LT!FPC under base flow conditions. The integration point for the WLM.A stoiTn sewer network is a location designated Outfall 200:

• Water Treatment at (!)utfall 200 - new water tieatment facility will be constmcted to treat discharges from (!)utfall 200 with a tieatment capacity of 3000 gpm (4.3 Mgd) of influent suiface water and storage capacity for stoiTnwater flows in excess of ti eatment capacity up to 2 million gallons to manage stream flows up to 40.000 gpm. Treatment operations will include 40.000 gpm capacity for grit removal, followed by chemical co-precipitation clarification (sulfide-functional polymer precipitation, and fenic chloride co-precipitation with clarification) and filtiation (multimedia filtiation) for influent surface water flows and stored stoiTnwater up to 3000 gpm plus recycle flows (e.g.. backwash water, filter press filtrate) up to 1000 gpm. Stormwater storage capacity of 2 million gallons will be prov ided using above-ground tanks or lined stoiTnwater retention basins. Peak flows from larger stoiTn events will bypass tlie tieatment facility, although the initial mnoff flow from tliese larger stoiTn events (i.e.. the "fust flush", defined as tlie mnoff from the 60-minute period sunounding the hydrograph peak) will be captured up to 40.000 gpm. StoiTnwater flows that exceed tlie tieatment and storage capacity will bypass tlie facility to LT!FPC witliout tieatment.


Remediation of the entire L^FPC watershed w ill be conducted in stages using a phased approach. The remedial actions presented in the LT!FPC Phase I R(!)D selected remedy, and as modified by this R(!)D amendment, constitute an initial phase, focusing on interim source control actions for remediation of mercury-contaminated soils, sediments, and groundwater that contiibute contamination to surface water. These actions are expected to reduce tlie mercuiy releases to LT!FPC and attain the interim goal established by tlie FF.\ parties for suiface water quality and to make substantial progress toward attainment of tlie long-term w ater quality goal. Subsequent phases of remediation w ill address additional contaminated soils and sediments, groundwater, and buildings.

The selected remedy, as modified by tliis R(!)D amendment, fits into the overall D(!)E-(!)RR cleanup strategy by remov ing and disposing of contaminated media to the extent practicable. The remedial actions implemented under tliis interim R(!)D will be completed, evaluated, and used as tlie basis for deteiTnining what, if any. additional remedial actions may be necessaiy to meet final goals. Decisions regarding final land use and final goals for suiface water, groundwater, and soils will be deteiTnined in future decision documents.

The LT!FPC Phase I R(!)D. as modified by tliis R(!)D amendment, is one component of an integrated multi-part stiategy to reduce mercuiy contamination at Y-12. The Strategic Plan for Mercurv Remediation at the Y-12 National Security- Center (D(!)E 2014c) describes D(!)E*s integrated plan to remediate mercuiy contamination at Y-12 and impacted suiface water downstieam from Y-12 using an adaptive management approach. .Adaptive management is an approach for natural resource management that promotes flexible decision making that can be adjusted in tlie face of uncertainties as outcomes from management actions and otlier events become better understood. .Adaptive management acknowledges uncertainty about how ecological and natural resource systems function and how they respond to management actions, and makes use of management inten entions and follow-up monitoring to promote understanding and improv e subsequent decision making tluough an iterativ e process. The Strategic Plan recognizes that the cleanup of mercuiy contamination and sources at Y-12 is a complex, multi-faceted problem that requires an equally multi-layered remediation approach. .As an adaptiv e plan, the Strategic Plan is expected to ev olv e as results of implemented actions are obtained and ev aluated, and modifications may be proposed as necessaiA.

4.0 BASIS FOR THE DOCUMENT

This section discusses tlie justification for reev aluating water ti eatment at (!)utfall 200 as an appropriate component of tlie remedy for LT!FPC and summarizes the inlbiTnation gathered during tlie remedial design process tliat prompted and supports significant differences from tlie LT!FPC Phase I R(!)D selected remedy.

.After a R(!)D is signed, new inlbrmation may be receiv ed that could affect the implementation of the remedy selected in tlie R(!)D or could prompt reassessment of tliat remedy. The selected remedy in tlie LT!FPC Phase I R(!)D was designed to achiev e tlie following Remedial .Action (!)bjectiv e (R.A(!)): *lo restore suiface water to human healtli recreational lisk-based values at Station 17." Tliis R.A(!) was selected witli tlie recognition tliat remediation of tlie LT!FPC watershed would be conducted using a phased approach, and tliat an ultimate long-teiTn goal would be tlie attainment of tlie .A\\"Q)C in-sti eam standard for mercuiy. The R(!)D also established an interim goal of 200 ngL (200 ppt) for mercuiy in LT!FPC suiface water to monitor progi ess tow ard attainment of tlie R.A(!).

Wliile considerable progress has been made in reducing mercury releases from tlie Y-12 site to LT!FPC surface water (Figure 3). mercuiy concentiations in suiface water continue to exceed botli the interim goal established in the LT!FPC Phase I R(!)D and the Tennessee ambient water quality criteria (.A\\"Q)C)

Amendment to the Record of Decision for Phase I Interim Source Control Actions in the Upper East Fork Poplar Creek Characterization Area, Oak Ridge, Tennessee, Water Treatment at Outfall 200


Page: 11

standard of 51 ng/L (for which an interim waiver was approved in the UEFPC Phase I ROD). As a result of the continued mercury concentrations at Station 17 in excess of the interim risk-based goal, the most recent CERCLA Five-Year Review concluded that the Phase I ROD is not currently protective for ecological receptors (DOE 2012b).

2.0

1.{

1.6

^ 1.2 _|

1 I 0.8 ra X

0.6

0.4-

0.2

0.0

' Water

Fish

^P^Recommende^FisJ^issu^bas^ Water Qu^ ^ Surface Water ROD Limit

AWQC/Surface Water

1988 1990 1992 1994 1996 1998 2000 2002 2004 2006 2008 2010 2012 2014 DDC614m

Figure 3. Mercury Concentration in UEFPC Surface Water at Station 17 and in Fish

While the UEFPC Phase I ROD did not include large-scale water treatment operations as part of the selected remedy, it did include a study to evaluate the viability of long-term and large-scale treatment of mercury-contaminated surface water to support a future surface water decision. A treatability study and conceptual design study for a treatment system to reduce discharge of mercury from the WEMA storm sewer system was initiated in 2012 to fulfill this requirement. These studies led to the development of a conceptual design for a water treatment system to treat discharges at Outfall 200, which was documented in a conceptual design report (UCOR 2014) and a Remedial Design Work Plan (DOE 2014d). The successful performance of the existing water treatment systems (BSWTS, CMTS) for mercury reduction also suggest that additional water treatment facilities could achieve further reductions of mercury in UEFPC surface water.

In support of the conceptual design, a series of pre-design studies were conducted to provide data to be used in the design of the water treatment facility and evaluate opportunities to reduce the volume of base-flow and stormwater sewer contributions reaching the new treatment facility. Pre-design studies to evaluate potential diversion of stormwater (UCOR 2015a, 2015b) or non-contaminated process water (UCOR 2015c) from entering the WEMA storm sewer network did not identify significant opportunities for reducing the quantity of water requiring treatment. Stormwater characterization (UCOR 2015d) and mercury flux modeling (UCOR 2015e) studies provide a better understanding of flow dynamics in UEFPC and the flux of mercury and total suspended solids during base-flow conditions and storm events. Approximately 68 percent of the mercury flux and only 18 percent of the total suspended solids flux occur during base-flow conditions (UCOR 2015e). These studies also observed that the initial runoff from storm events contains an elevated loading of both total mercury and total suspended solids, and that these concentrations drop off rapidly after peaking as the flow subsides to pre-storm levels. This initial runoff from storm events is referred to as the "first flush", and is defined as the runoff from the 60-minute period surrounding the hydrograph peak. During this early phase of a storm event, the concentration of dissolved mercury was observed to decrease (due to dilution by the increase in stream-flow) while the


concenti ations of total suspended solids and total mercuiT were observ ed to increase similar to the peak in stream-flow (due to an increase in particle-associated mercurv). .After tlie peak in stream-flow, the concentration of dissolv ed mercurv increases, while the concentrations of total mercurv and total suspended solids decrease as the flow subsides to pre-storm levels. This analysis estimated tliat approximately 65 percent of the total mercuiy and 69 percent of the total suspended solids mass released during a storm ev ent occurs during this 60-minute period during tlie early phase of tlie storm. This effect is thought to be due to the sudden increase in turbulent flow in tlie storm drain piping tliat mobilizes mercury-laden sediment residing in the system (UCOR 2015e).

In addition to achiev ing reductions in the ongoing mercuiy releases from (!)utfall 200 to LT!FPC. the proposed water treatment facility also will prov ide potential benefits for tieatment of mercury-contaminated water from other sources. In particular, this facility will prov ide additional protection from increased mercuiy releases during future demolition actions at the major mercurv process buildings at WENLA and remediation of underlying soils. That is. the WENLA foiTner mercuiy-use buildings are planned for demolition followed by remediation of underlying soils to remov e major sources of mercuiy contamination. Wliile these future demolition and remediation actions will include comprehensive contamination conti ol measures and best management practices to minimize any release of contaminants to LT!FPC suiface water, the new water treatment facility could prov ide an additional lev el of protection against potential contaminant releases to LT!FPC and tlie offsite env ironment. To maximize this protection, constmction and operation of tlie proposed mercuiy ti eatment facility is scheduled to be completed prior to tlie start of the Y-I2 building demolition.

4.1 CONCEPTl AL SITE MODEL

new site conceptual model study conducted in 2008-2010 ((!)RNL 2011) to update the site conceptual model for mercuiy releases to LT!FPC identified changes in key assumptions of the LT!FPC Phase I R(!)D:

• (!)f the known mercury inputs to LT!FPC suiface water. (!)utfall 200 (representing combined inputs from WENLA and otlier upstream areas) is the most important cunent source of mercury to the stieam. representing up to 70-80 percent of the mercury flux at Station 17. particularly under low to av erage flow conditions. This is a significant change from the conditions when the Phase I R(!)D was issued (i.e.. prior to the constmction of the BSWTS and the resulting reduction in mercuiy flux from (!)utfall 51) when (!)utfall 200 was thought to represent approximately 20 percent of the flux at Station 17.

• Expected responsiveness offish to reductions of mercuiy levels in surface water have not been obseiAed - i.e.. mercuiy concentiations in fish tissue have not declined at a rate similar to the mercuiy concentiations in suiface water at Station 17 - indicating a more complex relationship tlian prev iously thought.

MercuiT contamination is widespread at the Y-I2 site and has been identified in soil, sediment, suiface water, groundwater, buildings, drains, and sumps. Contamination is intioduced into groundwater tluough multiple paths including spills, pipeline leaks, and dissolution from contaminated soils and sediments. MercuiT continues to be released into LT!FPC from point (discrete) and nonpoint (diffiise) sources witliin the Y-I2 site. Mercuiy enters LT!FPC from direct erosion of contaminated soil, migration of dissolv ed mercury tluough stoim drains and outfalls, and tluough shallow groundwater. In addition to widespread mercury contamination tluoughout tlie LT!FPC watershed, several areas have been identified as significant sources of mercury releases. These areas include tlie WENLA (under and around Buildings 9201-4. 9201-5. and 9204-4). Building 81-10 .\iea. Building 9201-2 .\iea ((9utfall 51). Lake Reality , and LT!FPC streambed. These areas act as resen oii s for the release and migration of mercury contamination to shallow aroundwater and suiface water.


Surface water receives contamination tluough groundwater discharge, stormwater runoff, and process outfall discharges, and is a route of offsite contaminant migration v ia L^FPC. which is the only surface water exit pathway. Mercurv concentrations in L^FPC hav e been decreasing as a result of response actions taken to date, but they remain above tlie interim goal and regulatorv criteria. Mercurv concentrations in fish tissue also continue to exceed target levels in East Fork Poplar Creek (EFPC). Sediments in tlie storm drain system and L^FPC channel are contaminated as a result of historic releases from operations at tlie Y-12 site and continue to receive contaminant inputs from stoiTn sewer discharges and nonpoint source mnoff during stoiTn events. The LT!FPC stieambed and Lake Reality contain sediments witli elevated concentiations of mercuiy and otlier contaminants, which are subject to remobilization and or downstream transport. These contaminated sediments will be excav ated under the LT!FPC Phase I R(!)D. although tlie schedule for implementation of this remedial action has been rev ised under the ESD (D(!)E 2012a) to follow demolition of tlie WEM.A foiTner mercury-use buildings and remediation of underlying soils.

Flistorically. tlie primaiy point sources of mercuiy to LT!FPC at Y-12 have included mercuiy-contaminated water discharge from the basement sumps of foiTner mercuiy-use buildings (i.e.. Buildings 9201-2. 9202. 9201-4. 9201-5. and 9204-4). mercury-contaminated pipes (internal and external to buildings) and storm drains that ultimately discharge tluough (!)utfall 200. and the mercuiA-contaminated natural spring tlow suifacing near the historic mercuiy-use area and discharging v ia (!)utfall 51. Tlie point source at (!)utfall 51 is now tieated by tlie BSWTS and discharge from tlie WEM.A building basement sumps (witli the exception of Building 9201-5) is tieated at CMTS. Treatment of the discharge from the WENLA storm sewer system at (!)utfall 200 is tlie subject of tliis R(!)D amendment.

5.0 DESCRIPTION OF SIGNIFICANT DIFFERENCES

The prev ious section established tliat new inlbiTnation has been obtained since the signing of the LT!FPC Phase 1 R(!)D tliat wan ants a reassessment of larger-scale water ti eatment to achiev e furtlier reductions in mercurv releases from tlie Y-12 site to tlie offsite env ironment. This R(!)D amendment foiTnallv presents the constmction and operation of a new water treatment facility to tieat discharges from the WENLA storm sewer system at (!)utfall 200 as an additional component of tlie selected remedy under the LT!FPC Phase 1 R(!)D. This change is expected to improv e tlie peifoiTnance of the selected remedy in achiev ing the LT!FPC Phase 1 RA(9.

5.1 REMEDI.AL .ACTION OB.JECTIVES

Peiformance in achiev ing tlie R.\(!) specified in the LT!FPC Phase 1 R(!)D is monitored and reported in tlie annual Remediation Effectiveness Report (DOE 2015c). Suiface water quality metiics utilized to evaluate progress toward attainment of tlie Phase 1 R.\(!) include a 200 ng L (200 ppt) peifoiTnance metiic for mercury in suiface water at Station 17. Suiface water monitoring at Station 17 is conducted to gauge the cumulativ e effects of tlie v arious actions as tliey are completed with regard to tlie contaminants of concern. Biological monitoring is peifoiTned to assess lev els of mercury in fish tissue.

.As shown in Figure 3. mercuiy concentrations in LT!FPC suiface water have been decreasing over time but continue to exceed tlie interim goal and regulatoiy criteria. The temporaiy increase in mercuiy concentiations duiing 2011-2012 is tliought to be partially attiibuted to tlie discharge of mercuiy contaminated sediment tliat was disturbed duiing tlie 2011 W est End stoiTn drain cleanout project, as well as tlie relativ ely higli rainfall levels dui ing tliis period. Fligher mercuiy tlux at Station 17 relativ e to upstieam monitoiing locations is tliouglit to be partially attiibutable to entiainment of contaminated sediment from tlie LT!FPC stieambed in tliis reach. tlow augmentation program, where approximately 3 million gallons per day of


water from tlie Clinch Riv er was released to tlie L^!FPC channel just below (!)utfall 200 beginning in 1996 to improv e stream conditions for aquatic biota, was teiminated in 2014. resulting in a reduction of tlie base-tlow at Station 17 by a factor of approximately two-tliirds. The impact of this change on the dynamics of mercury flux in LT!FPC will be evaluated tluough ongoing monitoring.

Tliis R(!)D amendment modifies tlie LT!FPC Phase I R(!)D selected remedy to include tlie constiaiction and operation of a new water ti eatment facility designed to tieat discharges from tlie WENLA stoim sewer system at (!)utfall 200. to acliieve fuitlier reductions in mercuiT releases liom Y-12 and in tlie concentiations of mercuiT in LT!FPC surface water. Tliis additional response action would supplement tlie remedial actions already included in the selected remedy of tlie LT!FPC Phase I R(!)D (as amended) to support attainment of tlie Phase I R.\(!) and to make substantial progi ess toward attainment of tlie long-teim water quality goal of meeting tlie .\\\"Q)C in-stieam standard for mercuiy in LT!FPC suiface water. To supplement tlie LT!FPC Phase I R.\(!). sub-watershed R.\(!)s specific to tlie proposed water tieatment system are summarized in Table 1 and discussed below.

Tlie proposed water tieatment sy stem will be designed to captuie discharges from tlie WENLA stoim sewer sy stem to LT!FPC under base-flow and stoiTn-tlow conditions to mitigate uncontiolled releases of mercuiy (and otlier hazardous substances) into LT!FPC suiface water. Stoimwater storage capacity will allow tlie collection of flow exceeding tieatment capacity for future tieatment. and reduce tlie liequency witli wliicli (!)utfall 200 discharges would bypass the facility witliout tieatment. Stoimwater capture will be targeted to maximize mercuiy tlux reduction. Collected wastewater will be tieated to achieve reductions in mercuiy concentiations to 51 ng L total mercuiy in tlie tieated eftluent for discharge to LT!FPC. Treated eftluent from tlie proposed water ti eatment facility will be discharged in compliance witli .\R.ARs and at lev els tliat are protectiv e of tlie receiv ing water. Discharges li om (!)utfall 200 tliat exceed tlie facility *s ti eatment capacity and stoimwater storage capacity will by pass tlie facility witliout tieatment. In order to limit tlie total mercuiy flux to LT!FPC li om (!)utfall 200. mercuiy concenti ations in LT!FPC suiface water, including any water by passing tlie tieatment facility, will be limited to a daily maximum concentiation of2000 ng L total mercuiy and an annual rolling Ilux of 1 kg year total mercuiA. To prev ent acute toxicity to fish and aquatic life, mercuiy concentiations in LT!FPC stieam-flow. including any by pass water, will be limited not to exceed 1400 ng L dissolved mercuiT.

Amendment to the Record of Decision for Phase I Interim Source Document Number: DOE/OR/01-2697&D2 Control Actions in the Upper East Fork Poplar Creek Characterization Area, Oak Ridge, Tennessee, Water Treatment at Outfall 200 Page: 15

Table 1. Remedial Action Objectives

UEFPC Remedial Action Objectives in Phase I ROD (DOE 2002)

1. Restore surface water to human health recreational risk-based values at Station 17.

Sub-watershed Remedial Action Objectives Specific to the Outfall 200 Water Treatment Facility (DOE 2015b)

1. Capture discharges from the WEMA storm sewer system to UEFPC for treatment and/or storage under base-flow and storm-flow conditions to mitigate uncontrolled releases of mercury (and other hazardous substances) into UEFPC surface water Stormwater capture would be targeted to maximize mercury flux reduction.

2. Store captured wastewater with sufficient capacity to minimize mercury flux bypassing the facility without treatment (i.e., the mercury flux contained in stream-flow discharged at Outfall 200 that exceeds treatment capacity and stormwater storage capacity that would bypass the facility without treatment) to the extent practicable.

3. Treat collected water to achieve reductions in mercury concentrations to meet the AWQC standard of 51 ng/L total mercury in the treated effluent.

4. Discharge treated effluent in comphance with ARARs and at levels that are protective of the receiving water. 5. Minimize the total mercury flux discharged to UEFPC from Outfall 200. Mercury concentrations in UEFPC surface

water, including any water bypassing the treatment facility, must meet a daily maximum concentration of2000 ng/L total mercury and an aimual rolling flux of 1 kg/year total mercury. To prevent acute toxicity to fish and aquatic hfe, mercury concentrations in UEFPC stream-flow, including any water hypping the treatment facility, must not exceed 1400 ng/L dissolved mercury.

5.2 IDENTIFICATION OF NEW REMEDIAL ACTION ALTERNATIVES

This ROD amendment modifies the selected remedy in die UEFPC Phase I ROD to include the construction of a new water treatment facility designed to treat discharges from die WEMA storm sewer system at Outfall 200. As discussed previously, Outfall 200 is the integration point for the Y-12 storm sewer effluent entering UEFPC, which constitutes the largest source of mercury releases to UEFPC under base flow conditions. The storm sewer system adjacent to former mercury-use Buildings 9201-4, 9201-5, and 9204-4 is drained by Outfalls 150, 160, 163 and 169. This storm sewer system is badly deteriorated and has numerous leaks, despite previous attempts to reline the system, that result in the infiltration of mercury-contaminated groundwater and accumulation of mercury-contaminated sediment in the storm sewer that is a source for base flow discharges of mercury to UEFPC. Stormwater runoff through mercury-contaminated soils into the storm sewers and catch basins also results in the release of mercury-contaminated sediments to UEFPC during storm events. These storm sewer system flows converge at Outfall 200.

Remedial alternatives were developed and evaluated in the focused feasibility study (DOE 2015a) and proposed plan (DOE 2015b) for construction of a new water treattnent facility to treat discharges from die WEMA storm sewer system at Outfall 200. These remedial alternatives would supplement the actions already included in the Phase I ROD selected remedy (as amended) to achieve furdier reductions in mercury concentrations in UEFPC surface water and mercury releases to the offsite environment, but would not impact other components of the selected remedy. Multiple treatment facility configurations were evaluated with different levels of treatment capacity and stormwater storage capacity to manage UEFPC base-flow, various amounts of storm-flow, and potentially other inputs of mercury-contaminated water. In each case, storm-flow above the facility treatment and storage capacity would be bj^assed around the facility and released to UEFPC without treatment.

5.2.1 Alternative 1. No Further Action

The "No Action" altemative is required under CERCLA to provide a con^arative baseline against which other altematives can be evaluated. The no-action altemative does not initiate any action, and normally assumes that present security measures limiting access and use are not maintained, and that short- and


long-teiTn monitoring is eliminated. However, a tr ue no-action alternative is not relevant for this remedy modification, since some remedial actions have aheadv been peiformed under the L^FPC Phase I R(!)D selected remedy, and other actions are ongoing and planned. This supplemental action would not modifv existing components of the LT!FPC Phase I selected remedy. This "No Further .Action" alternativ e is defined to mean no change to tlie existing LT!FPC Phase I R(!)D selected remedy (as modified to date). Under this alternative. D(!)E would take no additional remedial action to supplement tlie selected remedy in tlie LT!FPC Phase I R(;)D (as amended).

5.2.2 .Alternativ e 2. W ater Treatment at Outfall 200

.Alternativ e 2 inv olv es the constmction of a new water treatment facility designed to tr eat dischar ges fr om (!)utfall 200 to reduce mercury releases to LT!FPC surface water. Multiple tr eatment facility configurations wer e ev aluated under this alternativ e witli differ ent lev els of tr eatment capacity and stormwater stor age capacity to manage LT!FPC base-fiow. v arious amounts of storm-flow, and potentially other inputs of mercury-contaminated water. In each case, storm-flow abov e tlie facility tr eatment and storage capacity would be bypassed around the facility and released to LT!FPC without tr eatment. The water tr eatment sy stem would be designed to achiev e a performance objectiv e of r educing mer cury concentrations to 51 ng L (i.e.. tlie .A\\"Q)C standard for mercury ) or less in system effluents. Tlie resulting mer cury removal would be expected to contr ibute substantially towar ds reducing tlie mer cury concentr ation in LTFPC surface water at Station 17 to meet tlie interim goal of200 ng L established in tlie LT!FPC Phase I R(!)D and tlie long-term goal of attaining tlie .A\\"Q)C in-str eam standar d of 51 ng L for total mer cury.

.All action alternativ es include the constrorction of a new water tr eatment facility at (!)utfall 200 designed to reduce mercury releases to LT!FPC surface water. These alternatives differ only in the level of treatment capacity and stormwater storage capacity to manage LT!FPC base-flow, v arious amounts of storm-flow, and potentially other inputs of mercury-contaminated water. The conceptual design for tlie treatment process would be similar for all alternatives, witli unit operations that include coarse solids (grit) removal, chemical co-precipitation clarification (suLfrde-functional polymer precipitation and ferric chloride co-precipitation with clarification), and dewatering witli multi-media filtration. Some alternatives include multiple parallel tr eatment tr ains of tliese unit operations. The system design also would be configur ed to maintain flexibility and expandability to accommodate scaling up of tr eatment capacity , tr eatment pr ocesses, and or stormwater retention, as needed and if warranted by futur e conditions. The tr eatment sy stem would be designed to achiev e a performance objectiv e of reducing mercury concentrations to 51 ng L (i.e.. the Tennessee .A\\"Q)C standard for mercury in waters classified for recr eational use) or less in sy stem effluents, to supporl the goal of r educing mercury concentr ations in LT!FPC surface water at Station 17 to tlie interim goal of 200 ng L and ultimately the .A\\"Q)C of 51 ng L.

W ater flowing from (!)utfall 200 would be div erled into the inlet channel of the headwor ks tluough an intake strorcture witli an adjustable weirv The inlet channel would contain bar racks or otlier coarse screen to remove oversize material from entering the grit removal chambers. Some alternatives would include multi-stage headworks sy stems to manage a greater range of flow conditions. Str eam flow in excess of the headworks design capacity would overflow the weir and continue to flow down LT!FPC without treatment.

The headworks system would include a manual bar screen, grit removal, grit classification and dewatering. and a pump station. Grit removal refers to the process of remov ing larger solids prior to the tr eatment processes that tar get remov al of colloidal and dissolv ed mer cury . Grit r emov al would be accomplished using vor1ex-grit chambers, which would remove grit parlicles larger tlian about 50 pm diameter and otlier high-density materials, potentially including any dr oplets of elemental mer cury of


sufficient size. Grit and any associated mercur\ would be periodically removed from the system using a grit pump for dewatering and disposal.

W ater that has completed the grit removal process would be pumped tluough a pump station to an equalization tank prior to further treatment. The equalization tank would be designed to prov ide a hydraulic retention time of approximately 1 hour. Effiuent from the equalization tank would be pumped to a pH contiol and dechlorination tanlc where acid or caustic reagents would be added to adjust pH to the required range (typically 7 to 9 for chemical precipitation of mercuiA). and agents would be added for dechlorination as requiied. before the effiuent is released to chemical precipitation tanks.

The effiuent from tlie pH control and dechlorination tank first would be pumped to a tank(s) where a sulfide-functional polymer and fenic chloride coagulant would be added in order to produce mercuiy-sulfiide bound solids and fen ic oxyhydroxides tliat adsorb or co-precipitate mercuiy witli otlier suspended solids. Dissolved mercuiy would be precipitated by the sulfide groups on the sulfide-functional polymer and adsorbed onto other species fonned during the precipitation process, and a fiocculation process would promote tlie fonnation of larger solids. Solids formed during the coagulation and fiocculation process, enlianced by the use of fen ic iion and organic polymers, would include colloidal and suspended mercuiy. The effiuent from tlie fiocculation and chemical precipitation process would tlien go to inclined-plate claiifiers to remov e tlie solids. portion of the clarifier sludge would be recycled back to the coagulant addition tank to promote growtli of denser precipitate solids, settling, fines capture, and driv e tlie precipitation process toward equilibrium. portion of the clarifier sludge would be sent to a sludge thickening tank to increase the solids concentiation to at least 5 percent prior to dewatering. The thickened solids from the sludge tliickening tank would be pumped to a filter press for dewatering. The filter cake generated from the filter press would be sent for disposal, while tlie filtiate would be recycled back into tlie tieatment process.

Clarifier effiuent would flow to a multi-media filtration process for additional solids remov al, with prov isions for pH adjustment and chemical addition prior to filtration. The multi-media filtration system would consist of a series of vessels containing appropriate filter media that would be operated in parallel witli indiv idual units being backwashed or taken offline as needed. .\t least one unit typically would be inactive at any time, so that it can be put into use when another unit reaches capacity and requires backwashing. Filter effiuent would flow to a tieated water clear well prior to discharge back to LT!FPC.

portion of tlie clean effiuent would be used for backwashing of the multi-media filters as needed.

Solid and liquid waste materials generated during facility constiuction and during tieatment operations would be appropriately characterized and disposed. W aste generated during facility constmction may include non-contaminated constiuction debris and asbestos-contaminated debris, as well as soil and debris that may contain mercuiy or other hazardous materials. The areas impacted by constmction of tlie new facility will be characterized for potential of contamination prior to constmction in accordance witli an approved Sampling and .Analysis Plan. Solid waste streams generated during treatment operations would include coarse debris from tlie bar screen, grit material from the grit removal system, filter cake from the filter press, spent media from the multi-media filters, laboratory sampling materials, personal protective equipment, and universal waste items. The predominant solid waste streams are expected to be grit material from tlie grit removal system, filter cake from tlie filter press, and spent media from the multimedia filters. Fiquid waste streams would include liquids from dewatering operations, spent laboratoiy chemicals, and equipment cleaning materials. Some liquid residuals, such as tliose generated during backwash operations and solids handling, would be pumped back into the equalization tank and reused in treatment system operations. Sampling and .Analysis Plan and W aste Handling Plan will be developed to characterize waste and deteiTnine appropriate disposition paths.


(!)peration of the new water treatment system would continue until mercur\ lev els in discharges from (!)utfall 200 have declined to levels tliat no longer require treatment - that is. planned remediation of mercury source areas at WEMA may result in reduction of mercurv releases to L^FPC to levels that no longer require treatment. For planning puiposes. a period of operations of 30 years is assumed.

Monitoring would include inlluent stream water entering the tieatment facility and tlie eftluent stream follow ing ti eatment. in order to ev aluate tlie effectiv eness of the ti eatment operations in attaining tlie peifoiTnance objective of 51 ng F. Tliis eftluent monitoring would be additional to the monitoring cunently required by tlie L^FPC Phase I R(;)D.

Sub-alternativ es dev eloped to ev aluate different lev els of treatment capacity and stoiTnwater storage capacity to manage L^FPC base-tlow. various amounts of stoiTn-flow. and potentially otlier inputs of mercury-contaminated water include the following.

5.2.2.1 .Alternative 2a. Water Treatment at Outtall 200 with 1500 gpm Treatment Capacity and No Stonnvvater Storage

.A new water tieatment facility would be constiaicted near (!)utfall 200 with tieatment capacity of 1500 gpm (2.2 Mgd) for inlluent surface water, and no capacity for retention and storage of stormwater flow in excess of tieatment capacity. This capacity would be sufficient to ti eat tlie av erage flow in LT!FPC. but most stonn events would exceed tliis tieatment capacity . Historical records indicate that approximately 19-24 percent of flow records exceed 1500 gpm (UC(!)R 2014). The conceptual design for the treatment sy stem would include the general process operations of coarse solids (grit) remov al, chemical co-precipitation clarification, and multi-media flltiation. .Alternative 2a is intended to represent a minimal sy stem capable of treating LT!FPC surface water under base-flow conditions only.

.Alternative 2a would have capacity for grit removal from influent surface water flows up to 3000 gpm. and capacity for other treatment operations for flows up to 1500 gpm of influent surface water plus 500 gpm of recy cle flows (recy cle of backwash water and filter press fllti ate). Influent flows greater tlian 1500 gpm but less than 3000 gpm would flow tlirough the grit removal system and then be released to LT!FPC without furtlier tieatment. while flows greater tlian 3000 gpm would oveillow tlie weir and continue to flow down LT!FPC without eitlier grit removal or chemical treatment. The headworks capacity of 3000 gpm is based on tlie 95"' percentile stieam-flow estimate for LT!FPC at tliis location (i.e.. 95 percent of stieam-flow records for LT!FPC do not exceed 3000 gpm). Therefore, stream-flow would exceed tlie facility headw orks capacity approximately 5 percent of tlie time on av erage and would exceed the treatment capacity even more frequently, and would bypass tlie facility without tieatment. Stieam-flow monitoring data indicate that such bypass would have occuned more than 100 times during 2010. which is considered a year of av erage flow conditions.

PieliminaiT peifoiTnance modeling (UCOR 2015e) estimates tliat this treatment system could remove approximately 1600 g year of mercuiy or approximately 52 percent of the mercury flux at (!)utfall 200 during a ty pical year, based on rainfall data from 2010 which is considered a year of av erage precipitation and stieam-flow. .Additional mercury reduction potentially may be achieved for stieam-flow between 1500 and 3000 gpm. which would be processed tluough tlie grit removal system only, but data are not cunently available to quantify any such reduction. Costs for .Alternative 2a are estimated at approximately $115 million capital cost, and operations and maintenance cost of approximately S2.2 million per year. The present v alue cost for constmction and 30 years of operation is estimated at approximately $142 million.


5.2.2.2 Alternative 2b. Water Treatment at Outtall 200 with 3000 gpm Treatment Capaeity and No Stonnwater Storage

A new water treatment facility would be constructed near (!)utfall 200 with treatment capacity for 3000 gpm (4.3 Mgd) of inlluent surface water plus 1000 gpm of recycle tlows (recycle of backwash water and filter press filtrate), and no capacity for retention and storage of stormwater flow in excess of treatment capacity. The conceptual design for tlie tieatment system would include tlie general process operations of coarse solids (grit) remov al, chemical co-precipitation with clarification, and multi-media filtration: and would be configured with two parallel treatment trains, each equivalent to that in .AJtemativ e 2a. .AJtemativ e 2b is intended to represent a system capable of ti eating LT!FPC surface water under base-flow conditions and up to tlie 95"' percentile stream-flow in LT!FPC at (!)utfall 200 (i.e. 95 percent of stieam-flow records for LT!FPC do not exceed 3000 gpm). Therefore, stream-flow would exceed tlie facility ti eatment capacitv approximately 5 percent of tlie time on av erage and bypass the facility without treatment. Stream-flow monitoring data indicate that such bypass would have occuned at least 88 times during 2010. which is considered a year of average flow conditions (UCOR 2015e).

The headworks and grit removal chamber for this facility would be identical to tliose in .Alternative 2a. and would be constmcted to manage flow up to 3000 gpm of influent surface water. Flowev er. tliis facility differs from .Alternative 2a in tliat the subsequent tieatment operations also are designed to treat flows up to 3000 gpm of influent surface water, which is approximately equiv alent to tlie 95"' percentile stream-flow estimate for LT!FPC at (!)utfall 200. The tieatment system also would have capacitv for up to 1000 gpm of recycle flows (e.g.. backwash water and filter press filtiate). Influent flows greater tlian 3000 gpm would oveiflow tlie weir and continue to flow down LT!FPC without eitlier grit removal or fuitlier ti eatment.

Pieliminaiy peifoimance modeling (UCOR 2015e) estimates tliat this treatment system could remove approximately 2100 g year of mercuiy or approximately 68 percent of the mercury tlux at (!)utfall 200 during a tvpical year (i.e.. 2010). Costs for .Alternative 2b are estimated at approximately $125 million capital cost, and operations and maintenance cost of approximately S2.7 million per year. Tlie present value cost for constmction and 30 years of operation is estimated at approximately $158 million.

5.2.2.3 .Alternative 2e. Water Treatment at Outtall 200 with 3000 gpm Treatment Capaeity and 2 Million Gallons Stonnwater Storage

.A new water tieatment facilitv would be constnicted near (!)utfall 200 with tieatment capacitv for 3000 gpm (4.3 Mgd) of influent surface water plus 1000 gpm of recycle flows and stored stormwater. and capacitv for retention and storage of 2 million gallons of stormw ater flow to manage stream-flow up to 40.000 gpm. The conceptual design for tlie tieatment system would include the general process operations of coarse solids (grit) remov al, solids precipitation clarification, and multi-media filtration. .Alternativ e 2c is intended to represent a ti eatment system capable of treating LT!FPC surface water under base-flow conditions and up to the 95"' percentile stieam-flow in LT!FPC at (!)utfall 200. and capable of capturing stoimwater in excess of these flow rates for future treatment. The 2 million gallon storage capacitv is designed to capture tlie initial mnoff from most stoim events (i.e.. tlie "first flush", defined as the mnoff from the 60-minute period surrounding the hydrograph peak) (UC(!)R 2015d) ev en where the total V olume exceeds tlie 2 million gallon storage capacitv . Stoimw ater flows in excess of this storage capacitv would bypass the treatment facilitv.

The headworks for tliis facilitv would be constmcted to manage a maximum flow of 40.000 gpm. using a two-stage weir system. Water flow ing from (!)utfall 200 would be div erted into tlie inlet channel of the headworks tluough an intake stmcture witli an adjustable weii" as in tlie prev ious alternatives. .Again, the


inlet channel would contain manual bar screens or otlier coarse screens to remove oversize material from entering the grit removal chambers. Under normal flow conditions, all influent water would be directed to tlie base-flow grit remov al chamber, as described for .Alternatives 2a and 2b. However, unlike .Alternatives 2a and 2b. this system would also contain a much larger grit removal chamber for stormwater flows, capable of treating influent flows up to 37.000 gpm (52 Mgd). Wlien stream-flow does not exceed the capacity of tlie smaller base-flow unit, all influent water would be processed for grit removal in the base-flow unit. The larger wet-weather unit would operate only during stoiTn events or when flows exceed base-flow conditions, and tlie stormwater tieated in tliat unit would be pumped to the stoiTnwater storage tanks following grit removal. Influent flows exceeding the headworks capacity of 40.000 gpm or tlie storage capacity would oveillow the weii" system and bypass the facility and continue to flow down LT!FPC without tieatment. Stieam-flow monitoring data indicate that such bypass would hav e occun ed approximately 14 times during 2010. which is considered a year of av erage flow conditions (UC(;)R2015e).

StoiTnwater storage would consist of one abov e-ground steel tank with a capacity of 2 million gallons. This tank would be equipped with mixers to keep in suspension any solids tliat remain after tlie grit removal processing. Stormwater stored in this tank would be pumped to the equalization tank for treatment during non-storm conditions at a flow rate up to 1000 gpm. During remedial design, alternative storage conflgurations may be ev aluated, including use of concrete retention basins v ersus the abov e-ground tank. The remainder of tlie tieatment sy stem would be equiv alent to that in .Alternativ e 2b. with two parallel tieatment tiains to prov ide a total tieatment capacity of 4000 gpm (3000 gpm for influent surface water plus 1000 gpm for treatment of recy cle flows and stored stormwater) for all treatment operations.

.Alternative 2c differs from .Alternative 2b primarily in: (1) tlie larger and more complex headworks system to feed botli the base-flow grit removal system and the stoiTn-flow grit removal system, as needed: (2) the inclusion of the large capacity (37.000 gpm) stoiTn-flow grit removal system: and (3) the inclusion of storage capacity for up to 2 million gallons of stoiTnwater flow. The remaining unit operations of the treatment sy stem are equiv alent to those under .Alternativ e 2b and double that of .Alternativ e 2a - that is two parallel tieatment tiains to prov ide a total tieatment capacity of 3000 gpm for influent surface water plus 1000 gpm for management of recy cle flows and stored stoiTnwater. Pieliminary perfoiTnance modeling (UCOR 2015e) estimates that this treatment system could remove approximately 2600 g year of mercury or approximately 84 percent of tlie mercuiy flux at (!)utfall 200 during a typical year (i.e.. 2010). Costs for .Alternative 2c are estimated at approximately $146 million capital cost, and operations and maintenance cost of approximately S 3.1 million per year. The present value cost for constmction and 30 years of operation is estimated at approximately $185 million.

5.2.2.4 .Alternative 2d. Water Treatment at Outtall 200 with 3000 gpm Treatment Capacity and 10 Million Gallons Stonnvvater Storage

.A new water tieatment facility would be constioicted near (!)utfall 200 with a treatment capacity for 3000 gpm (4.3 Mgd) of influent surface water plus 1000 gpm of recycle flows and stored stoiTnwater. and capacity for retention and storage of stormw ater flow up to 10 million gallons to manage flows up to 40.000 gpm. The conceptual design for tlie tieatment system would include the general process operations of coarse solids (grit) remov al, solids precipitation clarification, and multi-media filtration. .Altemativ e 2d is intended to represent a ti eatment sy stem capable of treating LT!FPC surface water under base-flow conditions and up to the 95"' percentile stieam-flow in LT!FPC at (!)utfall 200. and capable of capturing stoiTnwater in excess of these flow rates for future treatment. StoiTnwater storage of up to 10 million gallons would be prov ided using above-ground tanks. This storage capacity is designed to capture stormwater from a l-year 24-hour stoiTn (UC(!)R 2014). Peak flows from larger stoiTn ev ents


would still bypass tlie treatment facility, although the initial runoff flow from these storm events (i.e.. tlie "tlrst flush") would be captured. Influent flows exceeding the treatment and storage capacity would bypass the facility to L^FPC witliout treatment. Stream-flow monitoring data indicate that such bypass would hav e occun ed approximately 12 times during 2010. which is considered a year of av erage flow conditions (UC(!)R 2015e).

.Alternative 2d differs from .Alternative 2c primarily in the larger storage capacity for up to 10 million gallons of stoiTnwater flow. The headworks for tliis facility would be constmcted to manage a maximum flow of 40.000 gpm. using a two-stage weir system identical to that described for .Alternative 2c. StoiTnwater storage would consist of fiv e abov e-ground steel tanks witli a capacity of 2 million gallons each, for a total storage capacity of 10 million gallons. Each tank would be equipped with mixers to keep in suspension any solids that remain after tlie grit removal processing. StoiTnwater stored in these tanks would be pumped to the equalization tank for treatment during non-stoiTn conditions at a flow rate

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