1,2,4-Trimethylbenzene and 1,3,5-Trimethylbenzene (TMBs) are recalci- trant mono-aromatic compounds found in crude oil and constitute approximately 40% of C-9 distillation fractions added directly to gasoline. They are frequently encountered in soils at an abandoned oil refinery site in south-central Michigan. Total TMB isomer con- centrations in soils collected from a historical refined petroleum spill area range from 3,000 μg/kg to 30,000 μg/kg. During the summer of 2003, an ex situ Pilot Study was implemented to evaluate the effectiveness of treating soils (45 cubic yard “cells”) con- taminated with aromatic volatile compounds, including TMBs. The effectiveness of plant-assisted bioremediation, using both seeded and planted prairie grasses was evalu- ated in an enhanced rhizodegradation cell (test cell) through chemical and biological monitoring, and compared to a control (unplanted, but tilled or landfarmed) cell. Rhizodegradation is a phytoremediation method referring to destruction of contaminants in the rhizosphere due to direct bacterial degradation and enzymatic co-metabolic degradation. In 2003, soil analyses indicated a 99% TMB reduction in the test cell over a 28-week period. Confirmation of rhizosphere degradation was supported in the test cell through increased heterotrophic bacteria counts (HBC) from an average of 9.7 million colony- forming units (CFU) the first 14-weeks to 11.8 million CFU, and counts as high as 21 million CFU the remaining weeks of monitoring. Increased HBC were also observed in soils collected from the rhizodegradation test cell, and inoculated with various petro- leum hydrocarbons, suggesting that some microbial communities do well in the presence of certain compounds (124-TMB, xylenes, and acenaphthene) when grasses are present. In June 2007, both cells were monitored to evaluate plant diversity (including pres- ence of invasive plants), concentration of contaminants, and microbial activity. After 4 years, the predominant grass type on the test cell is Elymus (both Canadian and Virginia rye), which constitutes approximately 52% of the vegetative cover. Rye seed consisted of 30% of the original 2003 seeded mixture. Approximately 26% of the cell is also covered with planted prairie grasses, and the remaining 21% of cover with various invasive pio- neering species (white sweet clover, chicory, common blue lettuce, spotted knapweed, curly dock, wild carrot, various cone-flowers, cotton wood, Siberian elm and cherry). Bare soil constitutes less than 1% of the test cell cover. In addition to rye grass, drought- tolerant prairie drop seed (Sporobolus heterolepis) has successfully established growth on the test cell. Wildlife-friendly and aesthetic species big blue stem (Andropogon gerardii), little blue-stem (Schizachrium scoparium) and side oats grama grass (Bouteloua curti- pendula) failed to persist after 4 years. The average above grade plant mass on the test cell was 838 g/m2. The control cell consists of approximately 46% invasives noted above, and also Artemisia or wormwood, 36% bare soil and 18% non-specific non-planted grasses. The average above-grade plant mass on the control cell was 598 g/m~2.
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