Overcoming Challenging Site Conditions to Remediate High Perchlorate Concentrations in Groundwater Using In Situ Bioremediation

APTIM Authors/Contributors

Background/Objectives
At a former Army Ammunition Plant in Northeast, Texas, in situ bioremediation (ISB) was proposed to address elevated perchlorate concentrations in groundwater under a fixed price performance-based remediation (PBR) contract. The site is approximately ½ acre in size and consisted of a former pilot wastewater treatment plant used from 1984 to 1997 to treat explosives and perchlorate contaminated wastewater from sumps throughout the ammunition plant. Approximately 3,400 cubic yards of mercury and perchlorate contaminated soils were excavated from the site in 2009 and disposed of offsite. Subsequent groundwater investigations identified a plume of perchlorate contamination in the shallow groundwater under the excavated area. The 2016 Record of Decision (ROD) selected ISB and land use controls as the remedy for the perchlorate in groundwater. Native subsurface geology consists of mostly tight clays, silty sands, and silty clays. The shallow groundwater-bearing zone consists of a fine-grained silty sand with an irregular thickness occurring anywhere between 7 to 20 feet below ground surface. The site slopes gently towards a creek to the west and includes a drainage ditch that carries surface water runoff to the creek. Prior to the ROD, sampling in 2010-2011 indicated that perchlorate concentrations as high as 5,410 µg/L were present at the site. Additional sampling was performed in 2018 and 2019 to complete the westward delineation of the plume. One of the wells installed to delineate the plume contained a perchlorate concentration of 10,000 µg/L to the east of the excavated area, more than two times the maximum concentration detected in 2011.

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