BIOAVAILABILITY AND BIODEGRADATION OF WEATHERED DIESEL FUEL IN AQUIFER MATERIAL UNDER DENITRIFYING CONDITIONS

During the in situ bioremediation of a diesel fuel-contaminated aquife r in Menziken, Switzerland, aquifer material containing weathered dies el fuel (WDF) and indigenous microorganisms was excavated. This materi al was used to identify factors limiting WDF biodegradation under deni trifying conditions. Incubations of this material for 360 to 390 d und er denitrifying conditions resulted in degradation of 23% of the WDF w ith concomitant consumption of NO3- and production of inorganic carbon . The biodegradation of WDF and the rate of NO3- consumption was stimu lated by agitation of the microcosms. Biodegradation was not stimulate d by the addition of a biosurfactant (rhamnolipids) or a synthetic sur factant (Triton X-100) at concentrations above their critical micelle concentrations. The rhamnolipids were biodegraded preferentially to WD F whereas Triton X-100 was not degraded. Both surfactants reduced the surface tension of the growth medium from 72 to <35 dynes/cm and enhan ced the apparent aqueous solubility of the model hydrocarbon n-hexadec ane by four orders of magnitude. Solvent-extracted WDF added at a conc entration equal to that already present in the aquifer material, was a lso biodegraded by the microcosms, but not at a higher rate than the W DF already present in the material. The results show that the denitrif ying biodegradation of WDF is not necessarily limited by bioavailabili ty but rather by the inherent recalcitrance of WDF.