Distribution of Toxic Substances in Rivers

A statistical technique was used to determine if hydrophobic organic substances have vertically a uniform distribution in a river. At a river polluted with chlorinated organic pesticides, several water samples were collected at different depths to perform an analysis of variance on a nested design. To check if the data had to be normalized for the analysis of variance and hypothesis testing, the probability distribution of a grab sample was studied. The lognormal distribution gave in most cases a better description of the grab sample distribution than the normal distribution. Water and bottom sediment samples were taken at several points of the polluted river reach. Although statistically no attenuation in the pesticide concentration over distance was observed, upper limits for a first order decay at which decay would have been observed are reported, as well as the observed partition coefficients. The pesticides appeared to be fractioned in three categories; adsorbed on suspended sediments, adsorbed on fine sediments, and in solution.