Evaluation of Dioxin-Related Risks in Proposed Reuse of Concrete Masonry Unit Blocks for Airport Construction

The primary objective was achieved through the excavation of contaminated soil and sediment, thermal treatment using In-Pile Thermal Desorption (IPTD) technology, and the construction of an enclosed and insulated structure to hold the dioxin-contaminated material. The IPTD structure consisted of approximately 28,000 interlocking concrete masonry unit (CMU) blocks, which did not have direct contact with contaminated soil and sediment inside the structure. Following the successful completion of Phase 2 treatment in August 2017, the IPTD structure was dismantled, and the CMU blocks were sampled and removed from the Project site. The CMU blocks were then stockpiled in a temporary storage area on the west side of the Airport. Initial wipe samples collected from the CMU blocks found that some of the interior blocks had trace dioxin contamination above the project action level. Core samples were collected from the CMU blocks in May and June 2018 to characterize the depth of dioxin contamination and volumetric concentration. The results of the wipe and core sample analyses confirm that dioxin was present in only a portion of the CMU blocks that formed the innermost layer of the IPTD structure. Based on the configuration of the IPTD, USAID conservatively estimates that during treatment operations less than 10% of the total number of CMU blocks were contaminated with the identified trace amount of dioxin. The CMU blocks were not anticipated to be impacted by dioxin, and the results of the sampling and analysis confirm that the contamination was limited to a small portion of the blocks. The evaluation of alternative uses for the CMU blocks considered unrestricted reuse, storage area, and restricted reuse onsite. The unrestricted reuse option was not recommended due to the potential for dioxin contamination to be released into the environment. The storage area option was considered, but it was determined that the CMU blocks could be safely reused onsite as a retaining wall. The risk assessment for the CMU blocks in the retaining wall configuration considered the potential impact on recreational swimmers, humans consuming Sen Lake fish, and ecological risk to Sen Lake fish. The results of the risk assessment indicate that the CMU blocks in the retaining wall configuration pose a low risk to human health and the environment. The conclusions and recommendations of the report are based on the results of the sampling and analysis, the evaluation of alternative uses for the CMU blocks, and the risk assessment. The report recommends that the CMU blocks be reused onsite as a retaining wall, and that the risk assessment results be used to inform future decisions regarding the use of the CMU blocks. The report also provides a summary of the key findings and recommendations, as well as a list of references used in the preparation of the report. The report is a comprehensive evaluation of the dioxin-related risks associated with the proposed reuse of the CMU blocks for airport construction.