Carbon-normalized sediment concentration

Sedoc (organic carbon-normalized sediment concentration) = [Sedi-ment]//oc... 

Sorption. Capture of neutral organics by non-living particulates depends on the organic carbon content of the solids (9). Equilibrium sorption of such "hydrophobic" compounds can be described by a carbon-normalized partition coefficient on both a whole-sediment basis and by particle size classes. The success of the whole-sediment approach derives from the fact that most natural sediment organic matter falls in the "silt" or "fine" particle size fractions. So long as dissolved concentrations do not exceed 0.01 mM, linear isotherms (partition coefficients) can be used. At higher concentrations, the sorptive capacity of the solid can be exceeded, and a nonlinear Freundlich or Langmuir isotherm must be invoked. 

Comparison of the relative sediment toxicity of different SPs can be difficult as there are a variety of different test methods and endpoints evaluated, in addition to other confounding factors relating to sediment quality. Amweg et al. [28] determined the toxicity of six SPs to //. azteca in 10-day studies at 23 °C in natural sediments containing 1-6% OC. Toxicity data were reported as bulk sediment concentrations and normalized to the organic carbon content (Table 5). The results indicated that normalization removed some, but not all, of the variability between sediments. Other factors such as sediment texture may also affect bioavailability and hence apparent toxicity in sediment studies. 

Either for soil or for sediment, the BSAF is usually expressed as the ratio of the lipid-normalized concentration in the organism and the organic carbon normalized concentration in the sediment or soil ... 

The Level I calculation suggests that if 100,000 kg (100 tonnes) of benzene are introduced into the 100,000 km2 environment, 99% will partition into air at a concentration of 9.9 x 10-7 g/m3 or about 1 pg/rn3. The water will contain nearly 1% at a low concentration of 4 pg/rn3 or equivalently 4 ng/L. Soils would contain 5 x 10-6 pg/g and sediments about 9.7 x 10 6 pg/g. These values would normally be undetectable as a result of the very low tendency of benzene to sorb to organic matter in these media. The fugacity is calculated to be 3.14 x 10-5 Pa. The dimensionless soil-water and sediment-water partition coefficients or ratios of Z values are 2.6 and 5.3 as a result of a Koc of about 55 and a few percent organic carbon in these media. There is little evidence of bioconcentration with a very low fish concentration of 3.0 x FT5 pg/g. The pie chart in Figure 1.7.6 clearly shows that air is the primary medium of accumulation. 

The other major reactant in Equation 1 is sulfate (SO42 ). Sulfate concentrations are highly variable in lake waters, from 3 x 10 5 mol/L in soft-water lakes in crystalline-rock drainage basins to 1.6 mol/L in hypersaline lakes (2.). In productive, freshwater lakes, sulfate reduction typically goes nearly to completion (5.). As sulfate concentrations increase, amounts of organic matter eventually become insufficient for complete sulfate reduction to occur. This is the case in "normal" marine sediment where a linear relation between total reduced sulfur and organic-carbon concentrations is observed. Sea-water sulfate concentration is 0.028 mol/L and the ratio of total reduced sulfur to organic-carbon concentrations (often referred to as S/C) in marine sediment is 0.33 ( ). ... 

In conclusion, such a model is convenient to get an idea of the calcium oxalate concentration, CO2 pressure and conditions for potential precipitation of secondary calcium carbonate through oxalotrophic bacterial activity. It demonstrates that as long as calcium is available and oxalotrophic bacteria are present, transformation of oxalate into carbonate can occur under normal conditions found in soils and surficial sediments. Therefore, an oxalate-carbonate cycle, or at least pathway, must exist at the surface of continents (Verrecchia Dumont, 1996), explaining the absence of calcium oxalate accumulation in soils and the fossil record. 

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