Assessment of potential anaerobic biotransformation of organic pollutants in sediment caps

In situ capping is a remedial approach for reducing the risk of biota exposure to sediment contaminants. Biotransformation of contaminants in sand-based sediment caps, rarely considered in sediment cap design, could further reduce the exposure risk. The anaerobic biotransformation of benzene, toluene, ethylbenzene, xylenes (BTEX), monochlorobenzene, dichlorobenzenes and naphthalene was evaluated with sediments from Onondaga Lake in dilute sediment slurries and in sand-capped sediment laboratory-scale columns. The percentage of sediment samples demonstrating biotransformation under anaerobic conditions in slurries incubated at 12°C was greatest for BTEX, followed by monochlorobenzene, 1,4-dichlorobenzene, 1,2-dichlorobenzene and 1,3-dichlorobenzene. Only toluene biotransformation was observed in sand cap columns. The rate of toluene biotransformation diminished over time, which might be due to inhibition caused by hydrogen from the experimental setup. Results suggest potential for the biotransformation of toluene, and possibly other pollutants, in sand-based sediment caps under anaerobic conditions at low temperatures.

Introduction

Approximately 10% of sediments in surface water bodies in the United States are considered ‘sufficiently contaminated with toxic pollutants to pose potential risks to fish and to humans and wildlife that eat fish’ [1]. Chemicals of concern commonly encountered in sediments include organic chemicals such as pesticides, polychlorinated biphenyls (PCBs) and polycyclic aromatic hydrocarbons (PAHs), as well as metals such as nickel, lead and mercury. Common sediment pollutants can deleteriously affect the health of wildlife and humans through direct releases to the overlying water as well as food chain effects. Benthic organisms are ecological receptors at particularly high risk of exposure that represent a point of entry for contaminants into food webs. Contaminant transfer from the sediment to biota represents a crucial first step in a complete human exposure pathway.

Sediment capping is a remediation approach that involves the in situ placement of clean granular material atop the contaminated sediments to reduce contaminant release and benthic organism exposure. ‘Conventional’ caps typically consist of 0.3–1 m of natural materials, typically sand, topped by gravel or cobbles for armoring at the surface. ‘Active’ caps can include an agent for sequestration (e.g., activated carbon) or reaction (e.g., iron- and palladium-embedded activated carbon) 2, 3. Capping provides physical separation between pollutants and benthic organisms, thus preventing ingestion of contaminated sediments; it also provides stability against hydraulic disturbances, thereby preventing particle suspension that could lead to contaminant desorption in the water column. Over time, pollutants may migrate through the cap by advection and diffusion, but the concentrations to which benthic organisms are exposed are typically greatly reduced [4].

Sediment caps become colonized by microorganisms from the sediments and surface water and potentially become a zone of pollutant biotransformation [5]. This was demonstrated in laboratory column tests in which the PAHs naphthalene and phenanthrene were biotransformed in a sand cap under aerobic conditions [6]. However, such aerobic degradation occurs only near the solids–water interface in which benthic organisms are active and thus exposure to contaminants by benthos might still be significant. Biotransformation in the anaerobic zone of a cap, which typically extends well beyond the zone of benthic activity, could significantly reduce the risk of pollutant exposure.

The terminal electron acceptors (TEAs) of greatest significance in anaerobic sediments, in order of diminishing energy yield, are ferric iron, sulfate and carbon dioxide. Conceptually, TEAs are utilized sequentially, which leads to the creation of overlapping zones, each with a dominant TEA. The vertical translation of TEA zones from sediments to overlying sand caps has been demonstrated in laboratory microcosms [7] and suggests that the metabolic requirements of microorganisms in sand caps, including energy sources, electron acceptors and nutrients, can be readily met. The presence of microorganisms capable of catalyzing biotransformation reactions will dictate whether pollutant biotransformation occurs in a sediment cap.

The purpose of the current research was to examine the potential for contaminant biotransformation in conventional sand-based sediment caps under anaerobic conditions. Onondaga Lake sediment, which contains elevated concentrations of aromatic compounds including benzene, toluene, ethylbenzene, xylenes (BTEX), monochlorobenzene, dichlorobenzenes and naphthalene, was used in laboratory tests. Dilute sediment slurries were used to assess the potential for biotransformation by autochthonous sediment bacteria while sediment and sand columns were used to assess the potential for biotransformation in a sand cap. Improved characterization of contaminant biotransformation could aid in optimizing the design of caps for sediment remediation.