Measuring Freely Dissolved Water Concentrations of PCBs Using LDPE Passive Samplers and Performance Reference Compounds (PRCs)

Oral Presentation

Prepared by A. Joyce, R. Burgess
US EPA, 27 Tarzwell Dr., Narragansett, RI, 02882, United States


Contact Information: joyce.abbey@epa.gov; 401-782-3096


ABSTRACT

Low-Density polyethylene (LDPE) sheets are often used as passive samplers for aquatic environmental monitoring to measure the dissolved concentrations of hydrophobic organic contaminants (HOCs). These concentrations are then used to evaluate the potential for ecological and human health effects. HOCs that are freely dissolved in water (Cfree) will partition into the LDPE eventually achieving an equilibrium with the surrounding environmental phases (e.g., water, suspended particles, colloids). This partitioning from the water to the polymer allows for detection limits below the pg/L range for highly hydrophobic compounds. One major challenge associated with passive sampling is ensuring that equilibrium between the polymer and the other environmental phases has been attained for each HOC. Performance reference compounds (PRCs) are one way to address this challenge. PRCs are often stable isotopes labeled HOCs (e.g., 13C-PCB) not present in the environment that have been loaded into the passive sampler prior to deployment. Equilibrium conditions for targeted HOCs can be estimated from determination of a fractional equilibrium (feq) or sampling rate (Rs) which are calculated based on the percent of PRC released during a deployment. Once equilibrium concentrations have been estimated for the passive sampler, Cfree for each target contaminant can be estimated using a known partitioning coefficient. This work will focus on presenting a study in which PRCs were pre-loaded into several LDPE passive samplers of varying thickness and deployed in the water column at three stations in the PCB-contaminated New Bedford Harbor Superfund site (MA, USA). The deployed samplers were then used to evaluate several models for processing and interpreting PRC data in order to calculate PCB Cfree. Following the deployments, the percent of PRC lost ranged from 0-100%. Fractional equilibrium decreased with increasing PRC molecular weight as well as sampler thickness, allowing Cfree comparisons for co-deployed samplers at different feq. This large range of observed sampling rates in combination with the various sampler sizes deployed allowed for a critical comparison of the available PRC models used in the literature today giving further insight into the precision and utility of these models. Overall, a total of 28 PCBs were measured at Cfree concentrations varying from 0.05 pg/L (PCB 206) to about 200 ng/L (PCB 28) on a single LDPE sampler. The large sampling capacity of the passive samplers allowed for spatial comparisons of each of the 28 PCBs measured at all three sites sampled. Results of this evaluation provided novel insights into processing and interpreting passive sampler and PRC data for making informed environmental monitoring decisions.