Program

The freely dissolved concentration (Cfree) is a critical parameter for estimating bioavailability and improving risk assessment for hydrophobic organic contaminants (HOCs). However, Cfree is difficult to measure for HOCs such as PCBs and DDT and its derivatives (DDTs), because these compounds are preferentially sorbed to suspended solids or dissolved organic matter. A number of passive sampling techniques have been developed and used for Cfree measurement of HOCs, which include semipermeable membrane devices (SPMD), polyethylene devices (PED), polyoxymethylene, and solid-phase microextraction (SPME). These passive samplers all operate on the presumption of equilibrium partition between the sampler sorbent and water, so that Cfree may be derived from the concentration in the sorbent via the use of a partition coefficient. However, many studies show that for strongly hydrophobic compounds, it may take several months or even longer to reach equilibrium(ter Laak, 2008). This limitation is the largest technical barrier hampering the practical application of these samplers. An effective approach to circumvent this limitation is to preload passive samplers with performance reference compounds (PRCs), ideally isotope labeled analogues of the target analytes. This method is based on isotropic exchange between the native analytes and their isotope-labeled counterparts, where uptake of the target analytes into the sampler is approximated by the desorption of PRCs from the sampler, allowing for virtually in situ calibration during sampling (Chen and Pawliszyn, 2004; Adams, 2007). To date, PRC calibration has been successfully applied in PED and SPMD for the in situ measurement of Cfree of PAHs, PCBs, hexachlorobenzene, and DDTs. In this study, to explore the use of PRC-SPME for in situ environmental sampling, disposable PDMS fibers (35-μm and 100-μm coating) preloaded with stable isotope labeled analogues as PRCs were deployed at six stations (each with three depths) in the open ocean above the Palos Verdes Shelf Superfund site for 33 d to measure Cfree of DDT and its degradates. The observed values of fractional equilibration (feq) of PRCs were mostly < 0.85, suggesting nonequilibrium conditions even 33 d after deployment. The observed feqs for the samplers varied with compounds, sampling stations and sampling depths, validating the need for calibration to derive accurate Cfree. The Cfree values of DDE and DDD were successfully determined with PRC-SPME and were in good agreement with those previously measured by in situ large-volume water sampling or polyethylene devices. The highest Cfree in the water 5 m off the ocean floor was 750 pg/L for o,p'-DDE, 2170 pg/L for p,p'-DDE, 24 pg/L for o,p'-DDD, and 75 pg/L for p,p'-DDD. The fluxes of o,p'- and p,p'-DDE in the water column from bottom to upper surface at the contaminated site ranged from 3.8×10-6 to 7.67×10-5 ng/cm2/yr. Results of this study demonstrated the feasibility and advantages of using disposable PDMS fiber coupled with PRCs for in situ sampling under field conditions.