USE OF AEROBIC AND ANAEROBIC MICROCOSMS TO ASSESS BTEX BIODEGRADATIONIN AQUIFERS

The natural attenuation of soluble aromatic hydrocarbons (BTEX) in aqu ifer plumes is controlled by several factors, including the concentrat ion of BTEX and dissolved oxygen (DO) or other electron acceptors (EA) , hydrogeological factors (dispersion, dilution, aquifer reaeration ra te), and the presence of BTEX-degrading microbes. We have investigated three laboratory soil and/or well water systems (aerobic, anaerobic, and chronic low DO) which may represent field plume conditions. Microc osms prepared with sufficient DO always degraded hydrocarbons (less th an or equal to 10,000 ppb) at high rates (greater than or equal to 0.1 0/d) with no lag phase. Experimental simulations of chronic low DO wer e performed in a pressure transducer-controlled respirometer in which O-2 was maintained at constant low levels (e.g., 0.2 to 2 mg/L). BTX a lso biodegraded rapidly (0.3 to 0.7/d) at all DO levels. These results suggest that plumes which appear to have low DO but continuous O-2 in filtration significantly bioattenuate hydrocarbons. The anaerobic biod egradation potential of BTEX was determined in microcosms prepared wit h subsoil or well water samples amended with NO3-, Fe+3, SO4-2, or no added EA. Well water from the ''aerobic'' or ''anaerobic'' portions of one site plume showed losses in BTEX, but there was no correlation be tween the presence of reduced EA and degradation. Anaerobic slurries p repared from subsoil of another site showed a significant decrease (0. 02 to 0.25/d) in BTEX with lag times of 0 to 40 days. Reduced gases (C H4, H2S, H-2) were not produced, but high concentrations of soil-bound Fe+2 formed, suggesting that Fe+3 may have been a predominant anaerob ic EA. These data indicate that the presumed consumption/reduction of anaerobic EA may not always be coupled to BTEX losses in ground water plumes.