A case study of longitudinal dispersion in small lowland rivers

Contaminant transport in natural streams is considered on the basis of tracer studies in Moldovian small lowland rivers. Theories describing the spread of conservative and passive pollutants in the context of a longitudinal dispersion model, as well as attempts to estimate the discrepancy between this model and natural processes, are discussed. It is shown that the model agrees with experimental data, with an accuracy ranging from 15 to 20%, at least in the upper [C(t) > 0.5Cmax] concentration distributions. The process behaves in a quasi‐Fickian manner only at distances greater than 80 to 100 times the river width. Primarily, the nonuniformity of the average velocities over cross sections affects the process. An estimate of the Lagrangian spatial scales was proposed, and it was shown that those scales are as large as two to four times the length of alternate bars.