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Bioremediation - 8 - la September 1993 <br /> 11. Nonuniformity of ground water flow direction and velocity can influence communication <br /> between injection points and recovery wells <br /> 12. Fine-grained soil layers within coarse-grained soil layers (heterogeneous) can impede flow <br /> 13. The presence of co-toxicants such as heavy metals may inhibit biodegradation <br /> 14. It may be difficult to deliver electron acceptors (oxygen) to the contamination zone <br /> 15. Some contaminants convert to more toxic substances during the metabolic process <br /> 16. Large amounts of biodegradable organic material in top soil layers will deplete oxygen <br /> reserves in soil and slow down oxygen diffusion rates to deeper layers. <br /> 17. Degradation of hydrocarbons to ppm levels should be readily attainable, however, treatment <br /> to ppb levels may require manipulation of the system to encourage cometaholism or <br /> degradation of the contaminant in the presence of an added primary substrate. <br /> 19. Bioremediation cannot be used where quick startup is needed (bioremediation typically takes <br /> 4 to 6 weeks for acclimation) <br /> 19. Bioremediation is not successful in a start/stop mode - it must continue 24 hours/day, 7 <br /> dayslweek <br /> 21 ' d 800' oN £2:6 26, 91 daS ON l31 <br />