Steady state bioaccumulation

Polyethylene Higher Simulated bioaccumulation decreases substantially due to presence of plastic Model predicts decrease in steady-state bioaccumulation for log >5 when microplastics in system... 

Also bioaccumulating substances may in some situations call for a higher assessment factor. If accumulation is likely, the toxicity studies need to be of sufficient length to cover the accumulation period (e.g., the time to reach a steady-state concentration). If there is limited information on these aspects, it has to be considered to which extent this lack of information should affect the assessment factor. 

The accumulation of HOCs in phytoplankton plays an important role in food-web bioaccumulation. Both the increased times to steady state and the effect of dilution decrease accumulation in phytoplankton. This decrease results in a lower phytoplankton body burden and a decreased exposure in higher organisms. As a result, an equilibrium-based model will tend to overestimate concentrations in phytoplankton, and this overestimate will be evident throughout the food web. 

Bioaccumulation factor is defined as the ratio of the chemical concentration in the fish and in the water at steady-state equilibrium. 

The U.S. Environmental Protection Agency (Lee et al., 1993) designed Bedded Sediment Bioaccumulation tests to estimate the bioaccumulation of sediment-associated contaminants by benthic organisms. The bioaccumulation potential of a chemical in sediment usually is expressed as a Biota-Sediment Accumulation Factor (BSAF). The simplest test determines the BSAF as the ratio of the steady-state chemical concentration in the test organism and the chemical concentration in the sediment. 

Landrum et al. (1992) developed a kinetic bioaccumulation model for PAHs in the amphipod Diporeia, employing first-order kinetic rate constants for uptake of dissolved chemical from the overlying water, uptake by ingestion of sediment, and elimination of chemical via the gills and feces. In this model, diet is restricted to sediment, and chemical metabolism is considered negligable. The model and its parameters, as Table 9.3 summarizes, treat steady-state and time-variable conditions. Empirically derived regression equations (Landrum and Poore, 1988 and Landrum, 1989) are used to estimate the uptake and elimination rate constants. A field study in Lake Michigan revealed substantial differences between predicted and observed concentrations of PAHs in the amphipod Diporeia. Until more robust kinetic rate constant data are available for a variety of benthic invertebrates and chemicals, this model is unlikely to provide accurate estimates of chemical concentrations in benthic invertebrates under field conditions. 

These models use thermodynamic terms to describe the behavior of chemicals in the environment. Fugacity is a thermodynamic quantity that can be viewed as the "escaping tendency of a chemical substance from a phase" (Mackay and Paterson, 1982 Mackay, 1991). At steady-state, Campfens and Mackay (1997) described bioaccumulation in benthic invertebrates as follows ... 

Thomann et al. (1992) developed a steady-state food web bioaccumulation model that combines kinetic and bioenergetic parameters to quantify chemical uptake and elimination by zooplankton, benthic invertebrates and fish. First-order kinetic rate constants quantify uptake of freely-dissolved chemical from interstitial water and overlying water and total chemical elimination from gills and feces. Various physiological and bioenergetic parameters quantify chemical uptake from diet and growth dilution. 

In the sections below, four different cases for converting biomonitoring data to exposure dose using pharmacokinetic modeling are considered lipid-soluble, bioaccumulative chemicals at steady state lipid-soluble, bioaccumulative chemicals not at steady state shorter-half-life chemicals at... 

Human Modeling Case 1 Lipid-Soluble, Bioaccumulative Chemicals at Steady State... 

In Table 4, the equation to predict the bioaccumulation factor (BAFl) of relatively persistent chemicals in human (fat) is also presented [191, 192]. This equation is only valid for chemicals which are not or only minimal metabolized in human. It is also important to note that for super-hydrophobic chemicals, such as octachlorodibenzo-p-dioxin (OCDD) and Mirex, no steady-state BAF value is reached during the whole life. 

Bioaccumulation factor in human fat ( BAFl = ) diet / 115-740 mean 430 no steady-state reached during the whole life 4100 (after 80 years) 83,000-165,000"... 

A9.5.2.3.9.4 If the BCF in terms of radiolabelled residues is documented to be > 1000, identification and quantification of degradation products, representing > 10% of total residues in fish tissues at steady-state, are for e.g. pesticides strongly reeommended in the OECD guideline No. 305 (1996). If no identifieation and quantifieation of metabolites are available, the assessment of bioeoneentration should be based on the measured radiolabelled BCF value. If, for highly bioaccumulative substanees (BCF > 500), only BCFs based on the parent compound and on radiolabelled measurements are available, the latter should thus be used in relation to classification. 

In addition to these field studies, the bioaccumulation of NPs was investigated in controlled exposure assays. In littoral enclosures, bluegill sunfish (L. macrochirus) exposed to NPs exhibited a bioconcentration factor42 of 87. The bioconcentration factors reported from laboratory experiments vary from 75 to 741, according to the species and the exposure conditions60,61,63,78. Radiolabeled NP was used to evaluate the NP concentration in stickleback (Gasterosteus aculateus) at steady state in... 

It is often assumed that, particularly in the aqueous environment, there is a steady-state situation, i. e. that the concentrations of pollutants in the water and the suspended solids is in equilibrium. Hendriks [40] verified this assumption. He found that the ratios of concentrations in different organisms and those in suspended solids of a series of organic compounds were not significantly different fi-om the calculated ratios that were based on existing bioaccumulation and sorption relationships. The organisms that were studied were chironomidae, mollusca, Crustacea and a number of fish species. 

Many studies have focussed on the uptake and bioaccumulation from water, and have resulted in models. Most of these existing models for the steady-state BCF are valid only for non-ionised organic chemicals and less for ionised chemicals or organometallic compounds. For practical purposes, a kind of worst-case BCF can be estimated for non-ionised organic chemicals based on the published BCF-Kq correlations. 

The term bioaccumulation factor (BAF) is a coefficient used to quantify the net body burden of chemicals (given uptake, distribution, metabolism, and excretion rates) and is expressed as the ratio of the concentration of a compound in the organism (or normalized to the lipid weight of the organism) to the concentration in its food or, for benthic or soil invertebrates, in the sediment or soil [9] (also see Chapter 6). Although by definition, BAF is measured at steady state, due to the complex nature of multiplepathway exposure, steady-state measures of BAF are often difficult to obtain. 

SCCPs are persistent and bioaccumulative, and thus concentrations in the environment and biota are expected to increase with continued release to the environment. Standard risk assessment methods comparing effect levels to environmental concentrations may underestimate the risk of persistent and bioaccumulative substances, such as SCCPs. Persistent substances can take decades to reach a maximum steady state concentration in the environment, resulting in an underestimation of the potential exposure to these compounds if steady-state has not been achieved, and releases into the environment continue. Similarly, it can take a long time for persistent and bioaccumulative substances to reach a maximum steady-state concentration within an organism this is supported by the observations of Sochova et al., [62] who noted an increase in toxicity of SCCPs for longer exposure duration with nematodes. The durations of standard toxicity tests may be insufficient to achieve the maximum tissue concentration, resulting in an underestimation of the effect threshold. 

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