The accumulation of organic residues in fish is the result of competing rates of uptake and elimination, which can be modeled by pharmacokinetic techniques. Although first-order kinetics are usually assumed, they are rarely verified. Models with biphasic, second-order or Michaelis-Menten kinetics may prove to be better choices, depending on exposure level and mode of elimination. The well-known correlation between bimncentration factor and partition coefficient (P) derives from separate correlations for the uptake and elimination rate constants with P.While such correlations appear to be linear in the range of log P = 2-5 for organics that are not metabolized, they sometimes fail at higher values of log P. A drug transport model is proposed to account for the "non-ideal" bimncentration of a variety of organics in fish. According to this model, uptake is a nonlinear function of partition coefficient, water solubility and membrane permeability.

The aqueous solubility of several low mlecular weight linear, cyclic, and branched permethylsiloxanes was determined at room temperature. A method for preparing molecularly dispersed, colloid‐free saturated aqueous solutions is described. Solubilities, decreasing with increased molecular weights, ranged downward from one part per million to mere parts per trillion. The cyclic oligomers were slightly more soluble than their linear analogues. While the effect of branching was mixed, polar groups such as phenyl and hydroxyl moities sharply increased aqueous solubility. Semilog plots of the solubility/molecular weight yielded linear regressions, the extrapolation of which indicate the absence of any environmentally relevant water solubility (< 1 ppt) for the conventional higher molecular weight polydimethylsiloxanes of commerce.

The dynamics of DDT in lentic environments were studied in a farm pond and four artificial pools. A hypothesis that biological magnification of pesticides was dependent on passage of the residues through a food chain was rejected and a hypothesis that accumulation depends on adsorption and solubility differences was proposed. The mechanism proposed is based on the principle that compounds are exchanged between water and fats. Exchange in fish passes through two stages, from water to blood and from blood to fats, permitting a high degree of magnification in fish. The proposed mechanism accounts for the reported observations that pesticides are excreted by fish, that body load of pesticides increases as the fat content of fish increases, that pesticide magnification by fish is inverse to the water solubility of the compounds and that pesticides persist longer in oligotrophic than eutrophic lentic ecosystems.

The undimensioned Henry's law constant (H c) and volatilization rate constant ( ) of the silicone fluid octamethylcyclotetrasiloxane (OMCTS) were determined experimentally. The grand mean H c for five experiments conducted under dynamic conditions with initial concentrations ranging from 4 to 50 μg/L (20 °C; 48-h equilibration period) was 3.4 ± 1.37. An H c of >17 was observed under kinetically limited, static conditions where equilibrium was not reached in 96 h. H c increased about 10-fold when humic acids were added to OMCTS test solutions, suggesting typical environmental cosolutes will increase the Henry's law constant for OMCTS. The measured volatilization/re-aeration ratio ( ) for OMCTS was 0.57, a value similar to that measured for trichloroethylene (0.57) and benzene (0.56), indicating that OMCTS will readily volatilize from water.

The toxicity of octamethylcyclotetrasiloxane (OMCTS) to representative freshwater and marine fish and invertebrates was investigated. Testing procedures followed Toxic Substance Control Act (TSCA) guidelines with chemical‐specific adaptations as presented in the OMCTS Testing Consent Order (Docket OPTS‐42071A). The physicochemical properties and behavior of OMCTS (i.e., high volatility, low water solubility) precluded the use of conventional experimental practices and exposure systems. Procedures and systems developed during these investigations provided a mechanism that produced exposure levels equal to the maximum achievable (i.e., “functional”) solubility of OMCTS in natural dilution waters. The toxicant delivery systems and exposure chambers were designed to minimize volatilization by elimination of the air/water interface. The flow‐through systems maintained consistent exposure concentrations and adhered to U.S. Environmental Protection Agency (EPA) Guideline test performance criteria. The “functional” water solubility of OMCTS in freshwater and seawater ranged from 14 to 30 μg/L and from 6.0 to 9.0 μg/L, respectively. Functional water solubility appeared to vary slightly with test conditions and dilution water characteristics. Continuous exposures of 2 to 93 d were conducted during these investigations with daphnids (Daphnia magna), rainbow trout (Oncorhynchus mykiss), mysids (Mysidopsis bahia), and sheepshead minnow (Cyprinodon variegatus). The rainbow trout was determined to be the most sensitive species to OMCTS (14‐d LC50 = 10 μg/L). At levels equal to the functional water solubility, OMCTS was not acutely toxic to D. magna, mysids, or sheepshead minnow. The survival of D. magna was reduced by 16%, relative to the control organisms, after 21‐d exposures to 15 μg/L OMCTS; exposure to 7.9 μg/L OMCTS or less had no effect on daphnid survival or reproduction. No toxicity was observed at the highest concentration tested in a 93‐d exposure of rainbow trout early life stages. The no‐observed‐effect concentration for this study was 4.4 μg/L, the same as determined in a 14‐d extended acute study.