A Field Evaluation of Performance Reference Compound Based Estimates of Cfree Using Water Column Deployed Passive Samplers

Low-Density polyethylene (LDPE) sheets are often used as passive samplers for aquatic environmental monitoring to measure the freely dissolved concentrations of hydrophobic organic contaminants (HOCs). HOCs that are freely dissolved in water (Cfree) will partition into the LDPE until a thermodynamic equilibrium is achieved; that is, the HOC’s chemical potential in the passive sampler is the same as its potential in the surrounding environment. However, achieving equilibrium for high molecular weight HOCs can take several months or even years. One way to evaluate the equilibrium status or estimate the uptake kinetics is by using performance reference compounds (PRCs). PRCs are often isotopically labeled versions of target compounds and are partitioned into the LDPE prior to deployment. Based on the fraction of each PRC lost during deployment, a sampling rate (Rs) or a fractional equilibrium (feq) can be determined for target HOCs, under the assumption that PRC desorption from the passive sampler occurs at the same rate as the unlabeled target HOCs. In this study, LDPE passive samplers were pre-loaded with six, 13C-labelled PCBs as PRCs, and deployed in New Bedford Harbor, MA, USA. Triplicate samplers were collected after 30, 56, 99, and 129 day deployments. PRC-corrected Cfree concentrations were estimated for 27 target PCBs (log KOW ranging from 5.07 – 8.09) at each time point. Results allowed for calculation of desorption rates of PRCs as well as uptake rates for target HOCs and confirmed that kinetics are indeed isotropic for isomers. Results were fit to a traditional first order kinetic model, a sampling rate model, and a diffusion model to assess how well each predicted equilibrium Cfree. Samplers at equilibrium showed agreement within 20%. However, for PCBs with slower kinetics, as the fractional equilibrium achieved decreased in magnitude, the Cfree agreement between models and other time points also decreased. In general, results from the 30-day deployment illustrated the highest Cfree for PCBs with a log KOW greater than 6.5 or when a feq of 15% or less was achieved over the course of the deployment. These results provide a field-based evaluation of the usefulness of PRCs but also suggest caution should be used when correcting passive sampling data by a factor of 10 or more.