Chesapeake Ecotox Research Program: Research

Contaminants in Sediments

Trace Elements

A number of trace element contaminants are present in Chesapeake Bay sediments at concentrations that can potentially have harmful effects (Eskin et al. 1996). Trace element contaminants can be categorized into different groups depending on their chemical and toxicological behavior. However, simply knowing the concentrations of toxic trace elements, either in solution or in sediment, is insufficient to predict their behavior and effects; one must also know their geochemical form and context must also be known. One important group, consisting of silver (Ag), copper (Cu), Cadmium (Cd), nickel (Ni), lead (Pb) and zinc (Zn) form strong, highly insoluble precipitates in the presence of sulfide, a common, but temporally and spatially variable component of sediment (van den Hoop et al. 1997) . The "AVS-SEM" hypothesis concerning this group is that if the acid volatile sulfide present is greater than the molar sum of the SEM metals, the metals will be almost exclusively bound to sulfide, their bioavailability will be extremely low, and toxicity is unlikely (Di Toro et al. 1990; Ankley et al. 1991). While mercury does form a highly insoluble sulfide, and geochemically could be considered a member of the AVS-SEM group, its toxicological properties are uniquely defined by the formation of methylmercury (MeHg). MeHg is more toxic than inorganic Hg, and highly bio-accumulative (Lawson and Mason 1998). MeHg is formed from inorganic mercury by biological methylation (Gilmour and Henry 1991; Matilainen 1995). Arsenic (As) and selenium (Se) form another group unusual among trace elements in that they are anions under environmental conditions. Both exhibit complex geochemistry, including alternate redox states (e.g. Cutter 1992; Tokunaga et al. 1997; Riedel et al. 1997; Riedel et al. In Press).

Persistent Organic Chemicals

Organic chemicals in sediments may cause deleterious effects through direct toxicity to aquatic organisms (narcosis, chitin synthesis inhibition, neurotoxicity) or through biomagnification to apex predators, including humans. In the Chesapeake Bay Regions of Concern, hydrocarbons, including polycyclic aromatic hydrocarbons (PAHs), are the most likely organic chemicals causing ambient toxicity, while the persistent organochlorines are most likely of concern in biomagnification. Although a myriad of organic chemicals are produced and released to the Chesapeake Bay region, only those with sufficient persistence and particle-reactivity will accumulate in sediments. These organic chemicals may be classified by source or by their potential effects. Many organic chemicals found in Chesapeake Bay sediments are inadvertently produced through the combustion of fossil fuels (PAHs) and the incineration of industrial, medical, and municipal wastes (chlorinated dioxins and furans). Others are industrial and agricultural chemicals that enter the environment during manufacturing and shipping (chlordane in Baltimore Harbor, Kepone in the James River), through improper disposal practices, or through agricultural runoff.