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Statistical comparison of performance reference compound (PRC)-based methods for calculating Cfree.
Citation:
Burgess, R. AND A. Joyce. Statistical comparison of performance reference compound (PRC)-based methods for calculating Cfree. Tenth International Conference on Remediation and Management of Contaminated Sediments, New Orleans, Louisiana, February 11 - 14, 2019.
Impact/Purpose:
Passive samplers are an accurate and relatively inexpensive tool to monitor the concentration of pollutants at contaminated sites. The freely dissolved concentration (Cfree) of many organic chemicals can be used to calculate the degree to which that chemical may be biologically available to fish and other organisms. Currently, there are four methods for calculating Cfree based on passive sampling data. This variety of methods can create confusion for federal, state and tribal environmental managers as to which one to apply. In this study, these four methods were used to calculate Cfree for several PCB congeners and total PCBs in the water column of a contaminated sediment site. In general, two of the methods resulted in similar estimates of Cfree while two other methods appeared to underestimate Cfree. Based on studies from the scientific literature, the first two methods which showed good agreement are believed to generate accurate Cfree values. Knowing how best to calculate Cfree will help environmental managers to use passive samplers properly resulting in a better understanding of how much of the chemicals at contaminated sites are available to the fish and other organisms living there.
Description:
Objectives. Freely dissolved concentrations (Cfree) have proven useful for estimating exposure concentrations of nonionic organic contaminants in the water column and sediment interstitial waters. It is critical that Cfree be determined under actual or modelled equilibrium conditions. Performance reference compounds (PRCs) are applied to model equilibrium conditions and estimate fractional equilibrium (feq) used to calculate Cfree. Currently, there are four methods for using PRC data to calculate feq. This variety of methods can create confusion for inexperienced users as to which one to apply. The objective of this research was to perform a comprehensive statistical analysis of the methods comparing their estimates of Cfree to provide a recommendation of which method(s) is preferable to apply. Approach/Activities. Low-density polyethylene (PE) in four thicknesses was used as a passive sampler and 27 PCB congeners served as the target chemicals. Deployments were performed in triplicate in the water column of three stations at the New Bedford Harbor Superfund site in New Bedford, Massachusetts (USA). Following the 30-day deployments, the passive samplers were recovered, the polymers extracted, and the extracts analyzed for PCBs by GC/MS. The four methods were then used to estimate Cfree and these values were compared statistically with ANOVA followed by Fisher’s test using SAS. This analysis resulted in twelve comparisons of the multiple PCB congeners and total PCBs. Results/Lessons Learned. In general, the diffusion-based and exchange rate-based methods resulted in statistically similar estimates of Cfree while assuming equilibrium often under-estimated Cfree and using the first order model generated Cfrees statistically-different from the diffusion and exchange rate methods. For example, at the most contaminated site (NBH2), for the 12, 25, 51 and 76 µm PE thicknesses, the diffusion-based and exchange-rate methods Cfree values were statistically identical 68%, 36%, 68% and 32% of the time, respectively. Based on the comparability of the diffusion method Cfrees to conventional measures of Cfree from literature studies, this statistical comparison indicates the diffusion-based and exchange rate-based models are the preferred methods to use for calculating accurate water column Cfrees.