Background/Objectives: A non-equilibrium correction protocol using performance reference compounds (PRCs) has been developed to enhance the accuracy of the passive sampling method to assess the freely dissolved concentration, Cfree, during short-term field deployments. The protocol has matured through laboratory experiments and field trials performed by academic, industry, and government programs.PRC desorption anomalies have often been observed by multiple investigators at sites of different hydrodynamic conditions, while using different passive sampling materials, and while using different classes of PRCs. PRC desorption anomalies occur when a PRC is measured in a passive sampling device after retrieval from a field exposure at a concentration that is much higher than is expected given the hydrophobicity of the PRC and/or the behavior or other PRCs used in the same device. Such anomalies are outliers with respect to other PRC data from the same device, and their presence increases the error and uncertainty associated with estimation of Cfree values. This presentation will discuss examples of PRC desorption anomalies, potential explanations, and data analysis approaches to resolve anomalies.
Approach/Activities: Several PRC datasets from PDMS- and PE-based passive samplers spiked with deuterated PAHs and rare PCB congeners PRCs were evaluated for anomalies. These passive samplers were deployed at several contaminated sediment sites for various durations. Non-equilibrium correction factors were calculated by determining a passive sampler mass transfer coefficient (ke) for all PRCs and fitting the linear relationship between the ke and a physiochemical property such as Kow or the passive sampler-solution partition coefficient, KPDMS or KPE. Based on these linear relationships, PRC desorption anomalies were identified and evaluated with respect to their influence on Cfree values.
Results/Lessons Learned: On average, PRC anomalies made up 13-22% of the PRC data sets reviewed, and were present in a high proportion of PE sampler data (i.e., 40 to 100% of samplers indicated at least one PRC anomaly). At the Puget Sound Naval Shipyard site (Bremerton, WA), where eleven PCBs were selected for use as PRCs for PE deployment at 10 locations and 4 were used for PDMS samplers, PRC anomalies were observed at all sampling locations for penta-, hexa-, and hepta-chlorobiphenyls. At the West Branch of the Grand Calumet River, four deuterated PAHs were selected for use as PRCs for PDMS sampler deployment at 21 locations, and PRC anomalies were seen at 8 locations for mid to high hydrophobicity PRCs. Desorption anomalies were also observed at five of the seven sampled locations for all deuterated PAHs at Chattanooga Creek. Desorption anomalies were also found in 33% of PE samplers (10 PRCs per sampler) deployed in San Diego Bay (San Diego, CA). As indicated by these results, PRC desorption anomalies are neither site specific, PRC specific, laboratory specific, nor sampler material specific. Reducing the limitations of mass transfer could reduce observations of these anomalies, as a dataset with 6 PE samplers that were agitated in a mixer for 2 weeks exhibited no PRC anomalies. Options to enhance mass transfer could include enhanced novel passive sampler materials, ex situ application of passive samplers in mixed samples, sample supports with vibration capabilities, and passive sampler configurations with higher area to volume ratios. Until these anomalies can be addressed and resolved, it is recommended that data from PRC desorption anomalies be carefully considered and communicated during Cfree calculations.