Formatted Title
Sustainable In Situ Bioremediation with Hardwood Mulch Bioborings: A Nature-Based Approach for Reductive Dechlorination
Background/Objectives
Chlorinated ethenes are among the most widespread contaminants in the western world, with enhanced reductive dechlorination (ERD) being a well-known technology for managing both contamination sources and plumes of chlorinated aliphatic compounds in groundwater. Interest continues to grow regarding the use of wood mulch as a nature-based, low-capital-cost option for in situ ERD applications. Wood mulch serves as slow-release electron donor, and helps to maintain reductive redox conditions in the aquifer, which support the anaerobic dehalogenation of the chlorinated solvents. Wood mulch is a cheap and often locally available material, a secondary product of the wood processing industry, (e.g. landscaping, biofilters) and its use as bioremediation amendment may help improve the economic viability, circularity and environmental sustainability of a remediation project. This work summarizes the experience developed with a bench-scale and field pilot test application of wood mulch as an electron donor for ERD for a contaminated Site, with groundwater impacts by chlorinated ethenes.
Approach/Activities
After installing a P&T system near the downgradient Site boundary as per the Public Authority’s request, ERM has considered in situ bioremediation options to treat the contamination source with the ultimate goal of shutting down the existing hydraulic barrier, associated with intensive O&M activities and a high energy demand.
A microcosm bench-scale experiment was set up in the CNR-IRSA laboratory in Rome in order to assess the treatability of the contaminated soil and groundwater with different organic and inorganic amendments. Based on the results of the bench-scale test, wood mulch was observed to be capable of supporting the complete reductive dichlorination of TCE up to ethene, with low methane formation. Wood mulch was then selected as a suitable electron donor for ERD application.
A field pilot test was carried out by installing six large (800 mm) diameter bioborings up to 10 m b.g.l. in the main contamination hotspot identified via a previous MIP investigation. The bioborings were filled with a mixture of hardwood mulch, silica gravel, limestone gravel, and a commercial bioaugmentation inoculum. A groundwater and soil gas monitoring network was installed upgradient and downgradient the bioboring area in order of monitor the evolution of groundwater geochemistry, and concentrations.
Results/Lessons Learned
After the bioborings’ installation, decreasing trends of dissolved oxygen and redox potential were observed in downgradient multi-level monitoring wells, together with a depletion of nitrate and sulfate, and an increase in dissolved iron (II), chloride, TOC and methane, which globally suggest the occurrence reductive anaerobic conditions, and the geochemical precursors of reductive dichlorination process. A decrease in dissolved TCE concentration was observed in the monitoring wells downgradient the treatment area, together with an increase in 1,2-DCE and VC concentration. Ethene was detected in groundwater in concentration higher than 100 µg/L from 5 months after the mulch bioborings installation, which provided a proof of concept that complete reductive dichlorination could be achieved in the aquifer with the tested amendment.
However, the baseline chlorinated solvents concentrations in the pilot test area were as high as 100 ppm, which was believed to limit the extent of the biological reductive process triggered by wood mulch in the subsoil. Therefore, for the following full-scale phase of the project ERM has proposed to increase the availability of electron donors in the aquifer by (i) decreasing the spacing between bioborings and (ii) add some emulsified vegetable oil (EVO) product in the amendment mixture in the most impacted zones of the contamination source.