Eva Webster

Potential role of sea spray generation in the atmospheric transport of perfluorocarboxylic acids

The observed environmental concentrations of perfluorooctanoic acid (PFOA) and its conjugate base (PFO) in remote regions such as the Arctic have been primarily ascribed to the atmospheric transport and degradation of fluorotelomer alcohols (FTOHs) and to direct PFO transport in ocean currents. These mechanisms are each capable of only partially explaining observations. Transport within marine aerosols has been proposed and may explain transport over short distances but will contribute little over longer distances. However, PFO(A) has been shown to have a very short half-life in aqueous aerosols and thus sea spray was proposed as a mechanism for the generation of PFOA in the gas phase from PFO in a water body. Using the observed PFO concentrations in oceans of the Northern Hemisphere and estimated spray generation rates, this mechanism is shown to have the potential for contributing large amounts of PFOA to the atmosphere and may therefore contribute significantly to the concentrations observed in remote locations. Specifically, the rate of PFOA release into the gas phase from oceans in the Northern Hemisphere is calculated to be potentially comparable to global stack emissions to the atmosphere. The subsequent potential for atmospheric degradation of PFOA and its global warming potential are considered. Observed isomeric ratios and predicted atmospheric concentrations due to FTOH degradation are used to elucidate the likely relative importance of transport pathways. It is concluded that gas phase PFOA released from oceans may help to explain observed concentrations in remote regions. The model calculations performed in the present study strongly suggest that oceanic aerosol and gas phase field monitoring is of vital importance to obtain a complete understanding of the global dissemination of PFCAs.

Equilibrium Lipid Partitioning Concentrations as a Multi-Media Synoptic Indicator of Contaminant Levels and Trends in Aquatic Ecosystems

Abstract Large quantities of monitoring data have been gathered over many years for the Great Lakes basin in the form of contaminant concentrations in a variety of media including air, water, suspended and bottom sediments, wildlife (especially fish, birds, and birds’ eggs), soils, vegetation, foodstuffs, and human tissues. The quantity of data and the diversity of units used to report the concentrations are such that it is difficult to gain a comprehensive, ecosystem-wide, interpretation of contaminant levels and trends. It is suggested that the problems arising from the use of a diversity of units can be addressed, at least for an organic contaminant, by expressing all concentrations on a common equilibrium partitioning basis. The equilibrium lipid partitioning (ELP) concentration is suggested as being most appropriate for this purpose. It is shown that expressing monitoring data as ELP concentrations permits a more reliable determination of time trends in contaminant levels in multi-media systems, facilitates comparison between ecosystems, and contributes to an improved understanding of contaminant sources and fate. An “adjusted” ELP concentration may be deduced to compensate for differences in equilibrium status between media, thus consolidating all multi-media contaminant levels in a single, ecosystem-wide, synoptic indicator of trends in contaminant status. The approach is illustrated for polychlorinated biphenyls (PCBs) in the Lake Ontario and Lake Superior ecosystems showing a definite decrease in concentrations over the period 1977 to 1993 with estimated decay half-lives of 8.1 and 11.4 years, respectively.

Modeling the environmental fate of perfluorooctanoic acid and perfluorooctanoate: An investigation of the role of individual species partitioning

A multimedia multi-species environmental fate model was developed for the conjugate pair perfluorooctanoic acid (PFOA):perfluorooctanoate (PFO). The model allows assessment of the relative contribution of each individual species, in equilibrium with each other, to the overall environmental movement of the pair. The Lake Ontario (Canada/USA) watershed system was selected for this investigation and is simulated in a single-region, seven-compartment model, including a water surface microlayer, and aqueous aerosol generation and redeposition. Results indicate that in the equilibrated presence of both PFOA and PFO, the environmental fate of the pair can be accomplished by consideration of the physical properties of the neutral acid, which govern the intermedia distribution of the pair, coupled with processes of media advection, such as air or water flow. The role of the anion, while the most populous species in the aqueous phase, appears merely to be as a source of the neutral acid for subsequent partitioning. Thus, when only the bulk aqueous phase anion concentrations are of interest a multimedia fate model is not required because these concentrations are largely predictable from the magnitude of emissions to and the advection of the phase. With neutral species partitioning, all local field measurement concentrations of the conjugate pair, PFO(A), are explained by the model to within approximately an order of magnitude, with the exception of lake sediment solids. Model results indicate that bulk aqueous phase PFO acts as a net source for PFOA to the atmosphere, where it may be subject to long-range transport (LRT). Initial calculations suggest an atmospheric LRT potential for PFO(A) of thousands of kilometers, rendering it comparable to hexachlorobenzene.

TFA from HFO‐1234yf: Accumulation and aquatic risk in terminal water bodies

A next-generation mobile automobile air-conditioning (MAC) refrigerant, HFO-1234yf (CF(3) CF = CH(2)), is being developed with improved environmental characteristics. In the atmosphere, it ultimately forms trifluoroacetic acid (TFA(A); CF(3)COOH), which is subsequently scavenged by precipitation and deposited on land and water as trifluoroacetate (TFA; CF(3)COO(-)). Trifluoroacetate is environmentally stable and has the potential to accumulate in terminal water bodies, that is, aquatic systems receiving inflow but with little or no outflow and with high rates of evaporation. Previous studies have estimated the emission rates of HFO-1234yf and have modeled the deposition concentrations and rates of TFA across North America. The present study uses multimedia modeling and geographic information system (GIS)-based modeling to assess the potential concentrations of TFA in terminal water bodies over extended periods. After 10 years of emissions, predicted concentrations of TFA in terminal water bodies across North America are estimated to range between current background levels (i.e., 0.01-0.22 µg/L) and 1 to 6 µg/L. After 50 years of continuous emissions, aquatic concentrations of 1 to 15 µg/L are predicted, with extreme concentrations of up to 50 to 200 µg/L in settings such as the Sonoran Desert along the California/Arizona (USA) border. Based on the relative insensitivity of aquatic organisms to TFA, predicted concentrations of TFA in terminal water bodies are not expected to impair aquatic systems, even considering potential emissions over extended periods.

A perspective on environmental models and QSARs.

A general review is presented of the roles of QSARs and mass balance models as tools for assessing the environmental fate and effects of chemicals of commerce. It is argued that all such chemicals must be assessed using a consistent and transparent methodology that uses chemical property data derived from QSARs, or experimental determinations when possible and applies evaluative or region-specific environmental models. These data and models enable an assessment to be made of the key chemical features of persistence, bioaccumulation, potential for long-range transport and toxicity. The other key feature is quantity used or discharged to the environment. A taxonomy of environmental models is presented in which it is suggested that rather than develop a single comprehensive model, the aim should be to establish a set of coordinated and consistent models treating evaluative and real environmental systems at a variety of scales from local to global and including food web models, organism-specific models and human exposure and pharmacokinetic models. The concentrations derived from these models can then be compared with levels judged to be of toxic significance. A brief account is given of perceived QSAR needs in terms of partitioning, reactivity, transport and toxicity data to support these models.

Equilibrium modeling: A pathway to understanding observed perfluorocarboxylic and perfluorosulfonic acid behavior

Equilibrium distribution models of hydrophobic neutral partitioning of the perfluorinated carboxylic and sulfonic acids were shown, without the need for any physical chemical properties, to successfully predict the sediment-water distribution (D(SW) ) directly from independently measured equilibrium tissue distributions known as the bioconcentration factor (BCF). The constant of proportionality required by the models successfully predicted the correlation between the biotic and abiotic distributions of both sets of chemicals, thus demonstrating the applicability of the assumptions inherent in the models, that is, hydrophobically driven partitioning of the neutral species, and thus the applicability of the models themselves. Colloquially speaking, the models are thus validated as applicable to these chemicals. Subsequent application of the standard equilibrium models showed order of magnitude agreement for 83% of measured BCF values and 88% of measured D(SW) for the perfluorinated carboxylic acids, confirming the physical chemical properties used. The applicability of the models to perfluorooctane sulfonic acid (PFOSA) was shown by the successful prediction of D(SW) from BCF. Therefore, the measured D(SW) and BCF could be used to calculate the octanol-water distribution, D(OW) , and hence the corresponding pK(a):K(OW) solution set, thus providing independent experimentally based estimates of these properties. For both the perfluorinated carboxylic and sulfonic acids, the existing standard equilibrium models are shown to be applicable.

Can the unit world model concept be applied to hazard assessment of both organic chemicals and metal ions

A unit world model that has the potential to be used for the hazard assessment of both metal ions and organic chemicals is described and discussed, with an emphasis on the problems that arise when treating metal ions. It is based on the steady-state equilibrium criterion model that is designed to simulate the fate of organic chemicals in a 100,000-km(2) region and comprises four well-mixed compartments: Air, water, soil, and sediment. To be applicable to metal ions, modifications are required. The single soil and sediment layers should be replaced by two layers to accommodate aerobic and anaerobic conditions. The more complex and variable partitioning of metals resulting from dependence on pH, redox conditions, ionic oxidation state, and presence of sulfide also must be addressed, but preferably in a separate geochemical model, because these factors can result in nonlinearity. For metals, a dynamic as well as a steady-state model is desirable. It is shown that the resulting model can be applied to both organics and metals. Rather than seeking to apply the hazard criterion of persistence to metal ions, the model can be used to deduce a critical loading that results in a defined toxic end point, thus integrating the hazard criteria of persistence, toxicity, and possibly, bioaccumulation. This approach is applied illustratively to naphthalene as a typical organic substance and to four environmentally relevant metal ions. Results are discussed and recommendations made for further development. Specifically, the absence of metal degradation can result in large, steady-state quantities in soils and sediments corresponding to residence times of many centuries. Consequently, the dynamic calculations are more relevant for fate assessments of metals over a period of years, and more focus on the aquatic environment is justified.

The Evolution and Future of Environmental Fugacity Models

In this chapter we first review the concept of fugacity as a thermodynamic equilibrium criterion applied to chemical fate in environmental systems. We then discuss the evolution of fugacity-based models applied to the multimedia environmental distribution of chemicals and more specifically to bioaccumulation and food web models. It is shown that the combination of multimedia and bioaccumulation models can provide a comprehensive assessment of chemical fate, transport, and exposure to both humans and wildlife. A logical next step is to incorporate toxicity information to assess the likelihood of risk in the expectation that most regulatory effort will be focused on those chemicals that pose the highest risk. This capability already exists for many well-studied chemicals but we argue that there is a compelling incentive to extend this capability to other more challenging chemicals and environmental situations and indeed to all chemicals of commerce. Finally, we argue that deriving the full benefits of these applications of the fugacity concept to chemical fate and risk assessment requires continued effort to develop quantitative structure–activity relationships (QSARs) that can predict relevant chemical properties and programs to validate these models by reconciliation between modeled and monitoring data.

An exploration of the role of mysids in benthic‐pelagic coupling and biomagnification using a dynamic bioaccumulation model

A mass-balance model for the uptake of organic contaminants in the opposum shrimp (Mysis relicta) is developed. The model describes the concentration in the mysid as a function of time as it grows and its lipid content changes over a two-year life span. The model can describe uptake in varying proportions from pelagic and benthic sources. Four mysid scenarios are considered: entirely pelagic, entirely benthic, half-pelagic and half-benthic, and following the observed seasonal behavior patterns. The seasonal-scenario mysid yields results consistent with levels of total polychlorinated biphenyls reported for Lake Ontario (Canada/USA). The relative sediment and water fugacities are shown to play a critical role in determining the contamination level in mysids. Inclusion of mysids in a simple food-web model demonstrates higher concentrations in upper-trophic-level organisms by two effects: introduction of another trophic level in the food web, and increased benthic-pelagic coupling.

A cautionary note on implications of the well‐mixed compartment assumption as applied to mass balance models of chemical fate in flowing systems

A convenient, simple, and widely used approach for modeling the fate of a chemical in a flowing environmental or biological system is to simulate the system as comprising one or more well-mixed boxes, also known as continuous stirred tank reactors (CSTRs). In principle, any desired level of accuracy can be achieved by increasing the number of boxes. However, highly segmented systems require more input data, they are more computationally intensive, and the results may be more difficult to interpret. Thus there is a tendency to minimize the number of boxes, especially in screening level models. Whereas in the hydrology and engineering literature there is an appreciation of the mathematical errors associated with applying the well-mixed box concept, we believe that these errors are often underappreciated when modeling certain environmental systems. Here, we briefly review the implications of these errors in multimedia models, river and lake simulations, atmospheric transport, flow in soils, gastrointestinal absorption, and metabolism in the liver. The key conclusion is that if over 25% of the chemical entering a box is removed, applying this well-mixed assumption can lead to substantial error. We recommend that results obtained when this criterion is violated be treated with caution.