Many organic contaminants in groundwater are present at

concentrations below Cmin and would apparently go

unutilized. However, simultaneous utilization of several

different substrates is possible. Sometimes microorganisms

can metabolize these trace compounds in the presence of

other substrates, called primary substrates which support

the long-term biofilm growth. This process is termed

secondary utilization, or cometabolism. It is a mechanism

which allows microorganisms to degrade compounds that could

otherwise not provide enough energy to sustain the microbial

culture (McCarty et al., 1981).

 Studies have shown that the extent of biodegradation of

polychlorinated biphenyls was enhanced by adding sodium

acetate as a primary carbon source. The effect was

especially significant on higher-chlorinated isomers (Clark

et al., 1979). Marinucci and Bartha (1979) also found a

slight stimulation of 1,2,4-trichlorobenzene mineralization

was caused by the addition of primary substrates. Schmidt

and Alexander (1985) observed that the presence of acetate

has a negative effect on phenol degradation, and the delay

was lengthened by increasing acetate concentrations because

acetate is easier to degrade than phenol. Bouwer and

McCarty (1985) suggested that secondary substrate (i.e.

target contaminants) removal rates increase with time but

not with the increase of primary substrate concentrations

beyond a limiting concentration, and the overall residual