Bioavailability of contaminant

The Committee on Bioavailability of Contaminants in Soils and Sediments of National Research Council (NRC) of the National Academies USA chose to define the bioavailability process instead of bioavailability to avoid the confounding use of the term bioavailability (National Research Council 2003). According to the NRC report, bioavailability processes are the individual physical, chemical, and biological interactions... 

Mihelcic JR, Lueking DR, Mitzell RJ, Stapleton JM (1993) Bioavailability of sorbed- and separate-phase chemicals. Biodegradation 4 141-153 National Research Council (2003) Bioavailability of contaminants in soils and sediments processes, tools, and applications. The National Academies Press, Washington DC, USA... 

Eggleton J, Thomas KV (2004) A review of factors affecting the release and bioavailability of contaminants during sediment disturbance events. Environ Int 30(7) 973-980... 

The bioavailability of contaminants to wildlife and humans is also an area of critical importance, where contaminants can be taken up in pore water and by dermal contact, particle ingestion, or particle inhalation. The dynamics of sorption/desorption are not currently incorporated into exposure and risk assessment models for organic compounds, where availability, in most cases, is assumed to be 100% [224]. Recently, the following have been demonstrated and reported ... 

Ingersoll, C., Besser, J. and Dwyer, J. (1997) Development and application of methods for assessing the bioavailability of contaminants associated with sediments I. Toxicity and the sediment quality triad, Proceedings of the U S. Geological Survey (USGS) Sediment Workshop, U.S. Geological Survey, Columbia, Missouri (2003-12-22) ... 

Several researchers have confirmed that biodegradation can be limited by the slow desorption of organic compounds [22-25]. Though significant research has been conducted to study the sorption and desorption kinetics of organic compounds and their bioavailability, few studies have focused on the bioavailability of contaminants in soils containing only the desorption resistant fraction and how the degradation rates compare to those for freshly contaminated soils. 

ISO] International Organization for Standardization. 2008. Soil quality requirements and guidance for the selection and application of methods for the assessment of bioavailability of contaminants in soil and soil materials. Geneva ISO 17402 2008. 

Traditionally, environmental assessment has been directed mainly towards chemical characterization, which has resulted in the establishment of guidelines for regulatory purposes. Such parameters lack the dynamic toxicity information needed to determine the bioavailability of contaminants to the biota residing in an ecosystem (Munawar et al., 1989). 

Bioavailability of contaminants in waters and sediments also changes in temporal and spatial scales. For example, because sediments are at best mixtures, rather than solutions like most water-borne pollutants, the conditions that influence the bioavailability of a contaminant in two similar sites may change from day to day as well as be considerably different even though they are in close proximity to each other. In general, the characteristics of the water above the sediment (overlaying water) and the interstitial water (water in-between the particles of the sediment, often referred to as pore water) most influence the bioavailability of the contaminants in the sediment. It is believed that pore water has the greatest influence on the bioavailabihty of contaminants in sediments (Suter et al. 2000). One way to think of contaminants in water is that they are today s pollution issue, whereas contaminants in sediments are both today s and yesteryear s issues. 

Several tools have been investigated for assessing the bioavailability of contaminants in addition to the use of toxicity testing. Typically each of these methods is developed for specific purposes and may not be applicable directly outside of the original purpose of their development. It is also important to keep in mind that the total amount of a compound that can be extracted from sediment using analytical techniques does not correlate to the exposure or bioavailability of a toxicant. 

In addition to direct membrane analogs, many researchers have used Cg, Cis, and XAD resins to simulate the bioavailability of organic compounds. Lake et al. (1996) used Cig resins as a surrogate for benthic organism bioaccumulation of hydrophobic compounds from sediments. Gustafson and Dickhut (1997) and Ankley et al. (1991) also utilized resins to sorb compounds for the assessment of toxicity (and bioavailability) of contaminants in sediments. 

Characterization of the pore water of sediment samples is a simple way of assessing the potential bioavailability of contaminants in sediments and is one of the principal matrices used in the performance of sediment TIEs (USEPA, 1991a). Ankley et al. (1991) found that pore water is a better predictor of bioavailability of contaminants than sediment elutriates. Pore water typically has the toxicant freely dissolved, which is the most readily bioavailable fraction. The... 

Many toxicants are sequestered in sediments (such as sediments contaminated by wastewater) or they are naturally the source of the toxicant (such as for metal toxicity). In both of these examples the sediment would be termed the toxicant reservoir. Thus, an understanding of the processes affecting the bioavailability of contaminants in sediments is important for a comprehensive understanding of bioavailability in the aquatic environment. 

Chemical measurements have developed over the past decade. At present, sophisticated methods are available that can characterise the bioavailability of contaminants (Comelissen et ah, 2001 Burgess et al., 2003). These techniques may prove to be powerful in constructing lines of evidence between contamination and direct effects on organisms living in the sediment. 

Prediction of bioavailable heavy metal concentration appears to be more complex, and appropriate normalizing factors still have to be evaluated. Until predictive methods for determining bioavailability of contaminants in sediments can be validated, empirical measurements of body burden and effects as determined by the toxicity test and field monitoring provide the most direct approach for evaluating the impact of contaminated sediment in the aquatic environment (Fava et al., 1987). 

Sediment contact tests are biological methods for the determination of toxic effects induced by whole sediments in direct contact with test organisms, taking into account all possible pathways of contaminant uptake (particle contact, food, pore water). Sediment contact tests are highly relevant in order for an ecosystem approach to consider the actual bioavailability of contaminants sufficiently. 

In the following chapter, an outline of the influences of direct current (DC) electric fields on the bioavailability of contaminants and their bioremediation in soil will be developed based on the electrobioremediation tetrahedron (Fig. 18.1). 

The updated guidance document (EPA, 2001) includes refinements to the above equation to accoimt for the potential bioavailability of contaminants in the stratum comeum when exposure has ended and variable exposure times. Furthermore, the newer document discusses, in depth, the use of mathematical predictions of the permeability coeffident in dermal risk assessment. It is important to appreciate that the permeability coefficient should be determined experimentally using, ideally, a donor phase that mimics as closely as possible the existing environmental conditions. The use of permeability coeffidents predicted from theoretically derived equations adds a further imcertainty to the overall risk calculation. Although it has been suggested that the dermal permeability estimates are the most uncertain of the parameters in the dermal dose computation (EPA, 1992), it could be argued, given the refinement of in vitro techniques and the correlation between in vitro and in vivo measurements of human skin (Franz, 1978 Wester et al., 1992 van de Sandt et al., 2000 Cnubben et al., 2002 Zobrist et al., 2003 Colombo et al., 2003), that these measurements are the least assumptive and the most accurate of all the parameters used. 

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