Formatted Title
18 Years of Full-Scale Mulch Biowall System Performance to Address Chlorinated Solvents in Groundwater
Background/Objectives
The former Seneca Army Depot Activity is located in the Finger Lakes Region of New York near the town of Romulus. The Ash Landfill site is located in the former Depot and consists of five former solid waste management units related to the incineration of non-hazardous materials and disposal of the incinerator ash through burial. Operations and disposal practices at this site have resulted in chlorinated solvent impacts to soil and the formation of a plume that extended approximately 1,100 feet to the south, across the facility property line and onto neighboring private property. Remedies were implemented at the Ash Landfill starting in the mid-1990s with a source removal excavation and the installation of a zero-valent iron (ZVI) permeable reactive barrier (PRB) at the plume toe near the property line in 1999.
Approach/Activities
After the installation of the ZVI PRB, the Ash Landfill chlorinated solvent plume was assessed to determine how long the plume would likely be present and thus how long the ZVI PRB would be required to operate. The resultant model predicted that the plume lifespan was unacceptably long and additional plume control measures were conceptualized to accelerate plume reduction and site closure. Enhanced bioremediation in the form of bark mulch PRBs was selected for plume control after a pilot test was successfully completed onsite in 2005. A total of 2,840 linear feet of trenched-in bark mulch PRBs were installed in four separate biowall segments in 2006. The biowall segments were placed along the plume length, oriented perpendicular to groundwater flow and plume migration, approximately 200 feet or 3 years groundwater travel time apart. After approximately 10 years of highly successful performance, the biowalls were reaching depletion and remedy performance was starting to decline. In 2017, the Ash Landfill biowall system was refreshed with additional organic carbon and pH buffer mass to re-establish remedy performance. The biowall refresh was accomplished by installing injection wells in each biowall segment and directly injecting a mixture of emulsified vegetable oil mixed with pH buffer while extracting groundwater from adjacent biowall wells. Pairing extraction with coincident injection induced recirculation loops between extraction and injection wells, improving distribution and accelerating the refresh process.
Results/Lessons Learned
During the original biowall pilot study and full-scale design processes, the lifespan of the biowall system was estimated to be 10 to 15 years. The biowall system achieved the lower end of the life expectancy range before a carbon refresh was required to maintain system performance. Biowall depletion and the need for refresh was identified through multiple lines of evidence consisting of statistical analysis of geochemical shifts over time and reversals in parent (TCE) and daughter (cis-1,2-DCE, VC and ethene) product concentration trends. This line-of-evidence approach is applicable to any enhanced bioremediation application to identify depletion and the need for refresh. Eighteen years of performance data, including two depletion analyses will be presented in addition to refresh lessons learned and remedy life cycle cost analyses. In 2023 it was determined that the refresh is approaching depletion and a second refresh is being considered.