Participants

• Philip M. Gschwend, Ph.D. MIT, Principal Investigator

• John K. MacFarlane, MIT, Research Engineer

• Rachel G. Adams, MIT, Current Graduate Student

• Rainer Lohmann, Ph.D., MIT, Postdoctoral Associate

• Amparo Flores, MIT, M Eng

• Peter Israelsson, MIT, M Eng

• Ricardo Petroni, MIT, M Eng

Objectives

• To study the extent of polycyclic aromatic hydrocarbon, PAH, contamination in the water column of the Boston Inner Harbor and New York Harbor.

• To test the hypothesis that PAH concentrations will decrease as a result of the dredging and capping activities.

• To compare and contrast bed-to-water column PAH fluxes in Boston Inner Harbor's relatively protected water with sites undergoing resuspension such as New York Harbor to elucidate mechanisms by which PAHs are input into the water column.

Approach

Preliminary estimates indicate that the largest existing source of PAHs to the water column is the sediment-water exchange, with the contribution from river flow one order of magnitude smaller and other sources even smaller. If this is correct, then capping of contaminated sediments (which should reduce the sediment water flux) should lead to a significant improvement in water quality.

This hypothesis will be tested through two approaches: use of a box model to refine estimates of various terms in the PAH mass balance and examination of soot-bound PAH fractions in water and sediments.  For the harbors, a mass balance of the selected PAHs will be develped using a box model (Figure 1) which incorporates all major sources and sinks within the system (Table 1).

Figure 1. Conceptual Box Model.

A preliminary model of each PAH's mass balance in the harbors will be developed and used to design a sampling program.As the samples are analyzed and work progresses, the model will be calibrated to ensure that it provides an accurate representation of PAHs within the system.Using the box model that best represents current conditions, the impact of the dredging and capping activities in releasing and sequestering PAHs will be evaluated under different scenarios.As new data become available, for example from field sampling activities in the vicinity of the capped cells, the box model will be modified to incorporate new terms.The ultimate goal will be to predict changes in the water column PAH concentrations after the completion of the entire dredging and capping project.

The sampling program will focus on accurately measuring PAHs in water and sediments.Field data will be collected to improve the accuracy of the box model developed to estimate a mass balance for three PAHs.The sampling program will employ Polyethylene Devices (PEDs) which facilitate PAH measurement by concentrating water column PAHs. Methods will also be developed to determine the bioavailability of soot-bound PAHs, a potentially important component of the PAH fraction in harbor sediments.If PAHs are strongly bound and not bioavailable, remediation efforts, (e.g. dredging, capping and removal) may not be necessary.Similarly, if bioavailable PAHs are biodegrated quickly, PAH remediation may not be necessary.In addition, oxic surface sediments (~1cm) will be collected from three sites in Boston Harbor and biodegradation rates of the in situ PAHs measured in the laboratory.Initial PAH concentrations in the pore water, PAHs sorbed to solids, and the fraction of soot carbon present will be determined.Remaining PAH concentrations will be measured over time in order to determine biodegradation rates for each of the three PAHs of interest.This and several other laboratory experiments will help elucidate the role that soot plays on the bioavailability of PAHs.

Status Report

Polyethylene devices (PEDs), which rely on the partitioning of hydrophobic organic contaminants (HOCs) between water and polyethylene, were shown to be useful for the measurement of dissolved HOCs like polycyclic aromatic hydrocarbons (PAHs) in natural waters.These PEDs allow for the measurement of the fugacity or "fleeing tendency" of such chemicals in water.These dissolved concentrations are of ecotoxicological concern as they reflect the HOC fraction that is driving uptake by the surrounding organisms.Because PEDs require on the order of days to equilibrate in the field, their use provides time-averaged measurements.Laboratory-measured polyethylene-water partition coefficients for two PAHs were:17,000 ±1000 (mol/LPE)/(mol/Lw) for phenanthrene and 89,000 ± 6000 (mol/LPE)/(mol/Lw) for pyrene.These organic polymer-water partition coefficients were found to be comparable to other organic solvent-water partitioning coefficients.These large coefficients allowed for the measurement of dissolved concentrations as low as 1 pg/L for benzo(a)pyrene and 400 pg/L for phenanthrene in the lower Hudson Estuary.

Sampling performed in the lower Hudson Estuary during neap and spring tides revealed increased concentrations of dissolved pyrene and benzo(a)pyrene, but not phenanthrene, during increased sediment resuspension.These data suggest that resuspension events mostly influence the bed-to-water exchange of PAHs with greater hydrophobicities.PAH water concentrations predicted assuming dissolved and sorbed concentrations related via the product, fomKom, where fom is the fraction of organic matter in the suspended sediments and Kom is the organic-matter-normalized solid-water partition coefficient for the PAH of concern, were far from observed concentrations.Adding the influence of soot to the partition model via Kd = fomKom + fscKsc, where fsc is the weight fraction of soot carbon in the solid phase and Ksc is the soot carbon-water partition coefficient estimated form activated carbon data, yielded predicted concentrations that were much closer to the observed values when PAH partitioning to soot was included in the partitioning model.This finding suggests that soot plays an important role in controlling the cycling of PAHs in the aquatic environment.However, even when the soot partitioning of PAHs was included in the model, the predicted dissolved values were still larger than the measured values.This suggests that the time of particle resuspension is too short to allow for particle-water sorptive equilibrium.Using ratios of source indicative PAHs, it was estimated that 90% of the dissolved PAH fraction was derived from petrogenic sources.In contrast, the same source ratios for the total (dissolved and sorbed) PAH concentrations indicated that only 55% of the total were petrogenically-derived.The observations in this work suggest that efforts to regulate and remediate PAH-contaminated sediments must consider the potential impacts of soot associations of the PAHs.

Participant Information

• Philip M. Gschwend is a Ford Professor of Engineering at the MIT Parsons Lab.

• John K. MacFarlane is a Research Engineer at the MIT Parsons Lab.

• Rachel Adams is a doctoral student at the MIT Parsons Lab.

• Rainer Lohmann is a Postdoctoral Associate at the MIT Parsons Lab.

• Amparo Flores completed Masters with the MIT Civil and Environmental Engineering Masters of Engineering program.

• Ricardo Petroni completed a Masters with the MIT Civil and Environmental Engineering Masters of Engineering program.

• Peter Israelsson completed a Masters with the MIT Civil and Environmental Engineering Masters of Engineering program.