The release of colloids to groundwater was investigated in situ in an iron-oxyhydroxide-rich, sandy aquifer. Groundwater amended with various solutes was injected into and immediately withdrawn from the shallow aquifer. Turbidity and colloid composition were monitored in the retrieved injectate. The response of the aquifer material to the amendments generally mimicked that observed in an earlier study using packed columns containing the sediment, demonstrating the viability of the single-well method for testing colloid mobilization in situ. The decline of turbidity in the retrieved injectates with increasing withdrawal volume was analyzed to determine a reaction order n, describing the redeposition of mobilized colloids to the immobile matrix. Differences in the reaction order for the amendments tested presumably indicated the effectiveness of these amendments to generate repulsive colloid-immobile matrix interactions. ¿ 1999 American Geophysical Union