11R0002_Tetra_Tech_2004K_Agency_Review_Draft_OU-1_FS-February.pdf

DRAFT

REMEDIAL INVESTIGATION AND FEASIBILITY STUDY

FOR

OPERABLE UNIT 1

SULPHUR BANK MERCURY MINE SUPERFUND SITE

CLEARLAKE OAKS, CALIFORNIA

VOLUME 2 OF 2:

FEASIBILITY STUDY

Prepared by:

TETRA TECH EM INC.

Prepared for:

U.S. ENVIRONMENTAL PROTECTION AGENCY

Region IX

75 Hawthorne Street San Francisco, California 94105

February 2004 i

CONTENTS

Section Page

ACRONYMS AND ABBREVIATIONS . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . vi

EXECUTIVE SUMMARY . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . ES-1

1.0 INTRODUCTION . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1-1

1.1 REPORT ORGANIZATION . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1-1

1.2 SITE DESCRIPTION . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1-2

1.3 NATURE AND EXTENT OF CONTAMINATION . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1-2

1.4 PHYSICAL HAZARDS . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1-6

2.0 REMEDIAL ACTION GOALS AND OBJECTIVES AND GENERAL RESPONSE ACTIONS . . . 2-1

2.1 APPLICABLE OR RELEVANT AND APPROPRIATE REQUIREMENTS . . . . . . . . . . . . . . 2-1

2.1.1 Chemic al-Specific A pplicable or R elevant an d Appro priate Req uirements . . . . . . . . . 2-1

2.1.2 Location -Specific A pplicable or R elevant an d Appro priate Req uirements . . . . . . . . . . 2-4

2.1.3 Action-S pecific Ap plicable or R elevant an d Appro priate Req uirements . . . . . . . . . . . 2-6

2.2 PRELIMINARY REMEDIAL ACTION OBJECTIVES . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2-8

2.3 PRELIMINARY REMEDIATION GOALS . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2-8

2.4 GENERAL RESPONSE ACTIONS . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2-9

3.0 IDENTIFICATION AND SCREENING O F REMEDIAL TECHN OLOGIES

AND PROCESS OPTIONS . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3-1

3.1 NO ACTION . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3-1

3.2 NATURAL ATTENUATION . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3-1

3.3 INSTITUTIONAL CONTROLS . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3-2

3.3.1 Land U se Contro ls . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3-3

3.3.2 Access Restrictions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3-3

3.3.3 Groundwater Use Restrictions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3-4

3.3.4 Community Aw areness . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3-4

3.3.5 Water Management Practices . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3-4

3.4 CONTAINMENT . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3-5

3.4.1 Surface W ater Contro ls . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3-5

3.4.2 Source S urface Co ntrols . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3-6

3.4.3 Barriers . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3-8

3.5 REMOVAL, TRANSPORT, AND DISPOSAL . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3-8

3.5.1 Removal . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3-9

3.5.2 Transport . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3-9

3.5.3 On-Site Disposal . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3-10

3.5.4 Off-Site Disposal . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3-10

3.6 TREATMENT . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3-11

3.6.1 Physical/C hemical T reatment of Solid M edia . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3-11

3.6.2 Physical/Chemical Treatment of Water . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3-14

3.6.3 Biological Treatment of Water . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3-16

3.7 RESOURCE UTILIZATION . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3-17

3.8 SUMMARY OF POTENTIALLY APPLICABLE REMEDIAL TECHNOLOGIES AND

PROCESS OPTIONS . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3-18

4.0 DEVELOPMENT AND SCREENING OF WASTE SYSTEM REM EDIAL ALTERNATIVES . . . . . 4-1

4.1 HERMAN IMPOUNDM ENT SYSTEM . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4-4 ii

CONTENTS (Cont.)

Section Page

4.1.1 No Action . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4-5

4.1.2 Institutional C ontrols . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4-7

4.1.3 Surface C ontrols . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4-8

4.1.4 Containment of Solid Mine Wastes (Soil Cover or Soil Cap with a Liner System) . . 4-10

4.1.5 Excavation and Disposal of Solid M ine Wastes in an On-Site Repo sitory . . . . . . . . . 4-13

4.1.6 Excavation of Solid Mine Wastes and Wet Cover Disposal . . . . . . . . . . . . . . . . . . . . 4-15

4.1.7 Excavation of Solid Mine Wastes and Off-Site Disposal . . . . . . . . . . . . . . . . . . . . . . 4-17

4.1.8 Physical/Chemical Treatment of Solid Mine Wastes . . . . . . . . . . . . . . . . . . . . . . . . . 4-19

4.1.9 Containment of Surface Water – Enhanced Evaporation . . . . . . . . . . . . . . . . . . . . . . 4-20

4.1.10 Biological Treatment of Surface Water . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4-22

4.1.11 Physical/Chemical Treatment of Surface Water . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4-23

4.1.12 Containm ent of Grou ndwate r – Intercepto r Wells or B arrier Wa ll . . . . . . . . . . . . . . . 4-25

4.1.13 Physical/Chemical Treatment of Groundwater . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4-27

4.2 NORTHWEST PIT SYSTEM . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4-29

4.2.1 No Action . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4-29

4.2.2 Institutional C ontrols . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4-31

4.2.3 Surface C ontrols . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4-32

4.2.4 Backfilling in Conjunction with Development of an On-Site Repository for Mine Wastes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4-33

4.2.5 Containment and Removal of Surface Water . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4-36

4.3 ORE SYSTEM . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4-37

4.3.1 No Action . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4-37

4.3.2 Institutional C ontrols . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4-39

4.3.3 Surface C ontrols . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4-40

4.3.4 Containment of Ore (Soil Cover or Soil Cap with a Liner System) . . . . . . . . . . . . . . 4-42

4.3.5 Excavation and Disposal in an On -Site Repository . . . . . . . . . . . . . . . . . . . . . . . . . . . 4-44

4.3.6 Excavation of Mine Wastes and Wet Cover Disposal . . . . . . . . . . . . . . . . . . . . . . . . . 4-46

4.3.7 Excavation of Mine Wastes and Off-Site Disposal . . . . . . . . . . . . . . . . . . . . . . . . . . . 4-48

4.3.8 Physical/C hemical T reatment of Solid M edia . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4-50

4.4 TAILINGS SYSTEM . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4-51

4.4.1 No Action . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4-52

4.4.2 Institutional C ontrols . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4-53

4.4.3 Surface C ontrols . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4-55

4.4.4 Containment of T ailings (Soil Cover or Soil Ca p with a Liner System ) . . . . . . . . . . 4-56

4.4.5 Excavation and Disposal in an On -Site Repository . . . . . . . . . . . . . . . . . . . . . . . . . . . 4-59

4.4.6 Excavation of Mine Wastes and Wet Cover Disposal . . . . . . . . . . . . . . . . . . . . . . . . . 4-61

4.4.7 Excavation of Mine Wastes and Off-Site Disposal . . . . . . . . . . . . . . . . . . . . . . . . . . . 4-62

4.4.8 Physical/Chemical Treatment of Tailings . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4-64

4.5 WASTE ROCK SYSTEM . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4-66

4.5.1 No Action . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4-67

4.5.2 Institutional C ontrols . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4-68

4.5.3 Surface C ontrols . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4-70

4.5.4 Containment of Solid Mine Wastes (Soil Cover or Soil Cap with a Liner System) . . 4-72

4.5.5 Excavation and Disposal in an On -Site Repository . . . . . . . . . . . . . . . . . . . . . . . . . . . 4-74

4.5.6 Excavation of Mine Wastes and Wet Cover Disposal . . . . . . . . . . . . . . . . . . . . . . . . . 4-76

4.5.7 Excavation of Mine Wastes and Off-Site Disposal . . . . . . . . . . . . . . . . . . . . . . . . . . . 4-77

4.5.8 Physical/C hemical T reatment of Solid M edia . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4-79

4.5.9 Containm ent of Grou ndwate r - Interceptor W ells, Interceptor T renches, or Barrier W all . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4-81

4.5.10 Physical/Chemical Treatment of Groundwater . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4-82 iii

CON TEN TS (Co nt.)

Section Page

4.6 DISTURBED ROCK SYSTEM . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4-84

4.6.1 No Action . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4-84

4.6.2 Institutional C ontrols . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4-85

4.6.3 Surface C ontrols . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4-87

4.6.4 Containment of Disturbed Rock (Soil Cover or Soil Cap with a Liner System) . . . . . 4-89

4.7 SUMMARY OF RETAINED REM EDIAL ALTERNATIVES . . . . . . . . . . . . . . . . . . . . . . . . 4-91

5.0 DETAILED DESCRIPTION OF RETAINED COMPONENT REMEDIAL ACTIONS . . . . . . . . . . 5-1

5.1 CONTAINMENT WITH SOIL COVER OR SOIL CAP WITH MEMBR ANE LINER . . . . . . 5-1

5.2 EXCAVATION AND DISPOSAL IN AN ON-SITE REPOSITORY . . . . . . . . . . . . . . . . . . . . 5-3

5.3 EXCAVATION WITH WET COVER DISPOSAL . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5-4

5.4 SURFACE WATER TREATMENT AND DISCHARGE . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5-4

5.5 GROUNDWATER COLLECTION AND DISCHARGE . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5-6

5.6 GROUNDWA TER CONTAINMENT USING BARRIER WALLS . . . . . . . . . . . . . . . . . . . . . 5-7

6.0 DEFINITION OF CRITERIA USED FOR THE ANALYSIS OF ALTERNATIVES . . . . . . . . . . . . . . . . . 6-1

6.1 OVERALL PROTECTION OF HUMAN HEALTH AND THE ENVIRONMENT . . . . . . . . . 6-1

6.2 COMPLIANCE WITH ARARS . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6-1

6.3 LONG-TERM EFFECTIVENESS AND PERMANENCE . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6-1

6.4 REDUCTION OF TOXICITY, MOBILITY, OR VOLUME THROUGH TREATMENT . . . . 6-2

6.5 SHORT-TERM EFFECTIVENESS . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6-2

6.6 IMPLEMENTABILITY . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6-3

6.7 COST . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6-3

6.8 STATE ACCEPTANCE . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6-4

6.9 COMMUNITY ACCEPTANCE . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6-4

6.10 CRITERIA PRIORITIES . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6-5

7.0 DEVELOPMENT AND EVALUATION OF SITE WIDE REMEDIATION ALTERNATIVES . . . . . . . . 7-1

7.1 DETAILED EVALUATION OF WASTE SYSTEM REMEDIAL ALTERNATIVES . . . . . . . 7-1

7.1.1 Herman Impoundment System . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7-2

7.1.1.1 No Action . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7-2

7.1.1.2 Capping the Waste Rock Dam . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7-6

7.1.1.3 Soil Cove r for the Satellite W aste Rock Pile . . . . . . . . . . . . . . . . . . . . . . . . . 7-10

7.1.1.4 Mine Waste Repository for Containment of all Herman Impoundment

System M ine Wa ste . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7-14

7.1.1.5 Physical/Chemical Treatment of Surface Water . . . . . . . . . . . . . . . . . . . . . . 7-17

7.1.1.6 Containm ent of Surfac e Wate r Using G roundw ater Barrier W all . . . . . . . . . 7-22

7.1.2 Northwest Pit System . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7-26

7.1.2.1 Backfilling the Northw est Pit to Develop a Sludg e Repository . . . . . . . . . . 7-26

7.1.2.2 Backfilling the Northw est Pit to Develop a Sludg e Repository . . . . . . . . . . 7-29

7.1.2.3 Containment and Removal of Surface Water to Control Groundwater

Migration . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7-32

7.1.3 Ore System . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7-35

7.1.3.1 No Action . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7-35

7.1.3.2 On-site Repository . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7-38

7.1.3.3 Wet Cover Disposal in Herman Impoundment . . . . . . . . . . . . . . . . . . . . . . . 7-41

7.1.4 Tailings System . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7-43

7.1.4.1 No Action . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7-43

7.1.4.2 Earthen Cover . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7-47

7.1.4.3 Paving Bradley Mine Road . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7-50

7.1.5 Waste Rock System . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7-52 iv

CON TEN TS (Co nt.)

Section Page

7.1.5.1 No Action . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7-53

7.1.5.2 Earthen Cover or Earthen Cap with Geomembrane Liner . . . . . . . . . . . . . . . 7-56

7.1.5.3 Northwest W aste Rock D isposal in On-site Repo sitory . . . . . . . . . . . . . . . . 7-60

7.1.5.4 Interceptor Trench for Groundwater Containment . . . . . . . . . . . . . . . . . . . . 7-62

7.1.5.5 Ex-situ Treatment . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7-65

7.1.6 Disturbed Rock System . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7-69

7.1.6.1 No Action . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7-69

7.1.6.2 Earthen Cover for Disturbed Rock System . . . . . . . . . . . . . . . . . . . . . . . . . . 7-72

7.2 SITE-WIDE REMEDIAL ALTERNATIVES . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7-75

7.2.1 Site-wide Remedial Alternative 1 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7-76

7.2.2 Site-wide Remedial Alternative 2 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7-78

7.2.3 Site-wide Remedial Alternative 3 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7-80

7.2.4 Site-wide Remedial Alternative 4 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7-82

7.2.5 Site-wide Remedial Alternative 5 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7-85

7.2.6 Site-wide Remedial Alternative 6 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7-87

7.3 ESTIMATED COSTS OF SITE-WIDE REMEDIATION PROGRAM

ALTERNATIVES . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7-89

7.4 COMPARATIVE ANALYSIS OF SITE-WIDE REMEDIATION PROGRAM

ALTERNATIVES . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7-90

REFERENCES . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . R-1

Appendices

A Applicable or Relevant and Appropriate Requirements (included on attached CD)

B HELP Modeling of Soil Cover and Earthen Cap with Liner (included on attached CD)

C Water Treatment Technical Memorandum (included on attached CD)

D Cost Estimates Summary Sheets (included on attached CD)

LIST OF TABLES

Table

1-1 Range of D issolved Aluminu m and M ercury Levels in Surface and Ground waters

1-2 Contaminant Mass in Various Mining-Related Waste Piles

2-1 Chemical-Specific Applicable or Relevant and Appropriate Requirements and To-Be-Considered Guidance

2-2 Location-Specific Applicable or Relevant and Appropriate Requirements and To-Be-Considered Guidance

2-3 Action-Specific Applicable or Relevant and Appropriate Requirements and To-Be-Considered Guidance

2-4 Water Quality Criteria

3-1 Remedial Tec hnology and P rocess Option Scre ening Summ ary

4-1 Alternatives Screening S ummary

7-1 Site-Wide Remedial Alternatives

7-2 Preliminary Cost Estimate Site-wide Remedial Alternative 1

7-3 Preliminary Cost Estimate Site-wide Remedial Alternative 2 - Level 1 Water Treatment

7-4 Preliminary Cost Estimate Site-wide Remedial Alternative 2 - Level 2 Water Treatment

7-5 Preliminary Cost Estimate Site-wide Remedial Alternative 2 - Level 3 Water Treatment v

LIST OF TABLES (Cont.)

Table

7-6 Preliminary Cost Estimate Site-wide Remedial Alternative 3 - Level 1 Water Treatment

7-7 Preliminary Cost Estimate Site-wide Remedial Alternative 3 - Level 2 Water Treatment

7-8 Preliminary Cost Estimate Site-wide Remedial Alternative 3 - Level 3 Water Treatment

7-9 Preliminary Cost Estimate Site-wide Remedial Alternative 4 - Level 1 Water Treatment

7-10 Preliminary Cost Estimate Site-wide Remedial Alternative 4 - Level 2 Water Treatment

7-11 Preliminary Cost Estimate Site-wide Remedial Alternative 4 - Level 3 Water Treatment

7-12 Preliminary Cost Estimate Site-wide Remedial Alternative 5

7-13 Preliminary Cost Estimate Site-wide Remedial Alternative 6 - Level 1 Water Treatment

7-14 Preliminary Cost Estimate Site-wide Remedial Alternative 6 - Level 2 Water Treatment

7-15 Preliminary Cost Estimate Site-wide Remedial Alternative 6 - Level 3 Water Treatment

7-16 Site-Wide Remedial Alternative Cost Estimates

7-17 Comparative Analysis of Site-Wide Remedial Alternatives

LIST OF FIGURES

Figure

1-1 Site Location Map

1-2 Operable Un it 1 and Site Bounda ry

1-3 Physical Site Features at the Sulphur Bank Mercury Mine

5-1 Earthen Cover

5-2 Earthen Cap and Liner

5-3 Waste Rock Dam Regrading Plan

5-4 Waste Rock Dam Alternative Regrading Plan

5-5 North Waste Rock Pile Regrading Plan

5-6 South Waste Rock Pile Regrading Plan

5-7 Current Relationship of Clear Lake, Waste Rock Dam, and Herman Impoundment

5-8 Future Relationship of Clear Lake, Waste Rock Dam, and Herman Impoundment

5-9 Level 1 W ater Treatm ent Schem atic

5-10 Level 2 W ater Treatm ent Schem atic

5-11 Level 3 W ater Treatm ent Schem atic

7-1 Site-wide Remedial Alternative 1

7-2 Site-wide Remedial Alternative 2

7-3 Site-wide Remedial Alternative 3

7-4 Site-wide Remedial Alternative 4

7-5 Site-wide Remedial Alternative 5

7-6 Site-wide Remedial Alternative 6 vi

ACRONYMS AND ABBREVIATIONS

AMD Acid mine drainage

ARD Acid rock drainage

ARAR Applic able and relevant an d appro priate requ irements

CCR California Code of Regulations

CERCLA Comprehensive Environmental Response Cleanup and Liability Act

CFR Code of Federal Regulations

COC Contam inant of Con cern cy Cubic Y ard

DTSC Department of Toxic Substance Control

EPA U.S. Environmental Protection Agency

FS Feasibility Study gpm Gallon s per min ute

GRA General Response Actions

HELP Hydrologic Evaluation of Landfill Performance

LCRS Leachate Collection and Removal System

LDR Land Disposal Restrictions

MCL Max imum Contam inant Le vels mg/kg Milligrams per kilograms (equivalent mg/L Milligrams per liter (equivalent to parts per million)

NAGPRA Native American Graves Protection and Repatriation Act

NCP National Oil and Hazardous Substances Pollution Contingency Plan

NPDES National Pollution Discharge Elimination System

NPL National Priorities List

O&M Operation and Maintenance

OU Operab le Unit

PRG Preliminary Remediation Goal

PRAO Preliminary Remediation Action Objective

RACER Reme dial Actio n Cost E stimating and Re quirem ents

RCRA Resource Conservation and Recovery Act

RI Remedial Investigation

RI/FS Remedial Investigation/Feasibility Study

RO Revers e Osm osis

ROD Record of Decision

RWQCB Regional W ater Quality Co ntrol Board vii

ACRONYMS AND ABBREVIATIONS

(Continued)

SBMM Sulphur Bank Mercury Mine

SEG Southeast Geyser

SWRCB State Water R esources Co ntrol Board

TDS Total Dissolved Solids

TtEMI Tetra Tech EM Inc.

TMDL Total Maximum Daily Load

TSD Treatment, Storage, or Disposal µg/L Micrograms per liter (equivalent to parts per billion)

USC United States Code

WQO Water Quality Objectives

AGENCY REVIEW DRAFT

ES-1

EXECUTIVE SUMMARY

The Sulphur Bank Mercury Mine (SBMM) Superfund site near Clearlake Oaks, California was placed on the National Priorities List (NPL) in August 1990. Mining of sulfur and mercury at SBMM occurred periodically from 1865 through 1957. SBMM was at one time the fifth largest producer of mercury in the

United States. The inactive SBMM property is currently owned by the Worthen Bradley Trust and the

Bradley Mining Company, the last operator of the mine.

This feasibility study (FS) develops and evaluates remedial alternatives to clean up hazardous substances derived from past mining activities conducted at SBMM and that are currently located on the terrestrial portion o f the SB MM Superfu nd site. Th e U.S. E nvironm ental Pro tection A gency (EPA ) designa ted this terrestrial portion of the SBMM Superfund site Operable Unit 1 (OU-1). All work related to the FS for

OU-1 was funded, planned, and directed by the EPA with logistic and contract support from the U.S.

Army Corps o f Engin eers.

The SBMM-derived contamination located in the sediments of Clear Lake, at the Elem Tribal Colony, and in the North Wetlands are not included in OU-1 but will be addressed by EPA in subsequent

Comprehensive Environmental Response Cleanup and Liability Act (CERCLA) actions, including additional remedial investigation (RI) and FS activities. Some additional field work for the RI of the

Elem T ribal Colo ny area h as been p erforme d, and E PA ha s comm enced a n FS to d evelop a nd evalu ate remedial alternatives for this area of the SBMM site. EPA is currently developing a work plan to conduct field work for an RI for the hazardous substances that are currently located in the sediments of Clear Lake and in the North Wetlands areas of the SBMM site. EPA has commenced field work to characterize the nature and extent of contamination in these areas.

Clear Lake is located in Lake County, California, approximately 100 miles north of San Francisco. Clear

Lake is o ne of the b est bass fish ing locatio ns in the U .S. and th e largest fres h water la ke wh olly with in

California. The Lake County economy is significantly dependent upon recreation and tourism at Clear

Lake.

In 1986 the California Department of Health Services issued a fish consumption advisory for Clear Lake due to the presence of elevated mercury in fish tissue. SBMM was recognized as a potential source for the merc ury con taminatio n that is fou nd in C lear Lake .

ES-2

The RI for the terrestrial portions of the SBMM Superfund site was recently completed. The SBMM OU-

1 RI quantifies mercury discharge rates through the groundwater pathway, evaluates the mobility of mercury from mine wastes, presents atmospheric emissions of mercury from mine wastes and native materials on the site, and assesses associated risks to human health and the environment. Based o n these investiga tions, the R I character izes seven systems at SBM M that c ontain h azardou s substan ces within

OU-1 as follows:

C Herm an Imp oundm ent Sys tem, inclu ding the impou ndme nt, the W aste Roc k Dam , the Satellite

Waste R ock Dam , and associated su rface water and groundw ater.

C Northw est Pit Sy stem, inclu ding the old ope n pit and associated surface an d groun dwater.

C Ore System , including the E ast and W est Ore Piles and a ssociated surface w ater, groundw ater, and acid rock dra inage (A RD).

C Tailings System, including the Tailing Pile and associated surface water, groundwater, and ARD.

C Waste Rock System, including the Northwest Waste Rock Pile, the North Waste Rock Pile, the

South Waste Rock Pile and the West Waste Rock Pile and the surface water, groundwater, and

ARD associated with each waste rock pile.

C Disturbed Rock System, including an area of disturbed native rock north of the Herman

Impou ndme nt and th e surface w ater, grou ndwa ter, and A RD ass ociated w ith this distu rbed area .

C Native Ro ck System , including areas o f undisturbed n ative rock within the bound aries of OU-1 and the s urface w ater and g round water ass ociated w ith these un disturbed areas.

The FS for OU -1 deve lops and evaluates potential re medial a lternatives th at could b e implem ented to address the discharge, or threat of discharge, of hazardous substances from contam inated mine wastes, including the ore piles, the tailings pile, the waste rock piles and areas disturbed by mining activity;

contaminated surface water in the Herman Impoundment and Northwest Pit; contaminated groundwater beneath sources in OU-1; and ARD from sources in OU-1 at the SBMM Superfund Site. The FS for

OU-1:

• establishes remedial action objectives and goals for the SBMM OU-1 cleanup action based upon the data an d inform ation dev eloped d uring the RI;

• identifies general response actions that could be implemented to achieve the objectives and g oals, and scree ns a broa d range of remed ial techno logies an d proce ss option s for gene ral applica bility;

ES-3

• develops waste system specific remedial alternatives by evaluating in more detail each of the technologies and process options that were retained after the preliminary screening to determine the potential for application of the technology or process option, and combinations of technologies and process options, to control the potential discharge of hazardous substances from each of the OU-1 waste systems. EPA’s evaluation considers the expected implementability, effectiveness, and cost of the application of each retained technology or process option to control the poten tial discharg es;

• develops detailed waste system specific remedial alternatives retained after this initial screening;

• develops and evaluates site-wide remediation program alternatives that are comprised of sets of waste system specific remedial alternatives; and

• compares the expected performance of each of the site-wide remedial alternatives against the nine criteria specified in the National Oil and Hazardous Substances Pollution Contingency Plan

(NCP).

Remedial alternatives developed for the SBM M OU -1 FS rely on the following techno logies and process options:

C Surface con trols including rev egetation and th e construction o f surface water run -on and run -off control channels, C Containment of solid mine w astes in place using earthen covers, C Containment of solid mine w astes in place using earthen caps with liners, C Excavation of solid mine waste and disposal in an on-site repository, C Collectio n of con taminate d surface water an d discha rge to the H erman Im pound ment, C Collectio n of con taminate d groun dwater a nd disch arge to the Herma n Impo undm ent, C Containment of contam inated groundwater using hy draulic barrier walls, C Water le vel contro l of the He rman Im pound ment us ing surfac e water trea tment w ith discha rge to surface water, and

C Water level control of the Herman Impoundment using surface water treatment with discharge for industrial re -use, such as dischar ge to a po tential on-s ite or near-s ite geothe rmal injec tion well.

The remedial technologies and process options developed for SBMM OU -1 were combined into six site-wide remediation programs as described below:

ES-4

• Alternative 1 would rely upon a hydraulic barrier wall located adjacent to the Herman

Impou ndme nt to min imize gro undw ater flow fr om the im pound ment into Clear La ke. The W aste

Rock Dam would be re-graded and capped with a multi-layer liner. Both Ore Piles and the

Northwest Waste Rock Pile would be excavated and relocated to a repository located on the

South Waste Rock Pile. Shallow groundwater flowing north from the Northwest Waste Rock

Pile area w ould be intercepte d and p umpe d to the H erman Im pound ment. N o water tre atment is included , nor is capp ing of the other w aste rock p iles or the T ailings inc luded in this alternativ e.

• Alternative 2 would rely upon a water treatment facility to maintain water level within Herman

Impoundment low enough to prevent groundwater flow from the impoundment through the

Waste Rock Dam into Clear Lake. The Waste Rock Dam would be re-graded and capped with a multi-layer liner. Both Ore Piles and the Northwest Waste Rock Pile would be excavated and relocated to a repository located on the South Waste Rock Pile. Shallow groundwater flowing north from the North Waste Rock Pile area would be intercepted and pumped to the Herman

Impou ndme nt. No ca pping o f the other waste ro ck piles o r the Tailin gs is includ ed in this alternative .

• Alternative 3 would rely upon a water treatment facility to maintain water level within Herman

Impoundment low enough to minimize groundwater flow from the impoundment through the

Waste Rock D am into C lear Lake . The W aste Roc k Dam and the N orth W aste Roc k Pile w ould be re-graded an d capped w ith a multi-layer liner. Both Ore Piles and the Northwest Waste Rock

Pile wo uld be ex cavated and reloc ated to a rep ository lo cated on the Sou th Wa ste Rock Pile.

• Alternative 4 would rely upon a water treatment facility to maintain water level within Herman

Impoundment low enough to minimize groundwater flow from the impoundment through the

Waste Rock D am into C lear Lake . The W aste Roc k Dam and the N orthwe st Waste Rock P ile would be excavated and relocated to a repository located on the east side of the Tailings and south side South Waste Rock Pile. The Waste Rock Dam site would be reclaimed. The shoreline would be returned to the approximate pre-mining alignment and would include a narrow beach area. The N orth W aste Roc k Pile w ould be re-graded and cap ped w ith a multi-la yer liner. B oth

Ore Piles would be excavated and relocated into (quenched in) the Herman Impoundment. The remaining waste areas (the north side of the Tailings, the north side of the South Waste Rock Pile, and the W est and Satellite W aste Rock P iles) and the Disturb ed Rock A rea along the no rth shore of the H erman Im pound ment w ould be re-graded and cov ered with soil.

• Alternative 5 would rely upon a hydraulic barrier wall located adjacent to the Herman

Impou ndme nt to min imize gro undw ater flow fr om the im pound ment into Clear La ke. The W aste

Rock D am wou ld be re-graded and capped with a multi-laye r liner. Re-grading would include excava tion and relocation of mine waste alo ng the sh oreline to s mooth out the sh oreline an d to provide a narrow beach ar ea. The N orth W aste Rock Pile would be re-graded and capped with a multi-layer liner. The Northwest Waste Rock Pile would be excavated and relocated to a repository located on the South Waste Rock Pile. Both Ore Piles would be excavated and relocated into (que nched in ) the Herm an Imp oundm ent. The re mainin g waste a reas (the no rth side of the Tailings, th e north sid e of the So uth W aste Roc k Pile, and the We st and Sa tellite

Waste Rock Piles) and the Disturbed Rock Area along the north shore of the Herman

Impou ndme nt wou ld be re-g raded an d cover ed with s oil. No w ater treatme nt is includ ed.

ES-5

• Alternative 6 would rely upon a water treatment facility to maintain water level within Herman

Impoundment low enough to minimize groundwater flow from the impoundment through the

Waste Rock D am into C lear Lake . The W aste Roc k Dam and the N orth W aste Roc k Pile w ould be re-graded and capped with a multi-layer liner. Both Ore Piles and the Northwest Waste Rock

Pile wo uld be ex cavated and reloc ated to a rep ository lo cated on the Sou th Wa ste Rock Pile.

The remaining waste areas (the Tailings, the South, West and Satellite Waste Rock Piles) and the

Disturbed Rock Area along the north shore of the Herman Impoundment would be re-graded and covered with soil.

These six site-wide remedia tion prog rams w ere then c ompa ratively an alyzed in regards to overall protection of human health and environment; compliance with ARARs; long-term effectiveness and permanence; reduction in toxicity, mobility, and volume; short-term effectiveness, implementability; and costs. State and community acceptance will be evaluated at a later date once EPA receives input from the

State and the community during the public comment period on this SBMM OU-1 FS and the Proposed

Plan.

1-1

1.0 INTRODUCTION

The U.S. Environmental Protection Agency (EPA) is addressing threats to human health and the environ ment po sed by h azardou s substan ces at the S ulphur B ank M ercury M ine (SB MM ) Superfu nd Site for Operable Unit 1 (OU-1), the terrestrial areas of SBMM, under the Comprehensive Environmental

Respo nse Co mpen sation an d Liability Act (CE RCL A, also k nown as Supe rfund), an d the Na tional Oil and H azardou s Substa nces Po llution C ontinge ncy Pla n (NC P).

As part of this process, a two-volume remedial investigation and feasibility study (RI/FS) has been prepared. Vo lume I is the reme dial investigation (R I), which presen ts the data that characterize th e nature and exte nt of con taminatio n for sou rces with in OU -1. The R I also prese nts the ba seline risk a ssessme nt.

Volume II is the feasibility study (FS). The FS for OU-1 identifies and screens potential cleanup technologies for application at SBMM. The FS then develops, screens, evaluates, and compares potential remedial alternatives to clean up solid w astes (including o re piles, tailings, waste rock p iles), contaminated surface water, and contaminated groundwater from sources within OU-1 at SBMM.

1.1 REPORT ORGANIZATION

This FS is organized into the following sections: Section 1 provides the introduction and site background informa tion. Sectio n 2 briefly summ arizes the ap plicable o r relevant a nd app ropriate req uiremen ts

(ARA R), reme dial action objective s and go als, and ge neral resp onse actio ns for the S BM M clean up.

Section 3 identifies and screens a broad range of remedial technologies and process options for potential use in implementing the SBMM cleanup. Section 4 identifies potential remedial alternatives for each waste system that was characterized in the RI, and evaluates and screens these alternatives based on effectiveness, implementability, and cost criteria. Section 5 provides a detailed description of the retained alternatives and the assumptions used to develop preliminary cost estimates. Section 6 identifies the criteria that the EPA uses for the detailed analysis of the alternatives pursuant to the NCP. Section 7 provides a detailed analysis of the retained alternatives both on a system and site-wide basis. References follow th e text. Tab les and fig ures follow the text in ea ch section .

1-2

1.2 SITE DESCRIPTION

The SBMM Superfund site is located within Lake County, California on the eastern shore of Clear Lake near the community of Clearlake Oaks (Figure 1-1). OU-1 includes the inactive mercury mine, including the Herman Impoundment and the Northwest Pit (Figure 1-2), ore piles, tailings, waste rock piles, and the associated contam inated gro undw ater and su rface wa ter.

The Elem Tribal Colony (Sulphur Bank Rancheria) is located immediately adjacent to the mine site to the north of the SBMM m ine property. The cleanup of SBMM derived contamination currently located in the

Elem Tribal Colony area will be addressed in a later cleanup action and is not being addressed as part of

OU-1. The SBMM-derived contamination currently located in the sediments of Clear Lake and in the wetlands located to the north of the mine site will also be addressed in a later cleanup action as part of

OU-2 .

1.3 NATURE AND EXTENT OF CONTAMINATION

Numerous investigations have been previously conducted at the SBMM S uperfund site by federal, state, and local agencies and interested groups to document contaminant concentrations in mine wastes, lake sediments, surface waters and groundwaters. These studies indicate releases of hazardous substances containing elevated concentrations of mercury, arsenic and other metals (antimony, chromium, zinc and others) tha t may p ose risks to human health an d the env ironme nt.

General descriptions of waste sources and contaminated media are presented below for the seven systems described in the RI (TtEMI 2002). The RI defines physical boundaries for each system (Figure 1-3) and each system may contain components of solid materials, surface waters, groundwaters, atmosphere, and organisms that dynamically interact. Table 1-1 summarizes the quality of groundwater in the seven systems. Table 1-2 summarizes the area and volume of various waste piles at SBMM and the concentration and mass of contam inants within these waste piles.

Herman Impoundment System: This system consists of the Herman Impoundment, the Waste Rock Dam, and the Satellite Waste Rock Pile. The Herman Impoundment, an open pit that was excavated during mining and is now filled with a mixture of m eteoric and hy drothermal w ater, is a 2,500,000 cubic yard

1-3

(cy) impoundment with a total surface area of 23 acres. This impoundment acts as a hydrologic sink for most of the mine site. Water from the Herman Impoundment, which is acidic and rich in metals (See

Table 1-1), flows through the waste rock dam where it leaches additional metals, in particular mercury, from the dam materials and mixes with infiltrating meteoric waters before discharging to Clear Lake. The

Herman Impoun dment itself is a sink for m ercury that enters into it via incoming sto rmwater, hydrothermal waters, and groundwaters. The mercury precipitates from solution in the acidic waters of the impo undm ent and re mains as a heavy metal sed iment.

The Waste Rock Dam consists of an estimated 556,000 cy of waste rock that was placed on top of in-place rock between the Herman Impoundment and Clear Lake. The dam has a total surface area of approximately 24 acres. In 1992 the dam was remediated on an interim basis with an engineered revegetated soil co ver consisting o f a 2 foot thick layer o f soil obtained from a non-merc ury-bearing o ff-site borrow area.

The Sa tellite Wa ste Rock Pile, whic h consists of 33,00 0 cy of w aste rock c overing an area of roughly three acres, consists of waste rock that was moved during re-sloping and covering of the Waste Rock

Dam in 1992. The pile releases acid and metals into the underlying saturated zone due to oxidation and reaction w ith infiltrating meteoric water.

Contam inants of c oncern (COC ) in this syste m cons ist of merc ury, arsen ic, and an timony in the wa ste rock, metals (especially mercury and aluminum) and acid in Herman Impoundment water and groundwater, and dissolved nickel found in runoff from the Waste Rock Dam. The unique chemical and physical characteristics of this system are why it is considered to be the most important source and pathway for mercury and other hazardous chemicals migrating from SBMM.

Northwest Pit System: This system consists of a 150,000 cy ov al-shaped pit that is the result of past mining operation s at the site. Th e bottom of the pit is u sually co vered b y a shallo w poo l of acidic w ater.

There are also several hydrothermal gas vents located within this pit. Because the pit is a depression, surface water cannot flow out of the pit, and any water that enters the pit either evaporates, or infiltrates into the saturated zone beneath the pit. Infiltration of water from the pit into the underlying groundwater is a concern because the pit is located on a groundwater mound and groundwater in this area may flow north tow ards the w etlands, n orthwe st toward s the Elem Indian C olony , or west to wards C lear Lake .

1-4

Water in the Nor thwest P it is of mod erate wate r quality fo r the site and contains little mercu ry, althou gh it is quite acid ic.

Ore System: This sys tem con tains the E ast and W est Ore P iles, which cover an area of 2.9 acres and 1.4 acres, respectively. Because the primary material in the system is mercury ore, highly mineralized rock that contained concentrations of mercury high enough to be processed at a profit, this system contains the mine waste with the highest concentrations of mercury. Infiltrating rainwater oxidizes and reacts with the ore to produce metal-rich acid rock drainage (ARD) that leaves the system as surface water runoff or infiltrates into the underlying groundwater. ARD waters released from the ore system either travel to the

Herman Impo undment or to Clear Lak e. Paradoxically, although the ore contains the highest concen trations of m ercury in the solid p hase, AR D from ore piles is sim ilar in merc ury con centration to

ARD from oth er mine w aste piles at S BM M.

Tailings System: The 45,000 cy of tailings at SBMM cover an area of 13.6 acres on the south side of the

Herman Impoundment. Tailings are solid-matrix waste products from mineral (ore) processing or concentrating operations. In a manner similar to that described above for the ore, meteoric surface and infiltrating waters react with the tailings to generate acid and metals, although many of the acid- and metal-generating minerals in the tailings were removed during ore processing. In general, ARD generated from the tailings is neither as acidic nor as metal rich as ARD from waste rock. Surface water and groundwater in the Tailings System generally travel north into the Herman Impo undment, but may also be transported west to Clear Lake.

Waste Rock System: This sys tem con sists of wa ste rock m aterials that are not ore g rade bu t contain significant concentrations of hazardous substances such as mercury and arsenic. The waste rock was removed during mining activities and placed in several piles located around the SBMM site. The piles are designated as the Northwest (3.1 acres), North (20.5 acres), South (27.1 acres), and W est (11.4 acres)

Waste Rock P iles. The waste piles contain materials that, under oxidizing conditions, may release significan t quantities of mercu ry and h azardou s chemic als into gro undw ater and sto rm wate r runoff.

Depending upon the location of a particular waste rock pile, the associated runoff and ARD may flow toward the Herman Impoundment, Clear Lake, the Elem Tribal Colony, or the North Wetlands. An addition al concer n is the po tential for ex posure b y direct co ntact with the waste s by pers ons or an imals performing activities at the site. Because of differences in the nature and extent of contamination related

1-5 to each waste pile, and because of differences in the potential transport pathways, remedial concerns differ for ind ividual w aste piles.

• The Northwest Waste Rock Pile is the smallest of the four waste rock piles (21,000 cy), but the material contains high contaminant concentrations. That portion of the waste rock pile north of the Elem Colon y Roa d was re mediate d on an interim ba sis with an enginee red reveg etated soil cover. In addition, the Northwest Waste Rock Pile is located close to potential human and ecological receptors in the Elem Tribal Colony and in the North Wetlands. ARD discharges from the Northwest Waste Rock Pile flow toward the North Wetlands and, therefore, may be a disprop ortionate ly greater s ource o f methy l mercury discharg es to Clea r Lake.

• The North Waste Rock Pile contains moderate contaminant concentrations, but the large volume of this waste rock pile (878,000 cy) results in a total contaminant mass significantly greater than the other waste rock piles. ARD discharges from the North Waste Rock Pile flow toward the

North W etlands and, therefo re, may be a d isproportionately greater source of m ethyl mercury discharg es to Clea r Lake.

• The So uth W aste Roc k Pile ma terial conta ins low c ontam inant con centration s, but the rela tively large volume (730,000 cy) of the waste pile results in a high total contaminant mass. The discharg es from th e South Waste R ock Pile g enerally flo w into th e Herm an Imp oundm ent.

• The W est Was te Rock Pile is sma ller in total vo lume (2 60,000 cy) than either the N orth or S outh

Waste Rock P iles, but the m aterial with in this pile co ntains hig her con taminan t concen trations.

This results in the total contaminant mass for the West Waste Rock Pile being similar to that of the North and South Waste Rock Piles. Some of the ARD discharges from the West Waste Rock

Pile flow toward the Herman Impoundment and some flow toward Clear Lake.

Disturbed Rock System: This system consists of 18.8 acres of andesite, lake sediments, and Franciscan

Complex rocks located north and east of Herman Impoundment that were excavated or terraced during mining. Much of the rock in this system has been heavily altered by hydrothermal fluids and has been mineraliz ed. The D isturbed R ock Sy stem has very hig h mercu ry conc entration s in solid-p hase ma terial, but groundwater does not appear to be a significant pathway for contaminant transport. Groundwater was only observed in the Franciscan Complex water-bearing unit. The observed groundwater appeared to be a hydro thermal u pwellin g unaffe cted by the disturb ed rock . Contam inants from this system migrate directly into Herman Impou ndment and do n ot directly affect off-site receptors.

Native Rock System: This system covers the remaining 69 acres within OU-1, and includes two components: (1) the undisturbed slope of Native Franciscan Complex material south of the mine and

(2) ande site north o f Herm an Imp oundm ent that w as not ex cavated , terraced, or covered by wa ste material. The Native Rock System contains contaminant levels that are generally below levels in the

1-6 other systems in vestigated. M ercury and o ther hazardou s substances oc cur naturally in the g roundw ater, solid-phase material, storm water runoff, and atmospheric emissions in the areas at SBMM that were not disturbed by mining activity.

1.4 PHYSICAL HAZARDS

Abandoned or inactive mining features such as the Herman Im poundmen t, the Northwest Pit, highwalls, or abandoned structures can create physical hazards for humans and animals. Although physical hazards are usually not directly addressed as part of a CERCLA action, they may be remedied if they represent an imminent hazard or as a consequ ence of site cleanup actions.

Table…