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A second-order closure model describing the diffusion of a passive scalar from a small source is presented. The model improves upon the earlier work of Lewellen & Teske (1976) by ensuring the early stage of the release, the so-called meander phase, is accurately described. In addition to the mean concentration and scalar fluxes, a model equation for the evolution of the scalar variance is proposed. The latter introduces a new lengthscale which represents the scale of the concentration fluctuations. The model predictions are compared with the recent experimental data of Fackrell & Robins (1982a, b).