[en] A capillary injection system has been installed on Alcator C-Mod which allows the localized introduction of gaseous species at a variety of poloidal locations. An experimental program has been undertaken to observe impurity puffs with a CCD camera through appropriate optical filters. The comet-like shape of an ion line emission plume formed in the region of an injection graphically displays the direction of background deuterium flow as well as that of the poloidal impurity ion drift. Parallel and cross-field one-dimensional fluid models are used to characterize the plume shapes and to deduce the background plasma temperature, the parallel flow velocity and the poloidal drift velocity of the impurity ion. Model results are benchmarked against simulations of DIVIMP, a Monte Carlo code, and fast scanning probe measurements. The experiments present a novel diagnostic technique to elucidate local edge impurity transport. (orig.)