A self-organized "tipping bucket model" of driven flow in fractured media was employed to study the degradation of contaminant by a dynamic population of traps representing microbial activity in the subsurface. For divergent flows, an increase in the degradation rate by an order of magnitude decreased the contaminant concentration in the exiting drips by at most one-third, but for convergent flows, the increased degradation rate did not significantly reduce the contaminant concentration.