The vulnerability of a shallow aquifer in south-central Kansas to contamination by atrazine (2-chloro-4-ethylamino-6-isopropylamines-triazine) was assessed by analyzing unsaturated zone soil and sediment samples from about 60 dryland and irrigated sites using an ultrasensitive immunoassay (detection level of 0.02 µg/kg) with verification by gas chromatography/mass spectrometry (GC/MS). Samples were collected at depths of 0 to 1.2 m (i.e., the root zone), 1.2 to 1.8 m, and 1.8 to 3.0 m during two time periods-prior to planting and after harvest of crops. About 75% of the samples contained detectable concentrations of parent atrazine. At the shallow sampling depth, atrazine concentrations ranged from 0.5 to approximately 12 µg/kg. Atrazine concentrations at the intermediate (1.2-1.8 m) depth generally were <1.0 µg/kg, with most of the concentrations <0.10 µg/kg, which suggests substantial degradation of parent atrazine in the root zone. Likewise, atrazine concentrations front the deepest (1.8-3.0 m) depth ranged from <0.02 to 0.33 µg/kg. The metabolite deethylatrazine (2-amino-4-chloro-6- isopropylamine-s-triazine) was detected by GC/MS only in 2 of 60 samples with concentrations of 1.4 and 1.5 µg/kg. The reconnaissance survey shows that, in spite of atrazine use ranging from 1 to 5 or more years, there does not appear to he a significant buildup of parent compound below the root zone. Therefore, the unsaturated zone does not appear to be a major storage compartment of atrazine contamination for the underlying shallow aquifer.