Date of Award
Doctor of Philosophy
Determining accurate exposure estimates and subsequent risk of hydrophobic organic contaminants (HOCs) in aquatic sediments requires measuring the bioavailable and/or bioaccessible concentration in sediment; as total extractable concentrations have not been found to produce accurate results. Organic carbon normalization was originally proposed as a means of accounting for the bioavailable concentration by estimating the chemical activity of the contaminant expressed as the freely dissolved chemical concentration in the interstitial water, thus correcting exhaustive extractable concentrations for the sorbing phase of sediments. Organic carbon normalization often fails, however, to accurately reflect exposure as other environmental variables (i.e. organic carbon composition, aging time of contaminants in the environment) alter desorption, such that changes in chemical activity as represented by the interstitial water concentrations are not controlled by organic carbon alone. Desorption-based samplers, such as single-point Tenax extractions (SPTE), provide a clearer estimate of bioaccessibility than organic carbon normalization by serving as a sink for desorbing compound for the length of the extraction. In this way, SPTEs account for all the factors affecting desorption and the resulting interstitial water concentrations, providing estimates of the chemical concentration that will become available for exposure in a given time frame. The utility of SPTEs as an exposure metric has been demonstrated many times in the literature through estimates of bioaccumulation and development of toxicity benchmarks. The simplicity, accuracy, and robust nature of this technique suggests this tool could serve as an ideal means of evaluating exposure and risk of HOCs, and more specifically acutely toxic compounds, such as pyrethroids, during environmental sampling and risk assessments of aquatic sediments. However, the use of this method is limited within the scientific literature and absent from most risk assessment protocols. The reasons for its limited use are linked to poor methodological standardization, an absence of understanding of environmental and methodological variation on estimates of bioaccessibility provided by SPTEs, and only a vague idea of how Tenax extractions relate to other exposure metrics, such as passive samplers. Therefore, the dissertation goals were to: evaluate the effects of variation in the SPTE, specifically the Tenax mass to organic carbon mass (Tenax:OC) ratio, on exposure estimates of pyrethroids (Chapter Two); understand how methodological and environmental variation affect the relation of SPTEs to bioaccessibility represented by desorption of pyrethroids from the labile desorbing fraction (Frap) (Chapter Three); and, determine how bioavailability and bioaccessibility are linked through evaluation of chemical activity expressed as the freely dissolved chemical concentrations provided by SPTEs, passive sampler concentrations, and Frap (Chapter Four). The most variable aspect of the SPTE within the Tenax literature is the Tenax:OC ratio used during 24 h SPTEs. Yet, no study has evaluated how altering this ratio may affect 24 h SPTE concentrations and thus, biological exposure estimates provided by Tenax extractions. Manipulating the Tenax:OC ratio used during 24 h SPTEs of pyrethroids from laboratory-spiked and field-contaminated sediments revealed the effect of this variation was such that Tenax extractable pyrethroid concentrations varied between 0.85 to 3.91-fold between the highest and lowest ratios examined. The results of this experiment suggest most of the variation in toxicological endpoints derived using Tenax extractable concentrations is due to toxicokinetic and toxicodynamic variation in biological responses across sediments and not due to methodological variation of the Tenax extraction (Chapter Two). The utility of the SPTEs as an estimate of exposure is linked to the ability of SPTEs to reflect the chemical concentration that desorbs from sediment. As many factors, such as the organic carbon content, aging time of sediments, and hydrophobicity of the compounds, can impact desorption, understanding how these factors affect the relationship of SPTEs to biological exposure is needed to evaluate the consistency of the Tenax extraction. The relation of SPTE concentrations to Frap was proportional despite changes in organic carbon content of the sediment being extracted, the hydrophobicity of the pyrethroids, or the Tenax mass used during the extraction, such that the SPTE concentration was equal to 1.46 ± 0.03 times the pyrethroid concentration in Frap (Chapter Three). Only the aging time of the pyrethroids in the sediment significantly affected this relationship, as desorption from longer aged sediments slowed, reducing the 24 h SPTE concentration to Frap ratio by -0.0027/d (Chapter Three). The results of Chapters Two and Three demonstrate the consistency of the Tenax extraction as a representation of biological exposure of pyrethroids in sediment. However, other aspects limit the widespread use of the Tenax method, particularly the relation of this technique to more widely accepted bioavailability-based metrics, such as passives samplers. Tenax extractions are often disregarded in favor of passive samplers as the link between bioavailability-based metrics, chemical activity, and exposure is well understood. However, as SPTEs and passive samplers both demonstrate a clear relation to bioaccessibility through estimates of Frap, it was hypothesized that both exposure metrics represent the same chemical fraction of sediment, and as such could be considered complementary tools for evaluating biological exposure through estimates of the freely dissolved interstitial water concentration. This was confirmed when comparisons of the chemical activity expressed as the interstitial water concentration at equilibrium were done using the chemical concentration estimated by Frap, a passive sampler, and SPTEs. Strong linear relationships (p<0.0001) were found among all three metrics, such that Frap, passive sampler, or 24 h SPTE concentrations of pyrethroids from sediment provide comparable estimates of the freely dissolved interstitial water concentration in sediment. Thus, Tenax extractions and passive samplers, which describe the bioaccessible and bioavailable concentrations, respectively, describe the same chemical fraction in sediment; the labile desorbing fraction. This dissertation provides further concrete evidence that the SPTE offers a robust, rapid, and cost-effective means of evaluating exposure of acutely toxic compounds in sediment. With data that link this exposure metric to more widely accepted methods, such as passive samplers, and demonstrate the robust character of the SPTE, the research presented here should further the use of the SPTE within the scientific and risk assessment communities.
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