Fate of benzene in a stratified lake receiving contaminated groundwater discharges from a superfund site

Predicting the fate of benzene in aquatic environments, and estimating corresponding human exposures, is critically dependent on knowledge of this carcinogen's biodegradation rate under the site-specific conditions. We used three approaches for quantifying this key fate process: (1) short-term (hours) observations of benzene loss in laboratory incubations of representative water samples, (2) whole-lake benzene mass balance studies, and (3) modeling of the temporal evolution of benzene vertical profiles in the lake. Our field site, the Halls Brook Holding Area (HBHA), continuously receives benzene input (about 20 μM or 1.5 ppm) into its anoxic hypolimnion via discharge of saline groundwater from an adjacent Superfund site (Industri-Plex in Eastern Massachusetts.) Using summertime, lake water samples in the laboratory, we found benzene was degraded in three metalimnion samples at rates between 1 and 2.5 d^-1. An epilimnion sample yielded a similar result, but no degradation was observed in another epilimnion sample. Losses were ≤0.04 d^-1 in a sulfate-rich hypolimnion sample. Since benzene loss could be inhibited by filtration or with a mixture of poisons and antibiotics, it was apparently being biodegraded. In the whole-lake mass balance studies of benzene, it was found that approximately 80% of the benzene entering the lake was degraded during the water's residence in the lake. Vertical distributions of benzene in the HBHA water column indicated that the chief sink of benzene was located in the metalimnion. A two-month progression of summertime profiles of benzene concentration vs depth was fitted well using a dynamic model, CHEMSEE, and assuming that the only sinks were epilimnetic flushing, water-to-air exchange, and biodegradation in a 0.4 m-thick metalimnetic layer at 2 d^-1. The biodegradation rate derived from such whole-system study appears more dependable than rates deduced from grab samples, and we suggest that we must learn to predict these intact-system rates.