The objective of this study was to investigate the occurrence of herbicide active and inactive ingredients (primarily volatile organic compounds) at four selected sites in Iowa representing drain tiles, observation wells, or lysimeters. Water samples were collected monthly and bi-monthly before and after herbicide applications in 1991, respectively. They were analyzed for seven herbicides and 32 volatile organic compounds using methods recommended by the U.S. Environmental Protection Agency. Commercially available herbicide formulations also were obtained and analyzed for volatile organic compounds.

Herbicides were detected in 50% of water samples, ranging from 78% of water samples from the Ames site to 25% from the Walnut Creek site. Among herbicides detected, listed in decreasing order of frequency, were atrazine > alachlor > cyanazine > metolachlor > metribuzin. Volatile organic compounds were detected in 11% of water samples. Among the compounds detected, listed in decreasing order of frequency, were xylene > toluene > acetone. One sample contained a detectable amount of aliphatic compound(s), with the empirical formula of C₈H₁₈. Results from the Deer Creek site showed that herbicides were detected primarily in the top layer (1.2 m), whereas xylene and other alkylbenzenes were detected at 2.1 m or deeper. Apparently, physico-chemical and other factors are separating herbicides and volatile organic compounds in the shallow unsaturated zone.