A combination of hydrogeological, geochemical, and microbiological methods was used to document the biotransformation of trichloroethene (TCE) to ethene, a completely dechlorinated and environmentally benign compound, by naturally occurring microorganisms within a fractured dolomite aquifer. Analyses of groundwater samples showed that three microbially produced TCE breakdown products (cis-1,2-dichloroethene, vinyl chloride, and ethene) were present in the contaminant plume. Hydrogen (H2) concentra tions in groundwater indicated that iron reduction was the predominant terminal electron-accepting process in the most contaminated geologic zone of the site. Laboratory microcosms prepared with groundwater demonstrated complete sequential dechlorination of TCE to ethene. Microcosm assays also revealed that reductive dechlorination activity was present in waters from the center but not from the periphery of the contaminant plume. This dechlorination activity indicated that naturally occurring microorganisms have adapted to utilize chlorinated ethenes and suggested that dehalorespiring rather than cometabolic, microbial processes were the cause of the dechlorination. The addition of pulverized dolomite to microcosms enhanced the rate of reductive dechlorination, suggesting that hydrocarbons in the dolomite aquifer may serve as electron donors to drive microbially mediated reductive dechlorination reactions. Biodegradation of the chlorinated ethenes appears to contribute significantly to decontamination of the site.

Abstract-Naphthalene has been transported approx. 400 m via groundwater flow from buried subsurface coal tar to an organic matter-rich seep area where the water emerges at the foot of a hill in a field study site. We have tested several hypotheses for explaining why naphthalene persists in seep sediments. In aerobic laboratory flask assays, conversion of 14 C-naphthalene to 14 CO 2 occurred and was not stimulated by amendments 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. Oxygenlimited naphthalene biodegradation was demonstrated both by measuring no conversion of 14 C-naphthalene to 14 CO 2 in samples of seep sediments prepared anaerobically and by measuring naphthalene loss from anaerobic nitrate-amended slurry-phase incubations of the sediment only after O 2 was added. However, when H 2 O 2 was added as an O 2 source to site sediments in situ in a randomized block design, no discernible naphthalene loss occurred. The possibility that decreased bioavailability might contribute to naphthalene persistence was investigated by monitoring 14 CO 2 evolved by microorganisms added to ␥-ray sterilized sediments that had been exposed under aseptic conditions to 14 C-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 aqueous-phase naphthalene. We conclude that oxygen limitation is the most probable cause for lack of naphthalene biodegradation at our field study site. However, diffusion or sorption reactions may also play a role.

Field experiments utilizing randomized block designs were implemented to assess the mobility of both coal tar-derived aromatic hydrocarbons and bacteria capable of metabolizing these substances at a contaminated field site. Arrays of sorbent materials wrapped in fiberglass mesh fabric were inserted into organic matter-rich freshwater sediments in order to intercept mobile chemicals and bacteria carried by the prevailing hydraulic gradient. Polyurethane foam plugs served as a sorbent for aqueous-phase coal tar components while sterile sand from the site served as a substrate for colonization by bacteria. Replicate sorbents were removed from the sediments at varying intervals and assessed for organic compounds (via gas chromatography/mass spectrometry) and for numbers of aromatic hydrocarbon-degrading bacteria (via viable plate counts). Organic contaminants including naphthalene, methyl naphthalene, indenes, and substituted benzenes were detected in the foam sorbents. Contaminant concentrations reached a maximum after 15 days before diminishing. Both naphthalene-and phenanthrene-utilizing bacteria were mobile and reached peak titers of 10 4 and 10 3.3 , respectively, within 11 days. Thus, comigration of both contaminants and microorganisms occurred at the study site. Furthermore, the in situ abundances of contaminants and microorganisms reflect a dynamic balance between processes causing accrual and elimination.