TECHNICAL SUPPORT FOR ROCKY MOUNTAIN ARSENAL

Offpost Operable Unit Endangerment Assessment/Feasibility Study Final Report Volume VIofVIII (FS Sections 3.0, 4.0, 5.0, 6.0, 7.0)

November 24, 1992 Contract Number DAAA15-88-0021 Task RIFSI (Delivery Order 0001)

PREPARED BY Harding Lawson Associates

PREPARED FOR PROGRAM MANAGER FOR ROCKY MOUNTAIN ARSENAL

THIS DOCUMENT IS INTENDED TOCOMPLY WITH THE NATIONAL ENVIRONMENTAL POLICY ACT OF 1969. THE INFORMATION AND CONCLUSIONS PRESENTED INTHISREPORT REPRESENT THE OFFICIALPOSITION OFTHE DEPARTMENT OF THE ARMY UNLESS EXPRESSLY MODIFIED BY A SUBSEQUENT DOCUMENT. THISREPORT CONSTITUTES THE RELEVANT PORTION OFTHE ADMINISTRATION RECORD FOR THISCERCLA OPERABLE UNIT. 

# M9200039TABLE OFCONTENTS Page VOLUME I - INTRODUCTION LIST OFTABLES ..................................................... xix LIST OF FIGURES .................................................... xxv PREFACE ........................................................... P-1 EXECUTIVE SUMMARY .............................................. ES-I INTRODUCTION ..................................................... I-I GLOSSARY .......................................................... G-1 VOLUME 11 - ENDANGERMENT ASSESSMENT PREFACE ........................................................... 11 - P-1 1.0 IDENTIFICATION OF CHEMICALS OF POTENTIAL CONCERN ........... II - 1-1 1.1 GENERAL SITE DATA EVALUATION CONSIDERATIONS .......... 11- 1-4 1.2 STATISTICAL PROCEDURES FOR COMPARISON WITH BACKGROUND ............................................. Il - 1-8 1.3 SELECTION OFCHEMICALS OF CONCERN ...................... Il- 1-15 1.3.1 Groundwater ......................................... 11- 1-15 1.3.2 Surface Water/Sediment ................................. 11-1-17 1.3.3 Surficial Soil .......................................... 11- 1-21 1.3.4 Biota ................................................ 11

- 1-22 1.4 SUMMARY ................................................ 11- 1-24 1.5 UNCERTAINTIES ........................................... 11- 1-26 2.0 EXPOSURE ASSESSMENT ......................................... 11-2-1 2.1 PROPERTIES AND PROBABLE ENVIRONMENTAL FATES OF CHEMICALS OF CONCERN ................................ 11-2-1 2.1.1 Equilibrium Partitioning Properties ofChemicals ofConcern ..... 11-2-2 2.1.1.1 Information Sources ............................. 11-2-3 2.1.1.2 Data Evaluation ................................ 11- 2-5 2.1.1.3 Results ............... :*- - - - - ******* . II - 2-7 2.1.2 Offpost Environmental Fate ofChemicals ofConcern ........... 11-2-9 2.1.2.1 Organochlorine Pesticides ......................... 11- 2-9 2 .1.2.2 Benzene Compounds ............................. II - 2-15 2.1.2.3 Halogenated Aliphatic Compounds .................. Il- 2-19 20000,317.10 - 1-toc 0306111892 iTABLE OF CONTENTS (Continued) Page 2.1.2.4 Compounds Containing Sulfur ...................... Il - 2-23 2.1.2.5 Compounds Containing Phosphorus .................. 11-2-24 2.1.2.6 Ionic Chemicals ................................. II - 2-26 2.1.2.7 Miscellaneous Compounds ......................... 11- 2-30 2.1.3 Uncertainties ......................................... II - 2-31 2.2 CHARACTERIZATION OF EXPOSURE SETTING .................. Il - 2-32 2.2.1 Site Characteristics Affecting Contaminant Migration ........... II - 2-32 2.2.1.1 SiteDescription and History ....................... II - 2-32 2.2.1.2 Environmental Setting ............................ II - 2-33 2.2.1.3 Sampling Locations and Media ..................... II - 2-34 2.2.1.4 Summary of Offpost Operable Unit Contamination ...... 11- 2-36 2.2.2 Potentially Exposed Populations ........................... 11- 2-37 2.2.2.1 Residential Land Use ............................ 11- 2-38 d-..(-.2. 1.1 Procedures ........................... II - 2-38

n.z.1.'ý - 2-41

- Results .............................. 11 2.2.2.2 Commercial/Industrial Land Uses ................... Il - 2-42 Qualitative Assessment of theLikelihood ofOccurence of the Selection ofFuture Land Use forthe Offpost Endangerment Assessment/Feasibility Study ........... 11 - 2-44 2.2.2.4 Water Use ..................................... II - 2-45 2.2.2.4.1 Existing Groundwater Use ............... 11- 2-45 2.2.2.4.2 Projected Future Use ofGroundwater ....... 11-- 2-49 2.2.2.4.3 Surface-Water Use ..................... II - 2-50 2.3 EXPOSURE PATHWAYS ...................................... II - 2-51 2.3.1 Potential Sources, Release Mechanisms, and Transport Media ..... II - 2-51 2.3.1.1 Groundwater ................................... II - 2-51 2.3.1.2 Surface Water and Sediment ....................... 11- 2-54 2.3.1.3 Atmospheric ................................... II - 2-55 2.3.1.4 Biotic ........................................ II - 2-55 2.3.2 Potential Exposure Points ................................ 11- 2-56 2.3.3 Potential Exposure Pathways and Routes ..................... 11-2-57 2.3.3.1 Inhalation Route ................................ II - 2-57 2.3.3.2 Dermal Route .................................. 11-2-60 2.3.3.3 Ingestion Route ................................. 11-2-61 2.3.4 Uncertainties ......................................... 11 - 2-64 2.4 QUANTIFICATION OF EXPOSURE ............................. 11-2-64 2.4.1 Identification ofZones Having Distinct Exposure Concentrations and/or Pathways ....................................... 11 - 2-66 2.4.2 Exposure Assessment Procedures and Exposure Point Concentrations ........................................ Il - 2-71 2.4.2.1 Statistical Procedures Used to Evaluate Monitoring Data ......................................... 11 - 2-72
20000,317.10 - 1-toc 0306111992TABLE OF CONTENTS (Continued) Page 2.4.2.2 Model Used to Estimate 30-Year Groundwater Exposure Point Concentration ............................. II - 2-73 2.4.2.3 Models Used to Estimate Concentrations in Agricultural Products . ...... 11-2-74 2.4.2.3.1 6hic, k*

e,

n,ig gs, * ....... 11 - 2-75 2.4.2.3.2 Vegetables ............................ II - 2-75 2.4.2.3.3 Beef ................................ II - 2-81 2.9...4.2.3.4 Dairy ............................... II - 2-82 2.41.4 Summary ofNonchemical-specific Parameters ......... II - 2-83 14.2.4.1Groundwater Fraction Used for Irrigation (GWF) ....................... II - 2-83 2.4.2.4.2 Fraction Exposed (fe) ................... II - 2-83 2.4.2.4.3 Partition Coefficient Deposited from Spray (Kd.p)) . . . . . . . . . . . . . . . . . . . . . . . . . . II - 2-83 2.4.2.4.4 Cattle Water/Feed Intakes (IPWM and Ipwd) .... Il - 2-84 2.4.2.4.5 Cattle Feed/Soil Intake (1p.) ............... 11- 2-84 2.4.2.5 Sample Calculations and Comparison With the Available Monitoring Data ................................ II - 2-84 2.4.2.6 Results ....................................... II - 2-86 2.4.3 Estimation ofChemical Intakes ............................ 11 - 2-90 2.4.3.1 Chemical-specific Parameters ...................... II - 2-94 2.4.3.2 Exposure Factors ................................ 11 - 2-95 2.4.3.3 Results ....................................... II - 2-98 2.4.4 Estimation ofChemical Intakes for Commercial/Industrial Scenario .............................................. II - 2-99 2.4.4.1 Estimation ofChemical Intakes ..................... 11-2-100 2.4.5 Uncertainty Analysis (Residential) ......................... 11-2-100 2.4.5.1 Preliminary Sensitivity Analysis .................... 11-2-101 2.4.5.2 Additional Scoping Considerations .................. 11-2-102 2.4.5.3 Procedures .................................... 11- 2-103 2.4.5.4 Determination ofInput Parameter Distributions ........ II - 2-104 2.4.5.5 Results ............. 11-2-106 2.4.5.5.1 Input Parameters ....................... 11-2-106 2.4.5.5.2 Example Results ....................... II - 2-108 2.4.5.5.3 Summary ofResults .................... 11-2-110 2.5 SUMMARY OFEXPOSURE ASSESSMENT ........................ 11-2-112 2.5.1 Residential Scenario ...... 11- 2-113 2.5.2 Commercial/Industrial Scenario ........................... 11-2-116 2.6 UNCERTAINTIES ........................................... 11-2-116 3.0 TOXICITY ASSESSMENT .......................................... II - 3-1 3.1 HUMAN REFERENCE DOSES ................................. 11 - 3-1 3.2 POTENTIAL CARCINOGENICITY OF CHEMICALS OF CONCERN .... II - 3-5
20000,317.10 - 1-toc 0306111892M

TABLE OF CONTENTS (Continued) Page 3.2.1 Weight of Evidence ..................................... 11- 3-5 3.2.2 Slope Factor .......................................... Il - 3-6 3.3 NONHUMAN RECEPTOR TOXICITY REFERENCE VALUES ........ 11- 3-7 3.3.1 Vegetation ........................................... II - 3-8 3.3.2 Aquatic Organisms ..................................... Il - 3-8 3.3.3 Terrestrial Organisms ................................... II - 3-9 3.3'.3.1 Derivation of a No-Observed-Adverse- Effect Level ..... 11 - 3-10 3.3.3.2 Phylogenetic Differences .......................... Il - 3-13 3J.3.2.1 Intraspecies Differences ................. 11- 3-16 3.3.3.2.2 Genus and Species Differences ............ II - 3-16 3.3.3.2.3 Family/Order Differences ................ 11 - 3-18 3.3.3.2.4 Threatened or Endangered Species .......... II - 3-19 3.4 MAXIMUM ACCEPTABLE TISSUE CONCENTRATIONS ............ 11- 3-19 3.5 UNCERTAINTIES INTOXICITY ASSESSMENT .................... 11- 3-20 3.5.1 Human Toxicity Assessment .............................. II - 3-20 3.5.2 Toxicity Reference Values ............................... 11- 3-21
VOLUME III - ENDANGERMENT ASSESSMENT 4.0 HUMAN RISK CHARACTERIZATION ............................... 111-4-1 4.1 RESIDENTIAL SCENARIO .................................... Ill- 4-2 4.1.1 Hypothetical Future Exposures ........... ................ 111 -4-3 4.1.1.1 Hypothetical Carcinogenic Risks Related to Background . . 111 -4-4 4.1.1.2 Child Chronic Hypothetical Future Hazard Indices ...... III - 4-5 4.1.1.3 Adult Chronic Hypothetical Future Hazard Indices ...... 111-4-6 4.1.1.4 Hypothetical Future Carcinogenic Risk ............... 111-4-6 4.1.1.5 Zone-specific Risks ............................. 111-4-6 4.1.1.6 Acute/Subchronic Effects ......................... 111-4-10 4.1.2 Hypothetic CurrentRisks Versus Hypothetical Future Risks ...... 111-4-12 4.1.3 Health Risk at Most Likely Exposure Intakes ................. 111-4-12 4.1.4 Alternative Risk Estimate Based on Predicted Decline in Groundwater Concentrations .............................. Ill - 4-14 4.2 COMMERCIAL/INDUSTRIAL SCENARIO ........................ 111-4-14 4.2.1 Adult Chronic Hypothetical Future Hazard Indices ............. III - 4-15 4.2.2 Hypothetical Future Carcinogenic Risks 111 -4-15 4.2.3 Carcinogenic Risk at Most Likely Exposure Intakes ............. Ill -4-15 4.3 RISK CHARACTERIZATION SUMMARY ........................ Ill- 4-15 20000,317.10 -I-toc 0306111992 ivTABLE OF CONTENTS (Continued) Page 4.4 UNCERTAINTIES INTHE HUMAN HEALTH ASSESSMENT ......... 111- 4-18 5.0 ECOLOGICALASSESSMENT ....................................... 111- 5-1 5.1 SITE CHARACTERIZATION ................................... 111- 5-1 5.1.1 Study Area Definition ................................... III - 5-1 5.1.1.1 Terrestrial Systems .............................. III - 5-2 5.1.1.2 Aquatic Systems ................................ III - 5-3 5.1.2 Ecological Characterization ofthe Offpost Operable Unit ........ III - 5-4 5.1.3 Contamination Pertaining to Offpost Biota ................... 111- 5-5 5.1.4 Selection ofChemicals ofConcern forBiota .................. III - 5-6 5.2 EXPOSURE ASSESSMENT ..................................... III - 5-7 5.2.1 Potential Biological Receptors and Sensitive Subpopulations ...... III - 5-7 5.2.1.1 Terrestrial Receptors ............................. 111- 5-8 5.2.1.2 Aquatic Receptors ............................... 111- 5-10 5.2.2 Potential Exposure Pathways .............................. III - 5-11 5.2.3 Exposure Point Concentrations ............................ III - 5-14 5.2.4 Ecological Assessment Endpoints .......................... 111- 5-15 5.2.4.1 Terrestrial ..................................... III - 5-15 5.2.4.2 Aquatic ....................................... III - 5-16 5.2.5 Bioaccumulation Exposure ............................... III- 5-17 5.2.5.1 Terrestrial Food Web Ingestion Pathways .............. III - 5-17 5.2.5.2 Terrestrial Food Web Predicted Tissue Concentrations .... III - 5-19 5.2.5.3 Aquatic Food Web Ingestion Pathways ................ 111-5-20 5.2.5.4 Aquatic Food WebPredicted Tissue Concentrations ...... 111- 5-21 5.2.6 Direct Exposure toChemicals ofConcern .................... III - 5-21 5.2.6.1 Surface-Water Ingestion Pathway ................... III - 5-21 5.2.6.2 Direct Contact with Surface Water or Sediment ......... III- 5-23 5.2.6.3 Soil Ingestion .................................. 111- 5-23 5.2.6.4 Dietary Ingestion for Wildlife ...................... 111-5-24 5.2.6.5 Dietary and Soil Ingestion Pathways for Livestock ....... III - 5-24 5.3 RISK CHARACTERIZATION .................................. 111-5-26 5.3.1 Terrestrial Food Web Ecological Risk ....................... 111-5-26 5.3.2 Aquatic Food Web Ecological Risk ......................... 111- 5-29 5.3.3 Underwater Aquatic Life ................................ 111- 5-30 5.3.4 Agricultural Life ...................................... 111- 5-30 5.3.5 Ecological Risk Assessment Uncertainty ..................... 111- 5-31 5.3.5.1 Site Characterization ............................. III- 5-31 5.3.5.2 Exposure Assessment ............................ III - 5-32 5.3.5.3 Risk Characterization ............................ 111- 5-34 5.4 SUMMARY ................................................ 111- 5-34

20000,317.10 - 1-toc 0306111892 vTABLE OF CONTENTS (Continued) Pape 6.0 CONCLUSIONS .................................................. 111-6-1 7.0 REFERENCES ................................................... III - 7-1

20000,317.10 - 1-toc 0306111892 viTABLE OF CONTENTS (Continued)

VOLUME IV - ENDANGERMENT ASSESSMENT LIST OF APPENDIXES A RELEASE OF VOLATILE CONTAMINANTS FROM GROUNDWATER WITH SUBSEQUENT EXPOSURE IN BASEMENTS: SCREENING LEVEL ANALYSIS B ESTIMATE OF AMBIENT PARTICULATE MATTER IMPACT DUE TO WIND EROSION NEAR ROCKY MOUNTAIN ARSENAL C EXPOSURE CONCENTRATIONS D ESTIMATED REASONABLE MAXIMUM EXPOSURE INTAKES E UNCERTAINTY ANALYSIS F TOXICOLOGICAL PROFILES * ESTIMATED REASONABLE MAXIMUM EXPOSURE RISKS AND HAZARD INDICES * SUPPORTING INFORMATION FOR THE ECOLOGICAL ASSESSMENT HI - ANIMAL SPECIES OF POSSIBLE OCCURRENCE IN RMA OFFPOST AREA H2 - BIOACCUMULATION FACTORS, BIOMAGNIFICATION FACTORS, AND PREDICTED TISSUE CONCENTRATIONS

- TERRESTRIAL FOOD WEB

- AQUATIC FOOD WEB H3 - EXPOSURE POINT CONCENTRATIONS OF CHEMICALSIN SURFACE WATER AND PREDICTED CHEMICAL INTAKE FOR AVIAN SPECIES H4 - HAZARD QUOTIENT SUMMARY TABLES FOR CATTLE H5 - MAXIMUM ALLOWABLE TISSUE CONCENTRATION VALUES H6 - SPATIAL WEIGHING ADJUSTMENT FACTORS H7 - ONPOST HHRC SOFTWARE BLUE VERSION 1.0 PREPARED FOR THE PROGRAM MANAGER RMA EBASCO SERVICES, INC., MARCH 1992

20000,317.10 - 1-toc 0306111892 viiTABLE OF CONTENTS (Continued) Page VOLUME V - FEASIBILITY STUDY PREFACE ........................................................... V - P-1

1.0 FEASIBILITY STUDY PURPOSE AND ORGANIZATION ................. V- 1-1 1.1 STEP I- DEVELOPMENT OF REMEDIAL ACTION OBJECTIVES ..... V - 1-1 1.2 STEP 11 - DEVELOPMENT AND SCREENING OF ALTERNATIVES .... V - 1-2 1.3 STEP III - DETAILED ANALYSIS OF ALTERNATIVES ............. V - 1-2 1.4 STEP IV - SELECTION OF THE PREFERRED SITEWIDE ALTERNATIVE ............................................. V - 1-3 2.0 DEVELOPMENT OF REMEDIAL ACTION OBJECTIVES AND SCREENING OF TECHNOLOGIES .............................................. V - 2-1 2.1 OVERVIEW OF REMEDIAL ACTION OBJECTIVES DEVELOPMENT V - 2-1 2.2 NATIONAL CONTINGENCY PLAN AND ACCEPTABLE RISK RANGE CONSIDERATIONS .......................................... V - 2-2 2.2.1 Comparison ofCumulative RiskWith the Acceptable Risk Range . . V - 2-2 2.21.21 Identification ofMedia Requiring Remedial Action Objectives .... V - 2-4 2.2.3 Site-specific Factors Considered for Groundwater Remedial Action Objectives Development .................... V - 2-6 2.3 DEVELOPMENT OF GROUNDWATER REMEDIAL ACTION OBJECTIVES ............................................... V - 2-6 2.3.1 Development ofRemedial Action Objectives, Step 1: Identification ofChemicals ofConcern ............... V - 2-7 2.3.2 Development ofGroundwater Remedial Action Objectives Step 2: Identification ofPotential Exposure Pathways ................. V - 2-7 2.4 GROUNDWATER REMEDIAL ACTION OBJECTIVES ............... V - 2-8 2.4.1 Groundwater Remedial Action Objectives .................... V - 2-9 2.4.1.1 Human Health .................................. V - 2-9 2.4.1.2 Environmental Protection ......................... V - 2-9 2.5 DEVELOPMENT OF GROUNDWATER PRELIMINARY REMEDIATION GOALS ..................................... V - 2-10 2.5.1 Effects ofFuture Land Use on the Development of Preliminary Remediation Goals ........................... V - 2-11 2.5.2 National Contingency Plan and Point of Departure Considerations for Development of Preliminary Remediation Goals ............ V - 2-12 20000,317.10 - 1-toc 0306111892 viiiTABLE OF CONTENTS (Continued) Page 2.5.3 Types of Groundwater Preliminary Remediation Goals .......... V - 2-15 2.5.3.1 Identification ofApplicable or Relevant and Appropriate Requirements ............. V - 2-15 2.5.3.1.1 Chemical-specific Applicable or Relevant and Appropriate Requirements .... V - 2-17 2.5.3.1.2 Location-specific Applicable or Relevant and Appropriate Requirements ............ V - 2-18 15.3.1.3 Action-specific Applicable or Relevant and Appropriate Requirements .... V - 2-18 2.5.3.2 Health-based Criteria ............................ V - 2-18 2.5.3.3 Background Concentrations ........................ V - 2-20 2.5.3.4 Ecological Criteria .............................. V - 2-20 2.5.4 Development and Identification ofPreliminary Remediation Goals - Groundwater ............. V - 2-20 2.5.4.1 Groundwater Chemical-specific Applicable orRelevant and Appropriate Requirements V - 2-20 2.5.4.2 Groundwater Health-based Criteria .......... V - 2-21 Selection ofGroundwater Preliminary Remedia ion Goals V - 2-21 2.6 QUANTITIES OF GROUNDWATER REQUIRING REMEDIATION ..... V - 2-22 2.7 GENERAL RESPONSE ACTIONS ............................... V - 2-22 2.7.1 No Action ............................................ V - 2-23 2.7.2 Institutional Controls ................................... V - 2-23 2.7.3 Containment .......................................... V - 2-23 2.7.4 Removal ............................................. V - 2-24 2.7.5 Treatment ............................................ V - 2-24 2.7.6 Disposal ............................................. V - 2-24 2.8 IDENTIFICATION AND SCREENING OFTECHNOLOGY TYPES AND PROCESS OPTIONS .......................................... V - 2-25 2.8.1 Universal Screening .................................... V - 2-26 2.8.2 Evaluation and Screening ofSelected Process Options ........... V - 2-26 2.8.3 Nonmedia-specific Process Options ......................... V - 2-27 2.8.3.1 No Action ..................................... V - 2-27 2.8.3.2 Institutional Controls ............................. V - 2-28 2.8.3.2.1 Access Restrictions ..................... V - 2-28 2.8.3.2.2 Alternative Water Supplies ................ V -2-29 2.8.3.2.3 Monitoring ........................... V - 2-29 2.8.4 Groundwater Process Options ............................. V - 2-30 2.8.4.1 Removal ...................................... V - 2-30 2.8.4.1.1 Groundwater Extraction ................. V - 2-30 2.8.4.2 Disposal ............ ...... **...* V - 2-33 2.8.4.2.1 Subsurface Disposal ..................... V - 2-33 2.8.4.2.2 Aboveground Discharge ................. V - 2-35 2.8.4.3 In-vessel Treatment ofGroundwater ................. V - 2-37 2.8.4.3.1 Solids Removal ........................ V - 2-37 20000,317.10 - 1-toc 0306111892 ixTABLE OF CONTENTS (Continued) Page .ý..8.4.3.2 ........................ V - 2-39 Phase Transfer 2.8.4.3.3 Sorption ............................. V - 2-40 2.8.4.3.4 Oxidation ............................ V - 2-42 2.8.4.3.5 Radiation ............................ V - 2-44 2.8.4.3.6 Biological Treatment .................... V - 2-45 2.8.4.4 In Situ Treatment ............................... V - 2-46 2.8.4.4.1 Water Flushing ........................ V - 2-46 2.8.4.4.2- Enhanced Biological Treatment ............ V - 2-47 2.81.4.5 Containment ................................... V - 2-48 Z-8.4.5.1 Subsurface Barriers ..................... V - 2-48 2.9 SUMMARY OF PROCESS OPTION SCREENING ................... V - 2-50 2.10 SELECTION OF REPRESENTATIVE PROCESS OPTIONS ............ V - 2-50 2.10.1 Representative Process Options for Groundwater Remediation .... V - 2-51 VOLUME VI - FEASIBILITY STUDY 3.0 DEVELOPMENT OF REMEDIAL ALTERNATIVES ..................... VI - 3-1 3.1 FEASIBILITY STUDY OVERVIEW .............................. VI - 3-1 3.1.1 U.S. Environmental Protection Agency Guidance for Alternatives Development and Evaluation .................... VI - 3-1 3.2 ALTERNATIVES IDENTIFICATION ............................. VI - 3-3 3.2.1 Approach ...................... *,*,*,, :,**'*****''**' VI - 3-3 3.2.1.1 Groundwater Model Input Into Alternatives Development ................................... VI - 3-4 3.2.1.1.1 Chemicals ofConcern .................. VI - 3-4 3.2.1.1.2 Use ofthe Model in Alternatives Development .......................... VI - 3-5 3.2.1.2 Groundwater Alternative Types .................... VI - 3-9 3.3 LAND OWNERSHIP IMPLICATIONS FOR IMPLEMENTING REMEDIAL ALTERNATIVES ............................................ VI - 3-10 3.4 COMMONALITIES OF ALTERNATIVES ......................... VI - 3-10 3.4.1 Commonalities ofGroundwater Remedial Alternatives .......... VI - 3-11 3.4.1.1 Groundwater Alternatives Institutional Controls ........ VI- 3-11 3.4.1.1.1 Provision ofan Alternative Water Supply (Exposure Control) ..................... VI - 3-11 3.4.1.1.2 Monitoring Networks ................... VI - 3-12 3.4.1.1.3 Site Review ........................... VI - 3-12 3.4.1.2 Groundwater Pretreatment Process Options ............ VI - 3-13
20000,317.10 - 1-toc 0306111892 xTABLE OF CONTENTS (Continued) Page 3.4.1.3 Continued Operation ofthe Boundary Systems and Compliance With Offpost Operable Unit Preliminary Remediation Goals .............................. VI - 3-13 3.5 GROUNDWATER REMEDIAL ALTERNATIVES IDENTIFICATION ... VI - 3-13 3.5.1 Identification ofGroundwater Alternatives: North Plume Group ............ *,, ,**: ...*''''***'*'**''**** VI - 3-14 3.5.1.1 Alternative No. N-1: No Action .................... VI - 3-14 3.5.1.2 Alternative No. N-2: Continued Operation ofthe North Boundary Containment System With Improvements as Necessary ..................................... VI - 3-14 3.5.1.3 Alternative No. N-3: Land Acquisition and Use Restrictions .......... VI - 3-15 3.5.1.4 Alternative No. N-4: interim Response Action A ....... VI - 3-16 3.5.1.5 Alternative No. N-5: Expansion I to Interim Response Action A ..................................... VI
- 3-18 3.5.1.6 Alternative No. N-6: Expansion 2 to Interim Response Action A .......... VI- 3-19 3.5.1.7 Summary ofAlternatives Identified forthe North Plume Group ........................................ VI
-3-21 3.5.2 Identification ofGroundwater Alternatives: Northwest Plume Group ........ VI - 3-22 3.5.2.1 Alternative NW-1: No Action ...................... VI - 3-22 3.5.2.2 Alternative NW-2: Continued Operation ofthe Northwest Boundary Containment System ..................... VI - 3-22 3.5.2.3 Alternative No. NW-3: Land Acquisition With Use Restrictions ............ VI - 3-23 3.5.2.4 Alternative No. NW-4: Nori"e'si *P*lu* tie,*G* r*o*u*p*,, **** - Extraction/Recharge System VI - 3-24 3.5.2.5 Summary ofAlternatives Identified for the Northwest Plume Group .................................. VI - 3-25 4.0 SCREENING OF ALTERNATIVES ................................... VI - 4-1 4.1 M ETHODOLOGY ............................................ VI - 4-1 4.1.1 Effectiveness Evaluation ................................. VI - 4-1 4.1.2 Implementability Evaluation .............................. VI - 4-2 4.1.3 Cost Evaluation ........................................ VI - 4-3 4.2 SCREENING OF ALTERNATIVES .............................. VI - 4-4 4.2.1 Screening ofAlternatives - North Plume Group ............... VI - 4-4 4.2.1.1 Alternative No. N-1: No Action .................... VI - 4-4 4.2.1.1.1 Effectiveness .......................... VI - 4-5 4.2.1.1.2 Implementability ....................... VI - 4-6 4.2.1.1.3 Cost ................................ VI - 4-6
20000,317.10 - 1-toc 0306111892 xiTABLE OF CONTENTS (Continued) Page 4.2.1.2 Alternative No. N-2: Continued Operation ofthe North Boundary Containment System With Improvements as Necessarv ..... VI - 4-6 4.2.1.2.1 Effectiveness .......................... VI - 4-7 4.2.1.2.2 Implementability ....................... VI - 4-8 4.2.1.2.3 Cost ...... ......................... VI - 4-8 4.2.1.3 Alternative No. N-3: Land Acquisition and Deed Restrictions .................................... VI - 4-9 4.2.1.3.1 Effectiveness .......................... VI - 4-9 4.2.1.3.2 Implementability ....................... VI - 4-10 4.2.1.3.3 Cost .... VI - 4-11 4.12. 1.4 Alternative No. N-4: interim Response Action A ....... VI - 4-11 4.2.1.4.1 Effectiveness .......................... VI- 4-11 4.2.1.4.2 Implementability ....................... VI - 4-12 4.2.1.4.3 Cost ....... ........... :,*****,,I*I* VI - 4-13 4.2.1.5 Alternative No. N-5: Expansion I to Interim Response Action A .............................. VI - 4-13 4.2.1.5.1 Effectiveness .......................... VI - 4-13 4.2.1.5.3 Implementability ....................... VI - 4-14 4.2.1.5.3 Cost ....... ........................ VI - 4-15 4. 1.6 Alternative No. N-6: Expansion 2-to Interim Response Action A .............................. VI - 4-15 4.2.1.6.1 Effectiveness .......................... VI - 4-15 4.2.1.6.2 Implementability ....................... VI - 4-16 4.2.1.6.3 Cost ... VI - 4-17 4.2.1.7 Alternative Screening'iU'm* m*a*r*y*a*n*d**R*e'com*'m* e*n*d*a'tio*n* VI - 4-17 4.2.2 Screening ofGroundwater Alternatives - Northwest Plume Group ............................................... VI - 4-18 4.2.2.1 Alternative No. NW-I: No Action .................. VI - 4-18 4.2.2.1.1 Effectiveness .......................... VI - 4-19 4.2.2.1.2 Implementability ....................... VI - 4-20 4.2.2.1.3 Cost ................................ VI - 4-20 4.2.2.2 Alternative No. NW-2: Continued Operation ofthe Northwest Boundary Containment System With Improvements As Necessary ....................... VI - 4-20 4.2.2.2.1 Effectiveness .......................... VI - 4-21 4.2.2.2.2 Implementability ....................... VI - 4-22 4.2.2.2.3 Cost ................................ VI - 4-22 4.2.2.3 Alternative No. NW-3: Land Acquisition and Deed Restrictions .................................... VI - 4-23 4.2.2.3.1 Effectiveness .......................... VI - 4-23 4.2.2.3.2 Implementability ....................... VI - 4-24 4.2.2.3.3 Cost ................................ VI - 4-24 4.2.2.4 Alternative No. NW-4: Northwest Plume Group Extraction/Recharge System ....................... VI - 4-25 4.2.2.4.1 Effectiveness .......................... VI - 4-25 4.2.2.4.2 Implementability ....................... VI - 4-26 4.2.2.4.3 Cost ................................ VI - 4-27 4.2.2.5 Alternative Screening Summary and Recommendation .... VI - 4-27 20000,317.10 - I-toc 0306111892 xiiTABLE OF CONTENTS (Continued) Page 5.0 DETAILED ANALYSIS OFALTERNATIVES ........................... VI - 5-1 5.1 CRITERIA ................................................. VI - 5-2 5.1.1 Overall Protection ofHuman Health and the Environment ....... VI - 5-2 5.1.2 Compliance With Applicable orRelevant and Appropriate Requirements ............................... VI - 5-2 5.1.3 Long-term Effectiveness and Permanence .................... VI - 5-3 5.1.4 Reduction ofToxicity, Mobility, orVolume .................. VI - 5-3 5.1.5 Short-term Effectiveness ................................ VI - 5-3 5.1.6 Implementability ....................................... VI - 5-4 5.1.7 Cost ................................................ VI - 5-4 5.1.8 State Acceptance ....................................... VI - 5-4 5.1.9 Community Acceptance ................................. VI - 5-5 5.2 MODELING RESULTS SUMMARY ............................. VI - 5-5 5.3 FURTHER DEFINITION OFALTERNATIVES ..................... VI - 5-7 5.3.1 Alternative No. N- 1: No Action ........................... VI - 5-7 5.3.2 Alternative No. N-2: Continued Operation ofthe North Boundary Containment System With Improvements as Necessary ... VI - 5-8 5.3.3 Alternative No. N-4: Interim Response Action A .............. VI - 5-8 5.3.4 Alternative No. N-5: Expansion I toInterim Response Action A ............................................ VI - 5-10 5.3.5 Alternative No. NW- 1: No Action ........................ VI - 5-11 5.3.6 Alternative No. NW-2: Continued Operation ofthe Northwest Boundary Containment System With Improvements as Necessary VI - 5-11 5.4 DETAILED ANALYSIS OFALTERNATIVES ..................... VI - 5-12 5.4.1 Overall Protection ofHuman Health and the Environment ...... VI - 5-12 5.4.1.1 Alternative No. N-1: No Action ................... VI - 5-12 5.4.1.2 Alternative No. N-2: Continued Operation of the North Boundary Containment System With Improvements as Necessary ....................... VI - 5-13 5.4.1.3 Alternative No. N-4: Interim Response Action A ...... VI - 5-14 5.4.1.4 Alternative No. N-5: Expansion Ito Interim Response Action A VI - 5-15 5.4.1.5 Alternative No. NW-1: No Action ................. VI - 5-15 5.4.1.6 Alternative No. NW-2: Continued Operation ofthe Northwest Boundary Containment System With Improvements as Necessary ....................... VI - 5-16 5.4.2 Compliance With Applicable orRelevant and Appropriate Requirements .............................. VI - 5-16 5.4.2.1 Alternative No. N-1: No Action ................... VI - 5-17 5.4.2.1.1 Chemical-specific Applicable or Relevant and Appropriate Requirements ... VI - 5-17
20000,317.10 - 1-toc 0306111892 xiiiTABLE OF CONTENTS (Continued) Page 5.4.2.1.2 Action-specific Applicable or Relevant and Appropriate Requirements ............ VI - 5-17 5.4.2.1.3 Location-specific Applicable orRelevant and Appropriate Requirements ............ VI - 5-17 5.4.2.2 Alternative No. N-2: Continued Operation ofthe North Boundary Containment System With Improvements as Necessarv ........... VI - 5-17 5.4.2.2.1 Chemical- specific Applicable or Relevant and Appropriate Requirements .... VI - 5-17 5.4.1".2.2 Action-specific Applicable or Relevant and Appropriate Requirements ............ VI- 5-18 5.4.2.2.3 Location-specific Applicable orRelevant and Appropriate Requirements ............ VI - 5-18 5.4.2.3 Alternative No. N-4: Interim Response Action A ....... Vl- 5-18 5.4.2.3.1 Chemical-specific Applicable or Relevant and Appropriate Requirements ............ VI- 5-18 5.4.2.3.2 Action-specific Applicable or Relevant and Appropriate Requirements ............ Vl- 5-18 5.4.2.3.3 Location-specific Applicable or Relevant and Appropriate Requirements ............ Vl- 5-18 5.4.2.4 Alternative No. N-5: Expansion I to Interim Response Action A .............................. VI- 5-18 5.4.2.4.1 Chemical-specific Applicable or Relevant and Appropriate Requirements ............ Vl- 5-18 5.4.2.4.2 Action-specific Applicable or Relevant and Appropriate Requirements ............ VI- 5-19 5.4.2.4.3 Location-specific Applicable orRelevant and Appropriate Requirements ............ VI - 5-19 5.4.2.5 Alternative No. NW-I: No Action ................. Vl- 5-19 5.4.2.5.1 Chemical-specific Applicable orRelevant and Appropriate Requirements ............ Vl- 5-19 5.4.2.5.2 Action-specific Applicable orRelevant and Appropriate Requirements ............ VI- 5-19 5.4.2.5.3 Location-specific Applicable or Relevant and Appropriate Requirements ..... ****- VI- 5-19 5.4.2.6 Alternative No. NW-2: Continued Operation ofthe Northwest Boundary Containment System With Improvements as Necessary ........................ VI - 5-19 5.4.2.6.1 Chemical-specific Applicable or Relevant and Appropriate Requirements ............ Vl- 5-19 5.4.2.6.2 Action-specific Applicable orRelevant and Appropriate Requirements ............ VI- 5-20 5.4.2.6.3 Location-specific Applicable or Relevant and Appropriate Requirements ............ Vl- 5-20 5.4.3 Long-term Effectiveness and Permanence ................... Vl- 5-20 5.4.3.1 Alternative No. N-1: No Action .................... VI- 5-20 5.4.3.2 Alternative No. N-2: Continued Operation ofNorth Boundary Containment System With Improvements as Necessary ..................................... VI- 5-20 20000,317.10 - 1-toc 0306111892 xivTABLE OF CONTENTS (Continued) Page 5.4.3.3 Alternative No. N-4: Interim Response Action A ....... VI - 5-21 5.4.3.4 Alternative No. N-5: Expansion Ito Interim Response Action A ... VI - 5-21 5.4.3.5 Alternative No. NW-1: No Action .......... VI - 5-22 5.4.3.6 Alternative No. NW-2: Continued Operation ofthe Northwest Boundary Containment System With Improvements asNecessary ........................ VI - 5-22 5.4.4 Reduction ofToxicity, Mobility, orVolume .................. VI- 5-23 5.4.4.1 Alternative iýo. N-1: No Action ...... ............. VI- 5-23 5.4.4.2 Alternative No. N-2: Continued Operation ofthe North Boundary Containment System With Improvements as Necessary ..... .................. VI - 5-23 5.4.4.3 Alternative No. N-4: Interim Response Action A ....... VI- 5-23 5.4.4.4 Alternative No. N-5: Expansion I to Interim Response Action A .............................. VI - 5-24 5.4.4.5 Alternative No. NW-1: No Action .................. VI - 5-25 5.4.4.6 Alternative No. NW-2: Continued Operation of Northwest Boundary Containment System With Improvements as Necessary ........................ VI - 5-25 5.4.5 Short-term Effectiveness . . *...*- :*** ...**''******' **- * VI- 5-25 5.4.5.1 Alternative No. N-1: No Action VI- 5-26 5.4.5.1.1 Protection ofCommunity and Workers ...... VI- 5-26 5.4.5.1.2 Adverse Environmental Impacts ........... VI- 5-26 5.4.5.1.3 Implementation Period .................. VI - 5-26 5.4.5.2 Alternative No. N-2: Continued Operation of North Boundary Containment System ................ VI- 5-26 5.4.5.2.1 Protection ofCommunity and Workers ...... VI - 5-26 5.4.5.2.2 Adverse Environmental Impacts ........... VI - 5-26 5.4.5.2.3 Implementation Period .................. VI - 5-26 5.4.5.3 Alternative No. N-4: Interim Response Action A ........ VI - 5-27 5.4.5.3.1 Protection ofCommunity and Workers ...... VI- 5-27 5.4.5.3.2 Adverse Environmental Impacts ........... VI- 5-27 5.4.5.3.3 Implementation Period .................. VI- 5-27 5.4.5.4 Alternative No. N-5: Expansion I to Interim Response Actoin A .............................. VI - 5-27 5.4.5.4.1 Protection ofCommunity andWorkers ...... VI - 5-27 5.4.5.4.2 Adverse Enviornmental Impacts ........... VI - 5-27 5.4.5.4.3 Implementation Period .................. VI - 5-28 5.4.5.5 Alternative No. NW-1: No Action .................. VI - 5-28 5.4.5.5.1 Protection ofCommunity and Workers ...... VI- 5-28 5.4.5.5.2 Adverse Environmental Impacts ........... VI- 5-28 5.4.5.5.3 Implementation Period .................. VI - 5-28 5.4.5.6 Alternative No. NW-2: Continued Operation of Northwest Boundary Containment System With Improvements as Necessary ........................ VI - 5-28 5.4.5.6.1 Protection ofCommunity and Workers ...... VI- 5-28 5.4.5.6.2 Adverse Environmental Impacts ........... VI- 5-29 5.4.5.6.3 Implementation Period .................. VI- 5-29
20000,317.10 - 1-toc 0306111892 xvTABLE OF CONTENTS (Continued) Page 5.4.6 Implementability ............... *:****'*''*******'***'*' VI - 5-29 5.4.6.1 Alternative No. N-1: No Action .................... VI - 5-29 5.4.6.2 Alternative No. N-2: Continued Operation ofNorth Boundary Containment System With Improvements as Necessary ......... VI - 5-29 5.4.6.3 Alternative No. N-4: interim Response Action A ....... VI- 5-30 5.4.6.4 Alternative No. N-5: Expansion Ito Interim Response Action A .............................. VI - 5-30 5.4.6.5 Alternative No. NW-1: No Further Action ............ VI- 5-30 5.4.6.6 Alternative No. NW-2: Continued Operation of the Northwest Boundary Containment System With Improvements as Necessary ........................ Vl- 5-31 5.4.7 Cost ................................................ VI
- 5-31 5.4.7.1 Alternative No. N-1: No Action .................... VI- 5-31 5.4.7.2 Alternative No. N-2: Continued Operation ofthe North Boundary Containment System With Improvements as Necessary ..................................... Vl- 5-31 5.4.7.3 Alternative No. N-4: Interim Response Action A ....... Vl- 5-32 5.4.7.4 Alternative No. N-5: Expansion I to Interim Response Action A .............................. Vl- 5-32 5.4.7.5 Alternative No. NW-1: No Action .................. VI- 5-32 5.4.7.6 Alternative No. NW-2: Continued Operation ofthe Northwest Boundary Containment System With Improvements as Necessary ........................ VI- 5-32 5.5 COMPARATIVE ANALYSIS OF REMEDIAL ALTERNATIVES ........ VI - 5-33 5.5.1 Comparison ofAlternatives - North PlumeGroup .............. VI- 5-33 5.5.1.1 Overall Protection ofHuman Health and the Environment ................................... VI- 5-33 5.5.1.2 Compliance With Applicable orRelevant and Appropriate Requirements ........................ Vl- 5-34 5.5.1.3 Long-term Effectiveness and Permanence ............. Vl- 5-34 5.5.1.4 Reduction in Toxicity, Mobility, and Volume .......... VI - 5-34 5.5.1.5 Short-term Effectiveness .......................... VI - 5-35 5.5.1.6 Implementability ................................ VI- 5-35 5.5.1.7 Cost ...................... . ... VI- 5-36 5.5.2 Comparison ofAlternatives - Northwest Plume Group .......... VI - 5-36 5.5.2.1 Overall Protection ofHuman Health and the Environment ................................... VI- 5-37 5.5.2.2 Compliance With Applicable or Relevant and Appropriate Requirements ........................ VI - 5-37 5.5.2.3 Long-term Effectiveness and Permanence ............. VI - 5-37 5.5.2.4 Reduction in Toxicity, Mobility, and Volume .......... VI - 5-37 5.5.2.5 Short-term Effectiveness .......................... VI - 5-38 5.5.2.6 Implementability ................................ VI - 5-38 5.5.2.7 Cost .......................... .............. VI - 5-38 5.5.3 Ranking ofAlternatives - North Plume Group and Northwest Plume Group ................................. VI - 5-39 20000,317.10 - 1-toc 0306111892 xviTABLE OFCONTENTS (Continued) Page 6.0 SELECTION OFTHE PREFERRED SITEWIDE ALTERNATIVE ........... VI - 6-1 6.1 IDENTIFICATION OF THE PREFERRED SITEWIDE ALTERNATIVE .. VI - 6-1 6.1.1 Preferred Sitewide Alternative Description ................... VI - 6-3 6.1.1.1 Alternative No. N-4: Interim Response Action A ....... VI - 6-3 6.1.1.2 Alternative No. NW-2: Continued Operation ofthe Northwest Boundary Control System With Improvements as Necessary ........................ VI - 6-6 6.2 CONSISTENCY WITH THE REQUIREMENTS OFCERCLA AND THE NATIONAL CONTINGENCY PLAN .................... VI - 6-7 6.2.1 Principal Threats Addressed by the Preferred Sitewide Alternative VI - 6-7 6.2.2 'Consistency With the Statutory Requirements ofCERCLA in Section I'Irl I ........................ .................. VI - 6-7 6.2.2.1 Protection of Human Health and the Environment ....... VI - 6-7 6.2-2.2 Compliance With Applicable or Relevant and Appropriate Requirements ........................ VI - 6-8 6.2.2.3 Cost-effectiveness ........ VI - 6-8 6.2.2.4 Utilization ofPermanent Solutions tothe Maximum Extent Practicable .................................... VI - 6-8 6.2.3 Consistency With the National Contingency Plan ............... VI - 6-8 6.3 SUMMARY ................................................ VI - 6-9 7.0 REFERENCES ................................................... VI - 7-1

20000,317.10 - 1-toc 0306111892 xviiTABLE OF CONTENTS (Continued)

VOLUME VII - FEASIBILITY STUDY LIST OF APPENDIXES A APPLICABLE OR RELEVANT AND APPROPRIATE REQUIREMENTS (ARARs) ANALYSIS B WETLAND DELINEATION AND ASSESSMENT C DERIVATION OF HEALTH-BASED CRITERIA AND ECOLOGICALCRITERIA D SECOND LEVEL PROCESS OPTION SCREENING E GROUNDWATER MODELING RESULTS SUMMARY F COST ESTIMATES VOLUME VIII - RESPONSES TOCOMMENTS U.S. ENVIRONMENTAL PROTECTION AGENCY U.S. FISHAND WILDLIFE SERVICE COLORADO DEPARTMENT OFHEALTH SHELL OIL COMPANY

20000,317.10 - 1-toc 0306111892 xviiiLISTOF TABLES Table No. Volume II - ENDANGERMENT ASSESSMENT 1.0-1 Data Needs in Each Medium 1.3.1-1 Statistical Comparison With Background - Groundwater 1.3.1-2 Chemicals ofConcern - Groundwater 1.3.1-3 Summarv of Available Information on Toxicity and Essentiality ofCalcium, Iron, Potassium, Magnesium, and Sodium 1.3.2-1 Statistical Comparison With Background - Surface Water (First Creek) 1.3.2-2 Chemicals of Concern - Surface Water (First Creek) 1.3.2-3 Statistical Comparison With Background - Surface Water (Canals) 1.3.2-4 Statistical Comparison With Background - Surface Water (Barr Lake) 1.3.2-5 Statistical Comparison With Background - Sediment (First Creek) 1.3.2-6 Statistical Comparison With Background - Sediment (O'Brian Canal) 1.3.2-7 Statistical Comparison With Background - Sediment (Barr Lake) 1.3.3-1 Statistical Comparison With Background - Surficial Soil
2.1-1 Henrv's Law Constant (Dimensionless) forOrganic Groundwater Chemicals of Concern 2.1-2 Molecular Diffusivity in Air and Water (m2/day) 2.1-3 Solubility (mg/1) and Vapor Pressure (torr) for Organic Groundwater Chemicals ofConcern 2.1-4 Octanol/Water Partition Coefficients (log KOW) 2.1-5 Adsorption Coefficient (1/kg) forChemicals Elevated in Soil orSediment 2.1-6 Uptake to Roots, Kw, (1/kg), for Organic Groundwater Chemicals ofConcern 2.1-7 Uptake to Plants, KWP (1/kg), forOrganic Groundwater Chemicals ofConcern 2.1-8 Plant Uptake Coefficients, K,,, (Dimensionless), forChemicals ofConcern in Offpost Surficial Soil 2.1-9 Plant Uptake Coefficients, K,P' for Chemicals ofConcern in Offpost Surficial Soil
20000,317.10 - I-toc 0306111892 xixLISTOF TABLES (Continued)

2.1-10 BeefBioaccumulation Coefficients, KPM1 for All Chemicals of Concern 2.1-11 Milk Bioaccumulation Coefficients, Kpd, for All Chemicals ofConcern 2.1-12 Fish Bioaccumulation Coefficients, K,,f(1/kg), for Surface-Water Chemicals of Concern 2.2.21. 1.2-i Population by Land-Use Area 1.2-" Land Use by Land-Use Area (in acres) 2.2.2.1.2-3 Potentially Sensitive Subpopulations 2.2.2-A.I-I Well-Use Data From Colorado State Master Extract Register 2.2.2.4.1-2 Well-Use Data From Consumptive Use Phase I, II, and III Studies 2.2.22. 4.1-3 Preliminarv Estimate of the Number ofWells bv Well Use in the Offpost Operable Unit ý.2.2.4.1-4 Well Use Data From theTri-County Health Department Survey 2.2.2.4.1-5 Combined Estimate of the Number ofWells by Well Use inthe Offpost Operable Unit 2.3.3.1-1 Exposure Pathways: Inhalation Route 2.3.3.2-1 Exposure Pathways: Dermal Route 2.3.3.3-1 Exposure Pathways: Ingestion Route 2.4.2.4-1 Nonchernical-specific Parameters Used in Bioaccumulation Models 2.4.2.5-1 Concentration ofAldrin Plus Dieldrin in Zone 3 2.4.2.5-2 Chemical-specific Properties ofAldrin and Dieldrin 2.4.2.6-1 Exposure Concentrations, Groundwater, Zone 1 2.4.2.6-2 Exposure Concentrations, Groundwater, Zone 2 (Northern Paleochannel) 2.4.2.6-3 Exposure Concentrations, Groundwater, Zone 3 (96th Avenue and Peoria Street) 2.4.2.6-4 Exposure Concentrations, Groundwater, Zone 4 (First Creek Paleochannel) 2.4.2.6-5 Exposure Concentrations, Groundwater, Zone 5 (Northwest Boundary) 2.4.2.6-6 Exposure Concentrations, Groundwater, Zone 6

20000,317.10 - 1-toc 0306111992 xxLISTOF TABLES (Continued)

2.4.2.6-7 Estimated Future Exposure Concentrations for Aldrin and Dieldrin, Groundwater, Zones 3 and 4 2.4.2.6-8 Exposure Concentrations in Surface Water 2.4.2.6-9 Exposure Concentrations in Surficial Soil and Sediments (mg/kg) 2.4.2.6-10 Reasonable Maximum Exposure Concentrations in Vegetables: Maximum Concentrations Estimated in Any Zone 14.2.6-11 Reasonable Maximum Exposure Concentrations in Dairy and Meat Products: Maximum Concentration Estimated in Any Zone 2.4.2.6-12 Reasonable Maximum Exposure Concentration in Eggs (Zones 1,2, and 6) 2.4.3-1 Summary of Land Use Scenarios Used to Estimate Reasonable Maximum Exposure Intakes by Zone 2.4.3.1-1 Dermal Absorption from Soil/Sediment 2.4.3."-I Exposure Factors: Ingestion Rates 2.4.3.2-Ia Parameters Used in Time-weighted Average Intakes Calculation (Milk and Soil Ingestion) 2.4.3.2-2 Exposure Factors: Exposure Frequency and Duration 2.4.3.2-3 Exposure Factors: Physiology 2.4.4-1 Exposure Factors: Commercial/Industrial (Reasonable Maximum Exposure) 2.4.4-2 Exposure Factors: Commercial/Industrial (Most Likely Exposure) 2.4.5.1-1 Sensitivity Analysis Summary 2.4.5.5.3-1 Uncertainty Analysis Results 3.1-1 Hierarchy ofDocuments Referenced Regarding Toxicity Data 3.1-2 Toxicity Data for Rocky Mountain Arsenal Offpost Chemicals ofConcern 3.1-3 Noncarcinogenic Reference Doses and Potential Noncarcinogenic Effects 3.1-4 Cancer Slope Factors and Potential Carcinogenic Effects ofCarcinogenic Operable Unit Offpost Chemicals ofConcern 3.1-5 Regulatory Criteria for Rocky Mountain Arsenal Offpost Operable Unit, Chemicals ofConcern
20000,317.10 - 1-toc 0306111992 xxiLIST OFTABLES (Continued)

3.3.1-1 Reference Media Concentrations for Vegetation and Aquatic Organisms 3.3.3-1 Toxicity Reference Values for Avian and Terrestrial Vertebrate Species of Concern Identified at Rocky Mountain Arsenal 3.3.3-2 References forCritical Toxicity Studies Used to Derive Terrestrial Toxicity Reference Values Listed inTable 3.3.3-1 3.3.3.2.1-1 Nonprimate Mammalian Oral LD50 Data Volume III

4.0-1 Target Organ- or System-specific Chemicals ofConcern 4.1.1-1 Summary ofResidential Hypothetical Future Carcinogenic Risks: Reasonable Maximum Exposure 4.1.1-21 Summarv of Residential Child Chronic Combined Noncarcinogenic Hazard Indices (HIs >1) 4.1.1-3 Contribution ofDieldrin and Other Chemicals to the Child Acute Hepatic Hazard Index 4.1.1-4 Summary of Residential Hypothetical Future Reasonable Maximum Exposure Subchronic Hazard Indices: Children 4.1.1-5 Summary of Residential Hypothetical Future Reasonable Maximum Exposure Subchronic Hazard Indices: Adult Females 4. 1.1-6 Comparison ofResidential Reasonable Maximum Exposure and Most Likely Exposure Hypothetical Future Carcinogenic Risks by Zone
5.1.2-1 Wildlife Species Observed in Offpost Operable Unit: Winter 1989-90 5.1.4-1 Chemicals ofConcern and Tissue Chemicals ofConcern for the Offpost Ecological Risk Assessment 5.2.1-1 Terrestrial Species Selected as Indicators ofEcological Impacts 5.2.1-2 Aquatic/Wetland Species in Relation to Indicator Species Selection Criteria 5.2.2-1 Potential Exposure Pathways for Biota 5.2.3-1 Exposure Point Concentrations for Soil, Surface Water, and Sediment Used for the Offpost Ecological Assessment 5.2.5.1-1 Dietary Fractions for Each of the Species in the Terrestrial Food Web
20000,317.10 - I-toc 0306111892 xxiiLIST OF TABLES (Continued)

5.2.5.2-1 Predicted Tissue Concentrations (mg/kg) for the Terrestial Food Web Ecological Receptors 5.2.5.4-1 PredictedTissue Concentrations (mg/kg) for the Aquatic Food Web Ecological Receptors on the Basisof Surface-Water Concentrations 5.2.5.4-2 Predicted Tissue Concentrations (mg/kg) for the Aquatic Food Web Ecological Receptors on the Basis ofSediment Concentrations 5.3.1-1 Summary ofIntakes from Zone 3 Soil, Surface Water, and Dietfor the Bald Eagle 5.3.1-2 Summary of Intakes From Zone 3 Soil, Surface Water, and Diet for the American Kestrel 5.3.1-3 Summary of Intakes From Zone 3 Soil, Surface Water, and Diet for the Great Horned Owl 5.3.1-4 Summary of Intakes From Zone 3 Soil, Surface Water. and Diet for the Deer Mouse 5.3.1-5 Summary ofIntakes From Zone 3 Soil, Surface Water, and Diet for the Prairie Dog 5.3.2-1 Summarv of Intakes From Zone 3 Sediment, Surface Water, and Diet for the Great Blue Heron 5.3.2-2 Summary ofIntakes From Zone 3 Sediment, Surface Water, and Diet for the Mallard Duck 5.3.4-1 Summary ofChicken Hazard Quotient by Zone, Offpost Operable Unit
Volume V - FEASIBILITY STUDY 2.2.1-1 Offpost Operable Unit Carcinogenic Risks for Each Medium 2.3.1-1 Offpost Operable Unit Groundwater Chemicals ofConcern 2.5.2-1 Preliminary Chemical-specific Applicable or Relevant and Appropriate Requirements forGroundwater 2.5.2-2 Health-based Criteria for Groundwater Chemicals ofConcern Without Applicable orRelevant and Appropriate Requirements 2.5.2-3 Preliminary Remediation Goals for Offpost Operable Unit Groundwater 2.8.1-1 Screening ofProcess Options for Groundwater 2.10.1-1 Selection of Representative Process Options forGroundwater
20000,317.10 - 1-toc 0306111992 xxiiiLIST OFTABLES (Continued)

Volume VI 3.5.1-1 Groundwater Alternatives for the North Plume Group 3.5.1.7-1 Summary ofPreliminary North Plume Group Groundwater Alternatives 3.5.2-1 Groundwater Alternatives for the Northwest Plume Group 3.5.2.5-1 Summary ofPreliminary Northwest Plume Group Groundwater Alternatives 4.2.1.7-1 Summary ofScreening ofGroundwater Alternatives; North Plume Group 4.2.2.5-1 Summary ofScreening ofGroundwater Alternatives; Northwest Plume Group 5.5.3-1 Summary ofthe Detailed Analysis and Ranking ofGroundwater Alternatives for the North Plume Group 5.5.3-2 Summary of the Detailed Analysis and Ranking ofGroundwater Alternatives for the Northwest Plume Group

20000,317.10 - 1-toc 0306111892 xxivLISTOF FIGURES Fip,ure No. Volume I - INTRODUCTION I Location Map ofOnpost Operable Unit, Rocky Mountain Arsenal 2 Offpost Area, Rocky Mountain Arsenal 3 Distribution ofDiisopropylmethyl Phosphonate (DIMP) in the Offpost Unconfined Flow System 4 Distribution ofDicyclopentadiene in the Offpost Unconfined Flow System 5 Distribution ofDieldrin in the Offpost Unconfined FlowSystem 6 Distribution ofEndrin in the Offpost Unconfined Flow System 7 Distribution ofChloroform in the OffpostUnconfined Flow System 8 Distribution ofChlorobenzene in the Offpost Unconfined Flow System 9 Distribution ofDibromochloropropane in the Offpost Unconfined Flow System 10 Distribution ofTrichloroethene (TRCLE) in the Offpost Unconfined Flow System II Distribution ofTetrachloroethene in the Offpost Unconfined Flow System 12 Distribution ofArsenic in theOffpost Unconfined Flow System 13 Distribution ofChloride inthe OffpostUnconfined Flow System 14 Distribution ofFluoride in theOffpost Unconfined Flow System 15 Offpost Operable Unit Confined Denver Formation Monitoring Well Network 16 Distribution ofOrganochlorine Pesticides Detected in Offpost Soil 17 Distribution ofOrganochlorine Pesticides Detected in 96th Avenue Area Offpost Soil 18 Distribution ofOrganic Compounds Detected in Offpost Operable Unit Surface Water 19 Distribution ofOrganic Compounds Detected in Offpost Operable Unit StreamBottom Sediment

20000,317.10 - I-toc 0306111892 xxvLIST OF FIGURES (Continued)

Volume II - ENDANGERMENT ASSESSMENT 1.0-1 Preliminary Site Conceptual Model, Pre-Remedial Investigation, Off'post Operable Unit 1.2-1 OffpostOperable Unit Background Surficial Soil Sampling Locations 1.2-2 Offpost Operable Unit Designated Surficial Soil Data Sets forComparison With Background 1.2-3 Offpost Operable Unit First Creek Surface-Water Sampling Locations 1.2-4 Offpost Operable Unit First Creek Sediment Sampling Locations 1.2-5 Offpost Operable Unit Surface-Water Sampling Locations 1.2-6 Offpost Operable Unit Sediment Sampling Locations 2.2.2.1.1-1 Offpost Operable Unit Land Use Areas and Consumptive Use Phase 11Study Area 2.2.2.1.2-1 Offpost Operable Unit - Future Land Use 2.2.2.1.2-2 Off'post Operable Unit Study Area Selected Current Zoning Map, 1991 2.3.1-1 Site Conceptual Model Off'post Operable Unit 2.4.1-1 Off'post Operable UnitZones Major Pathways ofGroundwater Contaminant Transport 2.4.1-2 Offpost Operable Unit Endangerment Assessment Zones 2.4.2.6-1 Estimated Future Exposure Concentrations for Dieldrin, Groundwater, Zones 3 and4 2.4.2.6-2 Estimated Future Exposure Concentrations for Aldrin, Groundwater, Zones 3 and 4 2.4.3.3-1 Offpost Operable Unit Chronic Intake ofChloroform by Pathway and Groundwater Zone - Residential Scenarios 2.4.3.3-2 Off'post Operable Unit Intake ofDieldrin by Zone and Pathway - Residential Scenario 2.4.5-1 Offpost Operable Unit Histograms Describing the Distribution ofIngestion Rate ofLocally Produced Milk (IRd, kg/day) for the Child Chronic and Lifetime Scenarios - Residential
20000,317.10 -I-toc 0306111992 xxviLISTOFFIGURES (Continued)

2.4.5-2 Offpost Operable Unit Histograms Describing the Distribution ofIngestion Rate ofLocally Produced Vegetables (IRv, kg/day) for theChild Chronic and Lifetime Scenarios - Residential 2.4.5-3 Offpost Operable Unit Histograms Describing the Distribution ofIngestion Rate ofLocally Produced Meat (IRm, kg/day Fresh Weight) forthe Child Chronic and Lifetime Scenarios - Residential 2.4.5.5.2-1 Offpost Operable Unit Distribution of Lifetime Oral Intake ofDieldrin in Zone 3 (mg/kg/dav) - Residential 2.4.5.5.2-2 Offpost Operable Unit Uncertainty inthe Mean Concentration ofDieldrin in Groundwater (mg/1) in Zone 3 2.4.5.5.2-3 Offpost Operable Unit Uncertainty inthe Mean Concentration ofDieldrin in First Creek Surface Water (mg/1) 2.4.5.5.2-4 Offpost Operable Unit Uncertaintv in the Mean Concentration of Aldrin and Dieldrin in Surface SoilofZone 3 (mg/1) 2.4.5.5.2-5 Offpost Operable Unit Uncertainty in the Mean Concentration ofDieldrin in Vegetables (CFv, mg/kg, Fresh Weight) in Zone 3 2.4.5.5.2-6 Offpost Operable Unit Distribution ofChild Chronic Oral Intake ofDIMP in Zone4 (mg/kg/day) - Residential 2.4.5.5.2-7 Offpost Operable Unit Distribution ofLifetime Oral Intake ofChloroform in Zone 2 (mg/kg/day) - Residential 2.4.5.5.2-8 Offpost Operable Unit Distribution ofLifetime Inhalation ofChloroform in Zone 2 (mg/kg/day) - Residential 3.3.1-1 Methodology for Determining Water and Soil Reference Media Concentrations for Vegetation and Aquatic Organisms 3.3.3- I Methodology to Derive Toxicity Reference Values (TRVs) FromClass-specific Toxicity Data Volume III 4.1.1-1 Offpost Operable Unit Combined Carcinogenic Risks (All Pathways) by Groundwater Zone - Residual Scenarios 4.1.1-2 Offpost Operable Unit Child Resident Hazard Indices Described for Toxic Chemicals and Exposure Pathways - Residential Scenarios 4.1.1-3 Offpost Operable Unit Contributors to Carcinogenic Risks - Residential Scenario
20000,317.10 - 1-toc 0306111992 xxviiLISTOF FIGURES (Continued)

4.1.1-4 Carcinogenic Risks Described by Chemical, Medium, and Exposure Pathway - Zone IA, Lifetime Rural Resident, RME 4.1.1-5 Carcinogenic Risks Described by Chemical, Medium, and Exposure Pathway - Zone IB, Lifetime Rural Resident, RME 4.1.1-6 Carcinogenic Risks Described by Chemical, Medium, and Exposure Pathway - Zone IC, Lifetime Rural Resident, RME 4.1.1-7 Carcinogenic Risks Described by Chemical, Medium, and Exposure Pathway - Zone 2, Lifetime Rural Resident, RME 4.1.1-8 Carcinogenic Risks Described by Chemical, Medium, and Exposure Pathway - Zone 3, Lifetime Urban Resident, RME 4.1.1-9 Carcinogenic Risks Described by Chemical, Medium, and Exposure Pathway - Zone 4, Lifetime Urban Resident, RME 4.1.1-10 Carcinogenic Risks Described by Chemical, Medium, and Exposure Pathway - Zone 5, Commercial/Industrial, RME 4.1.1-11 Carcinogenic Risks Described by Chemical, Medium, and Exposure Pathway - Zone 6, Lifetime Rural Resident, RME 4.1.1-12 Child Acute Liver Hazard Indices 4.1.1-13 Subchronic Child Liver Hazard Indices, Maximum and Minimum ofRange 4.2.1-1 Noncarcinogenic Hazard Indices Zone 5; AdultCommercial/Industrial; Chronic RME 5.1.1-1 Offpost Operable Unit BiotaSampling Locations 5.1.1-2 Habitat Map for First Creek Portion ofOffpost Operable Unit 5.2.2-1 Ecological Site Conceptual Model Offpost Operable Unit 5.2.5.1-1 Offpost Operable Unit Terrestrial Food Web 5.2.5.3-1 Offpost Operable Unit Aquatic Food Web 5.2.6.5-1 Offpost Operable Unit Agricultural Food Web

20000,317.10 - 1-toc 0306111892 xxviiiLIST OF FIGURES (Continued)

Volume V - FEASIBILITY STUDY 2.5.1-1 Offpost Operable Unit Feasibility Study Contaminant Distribution Zones 2.6.1 Areas ofExceedances ofGroundwater PRGs inthe Offpost Unconfined Flow System 2.8-1 Groundwater Process Options 2.8.1-1 Results ofUniversal Screening of Groundwater Process Options 2.9-1 Results ofthe Second Level Screening ofGroundwater Process Options 2.10.1-1 Selected Representative Process Options for Development ofGroundwater Remedial Alternatives Volume VI 3.2.1-1 Offpost Operable Unit Groundwater Plume Groups 3.5.1.4-1 Alternative No. N-4: Interim Response Action A 3.5.1.5-1 Alternative No. N-5: Expansion I to Interim Response Action A 3.5.1.6-1 Alternative No. N-6: Expansion 2 to Interim Response Action A 3.5.2.4-1 Alternative No. NW-4: Northwest Plume Group Extraction/Recharge System

20000,317.10 - 1-toc 0306111892 xxiX3.0 DEVELOPMENT OF REMEDIAL ALTERNATIVES
Section 3.0 presents the development ofalternatives, using the RPOs presented previously in Section 2.10. The following sections present theapproach to alternatives development, commonalities ofalternatives, anddetailed descriptions ofeach alternative considered in theFS.
3.1 FEASIBILITY STUDY OVERVIEW Thisportion ofthe FS addresses thedevelopment and screening ofalternatives, that were developed by combining general response actions and RPOs into arange ofoptions addressing RAOs. The alternatives were subsequently screened (Section 4.0) to identify those alternatives to be evaluated in the detailed analysis of alternatives (Section 5.0). Consistent with EPA FS Guidance (EPA, 1988b), the development and screening of alternatives is a three-step process consisting ofthe following: 1. Developing a range ofremedial alternatives by assembling combinations ofselected RPOs (Section 2.10) thataddress containment, source control, and treatment ofmedia requiring remediation within the Offpost OU 2. Refining and screening identified alternatives on ageneral basis toevaluate their effectiveness, implementability, and cost(Section 4.0), which reduces the numberof alternatives while preserving arange ofoptions 3. Identifying remedial alternatives retained for further analysis in Section 5.0
EPA guidance regarding the development and screening ofalternatives is further discussed below.
3.1.1 U.S. Environmental Protection Agency Guidance for Alternatives Develot)ment and Evaluation Remedial alternatives were developed and evaluated according to guidelines set forth inthe following regulations andguidance:

- National Contingency Plan (NCP) 40 CFR Part 300

- EPA Guidance on Conducting Remedial Investigations andFeasibility Studies under CERCLA, Interim Final, October 1988 (EPA, 1988b)

- EPA CERCLA Compliance with Other Laws Manual, June 1988, OSWER Directive No. 9234.1-01 (EPA, 1988a)

20000,317.10(10) -FS 0609111192 VI - 3-1- Role ofthe Baseline Risk Assessment in Superfund Remedy Selection Decisions, April 1991, OSWER Directive 9355.0-30 (EPA, 1991)

- Guidance Document for Providing Alternate Water Supplies, OSWER Directive No. 944.3-03 (EPA, 1988c)

- Guidance on Remedial Actions forContaminated Ground Water at Superfund Sites, OSWER Directive No. 9238.1-2 (EPA, 1988d) Guidelines contained in thedocuments listed above were developed by EPA in response to CERCLA Section 121. TheNCP indicates that alternatives should be developed thatprotect human health and the environment by eliminating, reducing, and/or controlling risk posed through each contaminant exposure pathway at the site. CERCLA provides for selection ofa remedial action that is protective ofhuman health and the environment, is cost-effective, and attains federal and state ARARs. The NCP requires that, at a minimum, thefollowing remedial alternatives be developed:

- The No-Action Alternative

- One ormore alternatives that involve little orno treatment but provide protection of human health and the environment primarily by preventing or controlling exposure to hazardous substances, pollutants, orcontaminants through engineering controls (e.g., containment) and, as necessary, institutional controls (e.g., land use and access restrictions) to assure continued effectiveness ofthe response action

- Treatment alternatives thatreduce toxicity, mobility, orvolume astheir principal element

- A limited number ofremedial alternatives for media thatattain site-specific remediation levels within different restoration periods, using one or more different technologies

- Treatment alternatives that, at aminimum, treat the principal threats posed by thesite but vary in the degree oftreatment employed and the quantities and characteristics ofthe treatment residuals and untreated waste that mustbe managed

- Treatment alternatives thatremove or destroyhazardous substances, pollutants, or contaminants tothe maximum extent feasible, eliminating or minimizing the need for long-term management (including monitoring) at the site

- One ormore alternatives incorporating innovative treatment technologies (ifthose technologies offer the potential forcomparable or superior performance or implementability, fewer orless adverse impacts thanother available approaches, or lower costs for levels ofperformance similar to thatofdemonstrated treatment technologies)

20000,317.10(10) - FS 0609111192 VI - 3-23.2 ALTERNATIVES IDENTIFICATION Remedial alternatives for the Offpost OU were developed by (1) identifying the media in which COCs were detected at levels exceeding PRGs, (2) calculating the areas and volumes of media exceeding PRGs, and (3) assembling combinations ofRPOs into alternatives representing a range oftreatment and containment combinations thataddress the RAOs developed for groundwater. Remedial alternatives weredeveloped with the intentofproviding a range ofremedial options using various technology types and RPOs. Consistent with the NCP, arange of alternatives for media exceeding PRGs was developed fromno action tocomplete removal or destruction of contaminants. The range ofalternatives for the groundwater medium ispresented in Section 3.5. As discussed in Section 2.2, soil, surface water, air,biota, and sediment were not identified asmedia requiring remediation. Thus, remedial alternatives forthese media were not developed.
3.2.1 Ar)proach This section describes the approach used to develop groundwater remedial alternatives to address the RAOs and PRGs presented in Sections 2.4 and 2.5. Alternatives were developed for the groundwater medium, based on the assumption thatthe boundary control systems willachieve PRGs for groundwater migrating from RMA tothe Offpost OU. Due to differences in hydrogeology, contaminants present, and concentration levels, two groups ofcontaminant plumeshave been identified ingroundwater in the Offpost OU. The plume groups are identified as the North andNorthwest Plume Groups. Figure 3.2.1-1 presents the general areas encompassed by each plume group. In Section 3.0, groundwater alternatives
were developed separately for the twoplume groups, and the plume group alternatives were screened separately in Section 4.0. In Section 6.0, alternatives for the North and Northwest Plume Groups were combined into the sitewide groundwater alternative.

20000,317.10(10) - FS 0609111092 VI - 3-33.2.1.1 Groundwater Model Input Into Alternatives Develonment This section describes the general approach tothe integration ofthe groundwater modeling results intothe development ofgroundwater alternatives. A detailed description ofthe groundwater model development ispresented inAppendix E ofthe FS.
3.2.1.1.1 Chemicals ofConcern The groundwater COCs were presented previously in Section 2.3, Table 2.3.1-1. A total of 28 organic compounds are listed as COCs. The remaining COCs are either inorganic ions, such as chloride, or inorganic chemicals, such as arsenic. A total of 15 COCs are classified ascarcinogens,
as identified inthe Offpost EA (Volume 11, Section 1.0). The carcinogens were shown to contribute the largest portion ofthe risk from groundwater. The noncarcinogens contribute only slightly to the overall calculated potential risk. TheOffpost RI Addendum used data from sampling events performed under the RI Addendum, IRA A, and the fall 1989 and winter 1990-1991 groundwater CMPprograms. The above data sources were adequate toprovide interpretation of "plumes"foronly 6 ofthe 15 carcinogenic COCspresent inthe Offpost OU groundwater. These chemicals are arsenic, chloroform, dibromochloropropane, dieldrin, tetrachloroethene, and trichloroethene. The remaining nine carcinogens either have limited extent
or sporadic detections and thus were not interpreted to be present as plumes. In conducting the groundwater modeling, the sixcarcinogenic COCs were assessed with respect tothe following: contaminant concentrations, areal extent, relative magnitude of contaminant exceedance ofPRGs, and relative transport properties. A comparison ofthe groundwater PRGs with the concentrations detected for the six carcinogens indicates that there

are no exceedances for arsenic. Ofthe five remaining carcinogens, three are halogenated volatiles (chloroform, tetrachloroethene, and trichloroethene), dibromochloropropane isa halogenated hydrocarbon, and dieldrin is an OCP. A comparison ofliterature values ofthe transport properties ofthese five compounds indicates thatchloroform, tetrachloroethene, trichloroethene, and dibromochloropropane have similar ranges ofboth calculated and measured distribution

20000,317.10(10) - FS 0609111092 VI - 3-4coefficients (Kd), andthe distribution coefficients for dieldrin were generally notsimilar to this group (Ebasco, 1991). Dieldrin was chosen as acontaminant requiring transportmodeling because it isthe only OCPamong the sixcarcinogenic COCs defined as plumes in the Offpost OU, and because its areal distribution and presumed Kd aredifferent from the otherfive carcinogenic COCs defined as plumes in the Offpost OU. Dieldrin isalso important asa risk-driver based on its relative high toxicity.The areal extents ofchloroform, tetrachloroethene, trichloroethene, and dibromochloropropane in the Offpost OU are generally similar. These four compounds are found predominately in the northern paleochannel as opposed tothe First Creek paleochannel. Chloroform exhibits a larger areal extent and occurs at generally higher relative concentrations than tetrachloroethene, trichloroethene, and dibromochloropropane. Based on these observations, chloroform was chosen torepresent this group ofcarcinogenic COCs for the purpose ofmodeling dissolved chemical transport. DIMPwas chosen as the third compound to bemodeled primarily because ofitshistorical importance inunderstanding the contaminant transport issues at RMA even though it is not a carcinogen. 3.2.1.1.2 Use ofthe Model in Alternatives Develot)ment Two numerical models (North Plume Group and Northwest Plume Group) were prepared to simulate the groundwater flow and dissolved chemical transport in the Offpost OU. The model development, assumptions, and results are described indetail inAppendix E ofthe FS. The parameters controlling the hydrogeologic framework andcontaminant transport in the Offpost OU were analyzed using the numerical models. Groundwater velocity in the paleochannels was found to be important in terms ofcontaminant transport in the Offpost OU. The estimated average linear groundwater velocity isapproximately 1000 to 1500 feet peryear (ft/yr) in the northern paleochannel, approximately 500to 1500 ft/yr in the FirstCreek paleochannel, and approximately 2500 to 3500 ft/yrin the northwestern paleochannel (Figure 3.2.1-1), using
20000,317.10(10) - FS 0609111092 VI - 3-5hydraulic gradient, hydraulic conductivity, and representative porosity datapresented in Volume VII, Appendix E. Also, the estimated average linear groundwater velocities in zone 3 are approximately twice those estimated inzone 4 ofthe First Creek paleochannel. The controlling contaminant characteristics include spatial distribution and estimated retardation factors (R) ofthe contaminants. Chloroform is present primarily in thenorthern and northwestern paleochannels (see Volume 1,Figure 7), dieldrin is present in the highest concentrations inthe First Creek paleochannel and atconcentrations slightly greater than the CRL inthe northern and northwestern paleochannels (see Volume 1,Figure 5), and DIMPis present in the FirstCreek and northern paleochannels (see Volume I, Figure 3). Contaminant retardation factors, defined asthe ratio of average linear groundwater velocity divided by the average contaminant velocity, were estimated
as described in Volume VII, Appendix E, using a range corresponding to alower-bound (optimistic) and upper-bound (pessimistic) retardation factor. Model-estimated ranges for the time to attain PRGs were made corresponding tothe range ofretardation factors for each contaminant. In summary, the numeric model showed that the controlling factors were the relatively highernorthern and northwestern paleochannel groundwater velocities compared tothe First Creek paleochannel, the distribution ofdieldrin, and the higher range ofretardation factors for dieldrin compared tochloroform orDIMP. Simulations ofcontaminant transport, using the numeric groundwater modeling presented in Volume VII, Appendix E, were made corresponding tothe continued operation ofthe boundary systems and additional extraction and/or injection alternatives for both the North Plume Group and the Northwest Plume Group. Initial conditions were chosen toreflect the contaminant plume extent consistent with the data set described in the Introduction (Volume I) and to reflect contaminant removal at the NBCS and NWBCS consistent with attainment ofOffpost OU PRGs at the boundary systems. The model results indicated that dieldrin transport is slower relative to DIMP and chloroform, most notably in areas of slower groundwater velocity (i.e., the First Creek paleochannel). DIMP was shown todecrease in concentration, rapidly attaining PRGsin a short time. Chloroform concentrations were also reduced rapidly to attain PRGs quickly, except for an

20000,317.10(10) - FS 0609111092 VI - 3-6..... ........

area oflow hydraulic conductivity in the northern paleochannel near the O'Brian Canal. Dieldrin concentrations were reduced slowly in the First Creek paleochannel and rapidly in the northern and northwestern paleochannels (where the initial concentrations ofdieldrin areslightly abovethe CRL and the groundwater velocity is high). Based on these observations regarding the controlling hydrogeologic and contaminant characteristics from the initial model simulations and on data from analytical wellfield simulator studies inthe First Creek and northern paleochannels (HLA, 1991b), additional model runswere made to analyze the effect ofgroundwater extraction wells and recharge systems on contaminant concentration reduction. These additional modelruns varied both the placement ofextraction and recharge components and the flowrates ofthese components in a screening approach todevelop alternatives thatevaluated varying levels ofcomplexity while attaining acceptable estimated aquifer drawdown/mounding. Model screening runs arenot presented explicitly in the FSbecause the runs were preliminary working tools used to develop the array ofalternatives presented in the FS. Various combinations ofextraction and recharge components wereevaluated for the First Creek, northern, and northwestern paleochannels. The following discussion describes the different configurations evaluated for each paleochannel. Groundwater extraction and recharge configurations were evaluated for the First Creek paleochannel, using the results ofan analytical wellfield simulator for FirstCreek axial, transverse, and combination axial/transverse extraction and recharge systems (HLA, 1991b). The wellfield simulator wasused toassess the relative performance ofaxial versus transverse or combination well configurations forconditions (or probable ranges ofconditions) considered to exist inthe First Creek paleochannel. The wellfield simulations evaluated location and spacing of wells for extraction and recharge, total effluent flow rate, and significance ofgroundwater recycling from recharge components back toextraction wells. Based on these simulations, an axial system was selected for evaluation in the numeric groundwater transport model over the transverse and combination axial/transverse systems for the First Creek paleochannel tooptimize
20000,317.10(10) - FS 0609111092 VI - 3-7contaminant removal alongthe length ofthe paleochannel. Thepresence ofdieldrin (a contaminant with a relatively higher retardation factor) at high concentrations in the relatively slower groundwater velocity First Creek paleochannel make an axial system more effective in the First Creek paleochannel than atransverse or combination system. Thus, the numeric modeling presented in Volume VII, Appendix Efocused on studying the relative differences between axial groundwater collection and recharge systems inthe FirstCreek paleochannel. The northern paleochannel groundwater extraction and recharge configurations were similarly evaluated. Data derived fromthe initial numerical groundwater transport modeling indicated thatthe combination ofhigh groundwater flowvelocities inthe northern paleochannel and contaminant distribution (i.e., contaminant concentrations slightly above CRLs immediately downgradient ofthe NBCS) result inrapid attainment ofPRGs for contaminants in the northern paleochannel immediately downgradient oftheNBCS in theabsence ofremedial components other than the NBCS. Initial concentrations of dieldrin at levels slightly exceeding the CRL immediately downgradient ofthe NBCS are reduced to nondetect levels ina model-estimated timeframe ofone tothree years. Atthe northern edge ofthe paleochannel, however, contaminant concentrations in excess ofPRGs remain for amodel-estimated timeframe of 15 to 30 years, in the absence ofremedial components other than the NBCS. Because an axial system scenario would evaluate placement ofextraction wells in areas without groundwater PRG exceedances, additional screening model runs varying the placement and flow rates oftransverse extraction and recharge configurations were used to develop the array ofgroundwater alternatives presented in the FS. The contaminant distribution and hydrogeology in the northwestern paleochannel were similarly evaluated in the initial groundwater modeling runs. The model estimated attainment of contaminant PRGs with continued operation ofthe NWBCS inone to five years. To assess the impact ofextraction and recharge components downgradient ofthe NWBCS, axial and transverse extraction well and recharge components targeting the dieldrin plumes were then evaluated using numeric groundwater modeling. The results of these screening model runswere used to develop the array of alternatives presented inthe FS.

20000,317.10(10) - FS 0609111092 VI - 3-8Themethodology described above were used todevelop groundwater alternatives for the two plume groups. Groundwater modeling wasused forthe following purposes in developing groundwater alternatives: 1.Developing conceptual designs for sizing and locating groundwater extraction and recharge systems 2. Estimating system flow rates for the purpose of sizingtreatment systemcomponents 3. Evaluating the relative effectiveness ofproposed systems toensure a range of groundwater alternatives varying in the degree oftreatment employed and the timeframe required for reffiediation
In each case, the results ofthe transport simulations were evaluated withrespect to achievement ofthe PRGs and the estimation oftime required to achieve PRGs. The evaluation ofnumbers and placement ofextraction wells and recharge trenches/wells isintended to beused for comparison ofalternatives. The number and placement ofextraction/ recharge facilities actually required would be determined during the remedial design/remedial action (RD/RA) phase offinal remedy implementation for the Offpost OU. 3.2.1.2 Groundwater Alternative Tyr)es Groundwater alternatives have been assigned anumerical designation indiscussion and reference. The numbering scheme used to identifygroundwater alternatives employs abbreviations that identify the plume group and a successive numbering of alternatives within the identified plume group. The first identifier in the alternative number indicates the plume group thatthe alternative addresses (see Figure 3.2.1 -1). The abbreviations to identifythe plume group are asfollows: N = North Plume Group NW =Northwest Plume Group
The second identifier in the alternative numerical designation indicates the orderofthe alternative within the specified plume group. The order inwhich the alternatives are listed is

20000,317.10(10) - FS 0609111092 VI - 3-9M

consistent with the orderofRPOs presented in Volume V, Section 2.10. For example, Alternative No. N-2 isthe second alternative in the North Plume Group. 3.3 LAND OWNERSHIP IMPLICATIONS FOR IMPLEMENTING REMEDIAL ALTERNATIVES

Complications thatarise when proposing the implementation ofremedial alternatives on privately owned land wereconsidered. Considerations that may have animpact on the implementation ofremedial alternatives include thefollowing: 1. Potential shor' t-term loss oflivelihood for landowners, associated with restriction of activities (e.g., farming, grazing) ordisruption ofagricultural activityduring remedial action 2. Landowners' reluctance to allowremedial action to be conducted on theirproperty Implementation ofgroundwater alternatives may require negotiations with landowners when extraction wells, pipelines, containment systems, orother construction and long-term operation activities are proposed for location on private land. Solutions tothese issues canbe addressed through negotiation withthe landowner. Consideration ofland ownership directly affects the implementability ofproposed remedial alternatives. During this FS, land ownership considerations were included in the alternatives developed in Section 3.5, asfollows: 1. Land Acquisition: Shell Oil Company orthe Army purchases land that requires remedial action or acquires pipeline easements for certainremedial actions. 2. Temporary Relocation and LandUse Compensation: relocate landowner/residents during remedial action; relocate livestock during remedial action, when applicable or necessary; compensate landowners for monetary losses associated withremedial action (e.g., crop losses, rental fees incurred due to relocation ofresidents, boarding fees for relocated livestock). 3. Land Lease/Use Compensation: lease land while landowner/residents remain at residence; compensate landowner for any monetary losses incurred during remedial action. 3.4 COMMONALITIES OFALTERNATIVES Alternatives developed forthe Offpost OU, withthe exception ofthe No Action alternatives, contain commonalities, including institutional controls, pretreatment process options

20000,317.10(10) - FS 0609111192 VI - 3-10-- -- _------

appropriate to implementing many of the alternatives, and continuation ofboundary systems operation (NBCS, NWBCS, Irondale Containment System [ICS]).
3.4.1 Commonalities of Groundwater Remedial Alternatives The following subsections discuss commonalities of groundwater alternatives, including institutional controls, pretreatment process options, and continuation of boundary systems operation in conjunction with compliance with Offpost OU PRGs.
3.4.1.1 Groundwater Alternatives Institutional Controls Institutional controls, such as provision ofan alternative water supply, groundwater monitoring, and five-year site review, are used as appropriate to supplement engineering controls for short- and long-term management to prevent or limitexposure,to hazardous substances, pollutants, or contaminants. Although institutional controls do notreduce mobility, toxicity, or volume of contaminants, they will likely be necessary to maintain the integrity of any remedial action alternative selected and may reduce the potential forhuman exposure to contaminated groundwater.
3.4.1.1.1 Provision ofan Alternative Water Sut)Dly (Exposure Control) All groundwater alternatives, with the exception of the No Action alternative, include the potential provision ofan alternative water supply, as necessary, to any identified future users of groundwater containing COCs at concentrations that exceed PRGs (Table 2.5.2-3) (i.e., exposure control). Bottled water would be provided for domestic consumptive use and would be a temporary measure until an alternative water supply could be provided. At the present time, no domestic-use wells contain groundwater COCs at concentrations exceeding PRGs. The ongoing Army commitment to provide an alternative water supply, as necessary, is developed as a contingency program.

20000,317.10(10) -FS 0609111092 VI - 3-113.4.L1.2 Monitoring Networks The monitoring programs developed in this report are preliminary conceptual designs with the primary purpose ofproviding costestimates for each alternative. Thefinal monitoring program for the Offpost OU would be designed during the RD/RA program. The purpose ofthe groundwater monitoring program would beto assess changing aquifer conditions during and after remedial action. Groundwater monitoring is part of allremedial alternatives, including the No Action alternative. The following monitoring programs were developed (forcosting purposes) for the North and Northwest Plume Groups:

- North Plume Group
o All alternatives 36 wells sampled semiannually

- Northwest Plume Group
o All alternatives 13 wells sampled semiannually
The North Plume Group monitoring program would also include a total ofseven semiannual surface-water samples from First Creek and the O'Brian Canal toevaluate the effects ofgroundwater remediation onsurface-water quality. Monitoring programs would consist ofsemiannual water-level measurements and analysisof samples from Offpost OU wells for COCs. Inaddition to the monitoring programs discussed above, groundwater monitoring will continue onpost. This program will monitor the potential foronpost contaminant plumes approaching the RMA boundaries. Monitoring data from thisprogram will provide information that may beused to evaluate whether modifications are necessary tothe boundary systems. The design and implementation ofa monitoring program isan essential component ofallremedial alternatives.
3.4.1.1.3 Site Review In accordance with CERCLA, a site review would be conducted at least every five years until groundwater PRGs wereachieved to assure thathuman health andthe environment are protected during and after remediation. The sitereview would use monitoring-program data to
assess whether additional action would bewarranted.
20000,317.10(10) - FS 0609111092 VI - 3-12A



### 3.4.1.2 Groundwater Pretreatment Process Ot)tions Groundwater alternatives thatinvolve extraction, in-vessel treatment, and reinjection would require pretreatment toreduce the introduction offine-grained particles that maywash through the extraction system and into the treatment system. Particulates entering the treatment system may interfere withthe operation ofin-vessel treatment process options andcause excessive wear topumps. Influent groundwater filters, such asbag filters, would beused toguard againstthe introduction ofparticulates into thetreatment system. 3.4.1.3 ContinuedOperation ofthe Boundary Systems andComt)liance WithOffvost Operable UnitPreliminary Remediation Goals Thedevelopment and evaluationofall groundwater alternatives, with the exceptionofthe NoAction alternative, assumes thatboundary response actions would comply with OffpostOU PRGsat theRMA boundary. Compliance ofboundary response actions withOffpost OU PRGs would include the followingrequirements: I.Boundary containment systems would continue to operate. Approximately 125 million gallons per year atthe NBCS and 450milliongallons per yearatthe NWBCS are currently treated. Changing conditions caused by operation ofIRA A and/or onpostresponse actions maychange the system flow rates. 2. Offpost OU groundwater COCPRGswould notbeexceeded in boundary containment system effluent. 3. Additional boundary containment systemimprovements wouldbe implemented, as necessary, to achieve OffpostOU PRGs. 3.5 GROUNDWATER REMEDIAL ALTERNATIVES IDENTIFICATION Adescription ofalternatives for groundwater ispresented in this section. The identification ofgroundwater alternatives is divided intotwo sections corresponding to the North and Northwest Plume Groups identified in Figure 3.2.1-1. Alternatives developed for eachplume group are identified and described in the subsections that follow. Information presentedin each alternative description includes:

- Adescription ofthe RPOs that comprisethe alternative

- A briefdescription ofthe location, area, and COCstowhich the alternative willbe applied
20000,317.10(10) - FS 0609111192 

### VI - 3-133.5.1 Identification ofGroundwater Alternatives: North Plume Grout) The following subsections identify the alternatives developed forachievement ofgroundwater PRGs. Groundwater PRGs, as discussed inSection 2.5.4, use ARARs forcontaminants with promulgated standards, residential 10-6 RME HBC forcarcinogens withoutARARs, and residential RME HBC for noncarcinogens without ARARs. Table 3.5.1 -1 presentsthe alternatives corresponding to the North Plume Groupand identifies process options, number ofwells and trenches, flow rates, remediation timeframes, treatment plant location, andprocess residuals generated. 3.5.1.1 Alternative No. N- 1: No Action UnderAlternative No. N-1, the operation ofthe NBCS would be discontinued. Alternative No. N-1 would therefore notprovide for active remediation ofaffected groundwater within the North Plume Group. Ceasing operation oftheNBCS would likely cause an increase in contaminant concentrations withinthe North Plume Group. Natural fate processes, including degradation and attenuation, would continue to reduce contaminant concentrations withtime in groundwater within the North Plume Group. The components ofAlternative No. N-1 include the following:

- Long-term groundwater monitoring

- Five-year site reviews A long-term groundwater monitoring program would be implemented. The purpose ofthe groundwater monitoring program is toassess changing UFS and CFS aquifer conditions during and after remedial action. Aspart ofAlternative No. N- 1,asite review would be conducted at least every five years until PRGs were achieved. 3.5.1.2 Alternative No. N-2: Continued Oi)eration ofthe North Boundary Containment System With Improvements as Necessary Alternative No. N-2 would provide foractive remediation ofaffected groundwater approaching the north boundary ofRMA through continued remediation ofgroundwater at the
20000,317.10(10) - FS 0609111192 VI - 3-14NBCS. Also, with time, natural fateprocesses, including degradation and attenuation, would continue to reduce contaminant concentrations ingroundwater within the North Plume Group. Themajor components are asfollows:

- Continued operation ofthe NBCS

- Improvements tothe NBCS as necessary

- Long-term groundwater monitoring

- Five-year site reviews

- Exposure control Under Alternative No. N-2, the NBCS would continue tocontain, extract, treat, and recharge approximately 125 million gallons ofgroundwater per year. Improvements would be made to the NBCS ifit was determined thatthe system was allowing groundwater containing COCs at concentrations exceeding offpost groundwater PRGs to migrate from RMA tothe North Plume Group. A long-term groundwater monitoring program would beimplemented. Thepurpose ofthe groundwater monitoring program is to assess changing UFS and CFS aquifer conditions during and after remedial action. As part ofAlternative No. N-2, a sitereview would be conducted at least every five years until PRGs were achieved.
3.5.1.3 Alternative No. N-3: Land Acouisition and Use Restrictions Under Alternative No. N-3, areas within the Offpost OU thatexceed groundwater PRGs would bepurchased and institutional controls would beimplemented tolimit exposure to affected groundwater. The NBCS would continue tooperate. These measures would meet the following objectives:

- Acquisition ofland to facilitate implementation of institutional controls for groundwater

- Limitation ofpotential exposure tochemicals in groundwater
The major components are asfollows:

- Land acquisition
20000,317.10(10) - FS 0609111092 VI - 3-15- Access anddeed restrictions

- Continued operation ofthe NBCS

- Improvements tothe NBCS asnecessary

- Long-term groundwater monitoring

- Five-year site reviews

- Exposure control This alternative would be applied tothe North Plume Group where contaminants have been reported to exceed PRGs (see Figure 2.6-1). Section 2.6 (Quantities ofAffected Media) presents a detailed discussion ofthe contaminants present and exceedances ofPRGs. A total of approximately440 acres arejudged toexceed PRGs. Land acquisition is assumed, for the purposes ofthisreport, to include activities associated with and necessary to conclude negotiations on afair market value forparcels ofland under private ownership or controlled by municipalities. A deed restriction is awritten and recorded document kept on file inthe county recorder's office that regulates anumber ofrestrictions primarily related to activities on aproperty. For this alternative, deed restrictions would prohibit subsurface activities, such asdomestic wellinstallation, or any other activities that have a potential for direct contact with affected groundwater, ingestion of affected groundwater, oruse ofaffected groundwater for the irrigation ofcrops or watering oflivestock. Restrictions on drilling newdomestic wells would reduce the potential forexposure to affected groundwater.
3.5.1.4 Alternative No. N-4: Interim Response Action A. Under Alternative No. N-4, the NBCS would continue tooperate, and the Groundwater Intercept andTreatment System North ofRMA (IRA A) would be constructed and operated to remove, contain, treat, andrecharge affected groundwater in the FirstCreek and northern paleochannels downgradient ofthe NBCS. The elements ofthe alternative are presented in the Final Implementation Document for IRA A (HLA, 1991b).

20000,317.10(10) -FS 0609111892 VI - 3-16The major components are asfollows:

- Removal ofcontaminated UFS groundwater north ofthe RMA boundary inthe First Creek and northern paleochannels using IRAA groundwater extraction wells

- Treatment ofthe organic COCs present in thegroundwater using carbon adsorption

- Recharge oftreated groundwater tothe UFS using IRA A wells and trenches

- Continued operation ofthe NBCS

- Improvements to the NBCS and IRA A as necessary

- Long-term groundwater monitoring

- Five-year site reviews

- Exposure control Alternative No. N-4 remediates UFS groundwater inthe First Creek and northern paleochannels that iscontaminated with COCs at concentration levels exceeding PRGs, as shown in Figure 2.6- 1. Extraction wells have been selected as the RPOand were used in the IRA A design for the groundwater extraction technology type. Extraction wellsuse conventional equipment for the removal ofcontaminated groundwater. Extracted groundwater would be conveyed to the treatment facility viapolyvinyl chloride (PVC) pipelines. Based on the numeric groundwater modeling discussed in Section 3.2.1.1.2, the configuration ofextraction wells and recharge systems shown inFigure 3.5.1.4-1 would capture and remove contaminants (primarily dieldrin) axiallyin the FirstCreek paleochannel where the groundwater velocity is relatively slower than the northern paleochannel groundwater velocity. Also, inthe higher groundwater velocity northern paleochannel system, capture would be attained using a transverse system ofextraction and injection wells. Alternative No. N-4 would use a lineof 12extraction wells (see Figure 3.5.1.4-1), pumping atotal ofapproximately 300 gpm, at the leadingedge ofthe northern paleochannel for the removal and capture ofgroundwater contaminated with organic COCs at concentrations exceeding PRGs. In the northern paleochannel, treated groundwater would be injected via aline

20000,317.10(10) - FS 0609111092 VI - 3-17of24 recharge wellsdirectly downgradient ofthe extraction wells. A total offive extraction wells would be placed at the leading edge and along theFirst Creek paleochannel axis, pumping atotal ofapproximately 180 gpm, to allow contaminant massremoval in the FirstCreek paleochannel. Thus, Alternative No. N-4 would extract approximately 480 gpm. A total ofsix recharge trenches wouldbe placed both downgradient oftheextraction wells and along the outer boundaries ofthe First Creek paleochannel. In this manner, therecharge trenches would provide both lateral hydraulic containment ofthe FirstCreek paleochannel and water flushing forenhancing the removal ofcontaminants. Construction ofthis system began inNovember 1991 and will be completed by approximately January 1993. Extracted groundwater from bothpaleochannels would be conveyed by pipeline to acentral carbon adsorption treatment facility located on land in the Offpost OU that waspreviously purchased by Shell Oil Company. Treatment would achieve PRGs for allorganic COCsbefore disposal via the injection systems. An intensive short-term monitoring component would be included in this alternative as part of the long-term monitoring program. For costing purposes, itisassumed that the intensive short-term program would consist ofmonitoring 60wells ina network that would be finalized through implementation ofthe alternative. Two years ofdata would be collected during the period commencing with IRA A operations start-up. Such a program is necessary toevaluate the performance ofthe NBCS and IRA A systems and to provide increased understanding of contaminant transport, estimated time to achieve PRGS, and the potential necessity of improvements toIRA A. 3.5.1.5 Alternative No. N-5: Expansion I to-Interim Resnonse Action A Similar toAlternative No. N-4, this alternative addresses remediation ofthe First Creek and northern paleochannel groundwater downgradient ofthe NBCS. Based on the numeric groundwater modeling discussed in Section 3.2.1.1.2, the configuration ofextraction wellsand recharge systems shown in Figure 3.5.1.5-1 would place additional extraction wells inlocations where the limiting hydrogeologic and contaminant characteristics are controlling remediation 20000,317.10(10) - FS 0609111092 VI - 3-18timeframes. Alternative No. N-5 is an expansion ofIRA A thatevaluates placement oftwo additional extraction wells and four recharge trenches in the areaofrelatively slower groundwater velocity and high dieldrin concentrations inthe FirstCreek paleochannel toremediate the dieldrin plume faster than Alternative No. N-4 and an additional extraction well and two recharge trenches inthe area oflow hydraulic conductivity in the northern paleochannel to remediate the chloroform plume faster than Alternative No. N-4. The major components are as follows:

- Removal ofcontaminated UFS groundwater north ofthe RMA boundary in the First Creek and northern paleochannels, using IRA A groundwater extraction wells

- Expansion I ofIRA A

- Treatment oforganic COCs present in the groundwater using carbon adsorption

- Recharge oftreated groundwater tothe UFS, using wells and trenches

- Continued operation ofthe NBCS

- Improvements to the NBCS as necessary

- Long-term groundwater monitoring

- Five-year site reviews

- Exposure control The expansion ofIRA A is shown on Figure 3.5.1.5-1. The three additional extraction wells would each pump 30 gpm (90 gpm additional) and the additional trenches would recharge the
same volume. Thus, Alternative No. N-5 would extract and treat a total of600 gpmcompared to 480 gpm for Alternative No. N-4. See Section 3.5.1.4 (Alternative No. N-4) for adiscussion ofthe IRA A components. 3.5.1.6 Alternative No. N-6: Expansion 2 to Interim Resvonse Action A Alternative No. N-6 addresses remediation ofFirst Creek and northern paleochannel groundwater downgradient ofthe NBCS. Based on the numeric groundwater modeling discussed in Section 3.2.1.1.2, the configuration ofthe extraction wells and recharge systems shown in Figure 3.5.1.6-1 would place additional extraction wells in locations inthe First Creek
20000,317.10(10) - FS 0609111092 VI - 3-19paleochannel ofrelatively low groundwater velocity and high dieldrin concentrations. Additional recharge trenches would beplaced along the paleochannel margins to flush contaminants outof the low conductivity aquifer materials. This alternative places close tothe maximum possible practical number ofextraction wells and recharge trenches along the entire length ofthe First Creek paleochannel (see Figure 3.5.1.6- 1). The model-estimated welldrawdowns and recharge mounding are at or near the boundsof acceptable elevation differences using this configuration, making additional wells or trenches impracticable. This alternative isan expansion ofIRAA that evaluates placement of an additional fourextraction wells and eight recharge trenches in the First Creek paleochannel toremediate the dieldrin plume faster than IRA A alone and an additional three extraction wells and five recharge trenches inthe northern paleochannel toremediate the chloroform plume faster than IRA A alone. The major components are asfollows:

- Removal ofcontaminated UFS groundwater north ofthe RMA boundary inthe First Creek and northern paleochannels, using IRA A groundwater extraction wells

- Expansion 2 ofIRA A

- Treatment ofthe organic COCspresent inthe groundwater, using carbon adsorption

- Recharge oftreated groundwater to the UFS, using wells and trenches

- Continued operation ofthe NBCS

- Improvements to the NBCS as necessary

- Long-term groundwater monitoring

- Five-year sitereviews

- Exposure control Expansion 2 ofIRA A is presented onFigure 3.5.1.6-1. The seven additional extraction wells would each pump 30 gpm. The 13 additional recharge trenches would recharge thesame volume. Thus, Alternative No. N-6 would extract and treat a total of690 gpmcompared to 480 gpm for Alternative No. N-4 and 600 gpm for Alternative No. N-5. See Section 3.5.1.4 (Alternative No. N-4) fora discussion ofthe IRA A components.
20000,317.10(10) -FS 0609111092 VI - 3-203.5-1.7 Summary ofAlternatives Identified for the North Plume Grow Six alternatives have been identified for addressing the remediation ofgroundwater within the North Plume Group. Table 3.5.1-1 presents alternative process options for the North Plume Group, numbers ofwells and trenches required for groundwater remediation, flow rate to treatment plants, and remediation timeframes foreach alternative. Additional detail for process options incorporated ineach alternative is inTable 3.5.1.7-1. The range of alternatives presented in Table 3.5.1.7-1 is consistent with the range ofalternatives specified by the NCP. The treatment alternatives incorporating active remediation in the North Plume Group, Alternative Nos. N-4, N-5, and N-6, vary substantially inthe level ofcomplexity asdefined by the number ofextraction wells and recharge trenches required, approximate length ofrecharge trenching, and total flow rate tothe treatment plant. Sections 3.5.1.4, 3.5.1.5, and 3.5.1.6 describe the components ofeach alternative, rationale forplacement ofextraction wells and recharge trenches based on groundwater modeling, and the location ofcomponents for Alternative Nos. N-4, N-5, and N-6, respectively. The results ofnumerous groundwater model screening runs were used to develop these three alternatives allowing analysis of contaminant transport as numbers ofwells/trenches, well/trench placement, and flow rates were varied. Figures 3.5.1.4-1, 3.5.1.5-1, and3.5.1.6-1 show the increasing complexity asthe total number ofextraction wells, recharge trenches, and total flow rate increases from 17 wells, 6 trenches (1500 ft), and 480 gpm total flow for Alternative No. N-4, to 24 wells, 19 trenches (5400 ft), and 610 gpm total flow for Alternative No. N-6. The estimated time to attain PRGs wasevaluated using approximate groundwater models developed for FS analysis. Because ofthe approximate nature of the models and the considerable uncertainty in the conceptual model and hydrogeologic parameters, none ofthe model-estimated timeframes should be construed as accurate predictions. Rather, the model results should be viewed as tools for assessing relative differences among alternatives. Theestimated timeframe to attain PRGs was 15 to 30 years for Alternative No. N-4, 10 to 20years for Alternative No. N-5, and 10 to 20 years for Alternative No. N-6. Because ofspatial limitations (see Figure 3.5.1.6.-1) and estimated well

20000,317.10(10) - FS 0609111092 VI - 3-21drawdown and recharge trench mounding limitations, Alternative No. N-6 is the upper-bound level ofcomplexity practicable forinclusion as an alternative for the North Plume Group.
3.5.2 Identification ofGroundwater Alternatives: Northwest Plume Group The following subsections identify the alternatives developed for achievement ofgroundwater PRGs forthe Northwest Plume Group. Table 3.5.2-1 presents the alternatives corresponding tothe Northwest Plume Group and identifies process options, numbers ofwells and trenches, flow rate, rernediation timeframes, treatment plant location, and process residuals generated. 3.5.2.1 Alternative NW-1: No Action Under Alternative No. NW- 1,theoperation ofthe NWBCS extraction, treatment, and reinjection system would be discontinued. Alternative No. NW-1 would therefore notprovide active remediation ofaffected groundwater within the Northwest Plume Group. Ceasing operation ofthe NWBCS would likely cause anincrease in contaminant concentrations within the Northwest Plume Group. Natural fate processes, including degradation and attenuation, would continue to reduce contaminant concentrations with time ingroundwater within the Northwest Plume Group. A long-term groundwater monitoring program would be implemented. The purpose ofthe groundwater monitoring program is to assess changing UFS and CFS aquifer conditions during and after remedial action. As part ofAlternative No. NW-1, a site review would be conducted at least every five years until PRGs were achieved. The major components areas follows:

- Long-term monitoring

- Five-year site reviews 3.5.2.2 Alternative NW-2: Continued Overation oftheNorthwest Boundary Containment System Alternative No. NW-2 would provide for active remediation of affected groundwater approaching the northwest boundary ofRMA through continued remediation ofgroundwater at theNWBCS. Also, natural fate processes, including degradation and attenuation, would continue
20000,317.10(10) - FS 0609111092 VI - 3-22toreduce contaminant concentrations withtime in groundwater withinthe Northwest Plume Group. Themajor components are asfollows:

- Continued operation ofthe NWBCS

- Improvements to the NWBCS asnecessary

- Long-term groundwater monitoring

- Five-year sitereviews

- Exposure control Under Alternative No. NW-2, the NWBCS would continue tocontain, extract, treat, and recharge approximately 450 million gallons ofgroundwater per year. Improvements would be made to the NWBCS if itwas determined that the system was allowing groundwater containing COCs at concentrations exceeding offpost groundwater PRGs tomigrate from OnpostRMA tothe Northwest Plume Group. A long-term groundwater monitoring program would be implemented. The purpose ofthe groundwater monitoring program is to assess changing UFS and CFS aquifer conditions during andafter remedial action. Aspart ofAlternative No. NW-2, a sitereview would be conducted at least every five years untilPRGs were achieved. 3.5.2.3 Alternative No. NW-3: Land Acguisition With Use Restrictions UnderAlternative No. NW-3, areas within the Offpost OU thatexceed groundwater PRGs would be purchased, and institutional controls would beimplemented tolimit exposure to affected groundwater. These measures would meet the following objectives:

- Acquisition ofland tofacilitate implementation ofinstitutional controls for groundwater

- Limitation ofpotential exposure tochemicals in groundwater
The major components ofAlternative No. NW-3 are asfollows:

- Land acquisition

- Access and deed restrictions

- Continued operation ofthe NWBCS
20000,317.10(10) - FS 0609111192 VI - 3-23- Improvements to the NWBCS as necessary

- Long-term groundwater monitoring

- Five-year site reviews

- Exposure control See Section 3.5.1.3 (Alternative No. N-3) fora description ofthe deed/well restriction andthe land acquisition components. This alternative would beapplied tothe Northwest Plume Group, where contaminants have been reported toexceed PRGs (see Figure 2.6-1). Section 2.6 (Quantities ofAffected Media) presents adetailed discussion ofthe contaminants present and exceedances ofPRGs. A total of approximately 440 acres isjudged to exceed PRGs.
3.5.2.4 Alternative No. NW-4: Northwest Plume Group Extraction/Recharge System This alternative addresses active remediation ofthe dieldrin plumes in the northwestern paleochannel downgradient ofthe NWBCS. Based on the numeric groundwater modeling presented in Section 3.2.1.1.2, the configuration ofextraction (recharge systems shown in Figure 3.5.2.4- 1)would place extraction wells in the dieldrin plume inthe northwestern paleochannel. Placement ofthree extraction wells and five recharge wells (see Figure 3.5.2.4-1) was evaluated for faster dieldrin plume remediation than Alternative No. NW-2 provides. The three extraction wells would pump 50 gpm each for atotal ofan additional 150 gpm above Alternative Nos. 2 and 3 , and the five recharge wells would recharge 30 gpm each. Extracted groundwater would be pumped to the NWBCS for treatment. Followingtreatment, the groundwater would be pumped to the reinjection wellsfor disposal. The major components of Alternative No. NW-4 follow:

- Removal ofcontaminated UFS groundwater northwest ofthe RMA boundary, using three groundwater extraction wells

- Treatment oforganic COCspresent inthe groundwater, using carbon adsorption

- Recharge oftreated groundwater to the UFS, using five wells

- Continued operation ofthe NWBCS
20000,317.10(10) - FS 0609111092 VI - 3-24- Improvements to the NWBCS asnecessary

- Long-term groundwater monitoring

- Five-year sitereviews

- Exposure control 3.5.2.5 Summary ofAlternatives Identified forthe Northwest Plume Grour) Four alternatives have been identified foraddressing the remediation ofgroundwater within the Northwest Plume Group. A summary ofNorthwest Plume Group alternatives indicating the process options incorporated in each alternative is presented inTable 3.5.2.5-1. The range of alternatives presented in Table 3.5.2.5-1 isconsistent with the range ofalternatives specified by the NCP. The alternatives developed for the Northwest Plume Group were developed using the results of groundwater modeling screening runs, similar to the approach described for the North Plume Group in Section 3.5.1.7. Alternative No. NW-2 contemplates continued operation ofthe NWBCS, which treats atotal of850 gpm. Figure 3.5.2.4-1 presents the groundwater extraction and recharge configuration for Alternative No. NW-4. Alternative No. NW-4 contemplates three extraction wells and five recharge wells pumping a total of 1000 gpm in addition tothe continued operation ofthe NWBCS. The estimated time to attain PRGs was evaluated using approximate groundwater models developed for FSanalysis. As stated previously in Section 3.5.1.7, the modelestimated timeframes should be viewed as tools for assessing relative differences between alternatives. The estimated timeframe to attain PRGs is three toeight years for Alternative No. NW-2 and two to five years for Alternative No. NW-4. Based on the groundwater modeling results, Alternative No. NW-4 represents the upper-bound ofcomplexity required fordeveloping
a full range ofNorthwest Plume Group alternatives because contaminant PRGs are attained in approximately identical timeframes under both Alternative Nos. NW-2 and NW-4.

20000,317.10(10) - FS 0609111092 VI - 3-254.0 SCREENING OF ALTERNATIVES
The objective ofthe screening of alternatives is toreduce the number of alternatives identified in Section 3.0 thatwill undergo athorough and extensive analysis during the detailed analysis of alternatives (DAA) (Section 5.0) butat the same time to preserve a range ofoptions. Section 4.0 describes the screening process methodology, defines the screening evaluation criteria (Section 4.1), presents the screening ofalternatives, and summarizes the results ofthe screening process identifying alternatives retained foranalysis during the DAA (Section4.2).
4.1 METHODOLOGY All alternatives identified in Section 3.0 were evaluated on the basis ofthe following three criteria as specified by the NCP: (1) effectiveness, (2) implementability, and (3) cost. These three criteria are described in thefollowing subsections. Comparisons were made between similar alternatives; that is, those alternatives that address the same plume group and employ the same general response actions (e.g., treatment).
4.1.1 Effectiveness Evaluation In accordance with the NCP, the effectiveness ofeach alternative wasevaluated withrespect to the degree to which the alternative (1) reduces toxicity, mobility, or volume through remedial action; (2) minimizes residual risks and affords long-term protection; (3) complies with chemicalspecific ARARs; (4) minimizes short-term impacts; and (5) requires time to achieve protection. Reduction in mobility, toxicity, or volume refers to changes in at least one characteristic of the hazardous substances or contaminated media viatreatment that decreases the inherent threats or risks associated with the hazardous substances, pollutants, or contaminants. The degree towhich each alternative minimizes residual risk and affords long-term protection was assessed by evaluating the ability ofeach alternative to control or eliminate the exposure pathways identified in the Offpost EA. Theability ofeach alternative to comply with ARARs was assessed by considering the ability ofan alternative to achieve the chemical-specific ARARs identified in Section 2.5. The degree to which each alternative minimizes short-term impacts was qualitatively 20000,317.10 - FS 0309111092 VI - 4-1evaluated by considering the potential for emissions of sitecontaminants beyond thosethat occur
or may occur under the No Action alternative. Finally, the time required foreach alternative to achieve protection was evaluated by estimating the time required to control or eliminate the exposure pathways through achievement ofPRGs. Timeframe estimates were derived from groundwater-modeling results presented inAppendix E. In accordance with the NCP, alternatives that do notprovide adequate protection ofhuman health orthe environment orthat provide significantly less effectiveness than other more promising alternatives have been eliminated from further consideration.
4.1.2 Implementability Evaluation Implementability is ameasure ofboth the technical and administrative feasibility of constructing, operating, and maintaining a remedial alternative, given the site conditions in accordance with the NCP. Technical feasibility refers to (1) the ability to construct, reliably operate, and meet technology-specific requirements for process options until aremedial action is complete; (2) the reliability and level ofdemonstration ofprocess options included in an alternative; and (3) the ability toreplace, maintain, and monitorthe technical components of an alternative, ifrequired, after the remedial action iscomplete. Administrative feasibility refers tothe activities needed tocoordinate with other offices and agencies and the ability and time required toobtain any necessary approvals and permits from other agencies foroffsite activities. Administrative feasibility may also consider landowner acceptance ofproposed remedial alternatives. Evaluation of landowner acceptance has been evaluated to be a public acceptance issue and is an evaluation criterion in the DAA portion of the FS (Section 5.0). In accordance with NCP, alternatives that are technically or administratively infeasible or that would require equipment, specialists, or facilities thatare not available within a reasonable time have been eliminated from further consideration.

20000,317.10 - FS 0309111092 VI - 4-24.1.3 Cost Evaluation The costofeach alternative isan estimate ofconstruction and any long-term costs to operate and maintain the alternative in accordance with the NCP. The objective ofthe cost evaluation
was to eliminate alternatives that do notprovide substantially greater protection ofhuman health and theenvironment but have coststhat are approximately an order ofmagnitude greater than the costs ofother similar alternatives. The costevaluation included estimating capital costs, long-term operation and maintenance (O&M) costs, indirect costs, and present worth, given the site-specific conditions within the Offpost OU. The costestimates werebased on vendor information, cost estimating guides, and review ofpublished cost data at similar sites. The capital costofeach alternative (with the exception ofthe No Action alternative and the Land Acquisition and Deed Restriction alternative) was multiplied by a factor toestimate total indirect costs. This factor was based on the following percentages ofcapital costs:

- Engineering and design 15 percent

- Construction management 10percent

- Resident engineering 2.5 percent

- Regulatory oversight 10percent

- Contingency 30 percent Present-worth analysis wasused toevaluate expenditures thatoccur during different periods. Discounting all costs to acommon base year was used to compare the costs for different remedial alternatives on the basis ofa single figure foreach alternative. Present worth was calculated according to the following formula: Present Worth = Capital Cost +(O&M Cost) (P/A)
Annual O&M maintenance costs are multiplied by the uniform series present worth factor, (P/A) and are added tothe capital cost.

20000,317.10 - FS 0309111092 VI - 4-3where: (P/A) + + P = present single sum ofmoney A = the amount ofeach payment ina uniform series ofequal payments made at the end ofeach period i = discount rate per period
n = number ofinterest periods in the projectevaluation life
The value ofP/A foran interest rate of5 percent over a 30-year period is 15.37. An interest rate of 5percent was used on the basis ofthe EPA guidance (EPA, 1988e). In accordance with the NCP, cost isa factor that may be used to eliminate alternatives. Alternatives providing effectiveness and implementability similar to thatof another alternative by employing a similar method oftreatment and orengineering control, but at greater cost, may be eliminated in the screening ofalternatives. 4.2 SCREENING OF ALTERNATIVES The screening ofalternatives was divided into two sections corresponding to the alternative sets forthe North and Northwest Plume Groups developed in Section 3.0. Section 4.2.1 screens alternatives developed to achieve PRGs for the North Plume Group and Section 4.2.2 screens alternatives developed to achieve PRGs for the Northwest Plume Group. Each alternative was screened based on effectiveness, implementability, and cost. 4.2.1 Screening,ofAlternatives - North Plume Grour) The following subsections present the screening ofgroundwater alternatives developed for addressing groundwater remediation in the North Plume Group.
4.2.1.1 Alternative No. N-1: No Action Under Alternative No. N-1, the operation ofthe NBCS extraction treatment and reinjection system would be discontinued. Alternative No. N- I would therefore notprovide for active
20000,317.10 -FS 0309111092 VI - 4-4remediation ofgroundwater within the North Plume Group. A description ofthis alternative was presented previously in Section 3.5.1.1.
4.2.1.1.1 Effectiveness The effectiveness evaluation forAlternative No. N- I is presented below.

- Reduction in mobility, toxicity, or volume through remedial action
o There would be no reduction in mobility, toxicity, or volume from treatment because of

discontinuation ofthe operation ofthe NBCS. However, reductions in toxicity, mobility, and volume will occur through natural attenuation processes.

- Residual risk and long-term protection
o Under the No Action Alternative, noprovisions are made foraddressing exposure

pathways, reducing residual risk, or affording long-term protection topotential users of affected groundwater from theNorth Plume Group.

- Compliance with ARARs
o Affected groundwater inthe North Plume Group would not be treated and would not

comply with groundwater ARARs.

- Short-term impacts
o There would be no short-term impacts.

- Timeframe for achievement ofprotection
oNot evaluated. Under Alternative No. N-1, no action would be performed to reduce the mobility, toxicity,
or volume ofcontaminated groundwater within the North Plume Group. Operation ofthe NBCS would cease, and contaminant concentrations would correspondingly increase in groundwater entering the Offpost OU. Exposure pathways would not be addressed. Therefore, no provisions would be made for reducing residual risk and affording long-term protection. Under Alternative No. N- 1, natural attenuation processes would be the only mechanism to reduce COCconcentrations to PRGs levels. After ceasing operations at the NBCS, onpost groundwater would act as a
source of North Plume Group contamination. No data isavailable for evaluating the impact of ceasing the operations at the NBCS. Therefore, the time frame forachieving PRGs hasnot been estimated. Groundwater monitoring and periodic site review would indicate when contaminant

20000,317.10 - FS 0309111092 VI - 4-5levels in groundwater have attained PRGs. There would beno short-term impacts under this alternative. 4.2.1.1.2 Imvlementabilitv The implementability evaluation for Alternative No. N-1 is presented below.

- Technical feasibility
o Monitoring and site review are readilyperformed and well-demonstrated athazardous

waste sites.

- Administrative feasibility " No permits are required. " Equipment and technical personnel required for monitoring and site review are readily

available. Periodic groundwater monitoring and site review are the only actions that would betaken under Alternative No. N- 1. Groundwater monitoring wells identified for sampling under the proposed Groundwater Monitoring Program (GMP) have already been installed, andinstallation of additional wells would be readily implemented, if necessary. Groundwater monitoring data would be used in performing a site reviewevery five years untilPRGs are achieved. Alternative No. N-1 would not require permits.
4.2.1.1.3 Cost The capital costs, long-term O&M costs, and total present worth costs for Alternative No. N-1 are presented below:

- Total Capital Costs = $ - 0 -

- Total Long-term Operation and Maintenance Costs = $4.1 - 6.0 million

- Total Present Worth Costs = $4.1 - 6.0 million
4.2.1.2 Alternative No. N-2: Continued Or)eration ofthe North Boundary Containment System With 1mvrovements asNecessary Under Alternative No. N-2, groundwater exceeding PRGs inthe North PlumeGroup would continue to be extracted, treated, and reinjected through continued operation ofthe NBCS. The
20000,317.10 - FS 0309111192 VI - 4-6NBCS system would be upgraded asnecessary to achieve PRGs in the North Plume Group. A description ofthis alternative waspresented previously inSection 3.5.1.2.
4.2.1.2.1 Effectiveness The effectiveness evaluation for Alternative No. N-2 ispresented below.

- Reduction in mobility, toxicity, orvolume through remedial action " Continued operation ofthe NBCS through removal, treatment, and recharge ofapproximately 125 million gallons ofgroundwater per year would result in areduction in the toxicity, mobility, and volume ofcontaminated groundwater entering the Offpost OU. " The alternative contains no active measures to address the reduction ofthe toxicity,

mobility, or volume ofexisting contamination within the North Plume Group north of the NBCS; however, some reduction in toxicity, mobility, and volume would occur through decreases in contaminant concentrations attributable to natural attenuation and flushing with treated water from the NBCS.

- Residual risk and long-term protection " Continued operation ofthe NBCS would result in increased long-term protection and

reduced residual risk attributable to a reduction in contaminant concentrations entering the Offpost OU. "No provisions would be made for addressing exposure pathways, reducing residual risk,

or affording long-term protection associated with existing contamination withinthe North Plume Group north ofthe NBCS.

- Compliance with ARARs
o Treatment ofaffected groundwater, source control provided by the NBCS, and natural

attenuation processes in the North Plume Group would achieve compliance with groundwater ARARs.

- Short-term impacts
o There would be no short-term impacts.

- Timeframe for achievement ofprotection
o Reductions incontaminant concentrations would occur through natural attenuation

processes and flushing with treated water from the NBCS; monitoring and site review would indicate when PRGs were achieved; groundwater modeling indicates that PRGs would be achieved within 15 to 30-plus years. Under Alternative No. N-2, continued operation ofthe NBCS to achieve Offpost OU PRGs would reduce contaminant concentrations in groundwater entering the OffpostOU and correspondingly Teduce the toxicity, mobility, and volume ofgroundwater migrating from RMA to the

20000,317.10 - FS 0309111092 VI - 4-7North Plume Group. Through treatment ofapproximately 125 million gallons ofgroundwater per year at the NBCS, long-term protectiveness is substantially improved inthe North Plume Group

area. Under Alternative No. N-2, natural attenuation processes, source control provided by the NBCS, and flushing the North Plume Group with treated groundwater would reduce COC concentrations to PRGs within 15 to 30-plus years. Groundwater monitoring and periodic site review would indicate when contaminant levels in groundwater have attained PRGs. There would be no short-term impacts under this alternative.
4.2.1.2.2 lmr)lementability The implementability evaluation for Alternative No. N-2 ispresented below.

- Technical feasibility
oGroundwater monitoring and periodic site review are readily performed and well

demonstrated at hazardous waste sites.

- Administrative feasibility " No permits are required. " Equipment and technical personnel required forgroundwater monitoring and site review

are readily available.
No implementability considerations areassociated with continued operation ofthe NBCS. Groundwater monitoring wells have already been installed in the Offpost OU, and installation of additional wells would be readily implemented, ifnecessary. Groundwater monitoring data would be used in performing a site review every five years until PRGs were achieved. Alternative No. N-2 would notrequire permits.
4.2.1.2.3 Cost Thecapital costs, long-term O&M costs, and total present worthcosts for Alternative No. N-2 are presented below:

- Total Capital Costs = $ - 0 -

- Total Long-term Operation and Maintenance Costs = $30.6 - 32.5 million

- Total Present Worth Costs = $30.6 - 32.5 million
20000,317.10 - FS 0309111092 VI - 4-84.2.1.3 Alternative No. N-3: Land Acauisition an Deed Restrictions Under alternative No. N-3, areas within the Offpost OU that exceed groundwater PRGs would be purchased, and institutional controls would be implemented to limitexposure to affected groundwater. A description ofthis alternative waspreviously presented in Section 3.5.1.3.
4.2.1.3.1 Effectiveness The effectiveness evaluation for Alternative No. N-3 is presented below.

- Reduction in mobility, toxicity, or volume through remedial action " Continued operation ofthe NBCS through removal, treatment, and recharge ofapproximately 125 million gallons ofgroundwater per year would result ina reduction in the toxicity, mobility, and volume ofcontaminated groundwater entering the Offpost OU. "The alternative contains no active measures to address the reduction ofthe toxicity,

mobility, or volume ofexisting contamination within the North Plume Group north of the NBCS; however, some reduction in toxicity, mobility, and volume would occur through decreases in contaminant concentrations attributable to natural attenuation and flushing with treated water from the NBCS.

- Residual risk and long-term protection " Exposure pathways would be controlled and residual risk would be reduced by

minimizing exposure pathways through well installation/deed restrictions. " Continued operation ofthe NBCS would result inincreased long-term protection and

reduced residual risk attributable to areduction in contaminant concentrations entering the Offpost OU.

- Compliance with ARARs
o Treatment and source control provided by theNBCS and natural attenuation processes

in the North Plume Group would achieve compliance with groundwater ARARs.

- Short-term impacts
oThere would be no short-term impacts.

- Timeframe for achievement ofprotection Reduction in contaminant concentration would occur through natural attenuation processes and flushing withtreated water from theNBCS. Monitoring and site review would indicate when PRGs were achieved; groundwater modeling indicates PRGs would be achieved within 15 to 30-plus years.

20000,317.10 - FS 0309111092 VI - 4-9Alternative No. N-3 would reduce the mobility, toxicity, and volume ofcontaminated groundwater entering the North Plume Group. Continued operation ofthe NBCS would continue to reduce contaminant concentrations and would achieve Offpost OU PRGs . Exposure pathways would beaddressed through land acquisition and well installation/deed restrictions. Thus, hypothetical risk would be reduced by preventing the use ofcontaminated groundwater. Purchasing land overlying affected groundwater would aid incontrolling the use ofcontaminated ground-water. Source control actions provided by continued operation ofthe NBCS, combined with natural attenuation processes, would reduce COCconcentrations toPRGs within 15 to 30-plus years. Alternative No. N-3 would not create any short-term impacts.
4.2.1.3.2 Implementability The implementability evaluation forAlternative No. N-3 ispresented below.

- Technical feasibility " Construction isnot required " Well installation/deed restrictions are common components ofremedial alternatives at

hazardous waste sites.

- Administrative feasibility " No permits are required "Technical personnel and equipment are notrequired Implementing Alternative No. N-3 would be technically feasible. No construction would be required. Areas ofthe Offpost OU have already been purchased by Shell Oil Company, and additional property could be purchased by the Army or Shell OilCompany asnecessary. Well installation/deed restrictions are common components ofremedial action at hazardous waste sites. Enforcement ofuse and access restrictions would be simplified by acquisition ofthe affected
areas. No permits, technical personnel, orequipment would be required under Alternative No. N-3.

20000,317.10 -FS 0309111092 VI - 4-104.2.1.3.3 Cost The capital costs, long-term O&M costs, and total present worth costs for Alternative No. N-3 are presented below:

- Total Capital Costs = $5.3 million

- Total Long-term Operation and Maintenance Costs = $30.4 - 32.3 million

- Total Present Worth Costs = $35.8 - 37.7 million
4.2.1.4 Alternative No. N-4: Interim Resnonse Action A Under alternative'No. N-4, the NBCS would continue tooperate and IRA A would be constructed and operated toextract, treat, and recharge affected groundwater in the First Creek and northern paleochannels downgradient oftheNBCS. A description ofthis alternative was presented previously in Section 3.5.1.4.
4.2.1.4.1 Effectiveness The effectiveness evaluation for Alternative No. N-4 is presented below.

- Reduction in mobility, toxicity, or volume through remedial action " Continued operation ofthe NBCS through removal, treatment, and recharge of approximately 125 million gallons per year would result ina reduction ofmobility, toxicity, and volume of affected groundwater entering the Offpost OU. " Removal, treatment, and recharge ofapproximately 250 million gallons ofgroundwater

peryear at the IRA A facilities would result in areduction in the toxicity, mobility, and volume ofcontaminated groundwater inthe Offpost OU within the North Plume Group. In addition, some reduction in toxicity, mobility, and volume would occurthrough natural attenuation and flushing.

- Residual risk and long-term protection
o Continued operation ofthe NBCS and operation ofthe IRA A would result in increased

long-term protection and reduced residual risk attributable to areduction in contaminant concentrations entering the Offpost OU and within the North Plume Group.

- Compliance with ARARs
oExtraction, treatment, and recharge ofaffected groundwater associated with operation

of the IRA A and NBCS systems would achieve compliance with groundwater ARARs.

- Short-term impacts
oThere would be no short-term impacts.
20000,317.10 - FS 0309111092 VI - 4-11Timeframe for achievement ofprotection
oGroundwater modeling indicates that contaminant PRGS would be achieved within 15 to

30 years. Alternative No. N-4 would intercept, pump, treat, and recharge UFS groundwater, thereby reducing organic COC concentrations within the North Plume Group. Treatment ofgroundwater with carbon adsorption would reduce the toxicity and volume ofcontaminated groundwater. Alternative No. N-4 would reduce the mobility ofcontaminated groundwater by downgradient interception and extraction in the northern paleochannel and by axial interception and extraction in the First Creek paleochannel. Hypothetical risk wouldbe reduced substantially through a reduction ofCOC concentration levels inUFS groundwater. However, some risk would remain until groundwater PRGs have been achieved in UFS groundwater. Treatment ofgroundwater with carbon adsorption would achieve groundwater PRGs for all COCs. Alternative No. N-4 would not cause any short-term impacts tohuman health orthe environment. On the basis of groundwater modeling, itis expected thatPRGs would be achieved within 15 to 30 years.
4.2.1.4.2 Imolementability The implementability evaluation for Alternative No. N-4 ispresented below.

- Technical feasibility " Construction and operation ofthe facility could bereadily implemented. " Carbon adsorption is areliable and well-demonstrated process option for removal of

organic contaminants inRMA groundwater.

- Administrative feasibility
o No permits are required. The IRA A Implementation Documenthas been finalized.
Alternative No. N-4 would betechnically feasible. Construction and operation ofthe IRA A system could bereadily implemented. Final design has been approved. Carbon adsorption is areliable and well-demonstrated process option for removal oforganic contaminants from RMA groundwater. The administrative feasibility ofAlternative No. N-4 would be high because

20000,317.10 - FS 0309111092 VI - 4-12no permits would be required and the Implementation Document for IRA A has already been approved. 4.2.1.4.3 Cost The capitalcosts, long-term O&M costs and total present worthcosts for Alternative No. N-4 are presented below:

- Total Capital Costs = $16.7 million

- Total Long-term Operation and Maintenance Costs= $39.8 - 46.4 million

- Total Present Worth Costs = $56.5 - 63.1 million
4.2.1.5 Alternative No. N-5: Exr)ansion Ito Interim Resr)onse Action A Under Alternative No. N-5, an expansion ofIRA A was evaluated toremediate the dieldrin plume faster in the FirstCreek paleochannel and the chloroform plume faster inthe northern paleochannel. Section 3.5.1.5 presented adescription ofthis alternative.
4.2.1.5.1 Effectiveness The effectiveness evaluation for Alternative No. N-5 is presented below.

- Reduction in mobility, toxicity, orvolume through remedial action " Continued operation ofthe NBCS through removal, treatment, and recharge ofapproximately 125 million gallons ofgroundwater per year would result ina reduction of toxicity, mobility, and volume ofcontaminated groundwater entering the Offpost OU. " Removal, treatment, and recharge of approximately 250 million gallons ofgroundwater

per year at the IRA A facilities would result ina reduction in the toxicity, mobility, and volume ofcontaminated groundwater in the Offpost OU within the North Plume Group. In addition, some reduction intoxicity, mobility, and volume would occur through natural attenuation and flushing.

- Residual risk and long-term protection
oContinued operation ofthe NBCS and operation ofIRA A and Expansion I to IRA A

would result in increased long-term protection and reduced residual risk attributable to a reduction in contaminant concentrations entering the Offpost OU and within the North Plume Group.

20000,317.10 - FS. 0309111092 VI - 4-13- --- - ---

_

- Compliance with ARARs
o Extraction, treatment, and recharge ofgroundwater associated with the operation of

IRA A, Expansion Ito IRA A, and the NBCS would achieve compliance with groundwater ARARs.

- Short-term impacts
o There would beno short-term impacts.

- Timeframe for achievement ofprotection
o Groundwater modeling indicates thatPRGs would be achieved within 10to 20years.
Alternative No. N'-5 would intercept, pump, treat, and recharge UFS groundwater, therefore reducing COCconcentrations in the North Plume Group. Treatment ofgroundwater with carbon adsorption would reduce the toxicity and volume ofcontaminated groundwater. The additional wells and injection trenches provided for inExpansion I to IRA A would enhance the rate of contaminant removal by extracting and flushing groundwater from areas upgradient ofthe interception extraction well systems presented inIRA A Alternative No. N-3. Consequently, Alternative No. N-4 would increase the rate ofboth toxicity and volume reduction in contaminated groundwater. Hypothetical risk would be reduced through a reduction in COC concentrations in UFS groundwater; however, some hypothetical risk arising from the potential for the use ofUFS groundwater before completion ofremediation would remain until PRGs were achieved. Alterative No. N-5 would achieve groundwater PRGs. Alternative No. N-5 would not present any short-term impacts. On the basis ofgroundwater modeling results, it isexpected that protectiveness would be achieved within 10 to 20 years. 4.2.1.5.3 Imnlementability The implementability evaluation for Alternative No. N-5 is presented below.

- Technical feasibility " Construction and operation of afacility are readily implemented. " Carbon adsorption isa reliable and well-demonstrated process option for removal of

organic contaminants in RMA groundwater.

20000,317.10 - FS 0309111092 VI - 4-14Administrative feasibility " No permits are required. " Required equipment and technical personnel are readily available.
Alternative No. N-5 would be technically feasible. Construction and operation ofthe treatment and extraction facility would bereadily implemented. Carbon adsorption is a reliable and well-demonstrated process option forremoval oforganic contaminants from RMA groundwater. The administrative feasibility ofAlterative No. N-5 would behigh because no permits would be required. Equipment and technical personnel required to implement Alternative No. N-5 would be readily available. 4.2.1.5.3 Cost The capital costs, long-term O&M costs, and total present worthcosts for Alternative No. N-5 are presented below:

- Total Capital Costs = $19.4 million

- Total Long-term Operation and Maintenance Costs= $36.9 - 43.6 million

- Total Present Worth Costs = $56.2 - 63.0 million
4.2.1.6 Alternative No. N-6: Exvansion 2to Interim Resr)onse Action A Under Alternate No. N-6, adifferent expansion ofIRA A was evaluated toremediate the dieldrin plume faster in the First Creek paleochannel and the chloroform plume faster inthe northern paleochannel. This alterative was described previously in Section 3.5.1.7.
4.2.1.6.1 Effectiveness The effectiveness evaluation for Alternative No. N-6 ispresented below.

- Reduction in mobility, toxicity, or volume through remedial action " Continued operation ofthe NBCS through removal and recharge ofapproximately

125 million gallons ofgroundwater peryear would result in a reduction oftoxicity, mobility, and volume ofcontaminated groundwater entering the Offpost OU. " Removal, treatment, and recharge of approximately 250 million gallons ofgroundwater

per year at the IRA A facilities would result ina reduction in the toxicity, mobility, and 20000,317.10 -FS 0309111092 VI - 4-15

### volume ofcontaminated groundwater inthe Offpost OU within the North Plume Group. In addition, somereduction intoxicity, mobility, andvolume would occur through natural attenuation and flushing.

- Residual risk andlong-term protection
oContinued operation ofthe NBCS and operation ofIRA A would result in increased

long-term protection and reduced residual risk attributable toa reduction in contaminant concentrations entering the OffpostOU and within theNorth Plume Group.

- Compliance withARARs
oExtraction, treatment, andreinjection ofaffected groundwater associated withthe

operation ofthe NBCS, IRA A, andExpansion 2to IRA A would achieve compliance with groundwater ARARs.

- Timeframe forachievement ofprotection
oGroundwater modeling estimates thatPRGs wouldbe achieved in 10to 20years. Alternative No. N-6 would intercept, pump, treat, and recharge UFS groundwater in the NorthPlume Group. Treatment ofgroundwater withcarbon adsorption wouldreduce the toxicity and volume ofcontaminated groundwater. Theadditional wells andinjection trenches provided for inExpansion 2toIRA A would enhance the rate ofcontaminant removal by extracting and flushing groundwater from areas upgradient ofthe interception extraction well systemspresented in Expansion Ito IRA AAlternative No.N-5. Consequently, Alternative No. N-6 would increase therate oftoxicity and volumereduction incontaminated groundwater. Hypothetical risk would bereduced through a reduction inCOCconcentrations inUFS groundwater; however, some hypothetical risk arising from the potential forthe use ofUFS groundwater beforecompletion of remediation would remain until PRGs wereachieved. Alternative No. N-6 would achieve groundwater PRGs. Alternative No. N-6 would notpresentany short-term impacts. Onthe basis ofgroundwater modeling results, itis expected thatPRGs wouldbeachieved within 10to 20 years. 4.2.1.6.2 Implementability The implementability evaluation forAlternative No. N-6 ispresented below.

20000,317.10 - FS 0309111092 

### VI - 4-16Technical feasibility " Construction and operation ofthe facility are readily implemented. " Carbon adsorption is a reliable and well-demonstrated process option for removal of

organic contaminants in RMA groundwater. Administrative feasibility " Nopermits arerequired. " Required equipment and technical personnel are readily available.
Alternative No. N-6 would be technically feasible. Construction and operation ofthe treatment and extraction facility would bereadily implemented. Carbon adsorption is a reliable and well-demonstrated process option for removal of organic contaminants from RMA groundwater. The administrative feasibility of Alternative No. N-6 would be high because no permits would be required. Equipment and technical personnel required to implement Alternative No. N-6 would be readily available.
4.2.1.6.3 Cost The capital costs, long-term O&M costs, and total present worth costs for Alternative No. N-6 are presented below:

- Total Capital Costs = $22.0 million

- Total Long-term Operation and Maintenance Costs = $37.6 - 44.2 million

- Total Present WorthCosts =$59.6 - 66.2 million
4.2.1.7 Alternative Screening Summary and Recommendation Six alternatives were evaluated for remediation ofUFS groundwater withinthe North Plume Group. The results ofthis evaluation and recommendations for selection of alternatives to be carried forward tothe DAA are summarized in Table 4.2.1.7-1. The screening ofthe six alternatives, with respect to the effectiveness criterion, shows thatthe four alternatives with active extraction/treatment/recharge remediation components at the NBCS and/or within the Offpost OU afford the best reduction in toxicity, mobility, and volume, the best long-term protection, and
20000,317.10 - FS 0309111092 VI - 4-17the bestcompliance with PRGs. Withregard tothe estimated time to achieve protection, both Alternative Nos. N-5 and N-6 have slightly reduced timeframes, ascompared toAlternative No. N-4, and Alternative No. N-4 has a reduced timeframe compared to alternatives without active extraction and treatment in the Offpost OU (i.e., Alternative Nos. N- Iand N-2). Alternative No. N-6, however, affords noreduction in estimated timeframe above Alternative No. N-5. With respect tothe implementability criterion, there are noprohibitive implementability
concerns with any ofthe six alternatives. A comparison ofthe costs associated with the six alternatives is presented inTable 4.2.1.7-1. The costs range from $4.1 - 6.0 million forAlternative No. N- I to$59.6 - 66.2 million for Alternative No. N-6. Table4.2.1.7-1 indicates that on the basisof effectiveness, implementability, and cost, three alternatives were selected forevaluation in the DAA. In addition, as required by the NCP, the No Action alternative was selected forevaluation in the DAA. The four alternatives selected for further evaluation follow: Alternative No. N-1: No Action Alternative No. N-2: Continued Operation ofthe NBCS with Improvements as Necessary Alternative No. N-4: Interim Response Action A Alternative No. N-5: Expansion I to Interim Response Action A
4.2.2 Screening ofGroundwater Alternatives - Northwest Plume Grout) The following subsections present the screening of groundwater alternatives developed for addressing groundwater remediation in the Northwest Plume Group. 4.2.2.1 Alternative No. NW-1: No Action Under Alternative No. NW-1, the operation oftheNWBCS would be discontinued. Alternative No. NW- I would therefore not provide foractive remediation ofgroundwater within the Northwest Plume Group. This alternative was described previously in Section 3.5.2.1.

20000,317.10 - FS 0309111092 VI - 4-184.2.2.1.1 Effectiveness The effectiveness evaluation for Alternative No. NW- Iis presented below.

- Reduction in mobility, toxicity, or volume through remedial action
o There would be no reduction in mobility, toxicity orvolume from treatment because of

discontinuation ofthe operation ofthe NWBCS. In addition, reductions in toxicity, mobility, and volume will occur through natural attenuation processes.

- Residual risk and long-term protection
o Under the No Action Alternative, no provisions are made for addressing exposure

pathways, reducing residual risk, or affording long-term protection to potential usersof affected groundwater from the Northwest Plume Group.

- Compliance with ARARs
o Affected groundwater in the Northwest Plume Group would notbe treated and would

not comply with groundwater ARARs.

- Short-term Impacts
o There would be no short-term impacts.

- Timeframe forachievement ofprotection
o Notevaluated. Under Alternative No. NW-I, no further action would be performed toreduce the mobility, toxicity, or volume ofcontaminated groundwater within the Northwest Plume Group. Operation ofthe NWBCS would cease, and contaminant concentrations in groundwater entering the Offpost OU would increase. Exposure pathways would notbe addressed. Therefore, no provision would be made for reducing residual risk and affording long-term protection. Under Alternative No. NW-1, natural attenuation processes would bethe only mechanism toreduce COC concentrations toPRGs. After ceasing operations at the NWBCS, onpost groundwater would act
as a source ofNorthwest Plume Group contamination. No data areavailable for evaluating the impact ofceasing operations at the NWBCS. Therefore, the timeframe forachieving protection has notbeen estimated. Groundwater monitoring and periodic site review would indicate when contaminant levels in groundwater have attained PRGs. There would beno short-term impacts under Alternative No. NW-1.

20000,317.10 - FS 0309111092 VI - 4-194.2.2.1.2 Implementability The implementability evaluation for Alternative No. NW-1 is presented below.

- Technical feasibility
o Monitoring and site review are readilyperformed and well-demonstrated at hazardous

waste sites.

- Administrative feasibility " No permits are required. " Equipment and technical personnel required for monitoring and site review are readily

available. Periodic groundwater monitoring and site review are the only actions that would be taken under Alternative No. NW-1. Groundwater monitoring wells identified for sampling under the proposed GMP have already been installed in the Offpost OU, and installation of additional wells would be readily implemented, ifnecessary. Groundwater monitoring data would be used in performing a site review every five years until PRGs are achieved. Alternative No. NW- I would not require permits. 4.2.2.1.3 Cost The capital costs, long-term O&M costs, and the total present worth costsfor Alternative No. NW-1 are presented below:

- Total Capital Costs = $- 0 -

- Total Long-term Operation and Maintenance Costs = $0.6 - 1.3 million

- Total Present Worth Costs = $0.6 - 1.3 million
4.2.2.2 Alternative No. NW-2: Continued Overation ofthe Northwest Boundary Containment System With Improvements AsNecessary Under Alternative No. NW-2, groundwater exceeding PRGs in the Northwest Plume Group would continue tobe extracted, treated, and reinjected through continued operation ofthe NWBCS. The NWBCS would be upgraded asnecessary to achieve PRGs in the Northwest Plume Group. This alternative was described previously in Section 3.5.2.2.
20000,317.10 - FS 0309111092 VI - 4-204.2.2.2.1 Effectiveness The effectiveness evaluation for Alternative No. NW-2 ispresented below.

- Reduction in mobility, toxicity, orvolume through remedial action " Continued operation ofthe NWBCS through removal, treatment, and recharge of

approximately 450 million gallons ofgroundwater per year would result in areduction inthe mobility, toxicity, and volume ofcontaminated groundwater entering the Offpost OU. " The alternative contains no active measures to address the reduction ofthe mobility,

toxicity, orvolume ofexisting contamination within the Northwest Plume Group; however, some reduction in toxicity, mobility, and volume would occur through decreases in contaminant concentrations attributable tonatural attenuation and flushing with treated water from the NWBCS.

- Residual risk and long-term protection " Continued operation ofthe NWBCS would result in increased long-term protection and

reduced residual risk attributable to reduction in.contaminant concentrations entering the Offpost OU. " No provisions would be made foraddressing exposure pathways and reducing residual

risk or affording long-term protection associated with existing contamination within the Northwest Plume Group.

- Compliance with ARARs
o Treatment of affected groundwater and source control provided by the NWBCS and

natural attenuation processes in the Northwest Plume Group would achieve compliance with groundwater ARARs.

- Short-term impacts
o There would be no short-term impacts.

- Timeframe for achievement ofprotection
o Reduction incontaminants would occur through natural attenuation processes;

monitoring and site review would indicate when PRGs would be achieved; groundwater modeling indicates that PRGs would be achieved within three toeight years.
Under Alternative No. NW-2, continued operation of the NWBCS to achieve Offpost OU PRGs would reduce contaminant concentrations in groundwater entering the Offpost OU and correspondingly reduce the toxicity, mobility, and volume ofgroundwater migrating from RMA to the Northwest Plume Group. Through treatment ofapproximately 450 million gallons of groundwater per year at the NWBCS, long-term protectiveness is substantially improved in the
20000,317.10 - FS 0309111092 VI - 4-21Northwest Plume Group. Under Alternative No. NW-2, natural attenuation processes and source control provided by the NWBCS and flushingthe Northwest Plume Group with treated groundwater would reduce COCconcentrations to PRGs within three toeight years. Groundwater monitoring and periodic sitereview would indicate when contaminant levels ingroundwater have attained PRGs. There would be no short-term impacts under Alternative No. NW-2.
4.2.2.2.2 Imt)lementability The implementability evaluation for Alternative No. NW-2 ispresented below.

- Technical feasibility
o Groundwater monitoring and site review arereadily performed and well demonstrated at hazardous waste sites.

- Administrative feasibility " Nopermits are required. "Equipment and technical personnel required for groundwater monitoring and sitereview

are readily available.
No implementability considerations are associated with continued operation of the NWBCS. Groundwater monitoring wells have alreadybeen installed inthe Offpost OU and installation of additional wells would be readily implemented, ifnecessary. Groundwater monitoring data would be used in performing a site review every five years untilPRGs areachieved. Alternative No. NW-2 would notrequire permits.
4.2.2.2.3 Cost The capital costs, long-term O&M costs, and total present worth costs for Alternative No. NW-2 are presented below:

- Total Capital Costs = $ - 0 -

- Total Long-term Operation and Maintenance Costs = $12.4 - 13.1 million

- Total Present Worth Costs = $12.4 - 13.1 million

20000,317.10 -FS 0309111092 VI - 4-224.2.2.3 Alternative No. NW-3: Land Acauisition and Deed Restrictions Under Alternative No. NW-3, areas within the Offpost OU thatexceed groundwater PRGs wouldbe purchased and institutional controls would be implemented to limitexposure to affected groundwater. This alternative was described previously in Section 3.5.2.3.
4.2.2.3.1 Effectiveness The effectiveness evaluation for Alternative No. NW-3 ispresented below.

- Reduction in mobility, toxicity, orvolume through remedial action " Continued operation ofthe NWBCS through removal, treatment, and recharge of

approximately 450 million gallons ofgroundwater per year would result in a reduction in the mobility, toxicity, and volume ofcontaminated groundwater entering the Offpost OU. "The alternative contains no active measures to address the reduction ofthe mobility,

toxicity, or volume ofexisting contamination within the Northwest Plume Group; however, some reduction in toxicity, mobility, and volume would occur through decreases incontaminant concentrations attributable to natural attenuation and flushing with treated water from theNWBCS.

- Residual risk and long-term protection
o Exposure pathways would be controlled, and residual risk would bereduced by

minimizing exposure pathways through use and access restrictions.

- Compliance with ARARs
o Treatment ofaffected groundwater and source control provided by the NWBCS and

natural attenuation processes in the NorthwestPlume Group would achieve compliance with groundwater ARARs.

- Short-term impacts
o There would be no short-term impacts.

- Timeframe for achievement ofPRGs
o Reduction incontaminant concentrations would occur through natural attenuation

processes and flushing with treated groundwater from the NWBCS. Monitoring and site review would indicate when protectiveness is achieved; modeling indicates PRGswould be achieved within approximately three to eight years. Alternative No. NW-3 would reduce the mobility, toxicity andvolume ofcontaminated groundwater entering the Northwest Plume Group. Continued operation ofthe NWBCS would reduce contaminant concentration in groundwater entering the Offpost OU and would achieve
20000,317.10 - FS 0309111092 VI - 4-23Offpost PRGs. Exposure pathways would be reduced through land acquisition and use restrictions. Thus, hypothetical risk would be reduced by preventing the use ofcontaminated groundwater for crop irrigation, watering oflivestock, and domestic purposes. Purchasing land overlying affected groundwater would aid in controlling the use ofcontaminated groundwater. Alternative No. NW-3 would not create any short-term impacts. Source control actions provided by continued operation ofthe NWBCS in combination with natural attenuation processes would reduce COCconcentrations toPRGs in 3to 8 years. 4.2.2.3.2 Imolementability The implementability evaluation for Alternative No. NW-3 is presented below.

- Technical feasibility " Construction is not required. " Use and access restrictions are common components ofremedial alternatives at

hazardous waste sites.

- Administrative feasibility " No permits are required. "Technical personnel and equipment are notrequired.
Alternative No. NW-3 would be technically feasible. No construction would berequired. Property could be purchased as necessary. Use and access restrictions are common components of remedial action at hazardous waste sites. Enforcement ofuse restrictions would be simplified by acquisition ofthe affected areas. No permits, technical personnel, orequipment would be required under Alternative No. NW-3.
4.2.2.3.3 Cost The capital costs, long-term O&M costs, and total present worth costs for Alternative No. NW-3 are presented below:

- Total Capital Costs = $2.1 million

20000,317.10 - FS 0309111092 VI - 4-24- Total Long-term Operation and Maintenance Costs = $12.3 - 12.9 million

- Total Present Worth Costs = $14.4 - 15.0 million
4.2.2.4 Alternative No. NW-4: Northwest Plume Grout) Extraction/Recharpe System Under Alternative No. NW-4, two additional extraction wells and recharge trenches would be installed toremediate the dieldrin plume in a shorter timeframe than Alternative No. NW-2. This alternative was described previously in Section 3.5.2.4.
4.2.2.4.1 Effectiveness The effectiveness evaluation for Alternative No. NW-4 is presented below.

- Reduction in mobility, toxicity, or volume through remedial action " Installation and operation ofthe Northwest Plume Group extraction/recharge system

would reduce mobility, toxicity, and volume ofcontaminated groundwater within the Northwest Plume Group. " Continued operation of the NWBCS through removal, treatment, and recharge of

approximately 450 million gallons ofgroundwater per year would resultin a reduction inthe mobility, toxicity, and volume ofcontaminated groundwater entering the Offpost OU. In addition, some reduction in toxicity, mobility, and volume would occur through natural attenuation and flushing.

- Residual risk and long-term protection
o Continued operation of the NWBCS and operation ofthe Northwest Plume Group

extraction/recharge system would result in increased long-term protection and reduced residual riskattributable to a reduction in contaminant concentrations entering the Offpost OU and within the Northwest Plume Group.

- Compliance with ARARs
o Extraction, treatment, and recharge associated withthe operation ofthe NWBCS and the

Northwest Plume Group extraction/ recharge system would achieve compliance with groundwater ARARs.

- Short-term impacts
o There would be no short-term impacts.

- Timeframe for achievement ofPRGs
o Groundwater modeling indicates the approximate time to achieve PRGs is two to

five years.

20000,317.10 - FS 0309111092 VI - 4-25Alternative No. NW-4 would intercept, pump, and treat UFS groundwater, thus reducing organic COCconcentrations before reinjection. Carbon adsorption would reduce the toxicity and volume ofcontaminated groundwater through treatment. Alternative No. NW-4 would reduce the mobility ofcontaminated groundwater bydowngradient interception and extraction inthe northwest paleochannel. Hypothetical risk would be reduced through a reduction ofCOCconcentrations inUFS groundwater; however, some hypothetical risk would remain until groundwater PRGs have been achieved inUFS groundwater. Carbon adsorption would achieve groundwater PRGs for all COCs. Alternative No. NW-4 would notcause any short-term impacts to human health or theenvironment. Groundwater modeling indicates that PRGs would be achieved within two to five years. Additionally, groundwater modeling shows that there would be noCOCs exceeding PRGs north ofO'Brian Canal after approximately one year. 4.2.2.4.2 Imr)lementability The implementability evaluation for Alternative No. NW-4 is presented below.

- Technical feasibility
o Construction and operation offacility are readily implemented.
o Carbon adsorption is a reliable and well demonstrated process option for removal of

organic contaminants inRMA groundwater.

- Administrative feasibility " No permits are required. " Required equipment and technical personnel are readily available.
Alternative No. NW-4 would be technically feasible. Expansion ofthe NWBCS system would be readily implemented. Carbon adsorption isa reliable and well demonstrated process option for removal oforganic contaminants from RMA groundwater. The administrative feasibility ofAlternative No. NW-4 would behigh because no permits would berequired and because equipment and personnel required to install and operate the extraction and recharge system would be readily available.

20000,317.10 - FS 0309111092 VI - 4-264.2.2.4.3 Cost The capital costs, long-term O&M costs, and total present worth costs for Alternative No. NW-4 are presented below:

- Total Capital Costs = $2.8 million

- Total Long-term Operation and Maintenance Costs = $12.2 - 12.5 million

- Total Present Worth Costs =$15.0 - 15.3 million
4.2.2.5 Alternative Screeninp,Summary and Recommendation Four alternatives Were evaluated for remediation ofUFS groundwater within the Northwest Plume Group. The results ofthis evaluation and recommendations for selection ofalternatives to be carried forward to the DAA are summarized in Table 4.2.2.5-1. The screening ofalternatives, with respect to the effectiveness criterion, shows thatthe three alternatives that include NWBCS operation afford the best reduction in toxicity, mobility, and volume; the best long-term protection; and the best compliance with PRGs. There is noappreciable decrease in the estimated time to achieve PRGs provided by the additional offpost extraction, treatment, and recharge in Alternative No. NW-4. With respect tothe implementability criterion, there areno prohibitive implementability
concerns with any ofthe four alternatives. A comparison ofthe costs associated with the four alternatives is presented in Table 4.2.2.5- 1. The total present worth costsrange from $0.6 - 1.3 million for Alternative No. NW- I to $2.8 - 15.3 million for Alternative No. NW-4. Table 4.2.2.5-1 indicates that on the basis ofeffectiveness, implementability, and cost, Alternative No. NW-2, Continued Operation ofthe NWBCS withImprovements asNecessary, was selected for evaluation in the DAA. In addition, as required by the NCP, the No Action alternative was selected for evaluation in theDAA.

20000,317.10 -FS 0309111092 VI - 4-275.0 DETAILED ANALYSIS OF ALTERNATIVES
A range ofsite remedial alternatives was screened in Section 4.0, based on effectiveness, implementability, and cost. A decision was presented in Section 4.0 indicating whether the alternative wasretained or eliminated for the detailed analysis of alternatives (DAA). Four remedial alternatives were analyzed forthe North Plume Group:

- Alternative No.N- 1: No Action

- Alternative No. N-2: Continued Operation ofthe North Boundary Containment System (NBCS) With Improvements as Necessary

- Alternative No. N-4: Interim Response Action A

- Alternative No. N-5: Expansion Ito Interim Response Action A
Two remedial alternatives were analyzed for the Northwest Plume Group. These alternatives are:

- Alternative No. NW-1: No Action

- Alternative No. NW-2: Continued Operation ofthe Northwest Boundary Containment System (NWBCS) With Improvements asNecessary These remedial action alternatives were evaluated with respect to threshold and primary balancing criteria as required by the NCP. The criteria are as follows
Threshold Criteria

- Overall protection ofhuman health and the environment

- Compliance with ARARs Primary Balancinp,Criteria

- Long-term effectiveness and permanence

- Reduction oftoxicity, mobility, orvolume

- Short-term effectiveness

- Implementability

- Cost

20000,317.10 - FS 0309111092 VI - 5-1Evaluation ofmodifying criteria (i.e., the state and community acceptance) was deferred until completion ofthe stateand public comment period. The evaluation criteria aredefined and described below in Section 5.1. A summary ofthe groundwater modeling results, asit applies to alternatives evaluation, ispresented in Section 5.2. The detailed analysis ofthe four alternatives for the North Plume Group and the two alternatives for the Northwest Plume Group is presented in Section 5.3. Section 5.4 presentsthe comparative analysis ofalternatives, which identifies the advantages and disadvantages ofeach alternative relative tothe others. 5.1 CRITERIA Each groupofcriteria (i.e., threshold, primary balancing, and modifying) has itsown weight when itis evaluated. Threshold criteria must be satisfied. Primary balancing criteria are used to weigh trade-offs among alternatives. Modifying criteria may be used to alter aproposed remedial alternative. This section presents a briefdescription ofthe evaluation criteria and the items considered when assessing alternatives with respect to each criterion. 5.1.1 Overall Protection ofHuman Health and the Environment This evaluation criterion servesas a final check in assessing whether each alternative provides adequate protection ofhuman health and theenvironment. The detailed analysis conducted for long-term effectiveness and permanence, short-term effectiveness, and compliance with ARARs was used toevaluate the overall protection ofhuman health and theenvironment. This criterion was also used to evaluate how risks would be eliminated, reduced, orcontrolled through treatment, engineering, institutional controls, or other remedial activities. 5.1.2 Comr)liance With Armlicable or Relevant and At)t)rot)riate Recluirements This evaluation criterion isused to determine whether each alternative willattain federal and state ARARs. The detailed analysis summarizes which requirements are applicable or relevant and appropriate to each alternative and describes how each alternative exceeds, attains, or 20000,317.10 - FS 0309111992 VI - 5-2does not attain these requirements. Other information such as advisories, criteria, orguidance documents have been considered where appropriate, during the ARARs analysis, and is presented in Appendix A. 5.1.3 Lonp,-term Effectiveness and Permanence This criterion addresses the risk remaining at the site after response objectives have been met. Components ofthe criterion thatwere addressed foreach alternative are:

- Attainment ofPRGs at theend ofremedial activities

- Adequacy and reliability ofcontrols that are used to either manage treatment residuals or untreated materials that remain at the site
5.1.4 Reduction ofToxicity, Mobility, or Volume This evaluation criterion addresses the statutory preference forselecting remedial actions that permanently and significantly reduce toxicity, mobility, and/or volume ofhazardous materials at the site. This preference is satisfied when treatment isused to reduce principal risks through destruction or irreversible reductions oftoxicity, mobility, and/or volume. The criterion focuses on

- The degree ofexpected reduction in toxicity, mobility, and volume

- The degree ofirreversibility ofthe process

- The type and quantity ofresiduals remaining following treatment

- The statutory preference for treatment asaprincipal element

- The relative amount ofhazardous materialsthat will be destroyed or treated
5.1.5 Short-term Effectiveness Short-term effectiveness addresses the effects ofeach alternative inthe protection ofhuman health and the environment during the construction and implementation phase. The following factors were addressed during the evaluation process

- Protection ofthe community during remedial actions - This addresses any risk thatresults from implementation ofthe proposed remedial alternative, such asdust fromexcavation ortransportation ofhazardous material.

20000,317.10 - FS 0309111092 VI - 5-3- Protection ofthe workers during remedial actions - This factor assesses threats that may be posed toworkers and the effectiveness and reliability ofmeasures to be taken.

- Environmental impacts ofthe remedial action - This factor addresses the potential adverse environmental impacts thatmay result fromconstruction andimplementation ofa remedial alternative and evaluates the reliability ofmitigation measures, if necessary, to prevent orreduce potential impacts.

- Time lapse before achievement ofresponse objectives - This factor includes an estimate ofthe time required to achieve protection for the site.
5.1.6 Imr)lementability This criterion evaluates the technical and administrative feasibility ofimplementing each alternative, and itaddresses the availability ofrequired services and materials during its implementation. The following factors were addressed during the evaluation process:

- Construction and operation - This factor relates to the technical difficulties and the unknowns associated with the technology.

- Reliability ofthe technology - This factor focuses on the likelihood that problems associated with implementation may result in schedule delays.

- Implementing additional remedial action - This factor is not applicable tothis FSbecause the alternatives considered are notinterim measures.

- Monitoring theeffectiveness ofthe remedy - This factor addresses the abilityto evaluate the effectiveness ofthe remedy and includes an evaluation ofthe risks ofexposure should monitoring be insufficient to detect a system failure.

- Coordination with other offices and agencies needed to implement remedial alternatives

- Availability ofnecessary equipment, specialists, services and materials, and adequate offsite treatment, storage, and disposal services
5.1.7 Cost The cost analysis consists ofthe evaluation ofboth capital costs and any long-term costs to operate and maintain an alternative. Thebasis for the cost estimates wasexplained in Section 4.1.3. Detailed cost tables foreach alternative are presented in Appendix F.
5.1.8 State Accemance The intent ofthis section is to evaluate technical and administrative concerns thestate may communicate in its comments concerning each alternative. The state willreview the Revised

20000,317.10 - FS 0309111092 VI - 5-4DraftFinal EA/FS Report. The state'scomments will be evaluated and addressed after their review. 5.1.9 Community Accemance The preferred alternative for a site is presented tothe public in aProposed Plan, which provides a briefsummary ofall ofthe alternatives studied in theDAA ofthe FS. In accordance with the NCP, the public will havean opportunity toreview and comment on the selected remedial alternatives presented in the Proposed Plan. Thepublic's comments will be addressed in the responsiveness summary and ROD for the Offpost FS.
5.2 MODELING RESULTS SUMMARY To assist in evaluating the alternatives with respect tothe NCP criteria, two numerical models were developed. The models simulated alluvial groundwater flow and dissolved contaminant transport inthe Offpost OU for the North and Northwest Plume Groups. The models were used to evaluate the relative merits ofremedial alternatives developed in Section 3.0. The two models, referred to as theNorth and Northwest Models, were constructed using the U.S. Geological Survey (USGS) finite element codeSUTRA (Voss, 1984). The North Model, which encompassed the area between the NBCS and the South Platte River, accounted for flow and transport in the northern and First Creek paleochannels as well asa third pathway stemming from the west end ofthe NBCS (see Appendix E). TheNorthwest Model covered the area lying between RMA's northwest boundary in the vicinity ofthe NWBCS and the South Platte River. Due to theconcerns about lateral dispersion during transport simulation and the potential effects ofmodel boundaries on remedial schemes, the lateral boundaries were located considerable distances away from the plumes being modeled. Consequently, the two model areas overlap. The models prepared for the FS analysis are approximate innature. Because comparative evaluation ofthe benefits derived from each remedial alternative does notrequire highly accurate models, attempts have been made toproduce models thatincorporate general features of groundwater flow and associated transport phenomena in the Offpost OU. Nonetheless, the

20000,317.10 - FS 0309111092 VI - 5-5resulting models are sufficiently detailed thatpredicted flow and chemical transport phenomena agree with historical and current hydrogeologic data and observed contaminant distributions. Due to the approximate nature ofthe models and the considerable uncertainty in the conceptual model and hydrogeologic parameters, none ofthe modeling results should be construed asaccurate predictions offuture contaminant distribution. Rather, the models and modeling results should be viewed as tools for assessing the relative merits ofremedial alternatives. Although there are inherent uncertainties in thegroundwater model, it isthe tool being used by the FSfor evaluation ofalternatives, and predicted differences in remediation timeframes are considered with respect to evaluating alternative effectiveness. The flow modeling was limited to simulation ofgroundwater movement in unconsolidated alluvial materials in the Offpost OU. Consequently, regions where alluvial materials are not saturated were excluded from the North and Northwest Models (see Appendix E, Figure EI). In the North Model, the exclusion has resulted inthe confinement ofgroundwater flow into distinct and separate pathways. Chemical transport simulations were performed using dieldrin, DIMP, and chloroform to evaluate the effects ofcurrent remedial actions (i.e., the NBCS and NWBCS) as well aspotential additional remedial actions. Transport simulation results were compared on the basis ofmaximum concentration versus time plots. Simulations were conducted using the most recent plume configuration interpretations based on fall 1989, 1990, and 1991 data, as presented inthe Offpost RI Addendum (HLA, 1991a). Simulations were carried out for aperiod of 30 years. For the North Model, the following remedial action scenarios were simulated: (1) Continued Operation ofthe NBCS with Improvements asNecessary (Alternative No. N-2), (2) IRA A (Alternative No. N-4), and (3) Expansion I to IRAA (Alternative No. N-5). The resultsofthese simulations were evaluated on the basis ofestimated remediation times measured on the maximum concentrations versus time graphs. The range ofestimated remediation times wasbased on attainment of the PRGs forDIMP, chloroform, and dieldrin, using arange ofretardation factors (See Appendix E).

20000,317.10 - FS 0309111092 VI - 5-6Simulations performed for the North Model show generally decreasing estimated timeframes for attainment ofPRGs with increasing complexity ofremedial components. Ofthe three contaminants simulated, estimated remediation times forDIMP are the quickest, and estimated remediation times fordieldrin are the slowest. This appears tobe related to the lowerrange of retardation factors for DIMPrelative todieldrin. For the Northwest Model, the remedial action scenario forContinued Operation ofthe NWBCS with Improvements asNecessary (Alternative No. NW-2) was simulated. For reasons presented inthe description ofthe No Action alternative (Alternative No. NW-1) in Section 5.3.1, groundwater modeling was notperformed forAlternative No. NW-1. A qualitative comparison of the remedial systems described in Alternatives Nos. NW-1 and NW-2 indicates thatAlternative No. NW-2 would attain PRGs in a substantially shorter timeframe than Alternative No. NW-1.
5.3 FURTHER DEFINITION OF ALTERNATIVES The following subsections present additional information about the alternatives. Common to all alternatives are long-term groundwater monitoring and five-year site reviews.
5.3.1 Alternative No. N-1: No Action The No Action alternative wasdescribed previously in Section 3.5.1.1. The major components are asfollows:

- Long-term groundwater monitoring

- Five-year site reviews Continued sitegroundwater monitoring and five-yearreviews are the only components ofthis alternative. Groundwater modeling was notperformed for this alternative. Because this alternative contemplates the hypothetical situation where the NBCS is notoperated toextract, treat, and recharge groundwater, there is no groundwater monitoring data corresponding tothe scenario. Thus a hypothetical, assumed set ofcontaminant concentrations would be required as input tothe groundwater model for this alternative. Because there would be substantial uncertainty associated with the estimation ofgroundwater contaminant concentrations entering the
20000,317.10 - FS 0309111092 VI - 5-7Offpost OU inthe hypothetical situation ofnotoperating the NBCS, the modeling was not conducted. However, the analysis ofthe No Action alternative, asa baseline case, for comparison with respect tothe other alternatives is substantially unaffected.
5.3.2 Alternative No. N-2: Continued Operation ofthe North Boundary Containment System With Imt)rovements asNecessary This alternative was described previously in Section 3.5.1.2. The major components are as follows:

- Continued operation ofthe NBCS

- Improvements tothe NBCS asnecessary

- Long-term monitoring

- Five-year site reviews

- Exposure control Model simulations for Alternative No. N-2 assume that the concentrations ofcontaminants in the NBCS effluent would remain at values equivalent tothe average measured effluent concentrations. Dieldrin and chloroform were notdetected in theNBCS effluent andDIMP was detected at low concentrations. Effluent concentrations for dieldrin, DIMP, andchloroform used as input tothe model were set at 0.025 ug/I (one-halftheCRL), 3.00 ug/l, and 0.25 Ug/I (one-half the CRL), respectively. Modeling runs performed for Alternative No. N-2 are presented in the form ofmaximum contaminant concentrations in the UFS asa function oftime (see Appendix E, Figures E13, E15, and E17). 5.3.3 Alternative No. N-4: Interim Rest)onse Action A The description ofAlternative No. N-4 waspreviously presented in Section 3.5.1.4. The major components ofthis alternative include:

- Removal of contaminated UFS groundwater north ofthe RMA boundary in the First Creek and northern paleochannels using IRA A groundwater extraction wells

- Treatment ofthe organic COCs present inthe groundwater using carbon adsorption 20000,317.10 -FS 0309111092 VI - 5-8- Recharge oftreated groundwater to the UFS using IRA A wells and trenches

- Continued operation ofthe NBCS and IRA A as necessary

- Improvements toIRA A and the NBCS as necessary

- Long-term groundwater monitoring

- Five-year site reviews

- Exposure control Similar to Alternative No. N-2, the groundwater modeling assumed thatconcentrations of the contaminants in the NBCS effluent were set at 0.025pg/l, 3.00jug/l, and 0.25jug/ fordieldrin, DIMP, and chloroform, respectively. As shown in Figure 3.5-1.4- 1,IRA A consists oftwo areas ofgroundwater extraction and recharge. Five extraction wellsand six recharge trenches would belocated in the FirstCreek paleochannel. As estimated in the Final Implementation Document for IRA A (HLA, 1991b) for the system inthe First Creek paleochannel, 180 gallonsper minute (gpm) total flow was used for the model simulation. The flow was divided equally among the five extraction wells and six recharge trenches, with each extraction well pumping at 36 gpm and each recharge trench receiving 30 gpm oftreated effluent. The northern paleochannel portion ofIRA A consists ofa
row of 12 extraction wells and a row of24 recharge wellsforming ahydraulic barrier across the northern paleochannel. As estimated in the IRAA Implementation Document for the system in the northern paleochannel, 300 gpm total flow wasused for modeling simulations. The flow was divided equally among the 12 extraction and 24 recharge wells, with each extraction well pumping at 25 gpm and each recharge well receiving 12.5 gpm oftreated effluent. For model simulations, the concentrations ofcontaminants in the IRA A treatment plant effluent were assumed to be at the same concentrations as the treated effluent from the NBCS. Model simulations for the implementation ofIRA A are presented in the form ofmaximum concentration versus time graphs in Appendix E, Figures E18 toE20.

20000,317.10 - FS 0309111092 VI - 5-95.3.4 Alternative No. N-5: Exr)ansion Ito Interim Resvonse Action A Alternative No. N-5 . was previously described in Section 3.5.1.5. The major components of this alternative include:

- Removal ofcontaminated UFS groundwater north ofthe RMA boundary in the First Creek and northern paleochannels using IRA A groundwater extraction wells

- Expansion I ofIRA A

- Treatment oforganic COCspresent in the groundwater using carbon adsorption

- Recharge oftreated groundwater tothe UFS using wells and trenches

- Continued operation ofthe NBCS

- Improvements tothe NBCS asnecessary

- Long-term groundwater monitoring

- Five-year site reviews

- Exposure control As shown inFigure 3.5.1.5-1, Expansion ItoIRA A consists oftwo areas ofgroundwater extraction and recharge thatcorrespond tothe IRA A systems previously described in Section 5.3.3. Expansion Ito IRA A was configured inthe groundwater modeling to include two additional extraction wells and four additional recharge trenches added to the IRA A system in the First Creek paleochannel. In the northern paleochannel, Expansion I wasconfigured to include one additional extraction well and two additional recharge trenches added to the IRA A system. The additional extraction wells were included in the model at a rate of30 gpm, and the recharge trenches were prescribed to recharge 15 gpmeach. For the model simulations, the concentrations ofcontaminants in the treatment planteffluent were assumed to be at the same concentrations asthe treated effluent from the NBCS. Dieldrin and chloroform were not detected in NBCS effluent, and DIMP was detected at low concentrations. Effluent concentrations used as model input were prescribed tobe 0.025 Ag1l, 3.00,ug/l, and 0.25 jug/l fordieldrin, DIMP, and chloroform, respectively.

20000,317.10 - FS 0309111092 VI - 5-10Appendix E presentsthe groundwater modeling simulation results as maximum concentration versus time graphs (Appendix E, Figures E21 toE23).
5.3.5 Alternative No. NW- 1: No Action This alternative was previously described in Section 3.5.2.1. The components ofthis alternative are limited tocontinued groundwater monitoring and five-year site reviews. Groundwatermodeling wasnot performed for this alternative for the reasons outlined under Section 5.3.1, Alternative No. N- I(No Action for the North Plume Group). Themajor components are as follows:

- Long-term groundwater monitoring

- Five-year sitereviews 5.3.6 Alternative No. NW-2: Continued Ot)eration ofthe NorthwestBoundary Containment System WithImprovements asNecessary This alternative was previously described in Section 3.5.2.2. The major components of Alternative NW-2 include:

- Continued operation ofthe NWBCS

- Improvements tothe NWBCS as necessary

- Long-term groundwater monitoring

- Five-year site reviews

- Exposure control Groundwater modeling simulations for this alternative included 850 gpmtreated and recharged at the NWBCS. Average NWBCS effluent concentrations from 1986to 1990 were used
as influent concentrations tothe model. The concentrations were0.025 Ug/l and 13.8 Ug/I for dieldrin andchloroform, respectively. The groundwater modeling results for thisalternative are presented in Appendix E, Figures E29 and E30.

20000,317.10 - FS 0309111192 VI - 5-115.4 DETAILED ANALYSIS OF ALTERNATIVES The following subsections present an assessment ofeach ofthe alternatives carried forward into the DAA from the Screening ofAlternatives, Section 4.0, with respect to the threshold and primary balancing criteria mandated inthe NCP. Each alternative isdiscussed under subheadings foreach ofthe criteria. Additional detail necessary tocomplete the evaluation ofthecriteria is provided for each alternative, asrequired, in the following section.
5.4.1 Overall Protection of Human Health and the Environment In this section, each remedial alternative isevaluated in terms oftheextent ofprotection provided for human health and the environment. 5.4.1.1 Alternative No. N-1: No Action This alternative does not provide protection ofhuman health and the environment because it contemplates a set ofcircumstances whereby the extraction and treatment ofcontaminated groundwater, asa source control measure for theOffpost OU, would notbe performed by the NBCS. This is a hypothetical alternative because the Army has operated the NBCS since 1979 and has made the commitment to continue operation and, further, toimprove the NBCS as necessary to achieve Offpost OU PRGs. For this alternative, the evaluation ofperformance with respect to controlling, eliminating,
orreducing hypothetical risks is based ona hypothetical scenario ofnot operating the NBCS. However, the Offpost EA presented risks for the Offpost OU considering continued operation of the NBCS (baseline risks). Thus, the hypothetical risks would be higher for the No Action alternative than the baseline EA risks. The groundwater exposure pathways used in the calculation of the baseline risks presented in the Offpost EA (Volume II, Section 2.0) would not be addressed by the No Action alternative. Natural attenuation would be solely relied upon to reduce contaminant concentrations. Hypothetical risks associated with the domestic usepathways (direct ingestion and inhalation during showering) would be higher than the estimated baseline risks presented in the Offpost EA due to the lack of source control and contaminant removal provided by the NBCS.

20000,317.10 - FS 0309111092 VI - 5-12Similarly, the hypothetical risks associated with the agricultural pathways (beef, dairy products, poultry products, and crops) would also behigher than the estimated baseline risks presented in the Offpost EA for the same reason. 5.4.1.2 Alternative No. N-2: Continued Operation ofthe North Boundary Containment Syst m With Imorovements asNecessary This alternative provides overall protection ofhuman health and the environment through the continued operation of the NBCS to achieve Offpost OU PRGs. Because there are no active remedial components treating groundwater in the North Plume Group (north ofthe RMA boundary), direct remediation ofcontaminant plumes north ofthe NBCS is notprovided by this alternative in the near-term. Through the extraction, treatment, and recharge ofapproximately 125 million gallons ofgroundwater annually, thecontaminant concentrations entering the Offpost OU from RMA would continue tobe greatly reduced. The hypothetical risks attributable to contaminant concentrations inthe North Plume Group would be consistent with those calculated in the Offpost EA. However, through continued operation ofthe NBCS (with improvements
as necessary), contaminant concentrations within the North Plume Group would decrease over time and correspondingly decrease hypothetical risks. There would be no provisions for addressing the exposure pathways identified in the OffpostEA, withthe exception ofthe Army's continued commitment toprovide alternative water for domestic uses, should a userofgroundwater exceeding PRGs be identified in the future. Because there would be no active measures in this alternative to reduce the contaminant concentrations in the North Plume Group, the time to attain PRGs as estimated by the groundwater modeling would be greater thanalternatives employing extraction and treatment of groundwater within the North Plume Group. Natural attenuation mechanisms would be relied upon for contaminant reduction rather thanactive measures. The groundwater modeling estimate oftime to attain PRGs isapproximately 15 to 30-plus years, corresponding tothe range ofretardation factors used in the groundwater modeling (see Section 5.2 and Appendix E).

20000,317.10 - FS 0309111092 VI - 5-135.4.1.3 Alternative No. N-4: Interim Response Action A Under this alternative, implementation ofthe groundwater extraction, treatment, and recharge components ofIRAA to remediate North Plume Group groundwater would substantially reduce hypothetical risks and provide protection ofboth human health and the environment. IRA A is designed to treat approximately 95 million gallons in the FirstCreek paleochannel and 158 million gallons in the northern paleochannel annually for a total annual volume of253 million gallons. This is in addition tothe 125 million gallons treated annually at the NBCS. Through the combined operation ofthese two systems, the hypothetical risks calculated in the Offpost EA would be greatlyreduced. The Army's commitment toprovide alternative waterto any users of groundwater exceeding PRGs thatmay be identified in the future would serve as a measure of protection during the time required to achieve PRGs. The extraction and recharge configuration ofIRA A is an aggressive remediation component that, in combination with the treatment performed at the NBCS (and improvements to both systems as necessary), would substantially reduce contaminant concentrations in the North Plume Group and would reduce remediation timeframes. As with other alternatives, natural attenuation mechanisms would also provide contaminant concentration reduction. The groundwater modeling estimate oftime to attainment ofPRGs isapproximately 15 to30 years, corresponding tothe range ofretardation factors used in the groundwater modeling (see Section 5.2 and Appendix E). The intensive short-term groundwater monitoring component ofAlternative No. N-4, described in Section 3.5.1.4, would provide for increased understanding and increased accuracy in theestimation ofcleanup times through the acquisition ofalarge amount ofcontaminant concentration data in the North Plume Group. An approximate two-year period ofintensive groundwater monitoring commencing with the start-up ofthe IRA A system would provide the opportunity for collection offull-scale system operation data providing information on contaminant plume movement in response to operation ofboth IRA A and the NBCS. Such data would be invaluable topredict the expected performance ofAlternative No. N-4 more accurately.

20000,317.10 - FS 0309111092 VI - 5-145.4.1.4 Alternative No. N-5: Expansion I to Interim Response Action A Alternative No. N-5 would provide overall protection ofhuman health and the environment similar toAlternative No. N-4. This alternative is an expansion tothe groundwater extraction, treatment, and recharge components ofIRA A that would provide for a similar reduction in hypothetical risks through a reduction in contaminant concentrations. TheFirst Creek paleochannel component of this alternative would treat approximately 125 million gallons of groundwater annually and the northern paleochannel component would treat approximately 173 million gallons annually, totaling 298 million gallons. This is in addition to the 125 million gallons treated annually at the NBCS. The combined operation ofExpansion I to IRA A and the NBCS would substantially reduce the hypothetical risks calculated in the Offpost EA. The potential for exposure to affected groundwater during the estimated timeframe until achievement of PRGs would be reduced by theArmy's commitment toprovide alternative water to any future identified affected groundwater users. The groundwater modeling estimate oftime to attain PRGs is approximately 10 to 20years, corresponding to the range ofretardation factors used in the groundwater modeling (see Section 5.2 and Appendix E). Expansion I toIRA A is a small addition to theextraction and recharge components of IRA A. In combination with thecontinued operation ofthe NBCS (and improvements to both systems as necessary), North Plume Group contaminant concentrations would be substantially reduced and remediation timeframes would be reduced. Natural attenuation mechanisms would also contribute to contaminant concentration reduction.
5.4.1.5 Alternative No. NW-1: No Action Alternative No. NW- I does not provide adequate protection ofhuman health and the environment for the Northwest Plume Group because itcontemplates ceasing groundwater extraction, treatment, and recharge at the NWBCS, similarto Alternative No. N-1 for the North Plume Group. This isa hypothetical circumstance because the Army has operated the NWBCS since 1984 and has committed to continue operations and implement improvements asnecessary to attain Offpost OU PRGs.
20000,317.10 - FS 0309111092 VI - 5-15As discussed in Section 5.4.1.1 for Alternative No. N-1, hypothetical risks associated with this alternative would be higher than those presented in theOffpost EA because itwas assumed in the EA that the NWBCS operation would be continued. Also, natural attenuation would be the only mechanism for contaminant concentration reduction inthe absence ofactive remedial
measures atthe NWBCS orin the Northwest Plume Group.
5.4.1.6 Alternative No. NW-2: Continued Ot)eration ofthe Northwest Boundary Containment Syst m.With Imt)rovements as Necessary This alternative provides forboth protection ofhuman health and protection ofthe environment through groundwater extraction, treatment, and recharge at the NWBCS to meet Offpost OU PRGs. The NWBCS treats approximately 450 million gallons ofgroundwater annually. Continued operation ofthis system would allow the Northwest Plume Group groundwater to attain Offpost PRGs. Hypothetical risks calculated in the Offpost EA would be substantially reduced through implementation ofthis alternative. Through the Army's commitment to provide alternative water to any identified future users ofgroundwater exceeding PRGs, hypothetical risks during the period until PRGs are attained would be reduced. The groundwater modeling estimates ofthe time necessary to attain PRGs is approximately three toeight years, corresponding tothe range ofretardation factors used in the modeling (see Section 5.2 and Appendix E).
5.4.2 ComL)liance With Apolicant orRelevant and At)t)rot)riate Reauirements In accordance with CERCLA guidance, a remedial alternative is required to meet ARARs or be subject toa waiver. However, CERCLA recognizes thatARARs are not available for all circumstances encountered at Superfund sites. Federal or state advisories or guidance may be classified as TBC criteria. This section discusses the ability ofan alternative to meet ARARs and TBCs.

20000,317.10 - FS 0309111092 VI - 5-165.4.2.1 Alternative No. N- 1: No Action This alternative would likelynot achieve ARARS and PRGs because the groundwater extraction, treatment, and recharge currently provided by the NBCS would notbe provided.
5.4.2.1.1 Chemical-st)ecific ADWicable or Relevant and Apt)ropriate Reauirements The chemical-specific ARARs thatmust be complied with apply to potable drinking water supplies. The requirements are listedin Section 2.5, Table 2.5.2-1. This alternative is not expected tomeet thechemical- specific ARARs presented in Table2.5.2-1.
5.4.2.1.2 Action-sL)ecific Armlicable orRelevant and Avvrot)riate Reauirements Action-specific ARARs areactivity-based ortechnology-based requirements intended to set standards for the actions taken with respect to hazardous wastes orcontaminants at a site. Because there are no remedial actions except groundwater monitoring contemplated under this alternative, there are no action-specific ARARS. 5.4.2.1.3 Location-sL)ecific AnDlicable or Relevant and At)r)roL)riate ReQuirements Location-specific ARARs arerequirements placed on an area or the conduct ofactivities within anarea intended to protect such things as wetlands or sensitive ecosystems. Because there
are no remedial actions except groundwater monitoring contemplated underthis alternative, there would be no location-specific ARARs.
5.4.2.2 Alternative No. N-2: Continued Operation ofthe North Boundary Containment System With lmt)rovements as Necessary 5.4.2.2.1 Chemical-specific Applicable or Relevant and Appropriate Reauirement This alternative may achieve chemical-specific ARARs presented inTable 2.5.2-1, as estimated by the groundwater modeling timeframes to attain PRGs. An estimated time range of 15 to 30-plus years, corresponding to the range ofretardation factors used inthe modeling, is predicted for attainment ofgroundwater chemical-specific ARARs presented inTable 2.5.2-1. Thus, this alternative would meet chemical-specific ARARs for the low end ofthe modelestimated timeframe (corresponding tothe lowerretardation factors). However, for the high end
20000,317.10 - FS 0309111092 VI - 5-17ofthe estimated timeframe (corresponding to the higher retardation factors) chemical-specific ARARs would notbe attained within at least a 30-year timeframe.
5.4.2.2.2 Action-sr)ecific Awlicable orRelevant and Mvrot)riate Recluirements Appendix A presents the action-specific ARARs. 5.4.2.2.3 Location-st)ecific Applicable or Relevant and Apt)ropriate ReQuirements Appendix A presents the location-specific ARARs. 5.4.2.3 Alternative No. N-4: Interim Response Action A 5.4.2.3.1 Chemical-st)ecific AvOicable or Relevant andAwromiate Reauirements This alternative is expected tomeet orexceed all chemical-specific ARARs presented in Table 2.5.2- 1. Based on the estimates ofthe timeframe required forattainment ofPRGs as calculated by the groundwater modeling, ARARs would be attained in approximately 15 to 30 years. Thisrange inpredicted time to cleanup corresponds tothe lower and upper estimates of retardation factors used inthe modeling.
5.4.2.3.2 Action-svecific Aimlicable orRelevant and Arwromiate Recluirements Appendix A presents the location-specific ARARs.
5.4.2.3.3 Location-st)ecific ApOicable orRelevant andAwropriate Reauirements Appendix A presentsthe location-specific ARARs. 5.4.2.4 Alternative No. N-5: Expansion I to Interim Resvonse Action A
5.4.2.4.1 Chemical-si)ecific Armlicable orRelevant andAt)prot)riate ReQuirements Alternative No. N-5 is expected to meetor exceed all chemical-specific ARARs as listedin Table 2.5.2- 1. The timeframe for attainment ofgroundwater PRGs, asestimated bythe groundwater modeling, is 10to25 years. Thisrange ofestimated cleanup times corresponds to the lower and upperends, respectively, ofthe range ofretardation factors used inthe modeling.

20000,317.10 - FS 0309111192 VI - 5-185.4.2.4.2 Action-st)ecific AgWicable orRelevant and AI)r)ror)riate Reauirements Appendix A presents the action-specific ARARs.
5.4.2.4.3 Location-svecific Armlicable or Relevant and At)t)rot)riate Reauirements Appendix A presents the location-specific ARARs.
5.4.2.5 Alternative No. NW-1: No Action 5.4.2.5.1 Chemical-sr)ecific Am)licable orRelevant and AI)t)ror)riate Reauirements Alternative No. NW- I would likelynot achieve the chemical- specific ARARs listed in Table 2.5.2-1 because this alternative would notinclude operation ofthe NWBCS. The ARARs would notbe attained due to the lack ofgroundwater interception, treatment, and recharge contemplated under this alternative. As discussed previously, groundwater modeling was not performed for this alternative toestimate the time to attain PRGs. However, without continued operation ofthe NWBCS, the concentration ofcontaminants entering the Offpost OU fromRMA would exceed PRGs, and ARARs would notbe attained inthe Offpost OU.
5.4.2.5.2 Action-st)ecific Anglicable or Relevant and AI)t)rot)riate Reauirements Appendix A presents the action-specific ARARs.
5.4.2.5.3 Location-st)ecific At)r)licable orRelevant and Appropriate Recluirements Appendix A presents the location-specific ARARs.
5.4.2.6 Alternative No. NW-2: Continued Ot)eration ofthe Northwest Boundary Containment System With Imr)rovements asNecessary 5.4.2.6.1 Chemical-st)ecific Apolicable or Relevant and At)propriate ReQuirements This alternative is expected to meetor exceed all ARARs presented inTable 2.5.2- 1. The time required to attain groundwater PRGs, as estimated by the groundwater modeling, is three to eight years. This timeframe corresponds tothe lower and upper estimates ofthe retardation factors, respectively, used in the groundwater modeling.

20000,317.10 - FS 0309111092 VI - 5-195.4.2.6.2 Action-specific Agglicable orRelevant and Appropriate ReQuirements Appendix A presents theaction-specific ARARs. 5.4.2.6.3 Location-specific Agplicable orRelevant and Ar)propriate Reauirements Appendix A presents the location-specific ARARs. 5.4.3 Long-term Effectiveness and Permanence Under thiscriterion, alternatives were evaluated with respect tothe risk remaining at the site after the alternative has been implemented and the response objectives have been met.
5.4.3.1 Alternative No. N-1: No Action Alternative No. N-1 would not reduce the magnitude ofresidual risk associated with groundwater within the North Plume Group. No remedial action would be performed to reduce contaminant concentration levels orcontrol exposure pathways associated with groundwater within the North Plume Group. Because contaminated groundwater at the NBCS and withinthe North Plume Group would not be treated, the hypothetical risk and groundwater exposure pathways identified through the Offpost EA would continue to pose a threat tohuman health and the environment. 5.4.3.2 Alternative No. N-2: Continued Ot)eration ofNorth Boundary Containment System With Immovements as Necessary Alternative No. N-2 would reduce residual risk associated with groundwater within the North Plume Group through extraction, treatment, and recharge ofgroundwater exiting the RMA north boundary at the NBCS. Treated groundwater recharged at the NBCS would flush the North Plume Group portion ofthe UFS. Groundwater modeling indicates thatPRGs would be achieved within the North Plume Group in 15 to 30-plus years. Improvements to the system would be added asnecessary to correct system inadequacies. Monitoring and five-year site review would indicate when PRGs had been achieved. Residuals generated at the NBCS consist ofspent carbon, which would beremoved from the site and regenerated at aRCRA-permitted facility.

20000,317.10 - FS 0309111092 VI - 5-20Regeneration would destroy all adsorbed contaminants. Therefore, no risks would be associated with treatment residuals. 5.4.3.3 Alternative No. N-4: Interim Resvonse Action A Alternative N-4 would reduce residual risk associated with groundwater within the North Plume Group through extraction, treatment, and recharge ofgroundwater exiting the RMA north boundary at the NBCS and interception, extraction, and treatment ofgroundwater within the First Creek and northern paleochannels. Groundwater modeling indicates that the combined operation ofthe two systems would achieve PRGs within the North Plume Group in 15 to 30 years. Performance for the NBCS and IRA A systems would be monitored using the groundwater monitoring component ofthis alternative, and the data gathered during this program would be evaluated to allow for identification of any system inadequacies (e.g., contaminant bypass). Improvements tothe two systems would be implemented, as necessary, to correct system inadequacies. Long-term groundwater monitoring and sitereview would indicate whenPRGs had been achieved. Treatment residuals generated at the NBCS and IRAA treatment facilities would consist of spent carbon, which would beremoved from the site and regenerated at aRCRAapproved facility. Regeneration would destroy alladsorbed contaminants. Therefore, no risks would be associated with treatment residuals.
5.4.3.4 Alternative No. N-5: Expansion I to Interim Rest)onse Action A Alternative N-5 would reduce residual risk associated with groundwater within the North Plume Group through extraction, treatment, and recharge ofgroundwater exiting the RMA north boundary at the NBCS and interception, extraction, and treatment ofgroundwater within the First Creek and northern paleochannels. Groundwater modeling indicates that the combined operation ofthe two systems would achieve PRGs within the North Plume Group in 10to 20years. Performance for the NBCS, IRA A, and IRA A Expansion I systems would be monitored, andthe data gathered during the groundwater monitoring program would be evaluated to allow for identification of any system inadequacies (e.g., contaminant bypass). Improvements tothe two

20000,317.10 - FS 0309111092 VI - 5-21systems would be implemented, asnecessary, to correct system inadequacies. Long-term groundwater monitoring and sitereview would indicate when PRGs had been achieved. Treatment residuals generated at the NBCS and IRA A treatment facilities would consist ofspent carbon, which would be removed from the site andregenerated at a RCRA permitted facility. Regeneration would destroy all adsorbed contaminants. Therefore, no risks would be associated withtreatment residuals. 5.4.3.5 Alternative No. NW-1: No Action Alternative No. NW- I would not reduce the magnitude ofresidual risk associated with groundwater within the Northwest Plume Group. No remedial action would be performed to reduce contaminant concentration levels or control exposure pathways associated with groundwater within the Northwest Plume Group. Because contaminated groundwater at the NWBCS within the Northwest Plume Group would notbe treated, the hypothetical risk and potential groundwater exposure pathways identified in the Offpost EA would continue topose a threat to human health and the environment. 5.4.3.6 Alternative No. NW-2: Continued Overation ofthe Northwest Boundary Containment System With Improvements as Necessary Alternative No. NW-2 would reduce residual risk associated with groundwater within the Northwest Plume Group through extraction, treatment, and recharge ofgroundwater exiting the RMA northwest boundary at the NWBCS. Treated groundwater recharged at the NWBCS would flush the Northwest Plume Group portion ofthe UFS. Groundwater modeling indicates that PRGs would be achieved within the Northwest Plume Group in three toeight years. Improvements tothe system would be added as necessary tocorrect system inadequacies. Groundwater monitoring and five-year site review would indicate when PRGs had been achieved. Residuals generated at the NWBCS consist ofspent carbon, which would be removed from the site and regenerated at a RCRA permitted facility. Regeneration would destroy all adsorbed contaminants. Therefore, no risk would be associated withtreatment residuals.

20000,317.10 - FS 0309111092 VI - 5-22

### 5.4.4 Reduction ofToxicity. Mobility. orVolume Under CERCLA guidance for selecting remedial action alternatives, preference is given to remedial technologies that significantly reduce the toxicity, mobility, orvolume ofthe affected media through treatment. This evaluation criterion assesses the degree ofreduction oftoxicity, mobility, orvolume of the hazardous material. Considerations include an analysis ofthe extent of irreversibility ofthe treatment and the disposition oftreatment materials.
5.4.4.1 Alternative No. N-1: No Action Under Alternative No. N-1, no action would be performed to reduce the toxicity, mobility,
or volume ofgroundwater exceeding PRGs within the North Plume Group. Alternative No. N- I would notemploy any groundwater treatment process and thus would not address the statutory preference for treatment asa principal element. Thethreats associated withpotential groundwater exposure pathways would not be addressed.
5.4.4.2 Alternative No. N-2: Continued Overation ofthe North Boundary Containment System With Immovements asNecessary Continued operation ofthe NBCS wouldreduce the toxicity, mobility, and volume of groundwater migrating fromRMA to the Offpost OU. Groundwater mobility would be reduced by the slurry wall and extraction system. Groundwater toxicity would be reduced by treatment with activated carbon. Alternative No. N-2 would reduce the toxicity ofgroundwater and the volume ofgroundwater exceeding PRGs within the North Plume Group by flushing the UFS with water treated at the NBCS. Residuals remaining after treatment would consist ofspent carbon, which would be regenerated offsite. The regeneration process would destroy all adsorbed organic contaminants. Because Alternative No. N-2 would employ carbon adsorption, Alternative No. N-2 would satisfy the statutory preference for treatment asa principal element.
5.4.4.3 Alternative No. N-4: Interim Rest)onse Action A. Alternative No. N-4 would reduce the toxicity, mobility, and volume of groundwater within the North Plume Group and groundwater migrating fromRMA to the Offpost OU. The mobility
20000,317.10 -FS 0309111092 VI - 5-23ofcontaminated groundwater migrating fromRMA would be reduced by the slurry wall at the NBCS. The mobility ofcontaminated groundwater currently within the North Plume Group would be reduced through hydraulic containment in the northern andFirst Creek paleochannels. Alternative No. N-4 would reduce the toxicity ofgroundwater extracted at the NBCS and IRA A systems through carbon treatment. Performance monitoring at the NBCS indicates thatcarbon treatment iscapable ofreducing COCconcentrations to achieve PRGs. Thevolume ofcontaminated groundwater within the North Plume Group would bereduced by carbon treatment and recharge ofthe treated water at the NBCS and IRA A recharge systems. Residuals remaining after treatment would consist ofspent carbon, which would be regenerated offsite. Because Alternative No. N-4 would employ carbon treatment, Alternative No. N-4 would satisfy the statutory preference for treatment as aprincipal element. 5.4.4.4 Alternative No. N-5: Expansion Ito Interim Rest)onse Action A Alternative No. N-5 would reduce the toxicity, mobility, and volume ofgroundwater within the North Plume Group and groundwater migrating from RMA to the Offpost OU. The mobility ofcontaminated groundwater migrating from RMA would be reduced by the slurry wall at the NBCS. The mobility ofcontaminated groundwater currently within the North Plume Group would be reduced through hydraulic containment in the northern and First Creek paleochannels. Alternative No. N-5 would reduce the toxicity ofgroundwater extracted at the NBCS and Expansion I to IRA A Systems through carbon treatment. Performance monitoring at the NBCS indicates that carbon treatment is capable ofreducing COCconcentrations to achieve PRGs. The volume ofcontaminated groundwater within the North Plume Group would be reduced bycarbon treatment and recharge ofthe treated water at the NBCS, IRA A and Expansion I to IRA A recharge systems. Residuals remaining after treatment would consist ofspent carbon, which would be regenerated offsite. Because Alternative No. N-5 would employ carbon treatment, Alternative No. N-5 would satisfy the statutory preference for treatment as aprincipal element.

20000,317.10 - FS 0309111092 VI - 5-245.4.4.5 Alternative No. NW-1: No Action Under Alternative No. NW- 1,no action would be performed to reduce the toxicity, mobility, orvolume ofgroundwater exceeding PRGs within the Northwest Plume Group. Alternative No. NW- I would notemploy any treatment process options and thus would not address the statutory preference for treatment asa principal element. The threats associated with potential groundwater exposure pathways would not be addressed.
5.4.4.6 Alternative No. NW-2: Continued Operation ofNorthwest Boundary Containment System With Imi)rovements asNecessary Continued operation ofthe NWBCS would reduce the toxicity, mobility, and volume of groundwater migrating from RMA tothe Off'post OU. Groundwater mobility would be reduced by the slurry wall and extraction system. Groundwater toxicity would be reduced by treatment with activated carbon. Alternative No. NW-2 would reduce the toxicity ofgroundwater and the volume ofgroundwater exceeding PRGs within the Northwest Plume Group by flushing the UFS with water treated at the NWBCS. Residuals remaining after treatment would consist ofspent carbon, which would be regenerated offsite. The regeneration process would destroyall adsorbed organic contaminants. Because Alternative No. NW-2 would employ carbon adsorption, Alternative No. NW-2 would satisfy the statutory preference for treatment asa principal element.
5.4.5 Short-term Effectiveness The impacts ofthe alternative during the construction and implementation phase are assessed under thiscriterion. Factors tobe considered include protection ofthe community and workers during remedial operations, time required toimplement the alternative, and potential adverse environmental impacts that may result.

20000,317.10 - FS 0309111092 VI - 5-25

### 5.4.5.1 Alternative No. N-1: No Action 5.4.5.1.1. Protection ofCommunity andWorkers Short-term conditions atthe site wouldremain unchanged. Potential human health risks would continue to existand may increase dueto increased contaminant concentrations in the North Plume Group after operations atthe NBCS would have ceased. 5.4.5.1.2 Adverse Environmental Imt)acts Again, short-term conditions at thesite would remain unchanged. The concentration of contaminants ofaffected groundwater inthe North PlumeGroup would increase dueto discontinuing extraction and treatment ofgroundwater at theNBCS. 5.4.5.1.3 Implementation Period There is no implementation period for Alternative No. N- 1. 5.4.5.2 Alternative No. N-2: Continued Operation ofNorth Boundary Containment System 5.4.5.2.1 Protection ofCommunity and Workers Short-term conditions atthe sitewould remainunchanged. Potential human health risks would decrease as the concentration ofcontaminants inaffected groundwater decreased due to extraction, treatment, and recharge atthe NBCS. Minimal impacts are expected. 5.4.5.2.2 Adverse Environmental Impacts Short-term conditions atthe site would remain unchanged. Theconcentration of contaminants in affected groundwater would decrease over time dueto extraction and treatment ofgroundwater atthe NBCS. Minimal impacts areexpected. 5.4.5.2.3 Imolementation Period There isno implementation period for continued operation ofthe NBCS.

20000,317.10 

### -FS 0309111092 VI - 5-265.4.5.3 Alternative No. N-4: Interim Rest)onse Action A 5.4.5.3.1 Protection ofCommunity and Workers The short-term impact on the hypothetical risks tothe community and workers would be minimal during construction ofthe IRA A facility. Past experience atthe Offpost OU indicates that workers would be adequately protected by adhering to standard health and safety practices. Trenching operations would be performed using theone-pass method, and thus worker exposure would be minimized. Protection ofthe community and workers' health would be achieved through adherence to an approved health and safety plan.
5.4.5.3.2 Adverse Environmental Imr)acts Minimal adverse impacts to air quality would result fromdust generation during construction. Dust control measures would limit these impacts. No adverse environmental impacts associated with groundwater are foreseen during construction ofIRA A. Water generated during trench construction dewatering activities would be treated and recharged tothe UFS.
5.4.5.3.3 Implementation Period The implementation for Alternative No. N-4 would proceed as follows: (1) 10months to construct the treatment facility, (2) 4 months forthe extraction, recharge, and conveyance systems for the northern paleochannel, (3) 2 months ofsystem start-up, and (4) 4 months ofconstruction of First Creek paleochannel extraction wells and recharge trenches. Therefore, approximately 20 months would be required from initiation ofconstruction tofull facility operation.
5.4.5.4 Alternative No. N-5: Expansion I to Interim Resr)onse Action A 5.4.5.4.1 Protection ofCommunity and Workers Short-term conditions associated with this alternative are similar to those associated with Alternative No. N-4.
5.4.5.4.2 Adverse Environmental Imnacts Environmental impacts are the same as Alternative No. N-4.
20000,317.10 -FS 0309111092 VI - 5-275.4.5.4.3 Implementation Period Approximately one additional year would berequired for the implementation period of Alternative No. N-5 compared to Alternative No. N-4. The additional time would berequired for design and construction ofthe additional extraction wells, recharge trenches, and conveyance system. Therefore, the total implementation period for Alternative No. N-5 would be approximately 32 months. 5.4.5.5 Alternative No. NW-1: NoAction 5.4.5.5.1 Protection of.Community and Workers Short-term conditions at the site would remain unchanged. Potential human-health risks would continue to exist and may increase due to increased contaminant concentrations in the Northwest Plume Group after operations at the NWBCS would have ceased.
5.4.5.5.2 Adverse Environmental Imi)acts Short-term conditions at the site wouldremain unchanged. The concentration ofcontaminants in affected groundwater in the Northwest Plume Group would increase due to discontinuing extraction and treatment at the NWBCS. 5.4.5.5.3 ImOementation Period There is no implementation period for Alternative No. NW-1.
5.4.5.6 Alternative No. NW-2: Continued Or)eration ofNorthwest Boundary Containment System With Imnrovements asNecessary 5.4.5.6.1 Protection ofCommunity and Workers Short-term conditions at thesite would remain unchanged. Potential human-health risks would decrease as the concentration ofcontaminants in affected groundwater decreased due to extraction, treatment, and recharge at the NWBCS. Minimal impacts are expected.

20000,317.10 - FS 0309111092 VI - 5-28

### 5.4.5.6.2 Adverse Environmental Impacts Short-term conditions atthe sitewouldremain uncharged. The concentration ofcontaminantswithin the Northwest PlumeGroup would decrease overtime due toextraction, treatment, and recharge atthe NWBCS. Minimal impacts are expected. 5.4.5.6.3 ImWementation Period There isnoimplementation period forAlternative No.NW-2. 5.4.6 ImWementabilitv Twomajor issuesare addressed when assessing the implementability ofa remedialaction alternative: 1. Technical feasibility - Theability toimplement and constructthe technology, the reliability ofthe technology, and the abilityto monitorthe effectiveness oftheremedy 2. Administrative feasibility - The effort andresources required to obtain approval from regulatory agencies 5.4.6.1 Alternative No.N- 1: No Action The only technical aspectofthe NoAction alternative isthe implementation ofamonitoring programto re-evaluate the sitein five years. Groundwater monitoringwells currently exist, and
no additional wells wouldbe installedto monitorconditions inthe system. Theadministrative feasibility ofobtaining approval fromthe agencies is expected tobe minimalforthis alternative. 5.4.6.2 Alternative No. N-2: Continued Ot)eration ofNorth Boundary Containment SystemWith Improvements as Necessary The NBCS has treated approximately 125 million gallonsofgroundwater per yearwith activated carbon since 1979. Effluent datafrom the NBCS have shown thatcontaminant concentrations in the effluent attain OffpostOU PRGs. Operational assessments onthe effectiveness ofthe system are issued tothe Organizations andState annually. Therefore, technical implementability isjudged tobe high.

20000,317.10 - FS 0309111192 

### VI - 5-295.4.6.3 Alternative No. N-4: Interim Rest)onse Action A IRA A consists ofextracting and treating contaminated groundwater and recharging treated groundwater toprevent plume migration. Theability toextract groundwater with extraction wells, treat contaminated groundwater with carbon to remove organic compounds, and recharge treated groundwater has been demonstrated tobe a reliable technology at the NBCS, NWBCS, and ICS. The ability to monitor the effectiveness ofthese technologies by monitoring wells has also been demonstrated at the three existing boundary systems. An Implementation Document for IRA A (HLA, 1991b) has been finalized.
5.4.6.4 Alternative No. N-5: Exoansion I toInterim Resoonse Action A IRA A consists ofextracting and treating contaminated groundwater and recharging treated groundwater to prevent plume migration. The abilityto extract groundwater withextraction wells, treat contaminated groundwater with carbon toremove organic compounds, and recharge treated groundwater has been demonstrated tobe areliable technology at the NBCS, NWBCS, and ICS. The ability to monitor the effectiveness ofthese technologies by monitoring wells has also been demonstrated at the three existing boundary systems. The Implementation Document for IRA A has been finalized. However, an additional remedial design andconstruction phase would be required to implement the additional systems included under Alternative No. N-5.
5.4.6.5 Alternative No. NW-1: No Further Action The only technical aspect ofthe No Action alternative is the implementation ofa monitoring program to re-evaluate the site in five years. Groundwater monitoring wells currently exist, and
no additional wells would be installed to monitor conditions inthe system. The administrative feasibility ofobtaining approval fromthe agencies is expected to be minimal for this alternative.

20000,317.10 - FS 0309111092 VI - 5-305.4.6.6 Alternative No. NW-2: Continued Ot)eration ofthe Northwest Boundary Containment System With Imr)rovements asNecessary The NWCS has treated approximately 450 million gallons ofgroundwater per year with activated carbon since 1981. Effluent data fromthe NWBCS have shown that contaminant concentrations in the effluent attain Offpost OU PRGs. Operational assessments on the effectiveness ofthe system areissued annually. 5.4.7 Cost The following subsections present summary cost estimates for each ofthe alternatives. Detailed costestimates areincluded in Appendix F.
5.4.7.1 Alternative No. N-1: No Action Estimated costs for Alternative No. N-1 are developed in Appendix Fand are presented in Table Fl. Cost evaluation of Alternative No. N- I includes groundwater monitoring and five-year sitereview. The capital costs, long-term O&M costs, and total present worth costs for Alternative No. N-1 are presented below:

- Total Capital Costs = $ -0-

- Total Long-term Operation and Maintenance Costs = $4.1 - 6.0 million

- Total Present Worth Costs = $4.1 - 6.0 million
5.4.7.2 Alternative No. N-2: Continued Ot)eration ofthe North Boundary Containment System With Improvements as Necessary Estimated costs forthis remedial action alternative are developed in Appendix F and are presented in Table F3. Long-term O&M costs were extended for 30 yearsbecause it isunknown when groundwater approaching RMA boundaries will comply with Offpost PRGs. The capital costs, long-term operation and maintenance costs, and total present worth costsfor Alternative No. N-2 are presented below:

- Total Capital Costs = $ -0-

- Total Long-term Operation and Maintenance Costs = $30.6 - 32.5 million

- Total Present Worth Costs =$30.6 - 32.5 million
20000,317.10 - FS 0309111092 VI - 5-315.4.7.3 Alternative No. N-4: Interim Response Action A Estimated costsfor Alternative No. N-4 aredeveloped in Appendix F and are presented in Table F4. The capital costs, long-term operation and maintenance costs, and total present worth costs for Alternative No. N-4 are presented below:

- Total Capital Costs =$ 16.7 million

- Total Long-term Operation and Maintenance Costs = $39.8 - 46.4 million

- Total Present Worth Costs =$56.5 - 63.1 million
5.4.7.4 Alternative No,.N-5: Expansion Ito Interim Resoonse Action A Estimates costs for Alternative No. N-5 are developed inAppendix F and presented in Table F5. The capital costs, long-term operation andmaintenance costs, and total present worth costs for Alternative No. N-5 arepresented below:

- Total Capital Costs = $19.4 million

- Total Long-term Operation and Maintenance Costs = $36.9 - 43.6 million

- Total Present Worth Costs = $56.2 - 63.0 million 5.4.7.5 Alternative No. NW- 1: No Action Estimated costs for Alternative No. NW- I are developed inAppendix F and are presented inTable F8. The capital costs, long-term operation and maintenance costs, and total present worth costs for Alternative No. NW- I are presented below:

- Total Capital Costs = $ -0-

- Total Long-term Operation and Maintenance Cost = $0.6 - 1.3 million

- Total Present Worth Costs = $.6 - 1.3 million 5.4.7.6 Alternative No. NW-2: Continued Overation ofthe Northwest Boundary Containment System With Improvements as Necessary Estimated costs for Alternative No. NW-2 are developed in Appendix F and are presented in Table F8. The capital costs, long-term operation and maintenance costs, and total present worth costs for Alternative No. NW-2 are presented below:

20000,317.10 - FS 0309111092 VI - 5-32- Total Capital Costs = S -0-

- Total Long-term Operation and Maintenance Cost = $12.4 - 13.1 million

- Total PresentWork Costs = $12.4 - 13.1 million 5.5 COMPARATIVE ANALYSIS OF REMEDIAL ALTERNATIVES This section provides acomparative analysis ofthe remedial action alternatives evaluated using the criteria presented in Section 5.1. The purpose ofthis comparative analysis is to identify theadvantages and disadvantages ofeach alternative relative to one another. Critical tradeoffs
were identified to assist'in thedecision ofwhich alternative to select as the preferred remedy. State and community acceptance is not included in this analysis because these criteria cannot be assessed until the state and public comment period has been concluded. 5.5.1 Comparison ofAlternatives - North Plume Group A briefcomparison ofeach alternative with respect to the evaluation criteria is presented below. As previously discussed, each ofthe alternatives incorporates commonalities including the Army's commitment toprovide alternative water to any future identified usersofgroundwater exceeding PRGs (i.e., exposure control), groundwater monitoring, five-year site review, and continued operation ofthe boundary containment systems (with the exception ofthe No Action alternative). Accordingly, these components ofthe alternatives were notevaluated in the comparative analysis. A comparison ofthe alternatives follows. 5.5.1.1 Overall Protection ofHuman Health and the Environment Overall protection ofhuman health and the environment would be provided by all alternatives with the exception ofAlternative No. N-1. Alternative Nos. N-4 and N-5 would provide greater protection than Alternative No. N-2 because extraction, treatment, andrecharge systems within the North Plume Group would decrease contaminant concentrations andreduce hypothetical risks within ashorter time period. Although groundwater modeling indicates thatAlternative No. N-5 would achieve PRGs in a shorter time period than Alternative No. N-4, the two alter20000,317.10 - FS 0309111092 VI - 5-33natives are essentially equivalent withrespect toproviding protection ofhuman health andthe environment for the reasons thatfollow:

- The groundwater monitoring program proposed under Alternative No. N-4 would provide full-scale system performance information thatcould*be used to identify any necessary or beneficial improvements to the system and toprovide information on optimal location of these additional systems without incurring additional capital costs.

- Uncertainties inherent in thegroundwater modeling results make the prediction of remediation times imprecise. 5.5.1.2 Compliance With Apolicable or Relevant and Arwropriate Reauirements Compliance with chemical-specific ARARs would be achieved byall alternatives withthe exception ofAlternative No. N-1. Groundwater modeling indicates thatchemical-specific ARARs would be achieved in the shortest time by Alternative No. N-5, followed by Alternative No. N-4, followed by Alternative No. N-2. Compliance with location-specific and actionspecific ARARs selected in Appendix A will be achieved by all treatment alternatives. Because no remediation would take place under Alternative No. N- 1,there would beno federal and state location- or action-specific ARARs. 5.5.1.3 Lonp,-term Effectiveness and Permanence Comparison ofalternatives with respect to long-term effectiveness and permanence indicates that the three treatment alternatives are essentially equivalent. All ofthe alternatives with the exception ofthe No Action alternative wouldreduce hypothetical risk and address exposure pathways through reducing COCconcentrations in North Plume Group. Underthe No Action alternative, hypothetical risks would likelyincrease afterceasing operations at the NBCS. Dueto inherent uncertainties ingroundwater modeling, predicted differences between treatment alternatives in time to achieve PRGs may not be reliable. 5.5.1.4 Reduction inToxicity. Mobility. and Volume All alternatives with the exception ofthe No Action alternative would reduce the toxicity, mobility, and volume ofcontaminated groundwater entering the Offpost OU north ofthe NBCS. Groundwater contaminant concentrations under the No Action alternative would likelyincrease.

20000,317.10 - FS 0309111092 VI - 5-34The two alternatives with active remedial components inthe North Plume Groups, Alternative Nos. N-4 and N-5 would provide the greatest reduction in toxicity, mobility, and volume of contaminated groundwater, through extraction, treatment, and recharge. As stated previously, the uncertainty associated with the remediation timeframes estimated by the groundwater modeling suggests that, in practical terms, the estimated timeframes for both Alternative Nos. N-4 and N-5
are equivalent. Further, the groundwater monitoring component ofAlternative No. N-4 would allow forfull-scale performance data regarding the reduction ofcontaminant concentrations. Such data would be necessary toassess the need for and optimum location ofany modifications to Alternative No. N-4. 5.5.1.5 Short-term Effectiveness The assessment ofthe alternatives with respect to short-term adverse environmental impacts, implementation period, and protection ofthe community and workers shows that the No Action alternative and Alternative No. N-2 are slightly better than the alternatives with active remediation components. However, during the implementation period, Alternative Nos. N-4 and N-5 would be able to minimize adverse short-term impacts through standard engineering controls and adherence to standard health and safety practices.
5.5.1.6 Implementability All alternatives evaluated would betechnically feasible to implement. The No Action alternative and Alternative No. N-2 would be theeasiest to implement with respect to technical feasibility because the monitoring wellshave already been installed and the NBCS system is currently operational. Alternative Nos. N-4 and N-5 would bereadily constructed. However, Alternative No. N-5 would require additional design, and Alternative No. N-4final design has already been approved. All treatment alternatives would use carbon treatment, which hasbeen demonstrated at the boundary containment systems to be a reliable groundwater treatment process option. Groundwater monitoring is a component ofall four alternatives and would provide information regarding the effectiveness ofeach alternative.

20000,317.10 - FS 0309111092 VI - 5-35All alternatives withthe exception ofthe No Action alternative would be administratively feasible. itis unlikely that the regulatory agencies orthe public would accept shutdown ofthe NBCS as proposed under the No Action alternative. Additionally, the Army does notintend to
cease operating theNBCS. No permits would be required under the remaining treatment alternatives. Each ofthe three treatment alternatives would meet federal and state substantive requirements for recharging the treated groundwater tothe UFS. The No Action alternative and Alternative No. N-2 would not require additional equipment and services. The implementation ofAlternative Nos. N-4 and N-5 would not belimited with respect to availability of services and materials. Contractors with theequipment and knowledge to construct and implement these two alternatives are readily available.
5.5.1.7 Cost The total present worth costs range from $4.1 - 6.0 million for No Action to $56.5 - 63.1 million for Alternative No. N-4. The present worth costs are nearly identical for Alternative Nos. N-4 and N-5 because the additional capital expenditures required for Alternative No. N-5
are balanced by the additional O&M costs incurred through the estimated 10-yeardifferences in remediation timeframe for Alternative No. N-4. The additional capital expenditure ofapproximately $2.7 million for Alternative No. N-5 as compared to Alternative No. N-4 is probably notjustified inlight of the need to collect full-scale data on contaminant transport and actual plume remediation timeframes through the intensive short-term monitoring program. Such data would allow for more informed decision-making asto the potential need for and placement ofimprovements to Alternative No. N-4.
5.5.2 Comparison ofAlternatives - Northwest Plume Grout) A brief comparison ofeach alternative with respect tothe evaluation criteria is presented below. As previously discussed, Alternative No. NW-2 incorporates commonalities, including the Army's commitment to provide alternative water to any future identified user ofgroundwater exceeding PRGs (i.e., exposure control), groundwater monitoring, five-year site review, and

20000,317.10 - FS 0309111092 VI - 5-36continued operation ofthe NWBCS. The No Action alternative includes only monitoring and the five-year sitereview. 5.5.2.1 Overall Protection ofHuman Health and the Environment The No Action alternative would notbe protective ofhuman health andthe environment because operation ofthe NWBCS would be ceased. Overall protection ofhuman health and the environment would beprovided by Alternative No. NW-2. Alternative No. NW-2 would decrease contaminant concentrations and reduce hypothetical risks associated with groundwater entering the Offpost OU from RMA. Recharge ofgroundwater treated at the NWBCS would reduce the contaminant concentrations in the Northwest Plume Group UFS through flushing. Groundwater modeling estimates that Alternative No. NW-2 would achieve PRGs in approximately three to eight years. The No Action alternative would likely not achieve PRGs, because the NWBCS would notbe operated. 5.5.2.2 Comr)liance With Applicable or Relevant and Am)rovriate ReQuirements Compliance with chemical-specific ARARs would be achieved only by Alternative No. NW-2. Groundwater modeling indicates that chemical-specific ARARs would be achieved in approximately three toeight years for this alternative. Alternative No. NW-2 would comply with location- and action-specific ARARs selected in Appendix A. 5.5.2.3 Lonp-term Effectiveness and Permanence Comparison ofthe alternatives with respect to long-term effectiveness and permanence indicates that Alternative No. NW-2 reduces hypothetical risk and addresses exposure pathways through reducing COCconcentrations in Northwest Plume Group. Under the No Action alternative, hypothetical risks would likely increase after operations at the NWBCS cease. 5.5.2.4 Reduction inToxicity, Mobility. and Volume Alternative No. NW-2 would reduce the toxicity, mobility, and volumeofcontaminated groundwater entering the Offpost OU north ofthe NWBCS. Groundwater contaminant concen20000,317.10 -FS 0309111092 VI - 5-37trations under the No Action alternative would likely increase; thus toxicity, mobility, and volume would notbe reduced. 5.5.2.5 Short-term Effectiveness The assessment ofthe two alternatives with respect to short-term adverse environmental impacts, implementation period, and protection ofthe community and the workers shows that the No Action alternative and Alternative No. NW-2 are essentially equivalent except that the discontinued operation oftheNWBCS, as part ofthe No Action alternative, has an adverse environmental impact. Neither alternative, with theexception noted above, has significant short-term effectiveness issues. 5.5.2.6 Implementability Both alternatives evaluated would betechnically feasible to implement. The No Action alternative and Alternative No. NW-2 would be implementable with respect to technical feasibility because the monitoring wells have already been installed and the NWBCS iscurrently operational. Alternative No. NW-2would use carbon treatment, which has been demonstrated at the boundary containment systems to be a reliable groundwater treatment process option. Groundwater monitoring is acomponent ofboth alternatives and would provide information regarding the effectiveness ofeach alternative. The No Action alternative would not be administratively feasible. It isunlikely that the regulatory agencies orthe public would accept shutdown oftheNWBCS as proposed under the No Action alternative. Additionally, the Army does notintend to cease operating the NWBCS. No permits would be required foreither alternative. Alternative No. NW-2 would meet federal and state substantive requirements for recharging the treated groundwater tothe UFS. Neither alternative would require additional equipment and services.
5.5.2.7 Cost The total present worth costs range from $0.6 - 1.3 million for the No Action alternative to $12.4 - 13.1 million for Alternative No. NW-2. 20000,317.10 -FS 0309111092 VI - 5-385.5.3 Ranking ofAlternatives - North Plume Grow)and Northwest Plume Group A summary ofthe detailed analysis foreach alternative and overall ranking are presented in Tables 5.5.3-1 and 5.5.3-2.

20000,317.10 - FS 0309111092 VI - 5-39

L6.0 SELECTION OFTHE PREFERRED SITEWIDE ALTERNATIVE
The following sections select the preferred sitewide alternative, describe the components of the preferred alternative, illustrate consistency with the requirements ofCERCLA and the NCP, and present a summary statement. 6.1 IDENTIFICATION OFTHE PREFERRED SITEWIDE ALTERNATIVE The preferred sitewide alternative consists ofimplementation ofAlternative No. N-4 for remediation ofgroundwater inthe North Plume Group and Alternative No. NW-2 for remediation ofgroundwater in the Northwest Plume Group. Each ofthecomponents ofthe preferred sitewide alternative is described below in Sections 6.1.1.1 and 6.1.1.2. This section describes the rationale for the selection ofthepreferred alternative. Specifically, key components ofthe preferred alternative are identified, and compliance with CERCLA and the NCP aredocumented. The comparative analysis ofthe groundwater alternatives presented in Section 5.5 discussed the relative merits ofthe alternatives with respect tothe seven detailed analysis criteria for both the North and Northwest Plume Groups. Due tothe approximate nature ofthe models and because the Offpost OU is characterized by considerable conceptual model and parameter uncertainty, none ofthe modeling results should be viewed as accurate predictions offuture contaminant distribution. Rather, the models and modeling results should beviewed as tools for assessing the relative merits ofremedial alternatives during the detailed analyses. The detailed analysis of alternatives showed thatfor overall protectiveness, compliance with ARARs, effectiveness, and reduction oftoxicity, mobility, and volume, Alternative Nos. N-4 and N-5 are superior to Alternative Nos. N- Iand N-2. Alternative No. N-4 isequal toAlternative No. N-2 in implementability. Alternate No. 4 is more readily implementable than Alternative Nos. N- I and N-5 because Alternative No. N- I would notbe administratively feasible, and Alternative No. N-5 would require a second design and construction phase. Alternative Nos. N-4 and N-5 are approximately equal incost when compared toeach other andhigher in costwhen compared to
20000,317.10 -FS 0309111092 VI - 6-1Alternative Nos. N-I and N-2. Therefore, Alternative Nos. N-4 and N-5 were identified as being superior to Alternative Nos. N- I and N-2. Direct comparison ofAlternative Nos. N-4 and N-5 is presented below. Alternative No. N-4 was demonstrated to besuperior to Alternative No. N-5 with respect to the seven detailed analysis criteria for thereasons thatfollow:

- Groundwater modeling estimated that Alternative No. N-5 would achieve PRGs in a shorter time than Alternative No. N-4. However, the two alternatives are essentially equivalent with respect toproviding protection ofhuman health and the environment because " 71ýhe intensive'short-term groundwater monitoring program included under Alternative

No. N-4 would provide full-scale system performance information that could be used to identify any necessary orbeneficial improvements to the system and to provide information onoptimal location of theseadditional systems without incurring additional capital costs. " Uncertainties inherent in thegroundwater modeling results make the prediction of

absolute remediation timeframes imprecise.

- The additional capital expenditure ofapproximately $2.7 million for Alternative No. N-5 as compared to Alternative No. N-4 isprobably notjustified in light ofthe need to collect full-scale data on contaminant transport and actual plume remediation timeframes through the intensive short-term monitoring program. These data will allow for informed decisions as tothe possible need forand placement ofimprovements to Alternative No. 4.

- Alternative No. N-4 ismore readily implementable than Alternative No. N-5 because implementation ofAlternative No. N-5 would require additional remedial design and construction.

- Alternative Nos. N-4 and N-5 areessentially equivalent with respect toevaluation of compliance with ARARs, long-term effectiveness and performance, short-term effectiveness, and reduction in mobility, toxicity, or volume. Therefore, Alternative No. N-4 isthe selected alternative for the North Plume Group. Alternative No. NW-2 ranks above Alternative No. NW-1 in all criteria except cost; however, the additional costs are notprohibitive in light ofthe reduction intime for remediation. Therefore, Alternative No. NW-2 is the selected alternative for the Northwest Plume Group. In summary, Alternative No. N-4 and Alternative No. NW-2 are the Army's preferred alternatives for the North and Northwest Plume Groups, respectively.

20000,317.10 - FS 0309111092 VI - 6-26.1.1 Preferred Sitewide Alternative Descrit)tion A description ofthe preferred sitewide remedial alternative is presented in this section. The preferred alternative is composed ofthefollowing components addressing remediation of groundwater in the North and the Northwest Plume Groups: NorthPlume Group - IRA A (Alternative No. N-4) Northwest Plume Group - Continued Operation ofthe NWBCS with Improvements as Necessary, (Alternative No. NW-2) 6.1.1.1 Alternative No. N-4: Interim Remonse Action A The IRA A system designhas been approved, andconstruction began inNovember 1991, with system start-up scheduled forapproximately January 1993. Foradditional detail concerning design specifics, the reader is referred tothe Final Implementation Document for the Groundwater Intercept andTreatment System North ofRMA (HLA, 1991b). The major components of this alternative include:

- Removal ofcontaminated UFS groundwater north ofthe RMA boundary in the First Creek andnorthern paleochannels, using IRA A groundwater extraction wells

- Treatment ofthe organic COCspresent in the groundwater, using carbon adsorption

- Recharge oftreated groundwater tothe UFS, using IRA A wells and trenches

- Continued operation ofthe NBCS

- Improvements to IRA A and the NBCS and IRA A, as necessary

- Long-term groundwater monitoring

- Five-year site reviews

- Exposure control The groundwater intercept and treatment system comprising IRA A is an aggressive array of extraction wellsand recharge trenches inthe northern and First Creek paleochannels. Thesystem is configured toextract and treat UFS groundwater exceeding PRGs andrecharge the treated groundwater. Figure 3.5.1.4-1 presents the placement ofextraction wells and recharge wells in the northern paleochannel and the placement ofextraction wells and recharge trenches in the First
20000,317.10 -FS 0309111092 VI - 6-3Creek paleochannel. The location ofthe treatment plant is also shown. The northern paleochannel collection system consists ofa total of 12extraction wells oriented across the paleochannel perpendicular to the direction ofgroundwater flow and placed approximately 200 feet apart. The recharge system in the northern paleochannel consists of24 recharge wells spaced 100 feet apart and placed parallel to and approximately 300 feet downgradient ofthe collection system. The First Creek paleochannel collection system consists offive extraction wells spaced 200 to 500 feet apart. The wells are placed along the axisofthe groundwater flowpathway. Recharge trenches
are placed such thatfour ofthesix trenches are parallel to the flow axis and located on the margins ofthe paleochannel, with the remaining two trenches located downgradient ofthe extraction well system and oriented perpendicular tothe flow axis. The system is designed to extract and treat an average flow of300 gpm from the northern paleochannel, an average flow of 180 gpm from the First Creek paleochannel and a peak flow of 1.5 times theaverage flow. The treatment plant basic process flow includes influent storage, pumping, bag filtration for particulate removal, granular activated carbon (GAC) adsorption, multimedia filtration, treated-water storage, treated-water pumping, and final bag filtration. A total ofapproximately 250 million gallons peryear would be treated by the IRA A facility at the above-referenced average flows. Operation ofthe NBCS component ofthisalternative would treat approximately 125 million gallons per year. Thus, a total ofapproximately 375 million gallons ofUFS groundwater would betreated annually to attain Offpost OU PRGs (Table 2.5.2-3) under this alternative. An intensive short-term monitoring component would beincluded in this alternative as part ofthe long-term monitoring program. For costing purposes, it isassumed that thisprogram would consist ofa network ofapproximately 60 wells tobe sampled semiannually for two to three years, beginning with the IRA A system start-up. The intensive monitoring program would allow the collection and subsequent interpretation ofperformance data for the full-scale operation ofboth the IRA A system and the NBCS. Thedata will also be used toassess the need forany

20000,317.10 - FS 0309111092 VI - 6-4improvements tothe systems. The acquisition ofsuch data would allow for increased accuracy in assessing the response ofthe UFS groundwater to the NBCS and IRA A remediation systems. A brief description ofthe IRA A treatment plant process units and their purpose follows. The influent tank has acapacity of20,000 gallons and isused forflow equalization and storage. Centrifugal pumps are used toconvey the untreated groundwater fromthe influent storage tank through the remainder ofthe treatment plant. Twobag filters, each with the capacity ofthe average flow, are used forparticulate removal before thecarbon units. Three upflow carbon contractors are used, each with a design flow of360 gpm. One ofthe carbon units is held in
reserve for reliability and operational flexibility. Untreated groundwater is pumped upward through the contractors and 2000-pound "pulses"ofGAC are moved downward through the unit. Multimedia filters, composed of sand with anthracite, will be used after GAC treatment, to
remove carbon fines. A treated-water storage basin with a total capacity of66,000 gallons is provided for storage offireprotection water, backwashing capacity for the carbon and multimedia filters, and flow equalization. Another set ofbag filters isprovided for final particulate removal before treated-water recharge. The NBCS collection system consists ofa 6740-foot-long soil bentonite barrier and 54 extraction wellsupgradient from the soilbentonite barrier. The recharge system initially consisted of38 recharge wells downgradient from the soilbentonite barrier. The treatment system is made up ofa cartridge-type prefilter toremove particulates from the water, three 30,000-pound upflow pulsed-bed carbon adsorbers, a carbon transfer and storage vessel, and cartridge-type and bag-type postfilters. The NBCS system was constructed in four phases. The first phase consisted ofa pilotsystem thatbegan operation inJune 1978. The second phase was
an expansion ofthe pilot system and began operation in January 1982. The third phase consisted of an additional ten recharge trenches thatbegan operation in December 1988. The fourth phase
was the addition offive recharge trenches, with operation beginning May 1990.

20000,317.10 - FS 0309111092 VI - 6-5

### 6.1.1.2 Alternative No.NW-2: Continued Ot)eration ofthe Northwest Boundary ControlSystem With Imi)rovements as Necessary This section summarizes the Continued Operationofthe NWBCS withImprovements as Necessary alternative. For additional detailsofthe extraction/recharge systems, therecent upgrades to the system, and the treatment facility atthe NWBCS, thereader is referred to the following reports: Implementation Document for NWBCS Short-Term Improvements IRA (MKES, 1990a); NWBCS Long-Term Improvements IRA B(ii)Final Assessment Document(WWC, 1991a); Proposed Decision DocumentNWBCS RMA Long-Term Improvements IRA(WWC, 1991b); and Report ofFieldInvestigations, Assessment, and Final DecisionDocument forthe NWBCS Short-Term Improvements IRA (MKES, 1990b). The majorcomponents ofthis alternative include:

- Continued operation ofthe NWBCS

- Improvements tothe NWBCS asnecessary

- Long-term groundwater monitoring

- Five-year site reviews

- Exposure control The NWBCS was putintooperation in 1984. Thecollection system consisted of 15extraction wells and aslurry wall approximately 1600feet inlength. The recharge system consisted of 21 downgradient recharge wells. Theexisting slurry wall was extended 665 feet tothe northeast toprevent contaminant bypassin July 1990 (MKES, 1990a), andone additional extraction well and two additional recharge wells were added. Three additional extraction wells andfour additional recharge wells were addedto thesouthwest portion ofthe NWBCS in August 1991. The NWBCS treatment system consistsofacollection sump, inlettransfer pumps, inletbag filters, three upflow carboncontractors, effluent bagfilters, effluent sump, and effluenttransfer pumps (WWC, 1991a). Theapproximate treatment rate is850 gpm orapproximately 450million gallonsannually.

20000,317.10 -FS 0309111092 VI- 6-66.2 CONSISTENCY WITHTHE REQUIREMENTS OFCERCLA AND THE NATIONAL CONTINGENCY PLAN

The following subsections describe the principal threats addressed by the preferred sitewide alternative, compliance with the statutory requirements ofCERCLA, and consistency with the NCP. 6.2.1 Princival Threats Addressed by the Preferred Sitewide Alternative The preferred sitewide alternative will result inthe remediation ofthe Offpost OU consistent with RAOs (Section 2.4) and PRGs (Section 2.5). The principal threat posed by the site, namely contaminated groundwater in the North and Northwest Plume Groups, will be addressed by implementation ofthe preferred alternative through groundwater extraction, treatment, and recharge. 6.2.2 Consistency With the Statutory Reauirements ofCERCLA in Section 121 The statutory requirements ofCERCLA section 121, asdescribed below, and the statutory preference for treatment are met through implementation ofthis alternative. 6.2.2.1 Protection ofHuman Health and the Environment The groundwater remedial actions described above in Sections 6.1.1.1 and 6.1.1.2 (and in Sections 3.0 and 5.0) will permanently address the principal threats tohuman health and the environment for the Offpost OU through treatment to reduce the toxicity, mobility, and volume of groundwater contaminants. It should be recognized, however, that studies conducted at other sites (by EPA and by others) have indicated that itmay not always be possible to reach groundwater PRGs due tothe limitations oftechnology used toassess groundwater hydrogeological properties, the technology used to estimate aquifer remediation timeframes, and the technology used to extract and recharge groundwater. Ifitbecomes apparent during operation ofthe groundwater treatment systems that the UFS groundwater contaminant levels are remaining constant at levels higher than the Offpost OU groundwater PRGs, the design and operation ofthe systems may require reevaluation.
20000,317.10 -FS 0309111092 VI - 6-7

### 6.2.2.2 Comliance With At)t)licable or Relevantand Awromiate Reauirements Groundwater PRGsarebased onchemical-specific ARARs forthose chemicals having promulgated standards and onHBCforthose chemicals withoutARARs (Table2.5.2-3). The preferred sitewide alternative is expectedto attainor exceed chemical-specific ARARs. The preferred sitewide alternative will alsocomplywith location-specific and action-specific ARARs identified in Appendix A. 6.2.2.3 Cost-effectiveness Thesitewide preferred alternative is cost-effective in itsapproach to remediating Offpost OU groundwater. The groundwatermonitoring program willletthe Army more accurately assess the contaminant removal rates asafunction oftime, usingthe full-scale dataavailable during operation ofbothIRA A and the NBCS. Theanalysis ofthisdata will allowfor cost-effective decisions regarding anyfuture improvements thatmay berequired for theremedial systems. 6.2.2.4 Utilization ofPermanent SolPtions to the Maximum ExtentPracticable The individual components ofthe sitewide preferred alternative use technologies that are permanent solutions to remediation oftheOffpost OU. Specifically, the useofgroundwater extraction coupled withGAC treatment resultsin apermanent reduction in the concentrations of COCs inthe UFS groundwater. 6.2.3 Consistency With the National Continizency Plan The NCPrequiresthatthe following two features bepresent intheremedy selection process:

- The nine criteria used toevaluate alternatives in the detailed analysis are usedtoselect a remedy.

- Selected Superfund remedies mustemploy the nine criteriato makethefollowing four determinations " Each remedial action selected shallbeprotective ofhuman health and the environment. " Onsite remedial actions selected in anROD mustattain ARARs or provide grounds for invoking awaiver.

20000,317.10 - FS 0309111092 

### VI - 6-8o Each remedial action selected shall be cost-effective, provided that itfirst satisfies the threshold criteria (defined previously in Section 5.0)
o Each remedial action shall usepermanent solutions tothe maximum extentpracticable.
The preferred sitewide alternative is fully consistent with the NCP, asis the selection process used to arrive at the preferred alternative. Alternatives were developed and screened, and the detailed analysis of alternatives was performed ina manner consistent with the NCP.
6.3 SUMMARY The preferred sitewide alternative for remediation ofthe Offpost OU is Alternatives Nos. N-4 and NW-2. The preferred alternative was selected in accordance withthe requirements ofCERCLA and the NCP. The remedial actions that comprise the sitewide preferred alternative will permanently address the principal threats through groundwater extraction and treatment to reduce the toxicity, mobility, and volume ofcontaminants forprotection ofhuman health and the environment.

20000,317.10 - FS 0309111092 VI - 6-97.0 REFERENCES Adams County Planning Commission, 1990. AirportEnvirons Plan forthe Year 2000. C112M Hill, 1988. 96th Avenue/Quebec Street Area Transportation Study for Adams County, Commerce City, and CityofThorton. April. Ebasco Services, Inc., R. L. Stollar &Associates, Inc., Hunter/Environmental Science and Engineering, Inc., and Harding Lawson Associates, 1989. FinalWater Remedial Investigation Report. Prepared forU.S. Army Program Manager's Office for Rocky Mountain Arsenal Contamination Cleanup. Ebasco Services, Inc., Applied Environmental, Inc., CH2M Hill, DataChem, Inc., R.L. Stollar and Associates, Inc., 1991. Final Remedial Investigation Summary Report, Appendix E. Environmental Science and Engineering and Harding Lawson Associates, 1988. Rocky Mountain Arsenal Offpost Operable Unit Remedial Investigation and Chemical Specific Applicable or Relevant and Appropriate Requirements. Final Report Version 3.1. Volume 1. Office ofthe Program Manager for RMA, U.S. Army. Environmental Science and Engineering, Inc., and others, 1989. OffpostOperable Unit Endangerment Assessment/Feasibility Study with Applicable orRelevant and Appropriate Requirements. Draft Final Report. Version 2.1. March. Harding LawsonAssociates, 1989. Final Decision Documentfor the Groundwater Intercept and Treatment System North ofRocky Mountain Arsenal, Interim Response Action, Revised Draft Final Report. Prepared forOffice ofthe Program Manager, Rocky Mountain Arsenal Contamination Cleanup, July. Harding LawsonAssociates, 1991a. Draft Final Offpost Operable Unit Remedial Investigation Addendum. Prepared for the Program Manager for Rocky Mountain Arsenal. Harding LawsonAssociates, 1991b. Final Implementation Document for the Groundwater Intercept and Treatment System North ofRocky Mountain Arsenal Interim Response Action. January 25. Morrison-Knudsen, 1990a. Implementation Document for Northwest Boundary System ShortTerm Improvements IRA. June. Morrison-Knudsen, 1990b. Report ofField Investigations, Assessment, and Final Decision Document for the Northwest Boundary System Short-Term Improvements Interim Response Action, RMA. June. R.L. Stollar& Associates, Inc. 1989. Comprehensive Monitoring Program Annual Groundwater Report for 1988. June. R.L. Stollar & Associates, Inc. 1990a. Comprehensive Monitoring Program Annual Groundwater Report for 1989. Version 2.0. June. R.L. Stollar&Associates, Inc. 1990b. Comprehensive Monitoring Program FinalSurface-Water Data Assessment Report for 1988. May.

20000,317.10 - FS 0309111892 VI - 7-1R.L. Stollar &Associates, Inc. 1990c. Comprehensive Monitoring Program AirQuality Data Assessment Report for 1989. Contract number DAAA15-87-095. Prepared for U.S. Army Program Manager for Rocky Mountain Arsenal. Commerce City, CO. R.L. Stollar &Associates, Inc. 1991a. Comprehensive Monitoring Program Annual Groundwater Report for 1990. Version I.I. August. R.L. Stollar & Associates, Inc. 1991b, Surface-Water Comprehensive Monitoring Program Annual Report for 1989. November. U.S. Environmental Protection Agency. 1986. Superfund Public Health Evaluation Manual. Office ofEmergency and Remedial Response, Office of SolidWaste and Emergency Response, Washington, DC. EPA 540/1-86/060. U.S. Environmental Protection Agency. 1988a. CERCLA Compliance with Other Laws Manual Draft, OSWER Directive 9234.101. August. U.S. Environmental Protection Agency. 1988b. Guidance for Conducting Remedial Investigations and Feasibility Studies Under CERCLA, Interim Final, EPA/540/G-89/004. October. U.S. Environmental Protection Agency. 1988c. Guidance Document forProviding Alternate Water Supplies, Washington, DC. OSWER Directive 944.3-03. U.S. Environmental Protection Agency. 1988d. Guidance on Remedial Actions for Contaminated Ground Water at Superfund Sites, OSWER Directive 9283.1-2. August. U.S. Environmental Protection Agency. 1988e. Remedial Action Costing Procedures Manual, EPA/600/8-87/049. October. U.S. Environmental Protection Agency. 1989a. Prepared by U.S. Environmental Protection Agency Region VIII, U.S Department ofthe Army, U.S. Department ofthe Interior, Rocky Mountain Region, and the Agency for Toxic Substances and Disease Registry, In the Matterof Rocky Mountain Arsenal, Colorado, U. S. Department ofthe Army, Shell OilCompany, Federal Facility Agreement Persuant toCERCLA Section 120, Docket No. CERCLA VIII-89-13. U.S. Environmental Protection Agency. 1989a. Control ofAir Emissions From Superfund Air Strippers at Superfund Ground-Water Sites. OSWER Directive 92355.0-28. U.S. Environmental Protection Agency. 1989b. The Risk Assessment Guidance forSuperfund (RAGS). Human Health Evaluation Manual Part A, Interim Final. Office ofEmergency and Remedial Response. Washington, DC. EPA/540/1-89/002. U.S. Environmental Protection Agency. 1989c. Health Advisory on Diisopropyl Methylphosphonate (DIMP). Office ofDrinking Water. Washington, DC. U.S. Environmental Protection Agency. 1989c. The Superfund Innovative Technology Evaluation Program: Programs and Accomplishments, Office of SolidWaste and Emergency Response, Washington, DC. EPA/540/5-89/009. March. U.S. Environmental Protection Agency. 1990. National Oiland Hazardous Substances Pollution Contingency Plan: Final Rule, 40CFR Part 300. U.S. Environmental Protection Agency. 1991. Roleofthe Baseline Risk Assessment inSuperfund Remedy Selection Decisions, Washington, DC. OSWER Directive 9355.0-30.
20000,317.10 - FS 0309111892 VI - 7-2U.S. Environmental Protection Agency. 1992. Letter to Mr.Kevin Blose, Program Manager for Rocky Mountain Arsenal, fromMr. Connally Mears, regarding Endangerment Assessment/ Feasibility Study Issues. February. Voss, C.I. 1984. SUTRA, a finite element simulation model for saturated-unsaturated, fluiddensity dependent ground-water flow with energy transport orchemically-reactive single-species transport. U.S. Geological Survey, Reston, Virginia. Woodward-Clyde. 1991a. Northwest Boundary System Long-Term Improvements Interim Response Action B(ii) FinalAssessment Document. June. Woodward-Clyde. 1991b. Proposed decision document Northwest Boundary System Rocky Mountain Arsenal Long-Term Improvements Interim Response Action. June.

200W,317.10 - FS 0309111892 VI - 7-3Table 3.5.1-1: Groundwater Alternatives forthe North Plume Group Recharge RemediExtractions Wells/Trenches ation Treatment Wells (total number/ Total Flow Timeframe Plant Residuals Alternatives Process Options Paleochannel (total number) total lengtýj Rate (Izpm) (years) Location Generated N-1 NoAction Monitoring FC, N None None N/A Unknown N/A None five-year site reviews N-2 Continued Operation NBCS operation FC,N No additional No additional 240 15to30+ NBCS No additional ofthe NBCS with (slurry wall, carbon Improvements as adsorption) Necessary N-3 Land Acquisition and NBCS operation FC, N No additional No additional 240 15to30+ NBCS No additional Use Restrictions Land acquisition and Userestrictions N-4 Interim Response Carbon adsorption FC 5 6 trenches: 180 15to30 T2S, R67W, Spent carbon Action A NBCS operation N 12 1500 feet 300 Sec 14, NE 1/4 Sec N-6 Expansion I toInterim Carbon adsorption FC 7 10 trenches: 240 10to 20 T2S, R67W, Spent carbon Response Action A NBCS operation N 13 2700 feet 330 Sec 14, 2 trenches: NE 1/4 Sec 600feet N-6 Expansion 2to Interim Carbon adsorption FC 9 14 trenches: 300 10to 20 T2S, R67W, Spent carbon Response ActionA NBCS operation N 15 3900feet 390 Sec 14, 5 trenches: NE 1/4 Sec 1500 feet

a All alternatives include groundwater monitoring and five-year site reviews. FC= First Creek N = northern N/A = not applicable NBCS = North Boundary Containment System 20000,317.10 - FS 0311111092Table 3.5.1.7-1: Summary ofPreliminary North Plume Group Groundwater Alternatives

Seven Three Additionalla Additionala Extraction Extraction Wells; IRA A IRA A Wells; Six Thirteen System System Carbon Additional Additional Land Use NBCS Extraction Recharge Adsorption Recharge Recharge Acquisition Restrictions Operation Wells Wells/Trends Treatment Trenches Trenches N-1 NoAction N-2 Continued Operation of the X NBCS With Improvements asNecessary N-3 LandAcquisition and Use x X Restrictions N-4 Interim Response ActionA X X x X N-5 Expansion ItoInterim X X X X X Response Action A N-6 Expansion 2 toInterim X x X x X Response Action A

All alternatives include groundwater monitoring and five-year sitereviews.
a Additional extraction wells and recharge trenches refers tothenumber of wells/trenches in additiontothe IRAAsystem. IRA = interim response action NBCS = NorthBoundary Containment System 20000,317.10 - FS 0311111092Table 3.5.2-1: Groundwater Alternatives for the Northwest Plume Group
Recharge RemediExtractions Wells/Trenches ation Treatment Wells (total number/ Total Flow Timeframe Plant Residuals Alternativea Process Options Palleochannel (total number) total lenizthl Rate (gym) (years) Location Generated NW-1 NoAction Monitoring NW None None N/A Unknown N/A None five-year sitereviews NW-2 Continued Operation NWBCS operation NW No additional No additional 850 3to8 NWBCS No additional oftheNWBCS With Improvements asNecessary NW-3 LandAcquisition and Use Land acquisition NW No additional No additional 850 3to8 NWBCS No additional Restrictions Use restrictions NWBCS operation NW-4 Northwest PlumeGroup Carbon adsorption NW 3 5 wells 1000 2 to6 NWBCS Spent carbon Extraction/Recharge NWBCS operation System

aAll alternatives include groundwater monitoring and five-year site reviews. N/A not applicable NW northwest NWBCS = Northwest Boundary Containment System

20000,317.10 - FS 0311111092Table 3.5.2.5- 1: Summary of Preliminary Northwest Plume Group Groundwater Alternatives Carbon Land Use NWBCS Extraction Recharge Adsorption Acquisition Restrictions Operation Wells Wells/Trenches Treatment
NW- I No Action NW-2 Continued Operation ofthe X NWBCS with Improvements as Necessary NW-3 Land Acquisition and Use X x X Restrictions NW-4 Northwest Plume Group X X X x Extraction/Recharge System

All alternatives include groundwater monitoring andfive-year sitereviews. NW = northwest NWBCS = Northwest Boundary Containment System 20000,317.10 - FS 03112111092Table 4.2.1.7-1: Summary of Screening ofGroundwater Alternatives; North Plume Group (Page I of2)

Effectiveness Implementability Cost Time to Achieve Capital Long-term O&M Present Reduction Long-term Complies Short-term Protection Technically Administratively Cost Cost Worth Recommended Alternative ofMTV Protection With PRG8 Impacts (years) Feasible Feasible (x 103 dollars) fx 103dollarml (x 103 dollarel for DAA

N-I None None No None Unknown Yes Yes -0- 4,061 - 6,012 4,061 - 6,102 Retained for furtherevalNo Action uation onthe basis of NCPrequirements
N-2 Reduce@M, T, Provides some long-term produc- Yes None 15 to 30+ Yes Yes -0- 30,600 - 32,600 30,6M - 32,600 Retained forfurther Continued Operation of and V atthe tionthrough reducing COCcon- eyalsuation the NBCS With NBCS only centrations at the NBCS Improvements asNecessary N-3 Reduces M, T Controls exposures pathways Yes None 15 to30+ Yes Yes 5,348 30,400 - 32,330 35,800 - 37,700 Eliminated from further Land Acquisition and Deed and V at the through useand accessrestrictions evalaustion onthe basisof Restrictions NBCS only and provides soon long-term pro- effectiveness issues tection throughreducing COC concentrations at theNBCS. N-4 Reduces M, T, Addresses exposure pathways Yes None 15 to 30 Yet Yes 16,700 40,000 - 46.400 56,600 - 63,100 Retained for further InterimResponse Action A and V at the through reducing COC evaluation NBCS and concentrations attheNBCS and within theNorth within theNorth Plume Group Plums Group

N-5 ReducesM, T. Addresses exposure pathways Yes None 10to 20 Yes Yes 19,400 36,900 - 43,600 66.200 - 63,000 Retained forfurther Expansion I toIRA A andV at the through reducing COC evaluation NBCS and concentrations at the NBCS and withinthe North within the North PlumsGroup PlurneGroup

N-6 Reduce@ M, T, Address" exposure pathways Yes None 10to 20 Yes Yes 22,000 37,600 - 44,200 59,6W -66,200 Eliminated from further Expansion 2to IRAA and V at the through reducing COC evaluation on thebasis of NBCS and concentrations atthe NBCS and similar preformance

ithinthe North within the NorthPlume Group comparison with Alternalume Group tiveNo. N-5 at higher cost
20000,317.10 - FS 0311111092Table 4.2.1.7-1: SummaryofScreening of Groundwater Alternatives; North Plume Group (Page 2 of2)

COG = chemical ofconcern DAA = detailed analysis of alternatives MTV = mobility, toxicity, and volume NBCS = NorthBoundary Containment System NCP National Contingency Plan O&M Operation andMaintenance PRG preliminaryremediation goal

20000,317.10 - FS 0311111892Table 4.2.2.5- 1: Summary of Screening of Groundwater Alternatives; Northwest Plume Group (Page Iof 2)

Effectiveness Implementability Cost Time to Achieve Capital Long-term O&M Present Reduction Long-term Complies Short-term Protection Technically Administratively Cost Cost Worth Recommended Alternative ofMTV Protection With PRGs Impacts (years) Feasible Feasible (x 103 dollars) (x 103 dollars) (x 103 dollars) for DAA

NW-I None None No None Unknown Yes Yes -0- 608- 1,260 608 - 1,260 Retained forfurther evalNo Action uation onthe basis of NCP requirements
NW-2 Reduces M, T, Provides some long-term protec- Yes None 3 - a Yes Yes -0- 12,400 - 13,100 12,400 - 13.100 Retained for further evalContinued Operation of and V atthe tion through reducing COGcon- uation on thebasis of theNWBCS and Improve- NWBCS only centrations at theNWBCS effectiveness, ease ofimments as Necessary plementation, and low took in comparison with other Northwest Plume Group alternatives
NW-3 Reduces M, T. Controls exposure pathways Yes None 3-8 Yes Yes 2,100 12,300 - 12,900 14,400 - 16,000 Eliminated fromfurther Land Acquisition and Use nd V at the through useandaccess restrictions evaluation on thebasis of Restrictions NWBCS and provides some long-term pro- similar performance in tection throughreducing COG comparison with concentrations at theNWBCS Alternative No. NW-2 at highercost
NW-4 Reduces M, T, Addresses exposure pathways Yes None 2 - 5 Yes Yes 2.774 12,200 - 12,500 15,000 - 16,300 Eliminated from further Northwest Plume Group nd V at the through reducing COC concentra- evaluation on the basis of Extraction/Recharge NWBCS and tions at theNWBCS and within similar performance in System within the theNorthwest Plume Group comparison with AlternaNorthwest Plume tiveNo. NW-2 at higher Group cost

20000,317.10 - FS 0311111092Table 4.2.2.6-1: Summary of Screening ofGroundwater Alternatives; Northwest Plume Group (Page 2 of2)

COG = chemical ofconcern DAA = detailed analysis ofalternatives MTV = mobility, toxicity, and volume NBCS =North Boundary Containment System NCP National Contingency Plan O&M Operation and Maintenance PRG preliminary remediation goal

20000,317.10 - FS 0311111892Table 5.5.3- 1: Summary ofthe Detailed Analysisand Ranking of Groundwater Alternatives for the North Plume Group (Page Iof 3) Alternative No.N-2 Continued Operation ofthe North Boundary Alternative No. N-5 Alternative No. N-I Containment System With Alternative No. N-4 Expansion ItoInterim Criteria No Action Improvements as Necessary Interim Response Action A Response Action A Overall Protection of Human This alternative would notprovide This alternative provides limited Reduces hypothetical risk and Reduceshypothetical risk and Health and theEnvironment protection ofhuman health and the overall protectionofhuman health provides protection of both human provides protection ofboth human environment. and the environment by preventing health and the environment by health and the environment by migrationofcontaminants from remediating North Plume Group remediating North Plume Group RMA to theOffpost OU north of the groundwater. groundwater. NBCS. Hypothetical risk associated with groundwater in theNorth Plume Groupwould decrease over time. Compliance With ARARs This alternative isnot expected to Groundwater modeling indicatesthat Chemical-specific ARARs would Chemical-specific ARARs would be achieve chemical-specific ARARs. chemical-specific ARARs may be be attained in approximately 15 to attained inapproximately 10 to attained 15to 30-plus years. 30years, asestimated by 20years, asestimated by groundwater groundwater modeling. modeling. Long-term Effectiveness and This alternative would notreduce This alternative would reduce This alternative would reduce Through tretment, thisalternative Permanence the residual riskassociated with residualrisk associated with North residual risk associated with North would reduce residual risk associated groundwater exposure pathways. PlumeGroup groundwater by Plume Group groundwater, withNorth Plume Group preventing contaminant migration at through operation ofthe NBCS groundwater through operation ofthe the NBCS and continuing recharge of and the IRA A systems and NBCS, the IRA A, and the treated groundwater to flush improvements toboth systems as Expansion Isystems. contaminants inthe North Plume necessary. Group. Reduction ofMobility, Toxicity, This alternative would notemploy This alternative wouldreduce Through treatment, thisalternative Thisalternative would reduce the orVolume any treatment process options and toxicity, mobility, andvolume of wouldreduce toxicity, mobility, toxicity, mobility, and volume of would notreduce toxicity, mobility, groundwater migrating from RMA to and volumeofgroundwater within groundwater within North Plume or volume ofgroundwater within the Offpost OU. the North Plume Group and Groupand groundwater migrating the North Plume Groupor groundwater migrating from RMA from RMA to theOffpost OU groundwater migrating fromRMA to theOffpost OU. through treatment. tothe Offpost OU.

20000,317.10 - FS 0311111892Table 5.5.3-1: Summary ofthe Detailed Analysis and Ranking ofGroundwater Alternatives for the North Plume Group (Page 2 of3)
Alternative No. N-2 Continued Operation ofthe North Boundary Alternative No. N-5 Alternative No. N-I Containment System With Alternative No. N-4 Expansion Ito Interim Criteria NoAction Improvements as Necessary Interim Response Action A Response ction A Short-term Effectiveness Because no remedial action would be There would be no short-term Community and workers would be Community and workers wouldbe performed, there would be noshort- impactsbecause the NBCS is already protected byadhering tostandard protected during construction through term impacts. There would be no operating. There would be noimple- health and safetypractices. The adhering to standard health and implementation period. mentation period. implementation period would be safety practices. The implementation approximately 20 months. period would be approximately 32 months. Implementability Technical feasibility would be high. This alternative is readily This alternative is readily This alternative is readily The administrative feasibility would implementable, and administrative implementable. Technical and implementable. However, the be low. feasibility would be high. administrative feasibility would be construction would be conducted in high. two time periods due tothe design phase for the expansion. Technical and administrative feasibility would be high. Cost Total Capital Cost $ -0- Total Capital Cost $ -0- Total Capital Cost = $16.7 million TotalCapital Cost= $19.4 million Total Long-term O&M Cost Total Long-term O&M Cost $30.6 Total Long-term O&M Cost Total Long-term O&M Cost $4.1 to6.0 million to 32.5 million $39.8 to46.4 million $36.9 to43.6 million Total Present Worth Total Present Worth Total Present Worth Total Present Worth Cost =$4.1 to 6.0 million Cost =$30.6 to 32.5million Cost= $56.5 to 63.1 million Cost = $56.2 to 63 million
Overall Rank Based on 4 3 1 2 Achievement ofCriteria

20000,317.10 - FS 0311111092Table 5.5.3- 1: Summary ofthe Detailed Analysisand Ranking ofGroundwater Alternatives for the North Plume Group (Page 3of3)

ARAR = applicable or relevant and appropriate requirement IRA = Interim Response Action NBCS North Boundary Containment System O&M Operation and Maintenance OU= operable unit RMA = Rocky Mountain Arsenal

20000,317.10 - FS 0311111092'Table 5.5.3-2: Summary ofthe Detailed Analysis and Ranking of Groundwatei Alternati%,es for the Northwest Plume Group (Page Iof2)
Alternative No. NW-2 Continued Operation ofthe Northwest Alternative No. NW- I Boundary Contaminated System With Criteria NoAction Improvements ifNecessary
Overall Protection ofHuman Health and the Environment This alternative would notprovide protection ofhuman This alternative would provide protection ofhuman health and the environment. health and the environment by preventing migration of contaminants from RMA to the Offpost OU northofthe NWBCS. Potential risksassociated with the Northwest Plume Group groundwater would be substantially reduced through continued operation ofthe NWBCS and improvements as necessary. Compliance With ARARs This alternative is not expected toachieve This alternative is expected to meet orexceed chemical chemical-specific AIRARs. specific ARARs inapproximately 3 to 8 years, as estimated by groundwater modeling. Long-term Effectiveness and Permanence This alternative would notreduce the residual risk This alternative would reduce residual risk associated associated with potential groundwater exposurepathways. with groundwater within the Northwest Plume Group through preventing contaminant migration at the NWBCS and recharging treated groundwater toflush contaminants in the Northwest PlumeGroup. Reduction ofToxicity, Mobility, or Volume This alternative would not employ any treatment process This alternative would reduce toxicity, mobility, and optionsand would notreduce the toxicity, mobility,or volume ofgroundwater migrating from RMA to the volume ofgroundwater within the Northwest Plume Offpost OU. Groundwater contaminant concentrations Groupor groundwater migrating from RMA tothe would bereduced within the Northwest Plume Group by OffpostOU. flushingprovided by recharge oftreated water atthe NWBCS. Short-term Effectiveness Because no remedial action would beperformed, there There would be noshort-term impacts. would be noshort-term impacts. There would be no implementation period. Implementability The technical feasibility would be high. The administra- Thisalternative isreadily implementable. Technical and tive feasibility would below. administrative feasibility would be high.

20000,317.10 - FS 0311111092Table 5.5.3-2: Summary ofthe Detailed Analysisand Ranking of Groundwater Alternatives for the Northwest Plume Group (Page 2 of2) Alternative No. NW-2 Continued Operation ofthe Northwest Alternative No. NW- I Boundary Contaminated System With Criteria No Action - Improvements ifNecessary
Cost Total Capital Cost $ -0- Total Capital Cost $ -0- Total Long-term O&M Cost = $0.6 to 1.3 million Total Long-term O&M Cost=$12.4 to 13.1 million Total Present Worth Cost =$0.6 to 1.3 million Total Present Worth Cost = $12.4 to 13.1 million
Overall Rank Based on Achievement ofCriteria 2 1

ARAR = applicable or relevant and appropriate requirement IRA - Interim Response Action NBCS, North Boundary Containment System O&M Operation and Maintenance OU =operable unit RMA =Rocky Mountain Arsenal 20000,317.10 - FS 0311111092RMAI 1177 /V

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