Geochemistry of volatile organic compounds in seawater: Mesocosm experiments with 14C-model compounds

A series of ten radiotracer experiments were conducted in controlled experimental ecosystems (mesocosms) to investigate the behavior of volatile organic compounds (VOC) in seawater. Time-series measurements of the redistribution of ¹⁴C-activity within several major pools—dissolved, particulate, intermediate metabolite, and CO₂—in the ecosystem made possible an evaluation of the rates of processes—volatilization, biodegradation, Sorption and sedimentation—acting to remove VOC from seawater in summer. The behavior of the model ¹⁴C-VOC fell into three categories. Aromatic hydrocarbons (benzene, toluene, naphthalene) were subject to both volatilization and biodegradation, with mineralization dominating in summer. Chlorinated C₂-hydrocarbons (tetrachloroethylene) and chlorinated benzenes (chlorobenzene, 1,4-dichlorobenzene, and 1,2,4-trichlorobenzene) were affected only by volatilization and were relatively resistant to biodegradation. Of all the model VOC studied, only aliphatic hydrocarbons (decane and octadecane) were sorbed onto suspended paniculate matter; however, the primary route of loss from the water column appeared to be biodegradation rather than sedimentation. The mesocosm-derived removal rate constants were then applied to estimate summer VOC residence times in Narragansett Bay, Rhode Island of about 1 day for aromatic hydrocarbons, 1 week for the chlorinated VOC and about 1 day for aliphatic hydrocarbons. Residence times in winter might be on the order of 1 week for all VOC.