Final Decision Document for Other Contamination Sources Interim Response Action Shell Section 36 Trenches, Rma

798308 - R8 SDMS FINAL DECISION DOCUMENT FOR OTHER CONTAMINATION SOURCES INTERIM RESPONSE ACTION SHELL SECTION 36 TRENCHES, RMA Prepared by MK-Environmental Services Denver, Colorado 80203 Prepared for Shell Oil/Holme Roberts & Owen Denver, Colorado 80203 April 1990 4^ DEPARTMENT OF THE ARMY PROGRAM MANAGER FOR ROCKY MOUNTAIN ARSENAL coMMCNCE crrv. COLORAOO aoo22-2iao May 3, 1990 llfLT TO ATTtNTiea or: Interim Response Division Mr. George Roe Shell Oil Company P.O. Box 4320 Houston, Texas 77210 Dear Mr. Roe: This letter serves to advise you of the finalization of the Decision Documents for the Army Complex Disposal Trenches and the Shell Section 36 Trenches Interim 0U23 Response Actions at Rocky Mountain Arsenal Since no dispute was raised, the Draft Final Decision Documents that were issued March 30, 1990, shall serve as the Final Decision Documents. Enclosed with this letter are replacement white covers and text pages for your copies of the documents. Comments were received from the U.S. Environmental Protection Agency (EPA) and the State of Colorado. Neither of the two Decision Documents were disputed. A brief response to the general comments and concems is provided below. Specific comments provided wQl be discussed with the Organizations and State (OAS) during the design phase. Comments were received from both the EPA and State on the Army Complex Disposal Trenches document. The Army agrees that all-party discussions need to be held to discuss design issues prior to the release of the Draft Implementation Document. Text changes for the Final version, however, will not be made except for page 5-2 which will update the Chronology of Events section. The Army commits to assessing the northeastern flow path to the degree necessary to determine whether or not the trenches areas located east of the bedrock high are significantly contributing to groundwater contamination. This commitment includes well installation. However, the Army does not believe at this time that the requested site specific aquifer pump tests are necessary. We wish to discuss this issue with the OAS before committing to performing aquifer tests. As stated in the Draft Final Decision Document, the reevaluation process presented was only a suggested procedure to be followed, and would only be finalized during the design process with input from the parties. The Army, however, does not envision a process that has "trigger levels" for certain compounds that would automatically invoke a predetermined remedial action if the "trigger level" was detected. The Army's general concept of the reevaluation process is that if the overall site conditions change significantly from that already known, the decision node on Figure 1-1 G9000367 "Is there a significant long-term benefit (in terms of cost or accelerated cleanup) if an IRA is done now" would be reevaluated. If monitoring does not show significant changes from that known to date, a Reevaluation Report would be submitted to the parties basically documenting the data collected for the reevaluation period. In response to the EPA's request to accelerate the deadline for the Draft Implementation Document, the Army caimot agree to change the date from October 1, 1990 to July 15, 1990. The reason for this is the procurement process which the Army is required to follow. We currently do not have a contract with any of our contractors to prepare a Draft Implementation Document. Furthermore, the Army does not have a contract to actually perform the monitoring, well installations, etc. at this time, and should not begin that procurement process until after the Implementation Document is finalized. The Army will, however, attempt to accelerate the process and issue the document as soon as possible. We would like to discuss this issue at the next RMA Committee meeting with the OAS. The Army does not presently intended to perform additional trenches charaaerization other than through groundwater monitoring. The Army beheves that sufficient trenches characterization exists for the purposes of this IRA, If additional trenches characterization is deemed necessary by the Feasibility Study, the field work would be performed under that program (e.g. Task RIFS14 - FS Field Data Colleaion). In response to the State's general comments on the Army Complex Disposal Trenches IRA, the Army disagrees with the following three issues of the State: 1. inadequate trench characterization at the site; 2. monitoring does not constitute an' IRA; 3. the reevaluation procedure virtually insiu-es a monitoring-only scenario. As stated in our response to EPA's concerns on additional trench characterization, the Army believes adequate characterization exists for supporting the monitoring decision. If the Feasibility Study requires additional information, work would be performed under that program. The Army also disagrees with the State's second issue because monitoring is a viable decision for an IRA in accordance with the Decision Flow Chan for Other Contamination Sources IRAs, Figure 1-1 of the Draft Final Decision Document. With regards to the State's third issue, the Army's reevaluation procedure was only a suggested procedure and would be finalized during the design phase with input from the OAS. The suggested procedure was not developed with the intent to insure a monitoring-only scenario. It was developed with the intent to provide a mechanism by which any substantial change in site conditions would be documented and a reasoned and informed response be taken. It is the Army's belief, however, that additional data will not be substantially different from existing data and will not result in a significant benefit to be gained in performing any other type of action for this IRA. Conunents were made on the Shell Section 36 Trenches IRA by the EPA only. It appears the EPA still has concems regarding the relationship between the eluvial clay layer and the Denver Formation, and the clay layer's potential for prohibiting vertical migration of contaminants from the trenches into the Denver Formation. These concems are appreciated and will be considered during the design of this IRA- It should be noted that thei objective of "Remediation of Other Contamination Sources" IRAs are not intended to totally eliminate the release of contaminants from a site. As stated in the Technical Program Plan, the objective of these mterim response actions, once determined necessary, is to mitigate the threat of release of contaminants fiom the contamination source. The effectiveness of the eluvial clay and the subsurface barrier wall can only be measured by a groundwater monitoring program. This monitoring program will be provided to the parties as part of the Preliitiinary Engineering Package (PEP). EPA requested that the Final Decision Document state that the hydraulic barrier be a slurry wall compatible with the contaminants at the site. Compatibility testing is currently being plaimed. The scope of the testing will be presented in the PEP. Results of the compatibility testing and other design factors may prove that other types of barrier walls would be more proficient. The type of barrier wall to be installed will also be presented in the PEP, Additional field investigations for tracking DNAPL outside the barrier wall area will be performed. It is not yet known the method or program by which the investigations will be conducted (i.e. as part' of this IRA or a separate IRA), however, the investigative approach will be presented in the PEP. If you have any questions, contact Mr. J.D. Smith, (303) 289-0201. Suicerely, / Donald L Campbell Deputy Program Manager Enclosure Copies Furnished; Major Lawrence E. Rouse, Headquarters, Department of the Army, ATTN: DAJA-ELL, Pentagon, Room 1C480, Washington, D.C 20310-2210 (w/o end) Mr. Bradley Bridgewater, Acumenics Research and Technology, Inc., DOJ Litigation Support, 999-18th Street, Suite 501, North Tower, Denver, Colorado 80202 (w/o end) Mr. William Adcock, Shell Oil Company, 1 Shell Plaza, 900 Louisiana Street, Room 1426, Houston, Texas 77210 (w/encl) Mr. Chris Hahn, Shell Oil Company, c/o Holme Roberts and Owen, Suite 4100, 1700 Lincoln Street, Denver, Colorado 80203 (w/end) Mr. Edward McGrath, Hohne Roberts and Owen, 1700 Lincoln, Suite 4100, Denver, Colorado 80203 (w/encl) Acumenics Research and Technology, Inc., Room 132, Building 111 Rocky Mountain Arsenal, Commerce City, Colorado 80022 (w/o end) TABLE OF CONTENTS Section S&3S. 1.0 INTRODUCTION 1 2.0 SITE DESCRIPTION 3 2.1 LOCATION AND SITE HISTORY 3 2.2 HYDROGEOLOGY 3 2.3 NATURE AND EXTENT OF CONTAMINATION 4 3.0 INTERIM RESPONSE ACTION OBJECTIVE 6 4.0 INTERIM RESPONSE ACTION ALTERNATIVES 7 4.1 ALTERNATIVE STRATEGIES 7 4.1.1 No Action 8 4.1.2 Monitoring/Maintenance 8 4.1.3 Excavation 8 4.1.4 In-Situ Remediation 9 4.1.5 Containment 10 4.2 SYSTEM ALTERNATIVES ' 10 4.2.1 Constructing and Operating a Recovery Trench Downdip and Downgradient and Extracting Groundwater and DNAPLs 11 4.2.2 Encircling Shell Trenches with a Physical Barrier and Extracting Groundwater 12 4.2.3 Encircling Shell Trenches with a Physical Barrier and Constructing a Soil and Vegetative Cover 12 5.0 CHRONOLOGY OF EVENTS 14 6.0 DESCRIPTION OF THE INTERIM RESPONSE ACTION 16 7.0 IRA PROCESS 18 8.0 APPLICABLE OR RELEVANT AND APPROPRIATE REQUIREMENTS FOR THE REMEDIATION OF OTHER CONTAMINATION SOURCES (SHELL TRENCHES) INTERIM RESPONSE ACTION 21 8.1 INTRODUCTION 21 8.2 AMBIENT OR CHEMICAL-SPECIFIC ARARS 21 03/22/90 — i — TABLE OF CONTENTS (continued) 8.3 LOCATION-SPECIFIC ARARS 23 8.4 ACTION-SPECIFIC ARARS

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