OXYGEN LIMITATIONS AND AGING AS EXPLANATIONS FOR THE FIELD PERSISTENCE OF NAPHTHALENE IN COAL TAR-CONTAMINATED SURFACE SEDIMENTS

Naphthalene has been transported approx, 400 m via groundwater flow fr om buried subsurface coal tar to an organic matter-rich seep area wher e the water emerges at the foot of a bill in a field study sire. We ha ve tested several hypotheses for explaining why naphthalene persists i n seep sediments. In aerobic laboratory flask assays, conversion of C- 14-naphthalene to (CO2)-C-14 occurred and was not stimulated by amendm ents with vitamins or inorganic nutrients. Thus, neither toxicity nor nutrient limitation were the cause of naphthalene persistence. At the site, in situ sediment oxygen concentrations were below detection. Oxy gen-limited naphthalene biodegradation was demonstrated both by measur ing no conversion of C-14-naphthalene to (CO2)-C-14 in samples of seep sediments prepared anaerobically and by measuring naphthalene loss fr om anaerobic nitrate-amended slurry-phase incubations of the sediment only after O-2 was added. However, when H2O2 was added as an O-2 sourc e to site sediments in situ in a randomized block design, no discernib le naphthalene loss occurred. The possibility that decreased bioavaila bility might contribute to naphthalene persistence was investigated by monitoring (CO2)-C-14 evolved by microorganisms added to gamma-rap st erilized sediments that had been exposed under aseptic conditions to C -14-labeled naphthalene for periods ranging from 0 to 28 d. Resulting patterns in the extent and rate of naphthalene mineralization revealed an inverse relationship to the duration of contact with the sediment, but only when the mixed microbial inoculum had been enriched on aqueo us-phase naphthalene. We conclude that oxygen limitation is the most p robable cause for lack of naphthalene biodegradation at our field stud y site. However, diffusion or sorption reactions may also play a role.