U.S. Geological Survey Toxic Substances Hydrology Program--Proceedings of the Technical Meeting, Colorado Springs, Colorado, September 20-24, 1993, Water-Resources Investigations Report 94-4015

Modeling Steady-State Methanogenic Degradation of Phenols in Ground Water at Pensacola, Florida

Abstract

Field and microcosm observations of methanogenic phenolic-compound degradation indicate that Monod kinetics theory governs the substrate disappearance but overestimates the observed biomass. In this paper, we present modeling results from an ongoing multidisciplinary study of methanogenic biodegradation of phenolic compounds in a sand and gravel aquifer at Pensacola, Florida. The aquifer is contaminated with chemicals and wastes used in wood treatment. Field-disappearance rates of four phenols match those determined in batch microcosm studies performed by other workers in a related study. The degradation process appears to be at steady state because the transported contaminants still are disappearing within 150 meters downgradient of the source even after a sustained influx over several decades. The existence of a steady-state degradation profile of each substrate together with a low bacteria density in the aquifer indicate that the bacterial population is exhibiting no net growth. This is possibly because of the oligotrophic nature of the biomass population in which growth and utilization are approximately independent of the phenolic-compound concentration for most of the concentration range. Thus, a constant bacteria growth rate should exist over much of the contaminated area, which may, in turn, be balanced by an unusually high decay or maintenance rate caused by hostile conditions or predation.