ABSTRACTA natural gradient tracer test was conducted in an unconfined sandy aquifer at Borden, Ontario. N‐butyric acid, phenol, p‐chlorophenol, and dimethyl phthalate (DMP) were used as organic tracers at concentrations ranging from 70 to 100 ppm and chloride was used as a nonreactive tracer at a concentration of 243 ppm. Approximately 2,100 1 were injected below the water table and were monitored with a three‐dimensional network of bundle‐type, multilevel piezometers over a period of 60 days and once again after 204 days.The plume gradually split into two zones due to local heterogeneity. The centers of mass for the organic compounds migrated at essentially the same rate as chloride, but attenuated at different rates. Butyric acid and phenol attenuated the most rapidly. Chlorophenol attenuated only slightly faster than chloride and DMP attenuated at roughly the same rate as chloride. Sorption does not appear to be important in the system studied. Dispersion and biodegradation probably produced the observed attenuation of o