Loss rates of polycyclic aromatic hydrocarbons from sediment and deposit-feeder fecal pellets

Deposit feeders modify sediment by aggregating individual particles into fecal pellets. Loss of contaminants may be either more (enhanced bacterial degradation) or less (hindered diffusion and desorption) rapid for sediment incorporated into pellets. We studied the loss of 10 polycyclic aromatic hydrocarbons (PAHs) from historically contaminated estuarine sediment and the same sediment after it had been pelletized by Capitella sp. I, a polychaete worm typically found in disturbed environments. Fecal pellets initially had higher concentrations of organic carbon and PAHs (enrichment factors 1.03-1.21) due to selective feeding. Over 60 d, desorption of PAHs was minimal, with an average loss of 4 to 5%. Biodegradation accounted for an additional 28% loss. A two-component model fit the data well and indicated that the slowly desorbing fraction was close to one for all PAHs. For low molecular weight PAHs, biodegradation rate constants were greater than desorption rate constants; for high molecular weight PAHs, the opposite was found. Desorption rate constants were similar for fecal pellets (0.0002-0.037/d) and sediment (0.00020-0.31/d). Biodegradation rate constants were also similar for fecal pellets (0.0006-0.022/d) and sediment (0.0002-0.018/d). Thus, incorporation of sediment into robust fecal pellets (half-life on the order of decades) did not affect the loss rates of PAHs.