Formatted Title
Bench-Scale Testing of Enhanced Reductive Dechlorination and In Situ Chemical Reduction Amendments for Trichloroethene and Tetrachloroethene
Background/Objectives
Bench-scale batch testing is underway in support of planned remediation for a site with a dissolved phase chlorinated solvent plume consisting of trichloroethene (TCE) and tetrachloroethene (PCE), with minor concentrations of cis-1,2-dichloroethene. Total volatile organic compound (VOC) concentrations range from 0.54 micrograms per liter to 135 milligrams per liter. Site geology is composed of sandy contaminant mass transport zones within a matrix of silt- and clay-rich, low permeability contaminant mass storage zones. Contaminant mass in on-site mass storage zones is back diffusing into on-site transport zones, facilitating off-site migration. Installation of a permeable reactive barrier (PRB) via subsurface injection of amendments at the downgradient site boundary is planned to prevent off-site migration of contaminants via the mass transport zones. The PRB is designed to serve as a sustained reactive zone where degradation of chlorinated VOCs will be facilitated over the long term. Two injectable amendment formulations are under evaluation for use in the PRB; one will facilitate enhanced reductive dechlorination (ERD), and one will facilitate in situ chemical reduction (ISCR). For ERD, the anticipated primary degradation pathway is hydrogenolysis (biotic) stimulated by the fermentation of an organic electron donor. For ISCR, the anticipated primary degradation pathway is beta-elimination (abiotic), stimulated by the addition of zero valent iron (ZVI). Although each amendment is expected to stimulate one primary degradation pathway type, biotic or abiotic, both amendments are expected to facilitate synergistic degradation pathways via both types. The criteria for selecting an amendment for pilot testing based on bench test results are a) ability to achieve 90% reduction of VOC concentrations at the planned PRB location, b) environmental footprint, and c) cost effectiveness.
Approach/Activities
The test microcosms are composed of soil and groundwater collected from the on-site mass transport zone where the PRB is planned. Neat vegetable oil was selected over emulsified vegetable oil as the ERD amendment due to its longevity within a PRB scenario (i.e., it is anticipated to serve as a long-term, slow release electron donor). The ISCR amendment is a proprietary mixture of 60% ZVI by weight and a combination of natural and food-grade chemicals. Two doses of each amendment and one control are being tested in duplicate for a total of five trials. Microcosm sample results will be used to assess the ability of the amendments to achieve complete degradation of contaminants to non-toxic end products, estimate contaminant degradation rates, and assess amendment impact on aquifer geochemical and biological conditions. The results will also be used to evaluate the potential need for bioaugmentation, aquifer buffering, and/or nutrient addition.
Results/Lessons Learned
Multiple rounds of microcosm water samples have been collected and analyzed over 35 days of reaction time. Initial results for the ERD amendment indicate rapid decreases in TCE and PCE concentrations, as much as 41% for the low dose and 62% for the high dose, over the first 24 hours. Over the next 34 days, TCE and PCE concentrations for both doses steadily increased by a small amount. Daughter products have not been detected, and only very low concentrations of dissolved gases have been detected. Initial rapid concentration decreases are suspected to be caused by partitioning of TCE and PCE into the vegetable oil rather than by degradation. Slow dissolution of the oil and associated release of TCE and PCE back into groundwater is the suspected cause of the subsequent small concentration increases. Eventual degradation is expected as the test progresses, and the oil continues to dissolve and become an available electron donor for ERD. Initial results for the ISCR amendment indicate steady decreases in TCE and PCE concentrations over the 35-day period, as much as 80% for the low dose and 91% for the high dose. Daughter product and dissolved gas concentrations have been steadily increasing, indicating both biotic and abiotic TCE and PCE degradation. Bench testing is anticipated to be completed by February 2024 and all results will be presented, along with an evaluation of the amendments for field pilot testing.