Biota equilibrium partition

Sediment/biota equilibrium partitioning. A very important aspect of the assessment of the environmental fate of chemicals is the prediction of... 

The model applies equilibrium partitioning to estimate chemical concentrations in phytoplankton, macrophytes, zooplankton, and benthic invertebrates. Chemical concentrations in sediment and water, along with environmental and trophodynamic information, are used to quantify chemical concentrations in all aquatic biota. This model can be applied to many aquatic food webs and relies on a relatively small set of input parameters which are readily accessible. 

There is no mechanistic equilibrium-partitioning model for toxic metals available for the soil and sediment compartments. However, the free metal ion concentration in pore water that is considered relevant for uptake in biota (water exposure route) may experimentally or empirically be related to the total metal content of the soil, according to equation 2.1 (above). 

We will address the distribution of organic compounds in the environment by looking at equilibrium partitioning of organic compounds between environmental phases, which include air, water, soil, and biota. Taking these phases pairwise, we can define the various physical and chemical properties that control the partition coefficients between these phases ... 

For sediment-dwelling organisms, one important factor that determines the degree of exposure to xenobiotics in the sediment phase is the partitioning from the true sediment phase into interstitial water from which the xeno-biotic may then be accumulated by biota. Exposure of sediment biota to xenobiotics is, however, a complex process, since uptake may proceed either via particulate material or via interstitial water, or by both routes. In the equilibrium partition model the concentration of a xenobiotic in the interstitial water (Ciw) is given by the following relation ... 

Distribution of organic chemicals among environmental compartments can be defined in terms of simple equilibrium expressions. Partition coefficients between water and air, water and soil, and water and biota can be combined to construct model environments which can provide a framework for preliminary evaluation of expected environmental behavior. This approach is particularly useful when little data is available since partition coefficients can be estimated with reasonable accuracy from correlations between properties. In addition to identifying those environmental compartments in which a chemical is likely to reside, which can aid in directing future research, these types of models can provide a base for more elaborate kinetic models. 

Partition coefficients are used to describe the distribution of nonpolar organic compounds between water and organisms. It can be viewed as a partitioning process between the aqueous phase and the bulk organic matter present in biota (Schwarzenbach et al. 1993). The premise behind the use of equilibrium models is that accumulation of compounds is dominated by their relative solubility in water and the solid phases, respectively. Equilibrium models, therefore, rely on the following assumptions (Landrum et al. 1996) ... 

A commonly used partition coefficient is the 1-octanol-water partition coefficient, K(k which is the ratio of a chemical s concentration in 1-octanol to its concentration in water at equilibrium in a closed system composed of octanol and water (Bacci 1994). The 1-octanol is chosen to mimic biological lipids. For organic chemicals, log Kow ranges from -3 to 7. When log Kow exceeds 3, substances are considered hydrophobic (Elzerman and Coates 1987). The Kow partition coefficient has been extensively used as an estimate of the BCF. Under the assumptions of Landrum et al. (1996), together with an estimated lipid content of about 5% in biota and an assumed equal affinity of the compound for both body fat and octanol, the BCF can be calculated by the use of BCF = 0.048 Kow (Paasivirta 1991). This equation can vary depending on the species used. The relationship between log K()W and BCF can be viewed by scatterplot analysis (Figure 2.4). These plots show a clear relationship for... 

A given pollutant may penetrate in soil down to a specific depth, and therefore transport calculations need individual depth data. Owing to mass transport restrictions, residence times of many pollutants in soils are (unfortunately) much longer than those in the gas or liquid phases. In addition, partitioning effects in soils can be dramatic a case in point is the concentration effect that occurs with uranium, which sometimes reaches levels up to 104 times higher than its concentration in water with which the soil is in equilibrium. Biota plays a key role in the transport and mobilization of pollutants from soil, because for example, many of them bioaccumulate in vegetation and cattle (see Section 9.2). 

We report here on the distributions of several chlorobiphenyls In samples of water, sediment and biota of the Acushnet River Estuary - New Bedford Harbor, Buzzards Bay, Massachusetts, U.S.A. Our general objective Is to gain Information of generic utility In addition to providing specific data and Interpretations of assistance to remedial action at this Superfund site. Our specific objectives In this paper are to 1) document the composition of Individual chlorobiphenyls In biota normally harvested by commercial and recreational fishermen and discuss factors which could lead to the observed distributions and potential Implications for public health standards for PCBs In fish and 11) to Investigate, In a preliminary manner, the adherence of bloconcentratlon of PCBs to predictions based on equilibrium assumptions and octanol/water (Kg ) partition coefficients (, 22). 

The hypothesis has been advanced that changes in relative concentrations of lipid type compounds, when comparing aquatic biota and their habitat, can be explained in large part by an estimate of their tendency to partition into tissues which has been related to octanol/water partition coefficients - K s (, 22). Table Vll presents tabulated data for and water to biota bloaccumu-latlon concentration factors calculated from data in Tables III and IV. Representative data from Table Vll are plotted in Figure 6 in the manner of Mackay (21) and Chlou (22), who have reviewed data on bloaccumulatlon of neutral hydrophobic compounds in aquatic biota. The solid line Is the expected distribution of data based on Chlou s review (22) of predictability for equilibrium situations. Our data is different in an absolute sense than the data used by Mackay and Chlou, because they used concentrations in biota... 

The concept of bioconcentration is derived from that of distribution coefficients in physical chemistry in these, the equilibrium concentrations of a compound distributed between two phases are measured, for example, between water and a water-immiscible solvent such as hexane. If partitioning were a passive reaction, direct physicochemical measurements of the partition between an aquatic phase and a suitable model for the biological membrane would be possible. It would therefore be attractive to measure distribution coefficients in a chemically defined system and to seek a correlation between the values found and those obtained by direct measurements in biota. 

Indeed, OCPs, once released into the environment, are distributed into various environmental compartments (e.g., water, soil, and biota) as a result of complex physical, chemical, and biological processes. In order to perform appropriate exposure and risk assessment analyses, multimedia models of pollutant partitioning in the environment have been developed. Properties which are at the base of such a partitioning are water solubility (WS), octanol-water partition coefficient (Ko ), soil adsorption (K ), and bioconcentration factors (BCFs) in aquatic organisms, following these four equilibriums ... 

The suspended sediment/water partition coefficients have been measured for 19 chlorinated organics in 25 samples from the St. Clair, Detroit and Niagara Rivers. An excellent linear correlation (r = 0.87) between the organic-carbon corrected partition coefficient (Kqq) and the octanol/water partition coefficient (Kq, ) was found (log = 0.76 log + 1.66). Using this equation plus another equation developed in this paper it is shown that the percentage of chemical in the dissolved and particulate phases in the study rivers could be estimated from a chemical s to within a factor of two. The paper also discusses the time required for equilibrium to be achieved between the dissolved and particulate phases, and the potential importance of biota such as algae in the partitioning process. 

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