| dc.description.abstract | The effectiveness of soybean oil on the bioremediation of tetrachloroethylene (PCE) was determined in this bench scale microcosm study. The contaminated site is located at a former dry cleaning facility in Ft. Riley, Kansas, where the PCE leaked into the surrounding groundwater and soil. Permanganate was injected by the Army Corps of Engineers near the contaminated groundwater wells; however, the levels of PCE rebounded in the Area of Contamination 3 (AOC3). A pilot study was performed in AOC2 by the Army Corps, where CAP18, emulsified soybean oil, was injected into the contaminated area to stimulate biodegradation. The study proved successful, with lower concentrations of PCE and degradation intermediates detected during routine groundwater monitoring. Soil cores were collected from two different depths, “A” and “C” in AOC3, and from one depth in AOC2 where the injection of soybean oil was successful. Microcosms were filled with soil slurries from AOC3 Horizon A and C, AOC2 was used as a positive control, and a composite sample from AOC3 was autoclaved and used as the negative control. The microcosms also contained artificial groundwater, with nutrients added to support microbial growth, and EOS Pro emulsified soybean oil. Two starting concentrations of PCE were used in the microcosms, 50 ppb and 150 ppb. Air was evacuated from the microcosms and they were sealed to create an anaerobic environment. Sacrifice sampling was used for this bench scale study, which means the microcosms were discarded after sampling was performed. The sacrifice sampling was used to avoid the loss of volume from the microcosms during sampling, and the study lasted for 16 weeks, with sampling performed in Weeks 0, 1, 2, 3, 4, 6, 8, 12, and 16. Analysis by Gas Chromatography – Mass Spectrometry with Solid Phase Microextraction showed erratic results for PCE concentrations over time due to inconsistent partitioning of the PCE into the organic phase. Degradation intermediates – trichloroethylene (TCE), cis-Dichloroethylene (cis-DCE), trans-Dichloroethylene (trans-DCE), and vinyl chloride (VC) were detected in AOC3-A, and low levels were detected in AOC3-C. The moderate levels of vinyl chloride in AOC3-A indicated there was likely bioremediation occurring in the microcosms. DNA analysis by qPCR showed that active bacterial communities were present in AOC3-A, AOC3-C, and AOC2. In AOC3-A where bioremediation occurred, the DNA concentration increased from Week 0 to 3, but then decreased some when vinyl chloride was produced from Week 6 onwards. The vinyl chloride was likely toxic to the bacteria, which caused the decrease in bacterial concentration. Terminal electron acceptor concentrations (bicarbonate, nitrate, sulfate, phosphate, ferrous iron) were measured using Inductively Coupled Plasma – Optical Emission Spectra (ICP-OES) and Ion Chromatography. The levels of bicarbonate increased in all microcosms as the organic carbon from the soybean oil was converted to inorganic carbon. The levels of nitrate remained low in all microcosms and was difficult to distinguish from the detection limit, since only a small amount of nitrate was initially present in the artificial groundwater. Sulfate levels decreased for AOC3-A, where bioremediation was occurring, indicating there were sulfate reducing conditions present in the microcosms. Phosphate concentrations in solution decreased rapidly in all microcosms, indicating in was likely the growth limiting nutrient. Detectable ferrous iron concentrations were present in AOC3-A only in later weeks when bioremediation was happening, indicating microbial iron reduction was occurring. The AOC3-A microcosms had active microbial communities capable of bioremediation at the contaminated site. It is recommended that soybean oil be injected into the depth around the A horizon in AOC3. The terminal electron acceptors native to the site are sufficient to sustain microbial growth, however, phosphate concentrations should be monitored over time to see if supplemental addition of phosphate is necessary. Bioaugmentation with a strain of Dehalococcoides, which is known to degrade PCE is recommended to sustain bioremediation. Vinyl chloride will likely accumulate in the groundwater and soil because it is the rate limiting step in the bioremediation of PCE. The addition of methanol as a carbon source is recommended because it will promote the growth of methanogens which will more easily degrade the vinyl chloride to ethene gas, which is an environmentally acceptable end product. | |
