The first part of this paper will deal with the relationships between physicochemical properties and toxicity to fish, for two groups of chemicals. The results of this part will be used in discussing the toxicity of mixtures of aquatic pollutants to fish. The toxicity studies used in this paper were LCSO determinations with 2-to 3-month-old guppies (Poecilia reticulate), which were exposed to the toxicants in a static system, with daily renewal of the water, without aeration. Most experiments lasted 14 days, a number were restricted to 7 days (Konemann, 1979).
QUANTITATIVE STRUCTURE-ACTIVITY RELATIONSHIPSThe relationship between physicochemical properties and toxicity has been studied with the Hansch approach (Hansch, 1971). He used an empirical equation with several variables to describe quantitative Structure-activity relationships (QSARs). His general equation is log l/C = k, log P,,,, + k,(log P,,Jz + k,pK,, + kdE, + . + k;,,where in this equation C is the concentration to produce the standard response (e.g., LC50); P,,,.t. the partition coefficient of a chemical for the distribution between fr-octanol and water: K,,, the acid dissociation constant; E,, a steric parameter: the coefficients k,, are obtained by fitting the equation to the experimental data. P,,Ct is the dominating parameter in QSAR research. In this work we have always used P,,,., values which are calculated after Rekker (19771, using his hydrophobic fragmental constants. In our opinion these values are better than experimental ones for calculated log P,,Ct values >4 (Konemann et al., 1979).The first LC5Os determined in this study were those of benzene and its chloro derivatives.The most simple QSAR gave a good description of their toxicity:log IiLCSO = 0.845 log P,,,, ~ 4.63, II = 12, r = 0.980, s = 0.133,where in this and the following QSARs II is the number of chemicals for which the equation has been calculated, r is the correlation coefficient, and s is the standard deviation, indicating the distribution around the line. Hexachlorobenzene has been left out of the calculation, because it did not produce any mortality, even at very high concentrations.This QSAR appeared to
