Suspended particulate matter transport of polycyclic aromatic hydrocarbons in the lower Columbia River and its estuary

Analysis of suspended particulate material (SPM) collected from the Columbia River and its estuary in 2007– 2008 revealed the ubiquitous presence of polycyclic aromatic hydrocarbons (PAHs) from several distinct sources. The two dominant ones were: (1) a suite of non-alkylated, three to five-ringed compounds derived from high temperature combustion and (2) perylene, a compound of diagenetic origin. A particle-selective, hydrodynamic trapping process explains how both PAH types become concentrated on both a particle weight and organic carbon basis in the estuarine turbidity maximum (ETM) by as much as 10 times relative to the riverborne particle source. The ETM is a transient sedimentary feature at the land-to-sea interface of riverdominated estuarine systems which, in the case of our study region, is located remotely from the likely site of initial PAH input. Particle normalized concentrations for PAH of notable environmental concern, such as fluoranthene, chrysene, and benzo[a]pyrene, exceeded the EPA-defined threshold effects level in all cases and were typically at, or above, the probable effects level. Comparison with results from studies for other waterways around the world indicates PAH concentrations in ETM-trapped particles from the Columbia River estuary are higher than those documented for SPM in waters of many far more industrialized and populated regions. Our refined understanding of PAH behavior in the Columbia River and its estuary should prove valuable for reliably modeling the transport and dispersal mechanism that is characteristic of other hydrophobic, particle associated persistent organic pollutants prevalent in this system, and for other river-dominated estuarine systems. The use of natural and man-made chemicals has substantially contributed to worldwide economic development and growth. However, these benefits have been realized without the guidance of an effective chemical management strategy that can mitigate the true cost of releasing chemicals into the environment and the consequential degradation of air, soil, and water quality. In freshwater and marine aquatic environments, adverse impacts include fishery and recreational closures, a loss of biodiversity, and unsafe drinking water, thereby limiting the benefits these environments once provided to human society. In response, the Clean Water Act of 1972 initiated measures to mitigate point source discharges of contaminants into waterways. While these regulations have seen some success, water quality issues persist as significant levels of fertilizers, fossil fuel contaminants, consumer products, and industrial waste still find their way into many waterways. The approach to restoring water quality today needs to focus on managing pollutants discharged from non-point sources, such as urban runoff and atmospheric fallout deposition (U.S. Ocean Commission 2004). Effective management of persistent organic pollutant (POPs) inputs is one major challenge. POPs have been released into the environment for decades, owing to the central role they have played in pest control (e.g., dichlorodiphenyltrichloroethane or DDT) and industry (e.g., polychlorinated biphenyls or PCBs, polybrominated diphenylethers or PBDEs). Their molecular structure makes them resistant to processes of chemical and biological degradation which explains how they can be transported long distances, bioaccumulate in the fatty tissues of organisms, and in some cases biomagnify. With regard to management, the ubiquity of POPs is well known, but incomplete understanding remains regarding their mechanism of transport, exposure pathways, and fate in each ecosystem of concern. A more holistic understanding is a desirable goal because it would allow resource managers to develop contaminant transport models that can aid in determining the most appropriate monitoring stations, predict zones where organisms may be at high risk to contaminant exposure, and assist in mitigation planning (U.S. Ocean Commission 2004; Schwarzenbach et al. 2006; LCREP 2007). The lower Columbia River and its estuary, located between the states of Oregon and Washington, represents an *Correspondence: [email protected] 1935 LIMNOLOGY and OCEANOGRAPHY Limnol. Oceanogr. 60, 2015, 1935–1949 VC 2015 Association for the Sciences of Limnology and Oceanography doi: 10.1002/lno.10144 environment where POPs are widespread (McCarthy and Gale 2001; Johnson et al. 2007a, 2007b). Polycyclic aromatic hydrocarbons (PAHs), DDT, PCBs, and PBDEs have been detected there in the tissue and stomach contents of juvenile Chinook salmon. The highest concentrations are typically reported for samples from sites adjacent to industrialized regions, such as Portland, Oregon. In some of the juvenile salmon, concentrations of DDT, PAHs, and PCBs have approached or exceeded health effect thresholds—the point where sublethal impacts on growth, development, reproduction, immune function, and behavior become prevalent (Johnson et al. 2007b). Currently, it is unclear as to where and how these organisms have become exposed to POPs within the Columbia River and its estuary. Using PAHs as model contaminants and tracers, our study investigates how riverborne suspended particulate matter (SPM) transport and selective hydrodynamic trapping in estuarine turbidity maxima (ETM) functions as a mechanism of organism exposure to POPs. The ETM is a complex, transient sedimentary feature characterized by high levels of SPM that occurs in most riverdominated estuaries (Simenstad et al. 1994). The formation of this feature is shaped by tidal forces pushing in-flowing salt water beneath out-flowing river water, creating a hydrodynamic trap for settling particles (Jay et al. 2007; Park et al. 2008). The ETM typically does not occur at a fixed location in an estuary. It follows the leading edge of the salt wedge, moving up and down river on the flood, and ebb tide, respectively. The strength and magnitude of the ETM varies over time and is controlled by the tidal energy (i.e., spring vs. neap forcing), river discharge rate, and patterns of estuarine circulation and stratification (e.g., Small and Prahl 2004; North et al. 2005; Jago et al. 2006). For example, in San Francisco Bay, variations in SPM concentrations are primarily a result of the spring-neap tidal cycle, where values increase from enhanced resuspension during the spring tide and decrease due to deposition on the bed during neap tide (Schoellhamer et al. 2007). In many estuaries, the ETM hosts a relatively large zooplankton biomass compared to upriver (e.g., Morgan et al. 1997). This characteristic makes the ETM a feeding ground for larval and juvenile fish (Suzuki et al. 2008). For example, striped bass in the Chesapeake Bay system release eggs upriver, which are transported to the ETM by river-to-estuary circulation patterns. This process places juvenile fish in a zone of abundant zooplankton prey and optimal salinity for growth (North et al. 2005). In the Columbia estuary, the ETM traps particles and their associated chemical constituents during neap tide conditions and erodes them during spring tide conditions (Reed and Donovan 1994; Small and Prahl 2004). Depending on the river discharge rate, the trapped particles reside in the estuary for 2 weeks to 4 weeks, whereas the residence time of water is much shorter, typically only a few days (Crump and Baross 2000). These particles play an important role in the food web of the estuary because zooplankton consume the particulate organic matter as a primary food source (Morgan et al. 1997). The zooplankton are linked to higher trophic levels, such as fish, making the ETM a region of high biological activity (Simenstad et al. 1994; Crump and Baross 2000). It is possible and likely that POPs are attached to and concentrated in the particles trapped by the ETM. Consequently, consumers feeding within the ETM and subsequently their predators are at increased risk to contaminant exposure. The hypothesis of this study is that PAHs and, by analogy, other POPs introduced upstream within the Columbia River Basin are transported effectively via SPM to the estuary where they are trapped particle-selectively by the ETM, leading to their increased concentration on a bulk particle basis. The objectives are: (1) to characterize the overall composition of SPM-associated PAHs delivered by the Columbia River to its estuary; (2) to identify the primary sources contributing to the overall observed composition; (3) to assess the role that the ETM plays as a site for concentrating PAHs by means of particle selective, hydrodynamic trapping; and (4) to determine how the measured concentrations of the PAHs on SPM in this transient sedimentary feature fit within the sediment quality criteria thresholds set for them as recognized priority pollutants. Materials and methods Study area As part of the Center for Coastal Margin Observation and Prediction (CMOP) program, SPM samples were collected from eight sites during a 2 week cruise (16 August 2007–29 August 2007) aboard the RV Barnes (Fig. 1; Table 1). The sampling sites in the mainstem Columbia were located downriver from the port of Vancouver (B), downriver from the confluence with the Willamette River (C), at Columbia City (D) and at Beaver Army Terminal (or BAT, E). Those in the Columbia River estuary (CRE) were located in the south (F) and north (G) channels, and near the mouth (H). The sampling site in the Willamette River was located downriver from Portland Harbor (A). SPM sampling was done throughout neap (20 August 2007–24 August 2007) and spring (26 August 2007–29 August 2007) tidal cycles at all three locations within the estuary (F, G, H: Fig. 1). The North Channel is open to the ocean, but does not extend far upriver; whereas the South Channel, also open to the ocean, is an active shipping channel, maintained by a regular dredging program to assure commercial connection with upriver ports. Our sampling in the estuary focused on capturing SPM from bottom waters during periods of ETM formation and disintegration. The vessel was anchored on station prior to flood tide and profiles of conductivity—temperature—depth (or CTD) along with transmissometry were obtained every 15–30 min to Gregg et al. PAH transport by Columbia River particles