Rocky Mountain Arsenal Offpost Operable Unit Final Record of Decision Rocky Mountain Arsenal Commerce City, Colorado Prepared for Program Manager for Rocky Mountain Amenal Building 111, Rocky Mountain Arsenal Commerce City. Colorado 80022-2180 HLA Project No. 21905 402010 Contract No. DAAA05-92-D-0003 Delivery Order No. 0005 TIUS DOCUMENT IS IN7LNDED TO COMPLY WITH THE NATIONAL ENVIRONMENTAL POLICY ACT OF 1969. THE INFORMATION AND CONCLUSIONS PRESENTED IN TFUS REPORT REPRE- SENT THE OFFICIAL POSITION OF THE DEPARTMENT OF THE ARMY UNLESS EXPRESSLY MODIFMD BY A SUBSEQUENT DOCUMENT. TMS REPORT CONSTI- TUTES THE RELEVAN7 PORTION OF THE ADMINISTRATIVE RECORD FOR TIES CERCLA OPERABLE December 19, 1995 Harding Lawson Associates z 11.= 1z Engineering and Environmental Services 707 Seventeenth Street, Suite 2400 Denver, CO 80202 - (303) 292-5365 M9604811 Recycled Paper CONTENTS **'-**-*- DS-1 DECLARATION STATEMENT ................................... I.o SITE NAME, LOCATION, AND DESCRIPTION ................................. 1-1 ... 1-2 1.1 Environmental Setting ................................. 1-2 1.2 Geology ......................................... 1-3 1.3 Hydrogeology .......................................... 2-1 2.0 SIIT HISTORY AND ENFORCEMENT ACTIVITIES ............................. 2-1 2.1 Operational History ................................................ * * * * 2-2 2.2 Previous Investigations .......................................... 2-2 2.2.1 U.S. Environmental Protection Agency Study Area .................. 2-3 2.2.2 U.S. Department of the Army Investigation ....................... - - 2-4 2.3 Boundary Containment Systems ................................... 2-5 2.3.1 North Boundary Containment System ........................... 2-6 2.3.2 Northwest Boundary Containment System ........................ 2-6 2.3.3 Irondale Containment System ................................. * * 2-7 2.4 Interim Response Actions .......................................... 2-7 2.4.1 Offpost Interim Response Action ............................... 2.5 History of CERCLA Enforcement Activities ............................... 2-8 3-1 3.0 HIGHLIGM OF COMmUNrff PARTICIPATION .............................. 4-1 4.0 SCOPE AND ROLE OF THE OFFPOST OPERABLE UNIT ......................... s.o SUINIMARY OF SIT`E CHARACTERISTICS .................................... 5-1 5.1 Sources of Contam ination ............................................ 5-1 5.2 Nature of Contam ination ............................................ 5-1- 5.3 Contamination Migration Pathways .................................... 5-2 5.4 Extent of Contam ination .............................................. 5-3 5.4.1 Groundw ater .............................................. 5-3 5.4.2 Surface W ater ............................................. 5-4 5.4.3 Stream-bottom Sediments .................................... 5-5 5.4.4 Surface and Subsurface Soil .................................. 5-5 5.4.5 Biota .................................................... 5-6 5.5 Potential Routes of Human and Environmental Exposure .................... 5-7 '21905 402010 Harding Lawson Associates 1107121895 R02 6-1 6.0 SUMMARY OF SrM RISKS ................................. 6-2 .......................... 6.1 Human Health Risks 6-2 of Chemicals of Concern .......................... 6.1.1 Identification ... 6-3 6.1.2 Exposure Assessment ...................... ............. 6-3 Offpost Study Area Exposure Assessment Zones 6.1.2.1 6-5 Offpost Study Area Potential Exposure Points .............. 6.1.2.2 6-6 potential Exposure Pathways and Routes .................. 6.1.2.3 6-8 6.1.2.4 Estimation of Chemical Intake .......................... ... 6-8 6.1.3 Toxicity Assessment .......................... *,*** .... 6-9 6.1.4 Risk Characterization ............................... 6-9 6.1.4.1 Carcinogenic Risks ................................... 6-10 6.1.4.2 Noncarcinogenic Effects .............................. 6-11 of Potential Ecological Effects ............................... 6.2 Estimation 6-11 M ethod ................................................. 6.2.1 6-13 6.2.2 Results ................................................. ****'*** .... 6-14 6.3 Conclusion .......................................... ALTERNATIVES ............... 7-1 7.0 DESCRIPTION OF GROUNDWATER REMMIATION 7-4 7.1 Common Elements of Alternatives ..................................... .............. 7-6 identification of Groundwater Alternatives: North Plume Group 7.2 7-6 7.2.1 Alternative N-1: No Action ................................... 7.2.2 Alternative N-2: Continued Operation of the North ...... 7-6 Boundary Containment System With Improvements as Necessary System 7-7 7.2.3 Alternative N-4: Offpost Groundwater Intercept and Treatment 7.2.4 Alternative N-5: Expansion of the Offpost Groundwater Intercept and Treatment System .............................. 7-10 .......... 7-12 7.3 identification of Groundwater Alternatives: Northwest Plume Group 7-12 7.3.1 Alternative N-1: No Action .................................. 7.3.2 Alternative NW-2: Continued Operation of the Northwest Boundary Containment System With Improvements as Necessary ..... 7-13 8-1 8.o COMPARATIVE ANALYSIS OFALTERNATWES ............................... 8-2 8.1 Comparative Analysis of Remedial Alternatives ........................... 8-3 8.1.1 Overall Protection of Human Health and the Environment ............ 8.1.2 Compliance With AppLcable or Relevant and Appropriate Requirements - 8-4 8.1.3 Long-term Effectiveness and Permanence ......................... 8-5 8.1.4 Reduction in Toxicity. Mobility. or Volume ....................... 8-6 8.1.5 Short-term Effectiveness ..................................... 8-6 8.1.6 Im plementability ........................................... 8-7 8.1.7 Cost .................................................... 8-9 8.1.8 State Acceptance .......................................... 8-10 8.1.9 Community Acceptance ..................................... 8-11 8.2 Conclusions of the Comparative Analysis of Alternatives ................... 8-11 ii Harding Lawson Associates 21905 402010 1107121895 R02 9-1 9.o IDENTIFICATION OF THE SELECT REMEDY ............................... System ......... 9-1 9.1 Alternative N-4: Offpost Groundwater Intercept and Treatment 9.2 Alternative NW-2: Continued Operation of the Northwest Boundary 9-3 Containment System with Improvements as Necessary ...................... 9-5 9.3 Additional Components of the Selected Remedy ........................... 9-6 9.4 Cost of Selected Remedy ............................................ 9-6 9.5 Lim itations ....................................................... 9.6 Criteria for Shutting Down Boundary and Offpost. Containment Groundwater 9-7 System s ......................................................... 10-1 10.0 STATUTORY DETERMINATIONS ......................................... ....... 10-1 10.1 Consistency with the Statutory Requirements of CERCLA in Section 121 10-1 10.1.1 Protection of Hinnan. Health and the Environment ................. 10-2 10-1.2 Compliance With Applicable or Relevant and Appropriate Requirements 10-3 10.1.3 Cost Effectiveness ......................................... 10-4 10-1.4 Utilization cf Permanent Solutions to the Maximum Extent Practicable . 10-4 10.2 Consistency with the National Contingency Plan ......................... 10-5 10.3 Sum m ary ....................................................... 11-1 11.0 DOCUMENTATION OF SIGNIFICANT CHANGES ............................. 12-1 12.0 G LO SSARY .......................................................... 13-1 13.0 BIBU OGRALPHY ....................................................... TABLES 6.1 Offj--551 ();-'rdble Urlit Groundwater Chemicals of Concern 6.2 Offt"m ()I--rdble Unit Surface-Water Chemicals of Concern 6.3 DI--r-Alle Unit Sediment Chemicals of Concern in First Creek 6.4 0ifix,st ()I--rable Unit Sod Chemicals of Concern 6.5 Suruman of Ldnd-Use Scenarios and Exposure Routes by Zone 6.6 R- 1--r--n, - I)-,u-s and Slope Factors for Chemicals of Concern Route 6.7 Surrinjar% of Rt-asonable Maximum Exposure Carcinogenic Risks by Zone and Exposure 6.8 Sunimar% of Adult Reasonable Maximum Exposure Noncarcinogenic Han d. Indices by Target Orgd,l dn I Lxposure Assessment Zone Treatment 7.1 COntdinnient System Remediation Coals for the Offpost Groundwater Intercept and S,.,stem 7.2 Containment Svstem Remediation Goals for the North Boundary Containment System 7.3 Containment System Remediation Coals for the Northwest Boundary Containment System 7.4 Groundwater Alternatives for the North and Northwest Plume Groups 8.1 Summarv of the Detailed Analysis and Ranking of Groundwater Alternatives for the North Plume Group 8.2 Summary of the Detailed Analysis and Ranking of Groundwater Alternatives for the Northwest Plume Group iii '21905 402010 Harding Lawson Associates 1107121995 R02 Remedy 9.1 Estimated Costs of the Offpost Operable Unit Selected and Other Applicable -or Relevant and Appropriate 10.1 Summary Evaluation of Chemical-specific Requirements for the Offpost Operable Unit Applicable or Relevant and Appropriate Requirements 10.2 Summary Evaluation of Action-specific for the Offpo'st Operable Unit or Relevant and Appropriate 10.3 Summary Evaluation of Location-specificApplicable Requirements for the Oftpost Operable Unit FIGURES and Offpost Study Area 1.1 Rocky Mountain Arsenal Operable Units 2.1 Locations of Contaminant Source Areas 5.1 Contaminant Migration Pathways Zones 6.1 Offpost Study Area Exposure