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Investigating the Effect of Bioirrigation on In Situ Porewater Concentrations and Fluxes of Polychlorinated Biphenyls Using Passive Samplers Jennifer N Apell, David Shull, Alison Hoyt, and Philip M. Gschwend Environ. Sci. Technol., Just Accepted Manuscript • DOI: 10.1021/acs.est.7b05809 • Publication Date (Web): 26 Mar 2018 Downloaded from http://pubs.acs.org on March 26, 2018
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Environmental Science & Technology
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Investigating the Effect of Bioirrigation on In Situ
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Porewater Concentrations and Fluxes of
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Polychlorinated Biphenyls Using Passive Samplers
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Jennifer N. Apell1*†, David H. Shull2, Alison M. Hoyt1#, and Philip M. Gschwend1
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Institute of Technology, Cambridge, MA 02139
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2
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Washington University, Bellingham, WA 98225
R.M. Parsons Laboratory, Department of Civil & Environmental Engineering, Massachusetts
Department of Environmental Sciences, Huxley College of the Environment, Western
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ABSTRACT
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Polychlorinated biphenyl (PCB) fluxes from contaminated sediments can be caused by
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mechanisms including diffusion, bioirrigation, and resuspension, but it is often unclear which
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mechanisms are important. In the Lower Duwamish Waterway (Seattle, Washington), the
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presence of abundant benthic macrofauna suggests that porewater bioirrigation may be an
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important mechanism for PCB transport from the bed into the overlying water column. In this
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field study, the fluxes of PCBs due to bioirrigation were quantified by using (a) polyethylene
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(PE) samplers to quantify in situ and ex situ (i.e., equilibrium) PCB porewater concentration
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profiles and (b) measurements of the geochemical tracer 222Rn to quantify the rate of porewater
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exchange with overlying water. The results showed that bioirrigation caused sorptive
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disequilibrium with the surrounding sediment, which led to lower in situ porewater
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concentrations than expected from sediment concentrations. The combined fluxes of seven PCB
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congeners (Σ7PCBs) were 1.6-26 ng/m2/day for the three field sites, similar in magnitude to the
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upper limit estimates of diffusive fluxes calculated assuming water-side boundary layer control
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(Σ7PCBs=1.3-47 ng/m2/day). Moreover, the depleted in situ porewater concentrations imply
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lower diffusive flux estimates than if the ex situ porewater concentrations had been used to
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estimate fluxes (Σ7PCBs=89-670 ng/m2/day). These results suggest that non-diffusive PCB
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fluxes from the sediment bed are occurring and that quantifying in situ porewater concentrations
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is crucial for accurately quantifying both diffusive and non-diffusive PCB fluxes.
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KEYWORDS: bioirrigation, passive sampler, polychlorinated biphenyls, sorption, desorption,
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bed-water fluxes, porewater, sediment, radon, in situ concentrations
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INTRODUCTION
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Historical releases of hydrophobic organic compounds (HOCs) into aquatic environments
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has led to sediment beds becoming a reservoir for HOC contamination. Since porewater
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concentrations are often higher than in the overlying water column, the sediment bed is thought
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to be a major ongoing source of contamination to the overlying water for many HOC-
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contaminated sites.1, 2 However, researchers’ abilities to understand and quantify HOC fluxes
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from sediments have been limited by the difficult task of measuring freely dissolved HOC
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concentrations.3, 4
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Polymeric passive samplers, which accumulate HOCs such as polychlorinated biphenyls
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(PCBs) in proportion to their dissolved concentrations and polymer-water partition coefficients,
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can be used to measure both freely dissolved porewater and overlying water concentrations.5
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These samplers, made of materials like polyethylene (PE) and polydimethylsiloxane (PDMS),
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can also obtain substantially lower detection limits compared to traditional methods involving
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porewater isolation and liquid-liquid extraction.5-7 The samplers can be used both in the field and
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in the laboratory to measure porewater concentration profiles and concentration differences
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across the sediment-water interface.8, 9 With these features, passive sampling methods present an
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opportunity to quantify in situ and ex situ porewater concentrations, which are proportional to the
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chemical activities of the porewater and sediment, respectively. Therefore, porewater
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concentrations can be used to better understand the transport of HOCs from sediment beds. With
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an improved understanding of HOC transport, the relative importance of different transport
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pathways can be quantified and contaminant fate models could be developed to assess long-term
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transport more accurately and predict the potential impacts of remediation alternatives.10, 11
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Recently, a passive sampling study in the Lower Duwamish Waterway (LDW) Superfund
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site, located in the city of Seattle, called into question the assumption that PCB transport from
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the sediment bed was predominantly due to diffusion.8 In this study, passive samplers deployed
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in the field (in situ) consistently measured lower porewater concentrations of PCBs when
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compared to the porewater concentrations measured in the laboratory (ex situ) using the same
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surface sediments. The apparent disequilibrium between the in situ porewater and the sediment
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was not consistent with a diffusive transport mechanism. It was, however, consistent with
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previous observations of other compounds in marine porewater inhabited by benthic macrofauna.
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Benthic macrofauna are known to exchange sediment porewater with overlying water
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(bioirrigation), and this flushing, in turn, has been shown to affect the porewater concentrations
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and fluxes of nutrients and oxygen across the sediment-water interface.12-14 Since the LDW has
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been shown to have an abundant and ubiquitous benthic community, this suggested that
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bioirrigation by benthic organisms may play a major role in PCB transport from the LDW
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sediment bed.15, 16
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In laboratory studies, the presence of benthic organisms has been shown to significantly
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affect the magnitude of HOC fluxes from the sediment into the overlying water. In these studies,
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up to a 25-fold increase in HOC flux was observed depending on the type of organism, its
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density in the sediment, and local environmental conditions.17-20 Similarly, in a field study where
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the benthic chamber was kept oxygenated, up to a 74-fold increase in PCB flux was seen as
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compared to the anoxic benthic chamber, and this was attributed to the activities of the benthic
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communities.21 Even though it is clear that benthic organisms can cause increased HOC fluxes, it
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is difficult to quantify the bioirrigation rate using the characteristics of the benthic community
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because the relationship between the two can be complex.18, 20, 22, 23 However, the use of a
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geochemical tracer like 222Rn can be used to infer the rate of porewater exchange, which was
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thought to be predominantly caused by bioirrigation (as opposed to tidal pumping or wave
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action) because of the site location and ubiquitous benthic community..24-28 This porewater
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exchange rate can then be used together with in situ HOC concentrations to calculate fluxes of
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HOCs from the sediment bed.
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The aim of this study was to determine the effect of bioirrigation on the porewater
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concentrations and transport of freely dissolved PCBs from sediment beds. PE samplers were
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used both in situ (in the field) and ex situ (in the laboratory) to determine if, and to what extent,
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porewater concentrations were unequilibrated with the surrounding sediment. The rate of
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porewater exchange was quantified using measures of 222Rn. These data were then used to
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calculate the fluxes of PCBs caused by bioirrigation, which were compared to the results of a
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diffusive flux model. Additionally, the in situ desorption rate constants of PCBs from sediment
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particles were inferred from the data and compared to reported literature values. This study is the
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first to compare in situ and ex situ porewater profiles of an HOC as well as to combine those
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measurements with an independent geochemical tracer to calculate bioirrigation fluxes of an
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HOC at field sites.
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MATERIALS AND METHODS
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Materials.
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All solvents were UltraResi-Analyzed (Avantor, Central Valley, PA). Ultrapure water
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was created using water treated with reverse osmosis followed by ion exchange and activated
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carbon system (Aries Vaponics), and finally ultraviolet exposure (Aquafine). Glassware was
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combusted at 450°C for 12 h except for the sediment jars which were purchased certified clean
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(ESS QC class, San Leandro, CA). All extracts were stored in amber glass vials.
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Passive samplers consisted of a 25-µm thick PE sheet (Film-Gard). The PE was pre-
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cleaned by soaking twice in dichloromethane and twice in methanol with each soak lasting 24 h.
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Isotopically-labeled PCB congeners 28, 47, 54, 97, 111, 153, and 178, used as performance
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reference compounds (PRCs), were then loaded into the PE by soaking with a 40:60
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methanol:water mixture containing the PCBs (approximately 100 g of PE in 2 L solution).29
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After 1 week on a shaker table (60 rpm), additional water was added to the mixture to make it
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~20:80 methanol:water, and the solution and PE were shaken for another 5 weeks. The residual
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methanol was removed from the PE by soaking it twice in ultrapure water for 24 h. This method
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allows for efficient use of the PRCs with final concentrations in the PE (100-250 ng/g PE)
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accounting for >70% of the congener masses initially added to the methanol:water solution. The
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lowest value (74%) was observed for the least hydrophobic PCB (octanol-water partition
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coefficient= 105.21) while the more hydrophobic PCBs were all near 100% transferred into the
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PE. PRC concentrations were also relatively homogenous in PE strips (relative standard
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deviation ≤7%).
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Field Sampling.
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In situ passive samplers were prepared by loading the PE into aluminum frames for
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insertion into the sediment bed the day before deployment. The exposed PE area measured 50 cm
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long by 15 cm wide. The EPA Region 10 dive team deployed samplers at three stations in the
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LDW on June 6-8, 2016 and retrieved samplers on July 25-27, 2016. Two of the sites were
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located near the beginning and end of the Superfund Site (designated as upriver and downriver,
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respectively), and the other site was located in between these two (designated as mid-river)
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(Figure S1). The sediment-water interface was identified by markings the divers used during
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insertion and an oxidation line that formed on the PE and frame during deployment (Figure S2).
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Starting at the sediment-water interface, the sampler was cut at 5 cm intervals except for
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immediately below the sediment surface where it was cut at the 0-3, 3-6, and 6-10 cm depth
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intervals. The potential for diffusion within the PE and sediment mixing to impact the PCB
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profile measured by the PE was evaluated, but both were determined to be negligible compared
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to the minimum PE segment (3 cm) with possible diffusion being
