Evidence for Phytodegradation of MTBE from Coupled Bench-Scale and Intermediate-Scale Tests

This paper presents methodologies and demonstrates the need to couple bench-scale and intermediate tree-scale experiments, to fully understand the transport and fate of organic contaminants, specifically methyl tert butyl ether (MTBE), in mature trees. Bench-scale experiments showed MTBE to be optimally taken up by small poplar saplings with a transpiration stream concentration factor of approximately 1, little or no degradation in soils and, nearly $100\pm 20\%$ recovery in the coupled water-plant-air system, indicating no measurable phytodegradation at the bench-scale. A large $14\mathrm{ft}$ tree chamber was designed to evaluate MTBE transport and fate through intermediate-scale ( $12\mathrm{ft}$ tall) poplar trees. Abiotic MTBE volatilization tests conducted in the tree chamber showed $100\pm 20\%$ MTBE mass recovery, thereby demonstrating the integrity of the large chamber and its air monitoring technique. In contrast, replicate intermediate-scale experiments conducted with large $\left(12\mathrm{ft}\right)$ trees irrigated with a known mass of MTBE showed a deficit of MTBE mass recovery $(65\pm 20\%)$ in replicate soil-tree-air systems monitored over a 2-week period. More significantly, tert butyl alcohol (TBA), a degradation product of MTBE, was detected in increasing concentrations in leaf biomass while MTBE concentrations in leaf biomass decreased as the experiment progressed. The MTBE mass recovery deficit, coupled with the detection of increasing TBA in leaf biomass, provides preliminary evidence of MTBE degradation in mature trees.