Freely dissolved concentration

Uptake and elimination. Illustrative example of the uptake and elimination of a chemical substance in a bioconcentration test consisting of an uptake period, where the organism is exposed to a constant concentration of a chemical substance in the water phase, and an elimination period, where the organism is exposed to clean water. In this example, bioconcentration is described by a first order organism-water two compartment model. The concentration in the organism is CB, the freely dissolved concentration in the water is CWD. 

It is important to recognize that the concentration in the aqueous phase must be that in free solution (i.e., not including that sorbed on to organic matter in the water or on to the surface of the test vessel). In general, for chemicals that are not highly hydrophobic (log Kow < 5), the total aqueous concentration can be taken as equal to the freely dissolved concentration. However, for very hydrophobic chemicals this may not be the case (Voutsas et al., 2002). 

Ter Laak TL, Agbo SO, Barendregt A, Hermens JLM. 2006. Freely dissolved concentrations of PAHs in soil pore water measurements via solid-phase extraction and consequences for soil tests. Environ Sci Technol 40 1307-1313. 

The finding from WQC research that was potentially most difficult to address in sediment was that the freely-dissolved concentration of a toxicant was the best predictor of toxicity, not its total water concentration. This resulted in water criteria including adjustment factors related to water hardness and pH 11). 

To test the applicability of this theoretical framework in sediment, and to verify predicted exposures, bioassay(/5) and pore water sampling procedures (16J7) were developed and analytical methods were adapted to quantify the freely-dissolved concentrations (13J8). In laboratory exposures, sediment organic carbon in combination with compound were found to be good predictors of freely-dissolved chemical and organism response (19,20). 

Figure 3). Surficial sediment (0-2 cm) was collected at ten stations and to -0.5 m at one station. Aliquots of IW separated from sediment (16) were processed directly (total concentrations) or passed through untreated C-18 cartridges (DOM-bound concentrations). The freely-dissolved concentrations were determined as the difference between the total and bound concentrations (16J 8). De-watered sediment was sonicated in acetonitrile. Extracts were cleaned on treated C-18 columns (36), and quantified by mass spectrometer in the single ion monitoring mode Twenty-three PAHs and 22 PCB congeners were quantified. The PAHs consisted of 18 unsubstituted PAHs (see Table I) and 5 alkylated PAHs (1 and 2 methyl-, 2,6 dimethyl- and 2,3,5 trimethyl-naphthalene, and 1 methyl-phenanthrene). Prior to extraction all samples were amended with 6 deuterated PAH and 4 PCB compounds in a 10 1 ratio. 

II. Conversion of the sediment concentration-based assessment ( ESGTUfcv,23) to Total PAH by applying the 4.14 factor would designate seven of the ten stations as not acceptable the for protection of benthic organisms. None of the five assessments based on measured, freely-dissolved concentrations would fit this category. 

If only sediment concentrations were used in this assessment, the naval shipyard area of Sinclair Inlet could be considered a Site of Concern (57) as the proposed national ESG for PAH mixtures was exceeded at a majority of sites among the piers of that facility. However, when the assessments were made on the basis of IW determinations of freely-dissolved concentrations, none among the pier sediment exceeded the ESG. The magnitude of J ESGTUfcv.tot values would not have been sufficient to elicit significant mortality in short-term sediment bioassays which were not done. The consequences of actual values in the 1-3 range might... 

The large differences between conqiuted and measured freely-dissolved concentrations of PAHs suggest that the assunq)tion of equilibrium was violated for most of these compounds, but not PCBs. Given the greater uncertainties of s used to compute Q for PCBs, a ratio of 0.5 is indistinguishable from 1.0, whereas the PAH ratios are significantly greater. 

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